Technical Note Large-scale interface shear testing of sandbag dyke

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					                                                                                   Geosynthetics International, 2007, 14, No. 2




Technical Note

Large-scale interface shear testing of sandbag dyke
materials
T. Krahn1 , J. Blatz2 , M. Alfaro3 and R. J. Bathurst4
1
  Engineer, Built Alternatives Inc., P.O. Box 22, Anola, Manitoba, Canada, Telephone: +1 204 866 3521,
Telefax: +1 204 866 3287, E-mail: timkrahn@mts.net
2
  Associate Professor, Department of Civil Engineering, University of Manitoba, Winnipeg,
Manitoba R3T 5V6, Canada, Telephone: +1 204 474 9816, Telefax: +1 204 474 7513,
E-mail: blatzja@cc.umanitoba.ca
3
  Associate Professor, Department of Civil Engineering, University of Manitoba, Winnipeg,
Manitoba R3T 5V6, Canada, Telephone: +1 204 474 8155, Telefax: +1 204 474 7513,
E-mail: alfarom@cc.umanitoba.ca
4
  Professor and Research Director, GeoEngineering Centre at Queen’s-RMC, Civil Engineering
Department, Royal Military College of Canada, Kingston, Ontario K7K 7B4, Canada,
Telephone: +1 613 541 6000 ext 6479/6347/6391, Telefax: +1 613 545–8336,
E-mail: bathurst-r@rmc.ca

Received 23 October 2006, revised 12 January 2007, accepted 16 January 2007

ABSTRACT: This paper presents results of large-scale direct shear testing of six interfaces with
materials used to construct sandbag dykes for flood protection structures. Two additional test series
using intact sandbags were also conducted to examine the influence of sandbag-to-sandbag
interaction in typical stacked sandbag dyke construction and sandbag-to-sod (grass) contact
representing the interface condition at the dyke foundation. The interface test results are
summarised using Mohr–Coulomb shear strength parameters and interface shear stiffness
parameters. The data presented in this paper are necessary for stability analyses of instrumented
and monitored sandbag dyke structures that are currently under way as part of the development of
a new guidance document for the design of sandbag dykes for temporary flood protection works in
Canada.

KEYWORDS: Geosynthetics, Direct shear, Testing, Sandbags, Dykes, Flood protection

REFERENCE: Krahn, T., Blatz, J., Alfaro, M. & Bathurst, R. J. (2007). Large-scale interface shear
testing of sandbag dyke materials. Geosynthetics International, 14, No. 2, 119–126
[doi: 10.1680/gein.2007.14.2.119]




                                                                    temporary flood control is illustrated in Figure 1. The
1. INTRODUCTION                                                     stability of these structures can be related to six different
Temporary secondary dykes are an integral component of              interface types constructed with four different materials.
the flood control system for the City of Winnipeg, located          The materials include sandbags manufactured from woven
in the province of Manitoba, Canada. In 1997 a 90-year              slit film polypropylene (WSFPP) geotextile, polyethylene
flood of the Red River that flows through the city                  sheeting (PES) used as an internal water flow cut-off, a
seriously challenged the integrity of clay dykes con-               clean pit run sand used to fill the bags, and grass (sod) at
structed immediately prior to the floods and temporary              the base of the dyke. The base material is generally
sandbag dykes constructed by volunteers. This event                 variable, and during typical flood periods can be frozen
prompted a review of the design template used by the city           and include snow and ice. It can be argued that the
for temporary sandbag dykes and investigation of strate-            interface identified as WSFPP on sand is not a potential
gies to improve the stability of these systems, particularly        slip interface. However, during dyke construction sandbags
if anticipated larger floods occur in the future.                   may be damaged, and this interface condition could
   A cross-section of the standard sandbag dyke design for          develop.
1072-6349 # 2007 Thomas Telford Ltd                                                                                          119
120                                                                                                                   Krahn et al.
                     PES on WSFPP

                    PES on PES                   WSFPP on WSFPP
   1.20 to 4.25 m
   (typical)
                                                    WSFPP on sand




                          PES on sod    WSFPP on sod




Figure 1. City of Winnipeg sandbag dyke construction
template showing six interface surfaces (after City of
Winnipeg 1997) (Notes: PES polyethylene sheeting; WSFPP
  woven slit film polypropylene geotextile)



   A review of the literature showed that previous labora-             Figure 2. Large-scale (1.0 m 3 1.0 m 3 1.0 m) direct shear
                                                                       apparatus
tory investigations of sandbag performance are sparse.
Kalumba and Scheele (1999) conducted interface shear
tests on three different sands in contact with a single                than the vertical loads applicable to the sandbag structures
nonwoven needle-punched geotextile. They found that the                used as temporary flood protection works in Winnipeg.
interface shear strength was lower than the shear strength                The writers are currently engaged in a research pro-
of the soil tested alone under the same confining pressure.            gramme that has the objective to update the City of
   Specimen size has been cited as an important issue in               Winnipeg guidance document for rapid construction of
geosynthetic interface shear testing. Devices having mini-             sandbag dykes (Krahn et al. 2005). As a first step in this
mum dimensions of 300 mm 3 300 mm are recommended                      larger programme, a laboratory investigation was carried
for these tests to reduce boundary effects during shear                out to quantify the interface properties of the component
(Bove 1990). The ASTM D 5321 method of test for shear                  materials identified above. The test programme was
testing of soil/geosynthetic interfaces recommends                     carried out using a large-scale shear box apparatus (1 m 3
300 mm as the minimum horizontal shear box dimension                   1 m in plan area) (Figure 2). This apparatus allowed tests
for these tests, and this recommendation has been fol-                 to be carried out using full-size sandbag specimens and to
lowed by others (e.g. Criley and Saint John 1997).                     minimise specimen size effects when interface shear test-
   Lohani et al. (2006) and Aqil et al. (2006) investigated            ing was carried out with sheet materials, and with sheet
the load–deformation response of stacked soil bags sub-                materials and sod in contact. These parameters are being
jected to vertical compression and lateral shear under a               using to evaluate the local stability of sandbag dykes
range of normal loads. Aqil et al. commented on the very               during construction and loading. The results of this phase
low interface shear friction angle at the horizontal inter-            of the larger research programme will be reported in
face between vertically stacked bags due to the smooth                 future publications.
geotextile surfaces. Lohani et al. showed that the vertical
stiffness of stacked soil bags increased with increasing
strength and stiffness of the geotextile used to make the
soil bags. At large vertical deformations, however, the
effect of initial soil density of the infill soil was seen to
diminish. During lateral shear tests the shear strength of
                                                                       2. TEST MATERIALS
the soil infill was only partly mobilised owing to confine-            The combinations of materials used to form the interfaces
ment effects by the encapsulating geotextile. However, the             in this investigation are summarised in Table 1. A descrip-
normal loads applied in these tests were very much higher              tion of the individual materials follows below.


                     Table 1. Test programme showing interface shear surfaces investigated

                      Test materials      PES           WSFPP           Sand            Sod         Sandbags
                                                       geotextile

                      PES                  3              3                              3
                      WSFPP                3              3              3               3
                      geotextile
                      Sand                                3              3a
                      Sod                  3              3                                            3
                      Sandbags                                                           3             3

                     Notes: PES ¼ polyethylene sheeting; WSFPP ¼ woven slit film polypropylene.
                     a
                       Benchmark used for comparison purposes only

                                               Geosynthetics International, 2007, 14, No. 2
Large-scale interface shear testing of sandbag dyke materials                                                                                                                                                                                                                                                                                                                                                          121


2.1. Polyethylene sheeting (PES)




                                                                                                                                                                  Residual

                                                                                                                                                                                        7–8

                                                                                                                                                                                        6–8


                                                                                                                                                                                        6–7
                                                                                                                                                                                         9

                                                                                                                                                                                         4
                                                                                                                                                                                         2

                                                                                                                                                                                         5
                                                                                                                                                                                         3
                                                                                                                                                                                         1

                                                                                                                                                                                                               Notes: a 1 ¼ highest shear strength; b benchmark used for comparison purposes only; R2 ¼ coefficient of determination; PES ¼ polyethylene sheeting; WSFPP ¼ woven slit film polypropylene geotextile.
The PES material is a readily available 0.15 mm thick
(0.006 in) vapour barrier polyethylene sheeting material




                                                                                                                                (Normal stress ón ¼ 24–85 kPa)
that meets the Canada General Standards Board specifica-
tion for this class of material (CAN/CGSB 51.34-M86).




                                                                                                                                                                  150 mm displacement
                                                                                                                                              Ranka
2.2. Woven slit film polypropylene (WSFPP)




                                                                                                                                                                                        7–8

                                                                                                                                                                                        6–7


                                                                                                                                                                                        6–8
                                                                                                                                                                                         9

                                                                                                                                                                                         4
                                                                                                                                                                                         2

                                                                                                                                                                                         5
                                                                                                                                                                                         3
                                                                                                                                                                                         1
geotextile
The WSFPP geotextile bags used by the City of Winnipeg
for sandbag dykes are purchased in bulk, usually in
bundles of 1000. The current suppliers of these products
are located in China, and use several manufacturing
plants. The bags are generally made from ends and scraps




                                                                                                                                                                  Peak

                                                                                                                                                                                        6–8
                                                                                                                                                                                        7–9
                                                                                                                                                                                        6–8
                                                                                                                                                                                        3–4

                                                                                                                                                                                        7–9

                                                                                                                                                                                        3–4
                                                                                                                                                                                         1

                                                                                                                                                                                         5

                                                                                                                                                                                         2
left over from larger, more valuable textile products. This
means that there is limited information available on the
material properties of the bag material, and product
consistency is likely to be poor.




                                                                                                                                                                                        1.00
                                                                                                                                                                                        1.00
                                                                                                                                                                                        1.00
                                                                                                                                                                                        0.99
                                                                                                                                                                                        1.00
                                                                                                                                                                                        0.95
                                                                                                                                                                                        1.00
                                                                                                                                                                                        1.00
                                                                                                                                                                                        1.00
                                                                                                                                                                 R2
                                                                                                                                 Residual displacement
   Specimen sheets used for the large-scale tests were
obtained from WSFPP samples used to bundle the bags




                                                                                                                                                                 ö (deg)
for shipping. The local distributor for the City of Winni-




                                                                                                                                                                                        14
                                                                                                                                                                                        13
                                                                                                                                                                                        19
                                                                                                                                                                                        25
                                                                                                                                                                                        30
                                                                                                                                                                                        13
                                                                                                                                                                                        23
                                                                                                                                                                                        27
                                                                                                                                                                                        43
peg Public Works department indicated that it was com-
mon practice for the manufacturer to use the same
materials for packaging as that used to manufacture




                                                                                                                                                                  c (kPa)




                                                                                                                                                                                        10




                                                                                                                                                                                        12
                                                                                                                                                                                         5
                                                                                                                                                                                         3
                                                                                                                                                                                         2
                                                                                                                                                                                         8

                                                                                                                                                                                         7
                                                                                                                                                                                         9
                                                                                                                                                                                         8
cheaper products such as the sandbags. All the specimens
tested in this programme were taken from the same stock.
Visual inspection of WSFPP samples taken from the bags
and the packing material did not reveal any visual

                                                                                                                                                                                        1.00
                                                                                                                                                                                        1.00
                                                                                                                                                                                        1.00
                                                                                                                                                                                        1.00
                                                                                                                                                                                        1.00
                                                                                                                                                                                        0.95
                                                                                                                                                                                        0.99
                                                                                                                                                                                        1.00
                                                                                                                                                                                        1.00
                                                                                                                                                                 R2
                                                                                                                                 150 mm displacement



differences aside from overall dimensions. A single test on
a representative specimen of geotextile gave a mass per
                                                                                                                                      (post-peak)

                                                                                                                                                                 ö (deg)


unit area of 100 g/m2 (ASTM D 5261) and a wide-width
                                                                                                                                                                                        16
                                                                                                                                                                                        13
                                                                                                                                                                                        17
                                                                                                                                                                                        24
                                                                                                                                                                                        31
                                                                                                                                                                                        13
                                                                                                                                                                                        25
                                                                                                                                                                                        29
                                                                                                                                                                                        39
strip tensile strength of 5 kN/m (ASTM D 4595).
   It is also common practice for the manufacturers to treat
                                                                                                                                                                  c (kPa)




the WSFPP with a surfactant to reduce static electricity
                                                                                                                                                                                        10
                                                                                                                                                                                        10




                                                                                                                                                                                        13
                                                                                                                                                                                         4
                                                                                                                                                                                         4
                                                                                                                                                                                         4


                                                                                                                                                                                         7
                                                                                                                                                                                         7
                                                                                                                                                                                         9
produced by rubbing that occurs during shipping and
handling. In all cases the surfactant was still present on
the material used in this test programme.
                                                                                                                                 Peak at < 30 mm displacement




                                                                                                                                                                                        0.88
                                                                                                                                                                                        0.99
                                                                                                                                                                                        1.00
                                                                                                                                                                                        1.00
                                                                                                                                                                                        1.00
                                                                                                                                                                                        0.95
                                                                                                                                                                                        1.00
                                                                                                                                                                                        0.99
                                                                                                                                                                                        0.97
                                                                                                                                                                 R2




2.3. Granular material (sand)
A washed pit run sand from a local quarry near Kingston
                                                                                                                                                                 ö (deg)
                                                                         Table 2. Summary of Mohr–Coulomb strength parameters




                                                                                                                                                                                        13
                                                                                                                                                                                        15
                                                                                                                                                                                        19
                                                                                                                                                                                        25
                                                                                                                                                                                        34
                                                                                                                                                                                        12
                                                                                                                                                                                        23
                                                                                                                                                                                        22
                                                                                                                                                                                        24




was selected because it has a gradation that is similar to
the washed pit run sand materials used in the Winnipeg
area. The soil used in this investigation is classified as SP
                                                                                                                                                                  c (kPa)




according to the Unified Soil Classification System and
                                                                                                                                                                                        10




                                                                                                                                                                                        14
                                                                                                                                                                                         6
                                                                                                                                                                                         4
                                                                                                                                                                                         4
                                                                                                                                                                                         7

                                                                                                                                                                                         7
                                                                                                                                                                                         6
                                                                                                                                                                                         8




has the following particle sizes: D10 ¼ 0.09 mm, D30 ¼
0.4 mm, D50 ¼ 1 mm and D60 ¼ 1.3 mm. The soil was
placed at low moisture content (approximately 3%). In
practice, the water content at the time of filling of the
sandbags during a flood event will vary, because the sand
                                                                                                                                                                                        Sandbags on sandbags
                                                                                                                                                                                        WSFPP on WSFPP




is typically stockpiled at city public works yards. The
                                                                                                                                                                                        Sandbags on sod
                                                                                                                                                                                        WSFPP on sand
                                                                                                                                                                                        PES on WSFPP




effect of moisture content and soil density was not
                                                                                                                                                                                        WSFPP on sod
                                                                                                                                                                                        Sand on sandb
                                                                                                                                                                                        PES on PES




                                                                                                                                                                                        PES on sod




examined in this investigation. Direct shear tests were
carried out in the large-scale direct shear box on 1 m3
                                                                                                                                 Interface




specimens of this sand compacted to an initial density of
about 1800 kg/m3 (relative density of 60%). The cohesion
and friction angle at large deformations were 10 kPa and
                                                                                                                                 Test series




308, respectively (Table 2). These numbers are useful as a
                                                                                                                                                                                        1
                                                                                                                                                                                        2
                                                                                                                                                                                        3
                                                                                                                                                                                        4
                                                                                                                                                                                        5
                                                                                                                                                                                        6
                                                                                                                                                                                        7
                                                                                                                                                                                        8
                                                                                                                                                                                        9




benchmark for comparison with the interface shear
strength values discussed later.
                                         Geosynthetics International, 2007, 14, No. 2
122                                                                                                                Krahn et al.

                                                                 computer and ancillary control cards and custom-designed
2.4. Grass sod                                                   software.
There are a large variety of plants and grasses present
along the riverbanks where sandbag dykes are built in the        3.3. Specimen preparation
Red River valley. For simplicity, Kentucky blue grass sod        Sheets of 16 mm and 24 mm thick plywood were used to
was chosen for the large-scale direct shear tests conducted      secure the sheet specimens during testing. The bottom
for this project. Kentucky blue grass is a common variety        plywood sheets were seated on a timber crib that was used
used in the Winnipeg area, and this provided a known             to fill the lower half of the shear box. Plywood sheets
baseline that can be compared against other grass varieties      were cut to 1 m square size to fit snugly into the shear
in future work, should that be necessary.                        box. A minimum of three plywood sheets was used above
                                                                 and below the interface surfaces to support the test speci-
                                                                 mens and to ensure that the shear interfaces remained flat
3. EXPERIMENTAL PROGRAMME                                        and continuous during testing. Sheet metal flashing (1 mm
                                                                 nominal thickness) was used as a shim material between
3.1. General                                                     the plywood sheets to adjust the interface location so that
The large size of the direct shear apparatus used in this        it was level and parallel with the shear plane of the test
investigation allows interface properties to be quantified       apparatus.
for systems that exhibit discrete block behaviour, in this          The sheet materials tested (PES and WSFPP types)
case filled sandbags and filled sandbags in contact with         were cut into 1.2 m square pieces that were then trimmed
sod. It is not possible to capture field-scale interface         at the corners and folded over the plywood sheet support.
performance using conventional bench-scale direct shear          They were then attached to the plywood sheet by stapling
equipment (i.e. typically 50 mm 3 50 mm or 100 mm 3              the sheet on the reverse side of the plywood. This ensured
100 mm in plan area). An additional limitation of a              that the specimen sheets were taut over the shear surface.
traditional bench-scale direct shear apparatus is the influ-        For sandbag/sod interface shear testing, the sod was
ence of boundary effects on measured behaviour. Bound-           affixed to a sheet of plywood using deck screws placed on
ary effects are due to local distortion of the sheet products    a 100 mm square grid pattern. The top 6 mm of the screw
based on how the specimen materials are attached to the          head was left protruding from the plywood to engage and
test device. In the large-scale device used here, the inter-     hold the soil at the base of the sod layer. This arrangement
face area is large enough that local distortions near the        minimised slippage between the sod and the plywood
specimen edges that would otherwise influence macro-             interface without interfering with the interface surface at
scale response are reduced. Finally, it should be noted that     the top of the sod layer. Figure 3 shows the sod in the
the shear tests were carried out in general accordance with      bottom half of the direct shear apparatus before a test.
the ASTM D 5321 method of test.                                  Figure 4 shows filled sandbags placed over the sod layer
                                                                 with the top half of the shear box removed following
3.2. Large-scale direct shear test apparatus                     shearing.
The large-scale direct shear test apparatus used in this
testing programme is a custom-built device with a shear          3.4. Normal load and shear displacement rate
box 1 m 3 1 m 3 1 m in size. The box comprises equal             The normal loads used for this research were selected to
top and bottom halves constructed from 250 mm high               match the range of sandbag dyke heights used in typical
steel C-sections. The 1 m 3 1 m shear plane is located at        flood protection efforts in the Red River Valley (Figure 1).
the mid-height of the shear box. Steel plates at the sliding     Dykes smaller than 1.20 m are generally stable, and hence
interface surface between the top and bottom boxes are
treated with a specially formulated industrial coating to
minimise friction and abrasion. Horizontal load and stroke
are provided by a 222 kN capacity hydraulic actuator. A
stiffened rigid steel loading plate is used to apply a
vertical load to the specimen using a 222 kN capacity
hydraulic actuator. The bottom box is seated on a set of
rollers. The apparatus is instrumented to record displace-
ment (horizontal shear) of the lower box while the top box
is restrained horizontally. Vertical and horizontal loads are
recorded using load cells, one mounted at the end of the
vertical actuator piston, and the other opposite the top box
in line with the horizontal connection with the test frame.
Displacement transducers located at the corners of the
vertical loading plate are used to record vertical movement
of the test specimens. The apparatus configuration and
loading are identical to those found in a conventional
shear box apparatus but at a larger scale. Actuator control      Figure 3. Sod layer ready for interface testing in large-scale
and data acquisition are carried out using a personal            direct shear apparatus

                                         Geosynthetics International, 2007, 14, No. 2
Large-scale interface shear testing of sandbag dyke materials                                                                                                   123
                                                                                           50
                                                                                                                                                σn        125 kPa


                                                                                           40




                                                                     Shear stress (kPa)
                                                                                                                                                           75 kPa
                                                                                           30


                                                                                           20

                                                                                                                                                           25 kPa
                                                                                           10


                                                                                            0
Figure 4. Filled sandbags on sod after shearing in large-scale                                  0   3   5        10        15         20             25             30
                                                                                                            Horizontal shear deformation (cm)
direct shear apparatus
                                                                   Figure 5. WSFPP geotextile/geotextile interface shear stress–
                                                                   displacement curves
1.20 m was taken as the minimum dyke height in this
investigation. The maximum dyke height selected was
                                                                                           80
4.25 m, although there were cases during the 1997 flood
where higher dykes were constructed to protect some                                        70                                                   σn        125 kPa
residential homes. This value has been suggested as the
                                                                                           60
maximum temporary sandbag height for future flood
                                                                      Shear stress (kPa)
protection works in Winnipeg, with the expectation that,                                   50                                                              75 kPa
where higher structures are required, engineered clay
                                                                                           40
dykes will be built. The pressure at the base of the dyke
was calculated at the centre of the structure using an                                     30
                                                                                                                                                           25 kPa
estimated saturated unit weight for sandbags of 20 kN/m3 ,                                 20
which gives a range of foundation pressure from 24 to
85 kPa. Based on these calculations, each shear test                                       10
configuration in the laboratory test programme was car-                                     0
ried out at normal pressures of 25, 75 and 125 kPa.                                             0   3   5        10        15         20             25         30
                                                                                                            Horizontal shear deformation (cm)
   A shear rate of 5 mm/min was used in the large-scale
direct shear testing, which resulted in a test duration of         Figure 6. WSFPP geotextile/sod interface shear stress–
1 h to achieve 300 mm of horizontal displacement. The              displacement curves
ASTM D 3080 method of test states that this rate is
sufficiently slow to ensure that excess porewater pressures
for the worst case of fully saturated sand specimens are                                   80
                                                                                                                                                σn        125 kPa
dissipated during shear. None of the materials was fully                                   70
saturated in this investigation; however, as a precaution
                                                                                           60
and to employ a consistent test methodology the value of
                                                                     Shear stress (kPa)




5 mm/min was used for all tests.                                                           50                                                              75 kPa

                                                                                           40

4. TEST RESULTS                                                                            30
                                                                                                                                                           25 kPa

4.1. General                                                                               20

For brevity, only plots from selected interface shear test                                 10
results are presented here (Figures 5 to 8). The peak shear                                 0
stress, shear stress at 15 cm displacement and residual                                         0   3   5        10        15        20              25         30
stress values are indicated by the solid symbols in each                                                    Horizontal shear deformation (cm)

plot. Some of the stress–displacement plots show stick–
                                                                   Figure 7. Sandbag/sod interface shear stress–displacement
slip behaviour (e.g. at about 3 cm displacement for the test       curves
at 75 kPa normal stress in Figure 6 and at 5–10 cm
displacement in Figure 7). This behaviour did not influ-
ence the interpretation of the results, because target             larger shear box equipment is required for the type of
strength values were taken from the trace of the backbone          interface shear testing reported here.
curve in each test. The frequency of stick–slip events                The peak shear strength values were taken over the 0 to
during shear testing was observed to be very much higher           3 cm displacement range in each curve. In some cases the
for smaller specimen tests using a 60 mm 3 60 mm shear             maximum value occurred at larger deformations. The
box apparatus (Krahn 2005). This is further evidence that          3 cm displacement criterion was selected because it was
                                           Geosynthetics International, 2007, 14, No. 2
124                                                                                                                                       Krahn et al.

observed that under operational conditions the accumula-         geotextile reduces sliding resistance. However, after large
tion of 3 cm of lateral displacement at each layer in the        deformations between 3 and 15 cm displacement, the
dyke could result in excessive leakage and structure             shear strength for the sandbag/sandbag tests (series 9) was
distortion, resulting in a breach of the sandbag dyke            greater than for the sand alone (series 5) in the present
(Krahn et al. 2005).                                             investigation. The explanation for this relative behaviour
   Post-peak strength values are of interest to designers        is that the sand within the sandbags cannot be placed in a
because it is possible that critical mechanisms with a           compacted (dense) state compared with the initial com-
stepped geometry can develop in a sandbag dyke. Both             pacted density of the sand in the benchmark sand/sand
peak and large-deformation shear strengths can be mobi-          tests. However, after large sandbag-to-sandbag shear de-
lised along the shear paths defining these critical mechan-      formations the strength and stiffness of the encapsulating
isms. Furthermore, the critical failure mechanism may            geotextile more than compensates for the initial lower
change as the dyke deforms.                                      density of the sand, and increasing shear capacity of the
   In order to characterise the post-peak response, strength     sandbags is mobilised at least up to about 30 cm horizon-
values at 15 cm displacement and at the end of the tests         tal displacement (see Figure 8).
were extracted from the stress–displacement curves. The             The interface shear strength for the sandbag/sod inter-
value of 15 cm displacement is arbitrary. The shear              face is always less than for the sandbag/sandbag interface
strengths at the end of each test (typically at 25 to 30 cm      (compare series 8 and 9). However, the differences be-
displacement) are identified as residual shear strength          tween the sandbag/sod and WSFPP geotextile/sod strength
values in this paper.                                            values are not as great. This relative trend is ascribed to
   The strength values at each normal load using the             the amount of specimen dilation that occurs during a test.
criteria described above were used to generate Mohr–             The greatest dilation occurs when the interface is com-
Coulomb (M-C) strength parameters for each interface             posed of sandbags alone (i.e. the bags roll over each other
type. These data are summarised in Table 2. The regres-          after large horizontal deformation). In contrast, the inter-
sion coefficients varied from 0.88 to 1.00, with most            face shear surface with a planar geotextile in contact with
datasets corresponding to values of 1.00. Hence linear           a layer of sod is non-dilatant.
M-C interface shear envelopes corresponding to the range            The lowest shear strength values are recorded for those
of normal stresses applied are accurate models for the           interfaces that include PES. This may not be unexpected
interfaces investigated.                                         but, to the best of the writers’ knowledge, interface shear
                                                                 data for these materials have not been reported in the
4.2. Mohr–Coulomb interface shear strength                       literature. For sandbag dykes that are routinely constructed
The following observations can be noted from the data in         with PES as a water barrier, these materials may control
Table 2.                                                         dyke stability. PES materials are hydrophobic, so that even
   The peak apparent cohesion value (4 kPa) and peak             in the presence of water in a field case, the interface
friction angle (198) for the sandbag geotextile-to-geotextile    properties determined from the dry tests reported here are
interface shear tests were less than values measured for         applicable.
the sandbag-to-sandbag interface (14 kPa and 248). Hence            The interface shear strength data points from all tests
there was greater interface shear strength recorded for the      are summarised in Figures 9–11. To prevent visual clutter,
sandbag-to-sandbag test configuration than for the geotex-       the M-C envelopes for each dataset are not plotted.
tile material alone. The difference in shear strength is         Lower-bound envelopes have been identified in the figures
much larger when Coulomb strength values at 15 cm                that can be used as a preliminary (conservative) estimate
displacement and at residual values are compared. The            of interface shear strength for limit equilibrium-based
difference is due to the test configuration for the sandbag-     stability analyses of sandbag dykes. It can be noted that
to-sandbag tests, which results in system dilation as the
top row of sandbags moves up and over the lower row of
                                                                                        140
sandbags. This discrete block mechanism does not develop                                130
when the geotextile sheet material is tested in isolation.                              120                                              σn      125 kPa
   Also included in the table are shear strength rankings                               110
for the range of normal stresses at the bottom of a sandbag                             100
                                                                   Shear stress (kPa)




                                                                                         90
dyke based on the City of Winnipeg design template (i.e.
                                                                                         80
24 to 85 kPa). The ranking of interface shear strength                                   70                                                       75 kPa
values is very consistent once the 15 cm displacement                                    60
criterion is achieved.                                                                   50
   The results of all interface shear tests showed that the                              40
                                                                                         30
benchmark peak shear strength for the sand/sand interface                                                                                         25 kPa
                                                                                         20
was the largest (series 5) of all (initial) peak strength                                10
values. The influence of sand and geotextile materials on                                 0
                                                                                              0   3   5        10        15        20       25         30
interface shear strength can be seen by comparing results                                                 Horizontal shear deformation (cm)
of test series 3, 4 and 5. These tests reveal increasing
shear strength in the same order, indicating that, when          Figure 8. Sandbag/sandbag interface shear stress–displace-
continuous planar interfaces are tested, the presence of the     ment curves
                                         Geosynthetics International, 2007, 14, No. 2
Large-scale interface shear testing of sandbag dyke materials                                                                                                                              125
                      45                                                                                                   140
                                                                                                                                       WSFPP on WSFPP      Sandbags on sandbags
                                                                                                                           130         Sandbags on sod
                      40
                                                                                                                           120                Range for sandbags on sandbags
                      35                                                                                                   110
                                   Range for all tests with PES                                                                                Range for sandbags on sod
                                                                                                                           100
 Shear stress (kPa)




                                                                                                      Shear stress (kPa)
                      30                                                                                                    90
                                                                                                                            80       Range for WSFPP on WSFPP
                      25
                                                                                                                            70
                      20                                                                                                    60
                                                                                                                            50
                      15
                                                                                                                            40
                      10                             τ   5    σn tan (11°)                                                  30
                                                                                  PES on WSFPP                                                                             τ 12 σn tan (24°)
                                                                                                                            20
                      5                                                           PES on PES                                                                        τ 7 σn tan (22°)
                                                                                  PES on sod                                10                          τ   3    σn tan (18°)
                       0                                                                                                     0
                           0      20        40          60       80      100          120    140                                 0        20       40      60       80      100    120     140
                                                     Normal stress (kPa)                                                                                Normal stress (kPa)


Figure 9. Range of shear strength values for PES interfaces                                          Figure 11. Range of shear strength values for WSFPP on
and lower-bound envelope for design                                                                  WSFPP and sandbag interfaces and lower-bound envelopes
                                                                                                     for design



                      70                                                                             geotextile sheet to geotextile sheet M-C line. Hence using
                                 WSFPP on WSFPP
                      65         WSFPP on sand                                                       interface shear strength values for the geotextile sheet
                      60         WSFPP on sod                                                        materials alone is likely to be conservative for sandbag
                      55
                                                                                                     dyke design.
                      50               Range for WSFPP on sod
 Shear stress (kPa)




                      45
                               Range for WSFPP on
                      40
                                           WSFPP                                                     4.3. Interface stiffness
                      35
                      30                                                                             Mohr–Coulomb strength parameters are useful for limit
                      25
                                                                                                     equilibrium-based stability analysis of sandbag dykes.
                      20                                 τ    7    σn tan (22°)
                      15
                                                                                                     However, for more sophisticated analyses using finite
                      10                             τ   3    σn tan (18°)                           element model or finite difference numerical codes, stiff-
                       5                                                                             ness values for the interfaces are required. Interface shear
                       0                                                                             stiffness values were calculated by taking the average
                           0      20        40          60       80      100          120    140
                                                     Normal stress (kPa)                             secant slope through the three stress–displacement curves
                                                                                                     for each interface type at a value of 0.5 cm. The value of
Figure 10. Range of shear strength values for WSFPP                                                  0.5 cm was selected to represent the initial stiffness of the
geotextile interfaces and lower-bound envelopes for design                                           interface surfaces but was otherwise arbitrary. The com-
                                                                                                     puted values are summarised in Table 3. The data show
                                                                                                     that the range of values is narrow compared with stiffness
                                                                                                     values for geomaterials that can differ by orders of
the design envelope for WSFPP geotextile in contact with                                             magnitude. It is interesting to note that the relative
sod is the same as the envelope for the sandbag/sod                                                  stiffness values do not rank in the same order as shear
interface (i.e. ô ¼ 7 + ón tan 228). However, the sandbag/                                           strength values. For example, the sand/sand stiffness value
sandbag design envelope falls above the corresponding                                                ranking is relatively lower than the strength value ranking.


                                         Table 3. Initial secant shear stiffness (Ki ) values interpreted from interface shear
                                         stress–displacement plots.

                                           Test series Interface                                                           Ki (kPa/mm)                      Ranka

                                                 9           Sandbags on sandbags                                             3.66                           1
                                                 4           WSFPP on sand                                                    3.48                           2
                                                 8           Sandbags on sod                                                  3.43                           3
                                                 2           PES on WSFPP                                                     3.04                           4
                                                 5           Sand on sandb                                                    2.79                           5
                                                 3           WSFPP on WSFPP                                                   2.75                           6
                                                 7           WSFPP on sod                                                     2.70                           7
                                                 6           PES on sod                                                       2.58                           8
                                                 1           PES on PES                                                       2.30                           9

                                         Notes: a 1 ¼ highest shear stiffness; b benchmark used for comparison purposes only; PES ¼
                                         polyethylene sheeting; WSFPP ¼ woven slit film polypropylene geotextile

                                                                             Geosynthetics International, 2007, 14, No. 2
126                                                                                                                                Krahn et al.

Not unexpectedly, the stiffness values corresponding to                 REFERENCES
two of the interfaces with PES materials are the lowest.                Aqil, U., Matsushima, K., Mohri, Y., Yamazaki, S. & Tatsuoka, F. (2006).
                                                                              Lateral shearing tests on geosynthetic soil bags. Proceedings of the
                                                                              8th International Conference on Geosynthetics, Yokohama, Japan,
5. CONCLUSIONS                                                                Millpress, Rotterdam, Vol. 4, pp. 1703–1706.
A large-scale direct shear apparatus was used to quantify               ASTM D 3080. Standard Test Method for Direct Shear Test of Soils
                                                                              Under Consolidated Drained Conditions. ASTM International, West
the shear behaviour of sandbag dyke interface surfaces.                       Conshohocken, PA, USA.
The test results showed that the interface shear strength of            ASTM D 4595. Standard Test Method for Tensile Properties of
sandbags in contact with sandbags is very much larger                         Geotextiles by the Wide-Width Strip Method. ASTM International,
than that deduced from interface shear testing on sandbag                     West Conshohocken, PA, USA.
geotextile sheets. The difference is ascribed to the dilatant           ASTM D 5261. Standard Test Method for Measuring Mass per Unit Area
                                                                              of Geotextiles. ASTM International, West Conshohocken, PA, USA.
behaviour that occurs as the sandbags move over each                    ASTM D 5321. Standard Test Method for Determining the Coefficient of
other during large shear deformations.                                        Soil and Geosynthetic Friction by the Direct Shear Method. ASTM
   In terms of shear response, the weakest and least stiff                    International, West Conshohocken, PA, USA.
interfaces for sandbag dykes built according to the City of             Bove, J. A. (1990). Direct shear friction testing for geosynthetics in waste
Winnipeg 1997 template are those including a PES layer.                       containment. Geosynthetic Testing for Waste Containment Applica-
                                                                              tions, STP 1081, Koerner, R. M., Editor, ASTM International, West
Clearly, seating a sandbag dyke directly on a layer of PES                    Conshohocken, PA, USA, pp. 241–256.
may lead to base sliding of the structure. In current                   CAN/CGSB 51.34-M86. (1996) Vapour Barrier, Polyethylene Sheet for
practice, this arrangement is avoided. Provided that a                        Use in Building Construction. Canadian General Standards Board,
horizontal planar surface with PES is not present within                      Gatineau, Canada K1A 1G6.
the sandbag structure, then internal failure from the loaded            City of Winnipeg (1997). Building a dike. http://www.winnipeg.ca/
                                                                              Services/CityLife/HistoryOfWinnipeg/flood/flood_dike_construc-
wet side to the dry side of the sandbag dyke is likely to be                  tion.stm
preventable because of the multi-step internal shear me-                Criley, K. R. & Saint John, D. (1997). Variability analysis of soil versus
chanism that must develop because of the staggered                            geosynthetic interface friction characteristics by multiple direct
construction of these systems. Nevertheless, the margin of                    shear testing. Proceedings of Geosynthetics ’97, Long Beach, CA,
safety of temporary sandbag dykes using the current City                      Industrial Fabrics Association International, Vol. 2, pp. 885–897.
                                                                        Kalumba, D. & Scheele, F. (1999). Friction characteristics of sand/
of Winnipeg design template has not been established.                         geotextile interfaces for three selected sand materials. Proceedings
This is the topic of ongoing work by the writers.                             of the Regional Conference for Africa: Soil Mechanics and
                                                                              Foundation Engineering, G. R. Wardle, G. E. Blight and A. B.
                                                                              Fourie, Editors, A.A. Balkema, Rotterdam, Vol. 12, pp. 201–206.
ACKNOWLEDGEMENTS                                                        Krahn, T. J. (2005). Assessment of sandbag dike performance. MSc
                                                                              thesis, Department of Civil Engineering, University of Manitoba,
The authors are grateful for the financial support from the                   Winnipeg, Manitoba, Canada, 298 pp.
Natural Sciences and Engineering Research Council of                    Krahn, T., Blatz, J., Alfaro, M. & Bathurst, R.J. (2005). Evaluation of
Canada and the Secondary Diking Enhancements Program                          sandbag dike performance using a full-scale test facility. Proceed-
administered by the Government of Canada, the Province                        ings of 58th Canadian Geotechnical Conference, Quebec City,
of Manitoba and the City of Winnipeg. The authors would                       Saskatoon. Canadian Geotechnical Society, Alliston, Ontario, 8pp
                                                                              (CD).
also like to thank the staff at Bathurst, Clarabut Geotech-             Lohani, T. N., Matsushima, K., Aqil, U., Mohri, Y. & Tatsuoka, F. (2006).
nical Testing Ltd (BCGT) for providing both technical                         Evaluating the strength and deformation characteristics of a soil bag
assistance and the large-scale direct shear apparatus that                    pile from full-scale laboratory tests. Geosynthetics International,
was used for the testing reported here.                                       13, No. 6, 246–264.

          The Editors welcome discussion on all papers published in Geosynthetics International. Please email your contribution to
                                     discussion@geosynthetics-international.com by 15 October 2007.




                                             Geosynthetics International, 2007, 14, No. 2

				
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