Proceedings of Indian Geotechnical Conference

December 15-17, 2011, Kochi (Invited talk-9)

DEEP BASEMENT EXCAVATION

S.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.

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.

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.

ii. 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.

iii. 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.

iv. 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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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.

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

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.

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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.

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.

0

20

40

60

80

0 20 40 60 80 100

-1.5 m

-5.4 m

-7.6 m

Fig1. Influence of the excavated depth on the ground

settlement (after Zhu and Liu,1994)

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.

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.00 0.50 1.00 1.50 2.00

Fig2. Maximum movement due to contiguous and

secant bored pile wall (after Puller, 2003)

Distance from wall/wall depth

Set

tlem

ent

(m)

Distance from wall (m)

hori

zonta

l m

ovem

ent/

wal

l

dep

th(%

)

secant

Contiguou

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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.

II. Complete RCC diaphragm wall along the perimeter of

the building. This will also serve as permanent

basement wall. III. 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.

V. 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.

VI. 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.

VII. Construct the bottommost basement floor supported

on piles leaving a construction joint along the

unexcavated area.

VIII. Raise the columns and subsequent floor of the higher

basement in the central area.

IX. Use the completed basement floors in the central area

to provide lateral support to the diaphragm wall with

steel struts. X. Remove the unexcavated soil along the perimeter to

the foundation level.

XI. Extract the dowel bars from the diaphragm wall and

complete the bottommost floor upto the construction

join.

XII. Complete balance columns and floor area of higher basement along the perimeter.

XIII. Remove temporary strut between the central portion

and diaphragm wall.

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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