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
ii.
iii.
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.
iv.
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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.
V.
VI.
VII.
VIII.
IX.
X.
XI.
XII. XIII.
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
II.
III.
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