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

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Design and performance of a deep excavation for a wagon tippler S.R. Gandhi Professor in Dept. of Civil Engineering Indian Institute of Technology Madras, India srg@civil.iitm.ernet.

1 Design and performance of a deep excavation for a wagon tippler S.R. Gandhi Professor in Dept. of Civil Engineering Indian Institute of Technology Madras, India srg@civil.iitm.ernet.in bstract: s a part of bulk handling facility at one of the major ports on the east coast of India, a 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 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. INTRODUCTION t Paradip Port on the east coast of India, mechanized bulk handling system has been created which includes a wagon tippler with hopper and belt conveyor below the ground level to receive bulk material from railway wagon. The structure is located within the harbour where ground water table is very close to the ground level and strata is essentially cohesionless with high permeability. picture of the excavation is shown in Fig.. The main facility measures x 70m x deep and a perpendicular tunnel to carry the product to surface through a belt conveyor. The entire perimeter of this T shaped facility required a temporary support for excavation. fter construction of the diaphragm wall, dewatering was carried out followed by excavation of soil. Each stage of excavation was for m depth and horizontal struts were provided m c/c before excavating to the next stage. fter 0m depth, it was noticed that deep wells provided around the excavation were not adequate and the water level inside the excavation could not be lowered. To meet the construction schedule, the water level within excavation was lowered by placing 6 pumps into the excavation in small sumps. Direct removal of water within the excavation created a gradient between the outside ground water level and the water level within the diaphragm wall. This resulted in large quantity of seepage of water into the excavation. long 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 panels. 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.. Picture of excavation Foundation execution for such excavation required RCC diaphragm wall as temporary wall in view of the large depth and to reduce the number of struts for the construction convenience. In view of the stringent requirement of waterproofing, use of RCC diaphragm wall itself as vertical wall of the facility was not possible. It was therefore planned to complete the excavation with strutted diaphragm wall and then build the main structure within the excavated area. The base raft and vertical walls were cast-in-situ against formwork and provided with water proofing treatment.. DETILS OF EXCVTION The wagon tippler requires RCC watertight chamber measuring 70m x. The depth upto bottom of raft is about 7m for the central x area, whereas the depth at either end is about 8m. Plan of the structure is shown in Fig. and typical cross sections at shallow and deep portion are shown in Fig. and Fig. respectively. The penetration of 600mm thick diaphragm wall is generally m beyond the excavation level. This penetration was considered adequate in view of the steel struts provided m centre to centre. mong the tunnel wall the depth of diaphragm wall was gradually varied to ensure m

2 0 m 70m CONVEYOR TUNNEL C D E F Fa G H J K L M N P 600 THK DIPHRGM WLL OPEN PIT m m IN-SITU CONCRETE WLL Fig.. Plan of Wagon Tippler FSL.0 FILL RL.00 NGL FSL RL-9.00 RL-5.60 Fig.. Typical Section of THK DIPHRGM WLL centre to centre. mong the tunnel wall the depth of diaphragm wall was gradually varied to ensure m penetration below the excavation level. No diaphragm was provided at the junction between tunnel and the wagon tippler chamber. The ground level at the time of construction was +.0m which is to be raised to +.m after completion of the structure. The water table is almost at +.0m level.. STRT CONDITION The site area is fairly leveled having average elevation of +.m above mean sea level. The strata comprises essentially of silty fine to medium sand with an average permeability of 5.5X0-5 m/sec up to the explored depth of 8m. No rock or impervious layers has been encountered. The ground water table is about m below the ground level. Soil profile comprises of the following layers: Layer : From +.m to.m is clayey silt / silty clay having SPT(N) value in the range of to. Layer : From.m to 0.m is gray color silty fine to medium sand occasionally with clay binders. The (N) value in this layer varies from 5 to 0. Layer : From 0.m to 8.9m, the strata is clayey silt to silty clay having SPT(N) value in the range of 6 to 0. Layer : From 8.9m to explored depth of 8m, the strata is silty medium to coarse sand with traces of a mica and kankars. The SPT(N) value in this layer varies from 5 to 5.. DEWTERING SCHEME For the purpose of lowering ground water table, 8 deep wells were provided at location shown in fig.5. The depth of well varies from to m. Details of well depth, size of pump and discharge recorded is shown in Table. The size of pump varied to make use of the available equipments. The peak discharge of all the pumps together is about 78 m /hour. ased on the subsoil investigation carried out, the permeability of the silty sand strata is estimated as 5.5x0-5 m/sec. s per known theories of dewatering, the total discharge required for lowering the ground water table below is estimated as follows: πk q = l ( H h ) n R where, q = Total rate of discharge required in m /hour. k = Permeability constant in m/sec(5.5x0-5 ) H = Hydrostatic head of ground water level above the impervious layer () h = Hydrostatic head in the bore well after dewatering (m) R = Radius of influence for the given drawdown.

3 = Equivalent radius of the excavation = F = m π F =rea of excavation in m (800 m ) Table Dewatering well discharge details Sl. No. Depth of Well (m) Capacity verage Discharge Per Hr. (Ltrs) Remarks FSL.0 LL RL.00 FSL RL-9.00 RL RL-0.60 FLEXILE PIPE OPEN PIT 600 THK DIPHRGM WLL Fig.. Typical Section of - Substituting values the total discharge required for a drawdown upto works out to 65 m /hour. This is much higher compared to the total discharge achieved by the 8 deep wells provided around the excavation. The water level outside the wall could not be lowered below 5.0m level. To meet the construction schedule, it was decided to provide 6 pumps in open sumps made within the excavation to pump out the water directly from the excavation. This resulted in large seepage into the excavation and inflow of fine soil particles into the excavation. fter 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 excavation was discontinued. The programme of dewatering was reviewed to avoid such subsidence. 5. REMEDILMESURES s can be seen from Table, out of the 8 wells provided 5 wells were not operating due to various reasons. It was therefore decided to commission all the 8 wells. Some of the wells were found to be blocked due to the seepage of fines which were cleared by flushing and made operational. The capacity of pumps were increased in some of the wells from or 5 horse power to 5 horse power. With above changes it was possible to achieve a total discharge of 5 m /hour. No diaphragm wall was provided at the junction between the wagon tippler chamber and the conveyor tunnel. This area was considered to be a zone of heavy seepage due to lack of impervious barrier. It was therefore decided to provide a curtain grout by insitu injection of cement slurry. The grouting was carried out at the junction over the entire width of the tunnel between the excavated level of upto 7.. The width of the grouted area was -m. In addition, the outer side of the diaphragm wall, particularly along the X central area was grouted between elevation 0m to 9m to ensure plugging of any cavity that might have taken place around the diaphragm wall due to the seepage. lso, grouting pipes were inserted at the 5.00 m 0 No 0 0 HP HP 7 6 HP 5 0 No 6 HP HP HP COMP COMP HP 69 0 COMP 0 5 HP 800 No installed Not operating Not 0 HP HP Not 6 0 COMP HP COMP Not joints of diaphragm wall panels to ensure no seepage of water / soil through the joint. With enhanced pumping capacity the ground water table outside the diaphragm wall was lowered up to 9m (instead of achieved earlier). With reduced water table outside, it was then possible to adopt direct pumping within the excavation. This is because the hydraulic gradient now available was less and hence safe to prevent sand boiling. The settlement of diaphragm wall panels were continuously monitored and were found to have stabilized with the enhanced pumping. Dewatering through all the wells were continued till the completion of construction of the inner RCC chamber of the wagon tippler and back filling of the gap between the diaphragm wall and the wall of the wagon tippler. The backfilling was carried out after completion of water proofing treatment on the outerside of the wagon tippler walls. In view of the limited space available, back filling was done with clean sand placed in 00mm thick layers and compaction was achieved using surface vibrator. 6. DMGE TO DJCENT UILDING The subsidence created by heavy seepage into the excavation resulted in several cracks in the adjacent RCC uilding.

.90 7 70m.0 (NO PUMP) R 8mm P 56mm N 66mm (PUMP 0HP) NOTE:- (NEW ORE) (PUMP-HP) 8 6. 0 CONTROL UILDING 0 CONVEYOR TUNNEL.50 5.60.60 0.8 (PUMP 0HP) (PUMP HP) m (PUMP 6HP) LL DIMENSIONS R 5 6.

4 m.0 (NO PUMP) R 8mm P 56mm N 66mm (PUMP 0HP) NOTE:- (NEW ORE) (PUMP-HP) CONTROL UILDING 0 CONVEYOR TUNNEL (PUMP 0HP) (PUMP HP) m (PUMP 6HP) LL DIMENSIONS R m Q I 8mm m O G mm mm M E 7mm m C mm m 9mm 8.8 (PUMP-HP) 0.8 EXCVTION LEVEL EXCVTION LEVEL -.6 EXCVTION LEVEL (PUMP H J 5m 59mm H 9mm F 9mm D (PUMP 6 HP) (PUMP HP) (NEW ORE) (PUMP 6 HP) m (PUMP 5HP) 7 59mm K m L Fig.5. Lay-out of Dewatering Wells The two-storied RCC building is meant for housing of the control panels for the mechanized operation. The building measures about x0m in plan and supported on RCC piles with founding level upto m. The plan of the building with observed settlements of various columns is shown in Fig.6. s can be seen, the settlements are not uniform and due to the differential settlements the RCC framed building had severe cracks in the wall panel and RCC structure. picture with typical damage is shown in Fig.7. The settlements of the RCC building columns as well as of the diaphragm wall panels were recorded using surveying equipments. Target points were marked with oil paint on the structure and the RL of the target point were monitored with reference to a fixed bench mark about 00m away from the excavation. Similarly, the settlement in the adjoining ground were monitored using surveying staff. 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 points around each pile group of a column. The average depth of grout was 8-0m below the ground level. The cement consumption in each grout hole varied from 50 to 500 kg. fter 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 satisfactorily. Fig.6. Settlements in Control Room (in mm) construction. Unless adequate precautions are taken, uncontrolled seepage into the excavation can create lot of problems in excavation, construction and can also create large subsidence in the surrounding area. If the excavation is adjacent to existing structure, the design has to be carefully made to ensure no damage to the structure on account of subsidence. Whenever diaphragm wall is used as temporary support for the excavation, the water tightness of the joint plays an important role. The end stopper shall be designed such a way that the joint is fairly water tight. In case of doubt it is better to grout the joint prior to excavation and dewatering. lso penetration of the wall below excavation line shall be safe both for the passive resistance required as well as for reducing the hydraulic gradient to avoid soil boiling. 7. SUMMRY The execution of 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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