Proceedings of Indian Geotechnical Conference December 15-17, 2011, Kochi (Invited talk-9)

DEEP BASEMENT EXCAVATIONS.R. Gandhi, Dept of Civil Engineering, Indian Institute of Technology Madras

ABSTRACT: In view of the space constraint, most of the commercial buildings as well as residential buildings require multilevel basements for utilities like car parking, refrigeration unit, affluent treatment plant, etc. For several infrastructure projects like metro rail, parking lots in commercial area, shopping malls, etc underground structures are preferred to preserve the landscaping in the area. Excavations up to a depth of 15-20m are very common for most of the projects. To maximize the space available, the basement extends not only under the entire building area but also extends up to the property line. Some of these property lines are edge of a busy street with heavy traffic which makes the excavation and construction challenging. This paper describes common methods adopted for such deep excavation, common problem faced while executing the excavation and remedial measures that can be adopted. Few case studies have been described highlighting typical problems.

INTRODUCTION With recent upsurge in commercial/residential multi-storied buildings, there has been increasing requirements of car parking and other utilities. This requires 3 to 4 basements in most of the buildings with large floor area. Such buildings are situated at strategic points with congested roads around the site and hence execution of deep basement excavation poses several challenging problems. Conventional technique of sheet pile or diaphragm wall is often inadequate due to the large depth of the excavation. Also providing anchors or strut is difficult in view of presence of utility trenches outside and large scale construction activities within the excavation area which has to be completed on a very tight construction schedule.

DIFFICULTIES ASSOCIATED WITH DEEP EXCAVATION Following difficulties have to be addressed while planning the excavation scheme: i. In view of the large volume of soil to be removed, it is preferred to have mechanized excavation. This is carried out either with mechanical excavators or with dozers which operate within the excavation area. This will require provision of a suitable ramp/access for lowering these equipments to the final excavation level. The ground water table is often very high and requires large scale dewatering to reduce water pressure on the retaining walls and to make the excavation stable from sand boiling/piping failure. Such large scale dewatering can result in subsidence in the surrounding area due to the increased effective stress. In many countries, large scale dewatering for such construction propose is not permitted and the excavation scheme has to be designed considering the hydrostatic pressure on the retaining structure. The natural strata below the excavation level is often comprising of loose sand or soft marine clay deposit which do not provide adequate passive resistance to the retaining structure to act as a cantilevering wall and thereby requires either ground improvement or additional anchors/struts. The plan dimensions of some of the commercial buildings are very large exceeding 50 to 100m. Design of strut for such span with large l/r is not

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In view of the above, use of temporary retaining wall such as sheet pile is very difficult and open unsupported excavations is often not possible due to space constraints. This paper describes the difficulties in execution, alternative methods of executing deep excavations in above situation. Few case studies will be discussed during the lecture.

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S.R. Gandhi possible. Also presence of struts significantly affects the construction activities. v. It is one of the requirements that the basement floors are free from seepage of water. This requires fairly good waterproofing of the basement walls and floor. Even in case of RCC diaphragm wall, the joint between the panels has to be made water tight either using a PVC rubber stopper or extensive grouting along the entire depth of the joint. In several cases, the water tightness of RCC diaphragm wall is questioned and as a result permanent wall is made using in-situ concrete with formwork after excavating with temporary support. In such case, appropriate waterproofing treatment can be provided on the outer side of the wall before backfilling, but this increases the cost. execution of permanent structure within the excavated area. Due to much higher rigidity compared to steel sheet pile, this wall can cantilever for a large height. Also, the spacing of the strut or anchors can be reduced. It is also possible to use a T shaped section which can cantilever for a very large height.

i. Secant Pile Wall Bored-cast-in-situ piles, almost touching each other in a row have been used as a retaining structure. Depending on depth of excavation, the piles can be provided with intermittent support with anchors or struts. If the soil retained is cohesionless with high water table, the zone between the piles may need cement grouting or inserting additional pile to prevent escape of soil through the joint. The top of all the piles is normally connected with a common copping beam which makes all the piles as an integral wall.

RETAINING WALLS COMMONLY ADOPTED Following types of retaining elements are commonly adopted: Steel Sheet Pile Wall This has an advantage of easy installation and subsequent retrieval for reuse. It is ideally suited for temporary application where the bending moment expected is not very high. Beyond certain depth (3 to 4m) this will require either anchors or strut to reduce the bending moment. Large number of steel sections are available depending on the requirements. Extending length of the sheet pile by welding another section axially or removing excess length by gas cutting is very simple. RCC Diaphragm Wall Concrete diaphragm wall varying in thickness from 600mm to 1m is often used either for temporary use or for permanent use as basement wall. Unlike steel sheet pile, it is not possible to retrieve the concrete wall and hence this is attractive only where the wall forms part of a permanent basement wall. However there are cases where RCC diaphragm wall has been used as a temporary wall which is left buried in the ground after

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Deep Basement Excavation

Berlin Wall In this method wide flange steel sections are inserted along the excavation line with a centre to centre spacing of about 1m. The sections are either driven into the ground or they are lowered in a pre-bored hole. The gap between the bore hole wall and the section is filled with concrete from the bottom upto the excavation level. Beyond this the gap is filled with soil. The excavation is carried out in stages of 0.5 to 1m and as the excavation progresses, wooden plank or steel formwork plate is inserted between the steel sections to retain the soil. The horizontal thrust of retained earth is transferred to the steel section through the flange.

Distance from wall/wall depth0 0 20 40 60 80 20 40 60 80 100

Settlement (m)

-1.5 m -5.4 m -7.6 m

Fig1. Influence of the excavated depth on the ground settlement (after Zhu and Liu,1994)Nailed Wall As the excavation progresses, the vertical face of the excavation is supported by either steel plate or wooden plank which is nailed into the ground using long reinforcement rod. After nailing the plate, the excavation is advanced by further 0.6 to 1m and another plate/plank is placed and nailed. It is possible to retrieve the planks/plates as well as the nails for reuse. However unlike other methods, it is not possible to have a vertical cut. The face of the retained earth is normally inclined at 70 to 80 degrees with the horizontal. SOIL MOVEMENT DUE TO EXCAVATION Based on monitoring of foundation excavation, it is noticed that the soil behind the retaining wall undergoes vertical and lateral movement to a considerable distance. The movement has to carefully checked and corrective measures are required to be adopted to minimize this movement. Several case studies are reported where adjacent structures are found to be severally damaged due to excavation. Fig.1 shows typical settlement recorded behind the wall as the depth of the excavation increases from 1.5m to 7.6m. As can be seen, the settlement extends upto a distance of 60m from the wall. Similarly fig.2 shows the horizontal displacement of the ground with distance from the wall in a non dimensional form normalized with height of the wall.Not much published work is available in this area and it is preferable that settlement monitoring is carried out wherever such deep excavations are executed.

horizontal movement/wall depth(%)

Distance from wall (m)0.50 1.00 1.50

0.00

2.00

-0.04 -0.02 0.00 0.02 0.04 0.06 0.08

Contiguou secant

Fig2. Maximum movement due to contiguous and secant bored pile wall (after Puller, 2003)

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S.R. Gandhi ALTERNATIVE EXCAVATION SCHEMES Following alternatives can be considered for deep basements were struts cannot be provided in view of the large plan dimension of the building: Excavation with Peripheral Soil Support Excavation of the central area alone, leaving soil with slope along the perimeter to support the retaining wall. In this concept, it is possible to reduce the section of retaining wall but it has following disadvantages: Construction joint is required in the basement floors. For completion of balance excavation along the perimeter, it may not be possible to use excavators due to limited space and manual excavation only can be adopted which is time consuming. Top-Down construction In this concept, after completion of perimeter retaining wall (RCC Diaphragm) and pile foundation at column locations, the ground floor slab is cast first connected to the peripheral diaphragm wall and the piles. Openings are provided at required locations to remove the earth subsequently. These openings are normally at location of staircase, lift well or ramp for vehicle movement. The slab can be cast on the natural ground itself and hence no formwork is required. After this, the soil below the slab is excavated upto the next basement level. The slab already cast serves as strut to support the wall. The first basement floor is then cast leaving again openings for second level basement excavation and the procedure above is repeated. While the construction of basements is in progress, the work of raising the building above ground level can also been taken up simultaneously. CASE STUDY FOR EXCAVATION IN SOFT CLAY A typical case study is discussed where 3 basement excavations is required to be executed through soft marine clay. Even at the bottommost basement, the shear strength of the strata was very low and required pile foundation to support the structure. Following construction scheme was adopted: I. Provide cement injection grouting for a width of 2m on either side of the diaphragm wall to improve the stability of the diaphragm trench and to reduce the active pressure and to increase the passive resistance. Complete RCC diaphragm wall along the perimeter of the building. This will also serve as permanent basement wall. Complete pile construction within the building area. The piles are constructed from the existing ground level, but the concrete is poured only upto the required level of the bottommost basement. IV. The excavation is carried out for a depth of 4m throughout the building area. This is maximum height of excavation which the RCC diaphragm wall can permit as cantilever. Provide peripheral dewatering outside the diaphragm wall to lower the water table and reduce bending moment on the wall. Do not pumpout water within the excavation area. Leaving a berm of 4 to 5m width from the diaphragm wall, excavate the central area of the building with a convenient slope to the final founding level. At this level, the piles already constructed will project out. Chip-off the extra concrete to the required cut-off level. Construct the bottommost basement floor supported on piles leaving a construction joint along the unexcavated area. Raise the columns and subsequent floor of the higher basement in the central area. Use the completed basement floors in the central area to provide lateral support to the diaphragm wall with steel struts. Remove the unexcavated soil along the perimeter to the foundation level. Extract the dowel bars from the diaphragm wall and complete the bottommost floor upto the construction join. Complete balance columns and floor area of higher basement along the perimeter. Remove temporary strut between the central portion and diaphragm wall.

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Various steps involved in construction will be discussed during the lecture. REFERENCES 1. Berlie Zhu and Guobin Liu, (1994), elasto plastic analysis of deep excavation in soft clay, Proc of 13th International Conference in Soil Mechanics and Foundation Engineering, New Delhi, India. 2. Malcolm Puller (2003), Deep excavation a practical manual 2nd Edition, Thomas Telford Ltd, 1 Heron Quay, London E14 4JD

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