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Design and performance of a 15m deep excavation for a wagon tippler


									Design and performance of a 15m deep excavation for a wagon tippler

S.R. Gandhi
Professor in Dept. of Civil Engineering
Indian Institute of Technology Madras, India

Abstract: As a part of bulk handling facility at one of the major ports on the east coast of India, a 15m deep excavation was
carried out for construction of a wagon tippler and belt conveyor tunnel. RCC diaphragm wall has been provided as a
temporary support for 20m x 70m excavation. During execution, the dewatering system provided was found to be inadequate
in view of the high permeability of cohesionless strata. To meet construction schedule, direct pumping of water was adopted
from the excavation which resulted in large quantity of seepage into the excavation and settlement of the diaphragm wall
panels. This also resulted in considerable subsidence of the surrounding area resulting in severe distresses to a building
adjacent to the diaphragm wall. This paper describes the details of the structure, strata condition, design of excavation,
problems faced during execution and the remedial measures adopted.

1   INTRODUCTION                                                     After construction of the diaphragm wall, dewatering
                                                                was carried out followed by excavation of soil. Each stage
  At Paradip Port on the east coast of India, mechanized        of excavation was for 3m depth and horizontal struts were
bulk handling system has been created which includes a          provided 3m c/c before excavating to the next stage. After
wagon tippler with hopper and belt conveyor 15m below the       10m depth, it was noticed that deep wells provided around
ground level to receive bulk material from railway wagon.       the excavation were not adequate and the water level inside
The structure is located within the harbour where ground        the excavation could not be lowered. To meet the
water table is very close to the ground level and strata is     construction schedule, the water level within excavation was
essentially cohesionless with high permeability. A picture of   lowered by placing 6 pumps into the excavation in small
the excavation is shown in Fig.1. The main facility measures    sumps. Direct removal of water within the excavation
20m x 70m x 15m deep and a perpendicular tunnel to carry        created a gradient between the outside ground water level
the product to surface through a belt conveyor. The entire      and the water level within the diaphragm wall. This resulted
perimeter of this ‘T’ shaped facility required a temporary      in large quantity of seepage of water into the excavation.
support for excavation.                                         Along with seepage, large quantity of silt and fine sand
                                                                seems to have entered the excavation resulting in loss of
                                                                frictional resistance for the diaphragm wall and subsidence
                                                                of some of the diaphragm wall panels. This also resulted in
                                                                significant settlement of the surrounding area.
                                                                     Due to excessive seepage into the excavation, following
                                                                consequences were noticed:

                                                                     i)        Settlement of some of the diaphragm wall
                                                                     ii)       Slippage of struts and buckling of the
                                                                               members of strut due to eccentricity and
                                                                     iii)      Severe cracks in the control building adjacent
                                                                               to the excavation.

Fig.1. Picture of excavation                                    2.          DETAILS OF EXCAVATION

    Foundation execution for such excavation required RCC           The wagon tippler requires RCC watertight chamber
diaphragm wall as temporary wall in view of the large depth     measuring 70m x 20m. The depth upto bottom of raft is
and to reduce the number of struts for the construction         about 17m for the central 20m x 20m area, whereas the
convenience. In view of the stringent requirement of            depth at either end is about 8m. Plan of the structure is
waterproofing, use of RCC diaphragm wall itself as vertical     shown in Fig.2 and typical cross sections at shallow and
wall of the facility was not possible. It was therefore         deep portion are shown in Fig.3 and Fig.4 respectively.
planned to complete the excavation with strutted diaphragm          The penetration of 600mm thick diaphragm wall is
wall and then build the main structure within the excavated     generally 3m beyond the excavation level. This penetration
area. The base raft and vertical walls were cast-in-situ        was considered adequate in view of the steel struts provided
against formwork and provided with water proofing               3m centre to centre. Among the tunnel wall the depth of
treatment.                                                      diaphragm wall was gradually varied to ensure 3m


                                                                                             CONVEYOR TUNNEL

                           A    A       B        C          D                        E   F         Fa   G   H     J       K    B        L   M   N   P    6 0 0 T H K D IA P H R A G M
                                                                                                                                                                           W ALL


                                                                O P E N P IT                                                                             IN -S IT U C O N C R E T E
                                                                                                                                                                        W ALL




                            A                                                                                                  B

Fig.2. Plan of Wagon Tippler

     FSL 4.40                   1           2        3

     FILL                                                                                     Layer 2:
                 RL 2.00
                                                                                              From –4.4m to –10.4m is gray color silty fine to medium
     NGL                                                                                      sand occasionally with clay binders. The (N) value in this
                                                                                              layer varies from 15 to 40.
                                                                                              Layer 3:
       FSL -5.55                                                                              From –10.4m to –18.9m, the strata is clayey silt to silty
                                                                         20m                  clay having SPT(N) value in the range of 6 to 20.
                                                                               2 m

       RL-9.00                                                                                Layer 4:
                                                                                              From –18.9m to explored depth of –38m, the strata is
                           600 THK   DIAPHRAGM       WALL
                                                                                              silty medium to coarse sand with traces of a mica and
                                DEWATERING WELL                                               kankars. The SPT(N) value in this layer varies from 25 to
           RL-15.60                                                                           45.
                                     DEWATERING WELL
                                                                                              4.                DEWATERING SCHEME
Fig..3 Typical Section of A-A
                                                                                              For the purpose of lowering ground water table, 18 deep
centre to centre. Among the tunnel wall the depth of                                          wells were provided at location shown in fig.5. The depth
diaphragm wall was gradually varied to ensure 3m penetration                                  of well varies from 20m to 33m. Details of well depth,
below the excavation level. No diaphragm was provided at the                                  size of pump and discharge recorded is shown in Table 1.
junction between tunnel and the wagon tippler chamber. The                                    The size of pump varied to make use of the available
ground level at the time of construction was +2.0m which is to                                equipments. The peak discharge of all the pumps together
be raised to +4.4m after completion of the structure. The water                               is about 78 m3/hour. Based on the subsoil investigation
table is almost at +2.0m level.                                                               carried out, the permeability of the silty sand strata is
                                                                                              estimated as 5.5x10-5 m/sec. As per known theories of
3.                  STRATA CONDITION                                                          dewatering, the total discharge required for lowering the
                                                                                              ground water table below –15m is estimated as follows:
The site area is fairly leveled having average elevation of
                                                                                                                πk (H 2 − h 2 )
+1.1m above mean sea level. The strata comprises essentially                                            q=
of silty fine to medium sand with an average permeability of                                                          R
5.5X10-5 m/sec up to the explored depth of –38m. No rock or                                                        ln  
impervious layers has been encountered. The ground water                                                               A
table is about 1m below the ground level. Soil profile                                        where,
comprises of the following layers:                                                            q                 = Total rate of discharge required in m3/hour.
Layer 1:                                                                                      k                 = Permeability constant in m/sec(5.5x10-5)
From +1.1m to –4.4m is clayey silt / silty clay having SPT(N)                                 H                 = Hydrostatic head of ground water level above
value in the range of 1 to 4.                                                                                     the impervious layer (30m)
                                                                                              h                 = Hydrostatic head in the bore well after
                                                                                                                   dewatering (13m)
                                                                                              R                 = Radius of influence for the given drawdown.

                                                                   Table 1Dewatering well discharge details
A               = Equivalent radius of the excavation
                       F                                            Sl.      Depth    Pump       Average          Remarks
                =            = 24m                                  No.        of    Capacity    Discharge
                       π                                                     Well                 Per Hr.
F               =Area of excavation in m2 (1800 m2)                           (m)                  (Ltrs)

      FSL 4.40
                             FLEX IBLE PIPE                             1    20      No                        No        Pump
                         1           2     3
                                                                                     Pump                      installed
LL R L 2.00
                                                                        2    20      20 HP           16000
                               O PEN PIT                                3    30      3 HP             1714
                                                                        4    33      16 HP            2323
     FSL -5.55
                                                                        5    30      No                        Not operating
     R L-9.00                                                                        Pump

                                                      35.00 m
                                                                        6    21      3 HP              6261
                                                                        7    30      5 HP              1600
  R L-14.60
                                                                        8    30      3 HP              7579
                                                                        9    30      COMP              4800
                    600 TH K D IA PH R A G M W A LL
 -17.50                                                                              AIR
                    D EW A TER IN G W ELL                               10   30      COMP              6000
                         D EW A TER IN G W ELL
                                                                        11   30      15 HP             3692
    R L-30.60
                                                                        12   20      COMP                      Not
Fig.4. Typical Section of B-B                                                        AIR                       commissioned
                                                                        13   20      15 HP             4800
     Substituting values the total discharge required for a             14   30      3 HP              1920
drawdown upto –15m works out to 165 m3/hour. This is much               15   30      3 HP                      Not
higher compared to the total discharge achieved by the 18                                                      commissioned
deep wells provided around the excavation. The water level
outside the wall could not be lowered below –5.0m level.                16   20      COMP            13091
    To meet the construction schedule, it was decided to                             AIR
provide 6 pumps in open sumps made within the excavation to             17   20      20 HP             8000
pump out the water directly from the excavation. This resulted          18   20      COMP                      Not
in large seepage into the excavation and inflow of fine soil                         AIR                       commissioned
particles into the excavation. After noticing the large
subsidence in the surrounding areas and settlement of some of
the wall panels, the direct pumping from the 6 wells within the    joints of diaphragm wall panels to ensure no seepage of
excavation was discontinued. The programme of dewatering           water / soil through the joint.
was reviewed to avoid such subsidence.                                  With enhanced pumping capacity the ground water
                                                                   table outside the diaphragm wall was lowered up to –9m
5.              REMEDIALMEASURES                                   (instead of –5m achieved earlier). With reduced water
                                                                   table outside, it was then possible to adopt direct
As can be seen from Table 1, out of the 18 wells provided 5        pumping within the excavation. This is because the
wells were not operating due to various reasons. It was            hydraulic gradient now available was less and hence safe
therefore decided to commission all the 18 wells. Some of the      to prevent sand boiling. The settlement of diaphragm wall
wells were found to be blocked due to the seepage of fines         panels were continuously monitored and were found to
which were cleared by flushing and made operational. The           have stabilized with the enhanced pumping.
capacity of pumps were increased in some of the wells from 3            Dewatering through all the wells were continued till
or 5 horse power to 15 horse power. With above changes it          the completion of construction of the inner RCC
was possible to achieve a total discharge of 115 m3/hour.          chamber of the wagon tippler and back filling of the gap
    No diaphragm wall was provided at the junction between         between the diaphragm wall and the wall of the wagon
the wagon tippler chamber and the conveyor tunnel. This area       tippler. The backfilling was carried out after completion
was considered to be a zone of heavy seepage due to lack of        of water proofing treatment on the outerside of the wagon
impervious barrier. It was therefore decided to provide a          tippler walls. In view of the limited space available, back
curtain grout by insitu injection of cement slurry. The grouting   filling was done with clean sand placed in 300mm thick
was carried out at the junction over the entire width of the       layers and compaction was achieved using surface
tunnel between the excavated level of –5m upto –17.5m. The         vibrator.
width of the grouted area was 2-3m.
    In addition, the outer side of the diaphragm wall,             6.        DAMAGE TO ADJACENT BUILDING
particularly along the 20m X 20m central area was grouted
between elevation –10m to –19m to ensure plugging of any           The subsidence created by heavy seepage into the
cavity that might have taken place around the diaphragm wall       excavation resulted in several cracks in the adjacent RCC
due to the seepage. Also, grouting pipes were inserted at the      Building.

                                                                                                                                                                                                       R                  P               N
                                                                        3.90                                               (NO PUMP)                                                                   338mm           256mm          166mm
                                                                         17                                   3.0

                                                              (PUMP 20HP)                                             1
                                                                                                                                                            ALL DIMENSIONS AR

                                                 18                                                                                                                                                                                                    120mm
                          (NEW BORE)                                                                                                                                                                   355mm            251mm         123mm
                                                             16.2                                                                  20m
                                                                                     CONVEYOR                 3.50                                                                                     Q                  O              M               C
                                                                10                                                                 (PUMP 20HP)
                                                                                      TUNNEL                                                                                                           I                  G              E                       134mm A
                                                                                                                      2                                                                                428mm           322mm           172mm



                                                            CONTROL                                                                (PUMP 3 HP)                   30m
                                                            BUILDING                                                                                             (FLUSHING)
                        (FLUSHING)                                                                                         3         33m
                        (PUMP-3HP)                                                                                                                           5
                                                                                                                                   (PUMP 16HP)

                                   15                                                                                                      4



                         30m                           16


                                                                                                                                                                                                                                                               139mm   B
                                2.6       7.70                                                                      9.50                       15.5

                       30m                                                                                                                                         3.70 6
                                                 EXCAVATION                          EXCAVATION                      EXCAVATION

                 14                               LEVEL -5.55                         LEVEL -14.6                     LEVEL -5.55                                         30m
                                                                                                                                                                        (PUMP 3 H
                       10.8                                                                                                                                                                            J   455mm         359mm          239mm          239mm

                                                                                                                                                                                                                          H                   F          D
                                                                                                                                                                                                                          4                   6          7             8
                                                                                                                                                                                                       592mm                     5m               5m            5m

                                 4.4            8.7             10.6           8.6               13.2                8.5                       11.6          4.5




                                                                                                                         8                                                                         K               L
                                                                                                                      30m                                         21m
                                                              12        11        10                           9    (PUMP 3HP)                                   (PUMP 5HP)                        2
                                20m 13                                               30m
                               (PUMP 16 HP)                    30m       30m        (FLUSHING)                   30m                                                                                                                      20m
                                                            (NEW BORE) (PUMP 16 HP)                             (FLUSHING)

            Fig.5. Lay-out of Dewatering Wells                                                                                                                                      Fig.6. Settlements in Control Room (in mm)

     The two-storied RCC building is meant for housing of the                                                                                                                       construction. Unless adequate precautions are taken,
control panels for the mechanized operation. The building                                                                                                                           uncontrolled seepage into the excavation can create lot of
measures about 20mx10m in plan and supported on RCC piles                                                                                                                           problems in excavation, construction and can also create
with founding level upto –12m. The plan of the building with                                                                                                                        large subsidence in the surrounding area. If the
observed settlements of various columns is shown in Fig.6. As                                                                                                                       excavation is adjacent to existing structure, the design has
can be seen, the settlements are not uniform and due to the                                                                                                                         to be carefully made to ensure no damage to the structure
differential settlements the RCC framed building had severe                                                                                                                         on account of subsidence.
cracks in the wall panel and RCC structure. A picture with                                                                                                                              Whenever diaphragm wall is used as temporary
typical damage is shown in Fig.7.                                                                                                                                                   support for the excavation, the water tightness of the joint
     The settlements of the RCC building columns as well as of                                                                                                                      plays an important role. The end stopper shall be
the diaphragm wall panels were recorded using surveying                                                                                                                             designed such a way that the joint is fairly water tight. In
equipments. Target points were marked with oil paint on the                                                                                                                         case of doubt it is better to grout the joint prior to
structure and the RL of the target point were monitored with                                                                                                                        excavation and dewatering. Also penetration of the wall
reference to a fixed bench mark about 100m away from the                                                                                                                            below excavation line shall be safe both for the passive
excavation. Similarly, the settlement in the adjoining ground                                                                                                                       resistance required as well as for reducing the hydraulic
were monitored using surveying staff.                                                                                                                                               gradient to avoid soil boiling.
     Ideally, the structure should have been built only after
completion of the wagon tippler and backfilling of the same.
However, due to the tight construction schedule, it was
decided to take up the building construction along with the
wagon tippler construction. In spite of improved dewatering
system, it was feared that some more subsidence may take
place. Therefore no repair was carried out till the entire wagon
tippler construction was completed and the sides of the wall
were backfilled. Even after the cracks, the structure was found
to be otherwise safe for use with suitable strengthening.
Grouting was carried out in soil strata with minimum 4 points
around each pile group of a column. The average depth of
grout was 8-10m below the ground level. The cement
consumption in each grout hole varied from 150 to 500 kg.
After completion of the grouting, the settlements of the
column were found to have stabilized
Fig.7. Picture Showing Typical Damage
and thereafter the building repair was taken up. The building
could be put to operation and has been functioning

7.                            SUMMARY

The execution of 15m deep excavation for the wagon tippler
revealed the importance of proper dewatering scheme for such                                                                                                                        Fig.7. Picture Showing Typical Damage


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