Documents
Resources
Learning Center
Upload
Plans & pricing Sign in
Sign Out

2900 - PDF

VIEWS: 4 PAGES: 7

									FIELD AND LABORATORY TESTS INVESTIGATING SETTLEMENTS OF
FOUNDATIONS ON WEATHERED KEUPER MARL

Hornig, E.-D.
Bergakademie Technical University Freiberg, Institute of Geotechnical Engineering, Germany




ABSTRACT: Measured settlements of buildings on the weathered Keuper Marl appeared to be much smaller than calculated
settlements, which were based on stiffness modules from standard oedometer tests. Therefore both special triaxial K0-tests and
oedometer tests were carried out for an accurate determination of stiffness moduli. Modules obtained in the triaxial K0-tests were
at least two to three times the values obtained in the oedometer tests. To verify observations from the laboratory tests, the loads
and the settlements of two single footings on weathered Keuper mudstone have been measured during construction of a building
during one year. Also a large scale footing load test with measurements of deformations was conducted on the weathered Keuper
mudstone. The measured settlements of the two single footings and the tested foundation were compared with the settlements
based on conventional calculations with moduli from oedometer tests and triaxial K0-tests. Up to a foundation pressure of σv=500
kN/m² the calculated settlement with ES-modules from triaxial K0-tests was found to correspond well to the measured deformation.
For foundation pressure beyond 500 kN/m², the foundation response was highly non-linear and it could not be described any more
with the linear-elastic model. Therefore the footing load test was also simulated by FEM analyses.
Keywords: stiffness; mudstone; oedometer; K0-Triaxial; field-measurements


1. INTRODUCTION
Observations and measurements at actual
constructions prove that calculated settlements of
foundations in the Keuper mudstone, based on
oedometer tests, are usually too large. By the way,
the German standard DIN 4019 therefore provides
the possibility of reducing the calculated settlements
by a factor down to ½.                                                        Figure 1. Natural structure of Keuper Marl

One of the main problems of using standard                          Several series of such “K0-tests” have been
oedometer tests for getting a module of deformation                 performed for different soil samples. The first
is the disturbance of the soil samples when they are                results by Illner [1] and Schmidt/Illner [2] proved
put into the oedometer ring, as these soils are often               that the modules of deformation Es from K0 triaxial
laminated and brittle (Figure 1).                                   tests are three times higher than those gained by
                                                                    conventional oedometer tests.
A significant improvement can be gained by
conducting lab tests in a special triaxial apparatus,               In the research described in this paper, two parallel
namely a one-dimensional compression under                          tests were conducted on the same soil specimen in
oedometer conditions but with a free lateral surface                order to compare the stress strain behavior: one in
(without ring), which means that stress is controlled               the oedometer test and one in the triaxial K0-test. It
in such a way that lateral strain remains zero (K0                  appeared that modules obtained in the triaxial K0-
condition).                                                         tests were at least two to three times the values
                                                                    obtained in the oedometer tests.


                                                                1
A large scale footing test was performed in order to              After preparation an oedometer test was performed.
verify settlement predictions based on parameters                 The other, bigger, part of the drill core sample (h/d
obtained by K0-triaxial tests. In this context,                   = 12-20cm/10cm) was fitted without much
settlement analyses have been made using                          preparation into the Triaxial apparatus for a
conventional methods as well as numerical by using                compression test under K0 conditions.
the Finite Element Method (FEM).
                                                                  The oedometer tests were performed on a
Taking into account the load-displacement behavior                conventional basis by increasing the load step by
beyond the regular working load of the footing, the               step. The load was doubled after 24 hours. The
FEM analyses have been performed using different                  triaxial compression test was performed in a fully
types of constitutive models.                                     computer controlled testing unit described by
                                                                  Menzies [3]. In both tests a first loading and several
2. LABORATORY TESTS                                               unloading- reloading steps were carried out.
It is very difficult, if not impossible, to put a
representative soil sample out of the weathered
Keuper mudstone without disturbance of the
structure into the oedometer ring (Figure 2 left).
The reason is that for these structured soils the
dimensions of the oedometer ring are too small.


                                                                  Figure 4 + 5. oedometer testing unit (left) and K0-Triax testing
                                                                  unit (right)

                                                                  The horizontal deformations of the sample were
                                                                  measured by radial sensors with Hall Effect
                                                                  Semiconductors (Clayton [4]) (Fig. 4). The K0
                                                                  controlled compression tests were performed on
Figure 2 + 3. Disturbed sample in oedometer ring (left) and
undisturbed sample for K0-Triax (right)
                                                                  unsaturated, drained samples (corresponding to
                                                                  oedometer tests). They were programmed in such a
Alternatively triaxial K0-tests on samples with a                 way that during a certain time period the cell
diameter of 100 mm (size of the drilling core) and a              pressure increases continuously to a given
height of 120 - 200 mm can be performed with                      maximum or minimum.
relative ease (Figure 2 right).
Table 1. Index values of the tested weathered Keuper
mudstone:

       moisture content w [%]          13 - 14
       liquid limit wL [%]           33,6 – 38,3
       plastic limit wP [%]           13 – 17,8
       plasticity index IP [-]       14,2 – 16,6                      Figure 6 + 7. sample for K0-Triax with Radial sensors
       consistency index IC [-]       1,2 – 1,4
                                                                  2.2. Test Results
       consistency                semi-solid – solid              The measured deformations from the oedometer
       dry density ρd [g/cm³]        1,85 – 1,95                  were compared with the results gained from K0
                                                                  triaxial compression tests. Figure 8 and 9 show
                                                                  results of two typical samples of the weathered
2.1. Sample Preparation and Testing Procedure                     Keuper mudstone.
Out of the same drill core sample, one part was
fitted into the oedometer ring (h/d = 2,0cm/7,1cm).

                                                              2
         10                        100                   1000    σ 0 [kN/m2] 10000
                                                                                          For the initial loading of sample a mean ratio H* of
 0,00                                                                                     2.11 between stiffness modules of K0-Triax and of
                                              K0-Triax      sample 1 / Keuper             Oedometer was calculated. Over whole stress range
 0,01                                                                                     the ratio did not vary much. At reloading a mean
 0,02
                                                                                          ratio H* of 2.46 was gained, at which the ratio
               placement data
               Oedometer
                                                                                          decreased from 3.45 to 1.31 during the area of stress
                                     Oedometer
 0,03          Wn = 13.0 %                                                                increase.
               ρ d = 1,91g/cm³
                 K 0-Triax
 0,04          Wn = 13,0 %                                                                Table 3. Comparison Stiffness modules ES of Keuper 2
               ρ d = 1,96 g/cm³
                                                                                          between Oedometer and K0-Triaxial:
 ε[-
 0,05 ]

          Figure 8. Test results Keuper mudstone sample 1                                      stress range         first loading
                                                                                                              Oedometer K0-Triax        ratio
                                                                                                    σ0          ES (EB)       ES (EB)    H*
         10                       100                 1000      σ 0 [kN/m²]   10000
                                                                                                 [ kN/m2 ]    [ MN/m2 ] [ MN/m2 ]        [-]
    0
                                                                                                  100-200          9            33       3,7
 0,01         sample 2 / Keuper
                                                            K0-Triax                              200-400         15            44       2,9
 0,02
                                                                                                  400-800         20            80       4,0
 0,03                                                                                            800-1200         33            90       2,7
              placement data
 0,04           Oedometer
                                         Oedometer                                                                             mean      3,3
              Wn = 14,5 %
 0,05         ρ d = 1,84 g/cm³                                                                                        reloading
 0,06
                K 0-Triax                                                                                     Oedometer K0-Triax        ratio
                                                                                                     σ0
              Wn = 13,8 %
 0,07         ρ d = 1,96 g/cm³
                                                                                                               ES (WB)       ES (WB)     H*
 ε [-]                                                                                           [ kN/m2 ]    [ MN/m2 ] [ MN/m2 ]        [-]
 0,08
                                                                                                   50-100
          Figure 9. Test results Keuper mudstone sample 2                                         100-200         41          73        1,8
                                                                                                  200-400         55          84        1,5
Table 2. Comparison Stiffness modulus ES of Keuper 1                                              400-800         67          129       1,9
between Oedometer and K0-Triaxial:                                                                                           mean       1,7

          stress range                  first loading                                     For sample Keuper 2, which was got from a
                                  Oedometer K0-Triax               ratio                  different site, the corresponding mean value of H*
                  σ0                ES (EB)       ES (EB)           H*                    for initial loading was 3.3. Thereby H* did not vary
              [ kN/m2 ]           [ MN/m2 ] [ MN/m2 ]               [-]                   much, too. At reloading, however, a mean ratio H*
                50-100                13            25             1,92
                                                                                          was 1.7, the variation was small, the amount of H*
               100-200                17            40             2,35
               200-400                29            67             2,31                   was nearly the half the value of initial loading.
               400-800                42            80             1,90
              800-1600                58            120            2,07                   3. IN SITU MEASUREMNTS
                                                   mean            2,11
                                          reloading                                       To verify observations from the laboratory tests, the
                                  Oedometer K0-Triax               ratio                  loads and the settlements of two single footings on
                  σ0               ES (WB)       ES (WB)            H*                    weathered Keuper mudstone have been measured
              [ kN/m2 ]           [ MN/m2 ] [ MN/m2 ]               [-]                   during construction of a building in the City of
                50-100                36                                                  Stuttgart during one year. Figure 10 and 11 show
               100-200                29            100             3,45                  the placed indirect load measuring device in one of
               200-400                38            100             2,63                  the two columns. Parallel to the measurements on
               400-800                105           138             1,31                  site, samples taken close to the foundations have
              800-1600
                                                                                          been analysed to obtain soil parameters, especially
                                                     mean           2,46
                                                                                          stiffness moduli from Triaxial K0-tests and
H* = ratio of stiffness module gained from K0-Triax                                       Oedometer tests. Using these characteristic values
and from Oedometer resp. H* = ES(K)/ ES(O)
ES=stiffness module, EB=initial loading, WB=reloading, K=K0-Triax,                        the settlements of the foundations have been
O=oedometer                                                                               calculated.



                                                                                      3
                                                                      of 900 kN each were used in such a way that an
                                                                      influence on measurements could be excluded.
                                                                      During the whole test the settlements were
                                                                      measured at three points on top of the foundation,
                                                                      using a measuring scaffold, with bearings far away
                                                                      from the calculated influence area of the footing.

                                                                      Four (re)loading – unloading steps were performed
                                                                      up to the failure of the strip. Figure 14 shows the
                                                                      measured settlements under loading cycles.

                                                                                                 Large scale footing load test
                                                                                10                    100                     1000      σ 0 [kN/m²]    10000
                                                                           0

                                                                         50

                                                                        100

                                                                        150
Figure 10 + 11*. Placed rebar strain meter in column before
                                                                        200
placing of concrete *(Martin [5], Hornig et al. [6])
                                                                       s [mm]
                                                                         250



4. LARGE SCALE FOOTING LOAD TEST                                              Figure 14. Loading cycles and measured settlements

The large scale footing load test (Fig. 12 + 13) with
                                                                      5. CALCULATIONS
measurements of deformations taken at the test
foundation itself and at several depths, using                        5.1. Conventional Settlement Analyses
extensometers, was carried out on the weathered                       The measured settlements of the two single footings
Keuper mudstone.                                                      and the footing load test were compared with the
                                                                      settlements based on conventional calculations with
Also parallel to the test on site, samples taken                      modules from oedometer tests and triaxial K0-tests.
beneath and close to the test foundation have been
analysed to obtain soil parameters. Using stiffness                   Figure 15 shows the measured and the calculated
moduli from Triaxial K0-tests and Oedometer tests                     settlements of one single footing under construction
the settlements of the foundation have been                           of the building up to foundation pressure of σ0 =
calculated.                                                           300 kN/m².

                                                                                     comparison: measurements - analytcal calculation / foundation 2

                                                                               0          50          100          150          200      σ 0 [kN/m²]
                                                                                                                                              250        300
                                                                          0
                                                                          5
                                                                         10
                                                                         15
                                                                         20
                                                                         25
Figure 12 + 13*. Photo (left) and schematic view (right) of the          30
                                                                                     calculation / K0-Triax first-loading module
                                                                                     calculation / Oedometer first-loading module
large scale footing load test *(Schnürch [7], Hornig [8],                35          calculation / Oedometer reloading module
                                                                                     measurement building foundation 2
Hornig et al. [6])                                                     s [mm]
                                                                          40


                                                                        Figure 15. Measured settlements – calculated settlements
For the test a cylindrical, reinforced concrete
footing with a diameter of 1,8 m and a height of 0,8
                                                                      A comparison of the measured settlements in Fig.
m was used. The loading was done by three 3 MN
                                                                      15 with the calculated based on triaxial K0-test-
hydraulic presses. Ten grouted injection anchors
                                                                      modules and reloading modules out of Oedometer
with a length of 10.5 m and a max. bearing capacity

                                                                  4
tests shows a very good agreement. The calculated                                                  Table 4. Parameters for the calculations with the FE-Code
settlements, which were based on first loading                                                     PLAXIS
modules out of Oedometer tests, are much too large.
                                                                                                                                   Hardening-                Mohr-
                                                                                                                                      Soil                  Coulomb
The measured and the calculated settlements of the
                                                                                                                           φ’ [°]      30                      30
footing load test under loading up to foundation                                                                       c’ [kN/m²]      35                      30
pressure of σ0 = 800 kN/m² are shown in Figure 16.                                                                    E50 [MN/m²]      38                       -
                                                                                                                            EOed       30                       -
                footing load test: measurements - analytical calculation / linear elastic
                                                                                                                      Eur [MN/m²]      80                       -
                                                                                                                      Eref [MN/m²]      -                      46
            0     100      200       300       400      500       600       700 σ 0 [kN/m2 ]
                                                                                     800
      0
                                                                                                                          νur [-]     0,2                       -
                                                                                                                           ν [-]        -                    0,315
     10                                                                                                               pref [kN/m²]    150                       -
     20
                                                                                                                           m [-]      0,5                       -

     30
                                                                                                   To describe the geometrical situation of the
     40
                    measurement
                                                                                                   cylindrical foundation, the finite element mesh
     50             calculation / K0-Triax
                    calculation / Oedometer
                                                                                                   shown in Figure 17 with 15 node elements under
 s [ mm ]
      60
                                                                                                   axial symmetrical conditions was used. After
                                                                                                   preliminary examination for the influence of the
   Figure 16. Measured settlements – calculated settlements                                        anchors and the necessary vertical and horizontal
                                                                                                   expansion, calculations were made with a fine mesh
For stresses up to σ0 = 500 kN/m² a comparison of                                                  in the central zone and a coarse mesh in the outer
the measured settlements in Fig. 16 with the                                                       zone.
calculated based on triaxial K0-test-modules shows                                                           -2.000     0.000       2.000   4.000   6.000    8.000   10.000   12.000




a very good agreement. Since the foundation                                                        10.000




response was highly non-linear for foundation                                                                            A      A




pressure beyond 500 kN/m², this response could no                                                   8.000




longer be described with the linear-elastic model.
The calculated settlements with a working pressure                                                  6.000




up to σ0 = 500 kN/m², which were based on
oedometer test modules, are much too large.                                                         4.000




5.2. Finite Element Method (FEM) Analyses
As described in 5.1, beyond a working load of σ0 =
                                                                                                    2.000




500 kN/m² nonlinear effects increase. Therefore the
footing load test was simulated by FEM analyses
                                                                                                    -0.000




using the Finite Element Code PLAXIS (Vermeer
and Brinkgreve [9+10]) which applies two non-                                                      Figure 17. Cross section of the axial symmetrical loaded mesh
linear models: the Mohr-Coulomb model and the
Hardening-Soil model. The needed model                                                             The excavation and the test construction were
parameters in table 4 were mainly gained through                                                   simulated by stage constructions. The (re)loading –
the lab-tests, additionally some were chosen within                                                unloading steps were described by multipliers.
FEM simulations. The module EOed from triaxial
K0-tests was used.                                                                                 5.3. Results and Comparison
                                                                                                   Figure 18 shows the measured and the calculated
                                                                                                   deformations under loading, unloading and
                                                                                                   reloading up to the ultimate bearing pressure of σ0 =
                                                                                                   1550 kN/m². As described, for the calculations the
                                                                                                   Mohr-Coulomb model and the Hardening-Soil
                                                                                                   model were used.




                                                                                               5
               comparison: measurements - numerical simulations                     ACKNOWLEDGEMENTS
           0      200     400     600    800     1000      1200   σ 0 [kN/m²]
                                                                    1400
      0
                                                                                    The in situ measurements have been supported by:
                                            measurements
     20                                     FE-calculation: Hardening Soil

     40
                                            FE-calculation: Mohr Coulomb            Hallesche Nationale Krankenversicherung Stuttgart
     60
                                                                                    Ed. Züblin AG – Niederlassung Stuttgart
     80
                                                                                    Terrasond Gesellschaft für
    100
                                                                                    Baugrunduntersuchungen mbH & Co. KG
    120                                                                             Dr. G. Hafner, Büro für Ingenieurgeologie, Erd-
    140                                                                             und Grundbau, Stuttgart
    160                                                                             FH Stuttgart – FB Vermessung und Geoinformatik
    180
  s [mm]
     200

                                                                                    The large scale footing load test has been supported
          Figure 18. Measured and calculated settlements                            by:
Only the Hardening-Soil model could describe the                                    Daimler-Chrysler AG – Werk Sindelfingen
deformations under loading, unloading and                                           Ed. Züblin AG – Niederlassung Stuttgart
reloading realistically. Simulations using the Mohr-                                Bauer Spezialtiefbau GmbH
Coulomb model only represent the initial loading                                    Terrasond Gesellschaft für
well.                                                                               Baugrunduntersuchungen mbH & Co. KG
                                                                                    Bauunternehmen Hämmerle GmbH & Co. KG
6. CONCLUSIONS                                                                      FMPA Baden-Württemberg – Abteilung 4 –
                                                                                    Geotechnik
For base pressures of up to working load, the                                       Smoltczyk & Partner GmbH
settlements for the measured single footings and for                                FH Stuttgart – FB Vermessung und Geoinformatik
the tested large scale footing can be described very
well with the linear-elastic model within
                                                                                    REFERENCES
conventional settlement analyses. An accurate
determination of the deformation parameter is very                                   1.   Illner, C. (1997). Verformungsverhalten von
                                                                                          Gipskeuperböden. Forschungsbericht Fachhochschule
important for the investigated soils. The K0-test in                                      Stuttgart – Hochschule für Technik, Labor für
the triaxial apparatus is therefore very useful.                                          Geotechnik, unveröffentlicht
Alternatively the use of reloading-modules from
Oedometer tests can also give practical results.                                     2.   Schmidt, H.-H., Illner, C. (1998). Bestimmung von
                                                                                          Verformungsmoduln an Gipskeuperproben.
                                                                                          Tagungsband der 25. Baugrundtagung, Stuttgart.
Deformations beyond base pressures of σ0 = 500
kN/m² should be simulated numerically by using                                       3.   Menzies, B. K. (1988). A Computer Controlled
non-linear models. In this case, the stiffness                                            Hydraulic Triaxial Testing System. American Society
modules out of triaxial K0-tests should be used as                                        for Testing and Materials. STP 977, Philadelphia, 82-
well. Calculations with the Finite Element Code                                           94.
PLAXIS (Vermeer and Brinkgreve [9+10]), which                                        4.   Clayton, C. (1989). The Use of Hall Effect
are based on the Mohr-Coulomb and the Hardening-                                          Semiconductors in Geotechnical Instrumentation.
Soil models, achieve good results.                                                        Geotechnical Testing Journal. Vol. 12, No. 1, 69-76.

                                                                                     5.   Maritn, S. (2001). Messung und Berechnung von
                                                                                          Setzungen an Einzelfundamenten im Gipskeuper.
                                                                                          Diplomarbeit, Institut für Geotechnik, Universität
                                                                                          Stuttgart, unveröffentlicht

                                                                                     6.   Hornig, E.-D., Buchmaier, R. F., Schmidt, H.-H.
                                                                                          (2002). Eindimensionale Kompression
                                                                                          überkonsolidierter bindiger Böden am Beispiel des
                                                                                          Gipskeupers. Forschungsbericht, Fachhochschule
                                                                                          Stuttgart – Hochschule für Technik.


                                                                                6
7.   Schnürch, R. (1998). Experimentelle Ermittlung
     geotechnischer Kenngrößen für die
     Setzungsberechnung bei überkonsolidierten Böden am
     Beispiel des Gipskeupers. Diplomarbeit,
     Fachhochschule Stuttgart – Hochschule für Technik,
     unveröffentlicht

8.   Hornig, E.-D. (2000). Fundamentprobebelastung auf
     verwitterten Gipskeuperböden, Spezialsitzung „Forum
     für junge Geotechnik-Ingenieure“. 26.
     Baugrundtagung, Hannover, 60-61.

9.   Vermeer, P. A., Brinkgreve R.B.J. (1998). PLAXIS
     Bedienungshandbuch Version 7.
     A.A. Balkema, Rotterdam.

10. Vermeer, P. A., Brinkgreve R.B.J. (1998). PLAXIS
    Material Model Manual Version 7.
    A.A. Balkema, Rotterdam.




                                                           7

								
To top