Case study on failure of Retaining wall

1. Proceedings of Indian Geotechnical Conference
December 22-24,2013, Roorkee
CASE STUDY OF FAILURE OF RETAINING WALL AT DWARAKANAGAR,
VISAKHAPATNAM
S.V. Abhishek, PG Student, A.U. College of Engineering, Visakhapatnam, svabhi.92@gmail.com
V. Tarachand, PG Student, A.U. College of Engineering, Visakhapatnam, vtarachandg@gmail.com
C.N.V. Satyanarayana Reddy, Professor, A.U. College of Engg., Visakhapatnam, cnvsnreddy@rediffmail.com
ABSTRACT: A 6.1 m high cantilever basement retaining wall of a proposed multi-storeyed structure failed during
heavy rains caused by tropical storm “Neelam” on November 3, 2012 at Dwarakanagar, Visakhapatnam. The
retaining wall was designed by a structural engineer analogous to a framed structure using incorrect backfill
properties and was constructed with inadequate weep holes. The walls on all four sides of the boundary eventually
yielded in, with severe cracking at the corners. To investigate into the failure, samples of the backfill are collected
and analysed in the laboratory for its properties. Based on the properties obtained, the retaining wall is redesigned
for the expected lateral earth pressure and thereafter compared comprehensively with the design given by the
structural engineer. The causes for failure of the retaining wall are determined and suitable measures are suggested
to prevent the possible recurrence of such failures in the future.
INTRODUCTION
Most of the failures of retaining walls are due to
adoption of incorrect design parameters, improper
execution/construction or a combination of both.
Although the design of retaining walls is
considered to be the job of structural engineers,
geotechnical engineers play a significant role with
regard to selection of appropriate backfill, design
of wall for surcharge loads and suggestion of
measures for drainage of the backfill if suitable
materials are unavailable. Increased land costs and
lack of sufficient space has resulted in construction
of
many
multi-storeyed
structures
with
underground retaining walls to facilitate cellar and
sub-cellar parking. The construction of these
retaining walls needs proper attention if they had
been initially designed akin to a framed structure,
i.e., in conjunction with beams, columns and slab.
The present paper deals with the failure of a
basement retaining wall of a proposed multistoreyed
building
at
Dwarakanagar,
Visakhapatnam. The failure occurred on November
3, 2012 after the onset of tropical storm “Neelam”.
The building consists of eight storeys
accommodating two basement floors, one stilt floor
with five upper floors and is proposed to be used
partly for residential purpose and partly for
commercial establishments. The retaining wall is
6.1 m high and is of cantilever type. The retaining
wall was designed by a structural engineer of a
private firm, similar to a framed structure using
incorrect backfill properties. During the site visit, it
is observed that insufficient weep holes are
provided in the retaining wall and the walls on all
four sides of the boundary yielded in with severe
cracking at the corners (Fig. 1).
Crack at corner
Fig. 1 Failure of retaining wall with crack at corner
To investigate into the failure, samples of the
backfill are collected and analysed in the
laboratory for its properties. The retaining wall has
been redesigned based on these properties and
thereafter compared comprehensively with the
design given by the structural engineer. The causes
for failure of the retaining wall are determined and
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2. S.V. Abhishek, V. Tarachand & C.N.V. Satyanarayana Reddy
specific measures are suggested to prevent the
possible recurrence of such failures in the future.
SUBSOIL PROFILE
Prior to construction of the retaining wall, field
investigation in the form of standard penetration
test was conducted in five boreholes by a private
soil exploration agency in Visakhapatnam. Core
drilling using double core barrels was carried out
on encountering rocky strata and rock cores were
obtained. In general, the subsoil profile at the site
consisted of yellowish brown clayey sand in the
top 4.5 m with Standard Penetration Resistance (N)
of 12, overlying a 2.0 m thick layer of soft
disintegrated rock (SDR) with N>100. About 3.0 m
of soft rock with Core Recovery (CR) of 53% lies
below the SDR layer. This in turn is underlain by a
thick layer of hard rock having Core Recovery of
62% and Rock Quality Designation (RQD) of 33%.
The ground water table was not encountered within
the depth of exploration.
Properties of Backfill
During investigation of failure of the retaining
wall, samples of the backfill are collected and
laboratory tests are conducted as per IS:2720 [1].
The properties of the backfill obtained are
presented in Table 1.
Table 1 Backfill Properties
S. No. Property
1.
Specific Gravity
2.
Particle Size Distribution
a) Gravel (%)
b) Sand (%)
c) Fines (%)
3.
Plasticity Characteristics
a) Liquid Limit (%)
b) Plastic Limit (%)
c) Plasticity Index (%)
4.
IS Classification Symbol
5.
Shrinkage Limit (%)
6.
Natural Moisture Content (%)
7.
In-Situ Density (g/cc)
8.
Shear Parameters
a) Cohesion (kN/m2)
b) Angle of Internal Friction
Based on the particle size distribution and
plasticity characteristics, the backfill is classified
as clayey sand (SC) as per Indian Standard Soil
Classification System. For an in-situ density of
2.16 g/cc and natural moisture content of 18.2%,
the in-situ dry density of the backfill works out to
be 1.83 g/cc. The in-situ density is treated as
saturated density since the natural moisture content
is greater than liquid limit. The shear parameters
reported in Table 1 correspond to the saturated
state of the backfill.
DESIGN OF RETAINING WALL
Retaining Wall Designed as Framed Structure
Figure 1 shows the cross section of the retaining
wall and the detailing of reinforcement according
to the structural engineer’s design. The retaining
wall was designed considering the friction angle
and unit weight of the backfill as 370 and 19 kN/m3
respectively. Bending moments in the retaining
wall were calculated using STAAD software and
the area of reinforcement was fixed accordingly.
Maximum bending moment at the base of the stem
was 40 kNm.
Value
2.67
1
62
37
26.5
18.0
8.5
SC
16.4
18.2
2.16
10
260
(a) Cross section of retaining wall
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3. Case study of failure of retaining wall at Dwarakanagar, Visakhapatnam
shear failure, the safe bearing capacity estimated
from Teng’s equation [2] is 90 t/m2. But for an
allowable settlement of 25 mm, the safe settlement
pressure obtained from the equation specified by
IS:8009 (Part 1) [3] is 25 t/m2. As a result,
allowable bearing capacity of 25 t/m2 is adopted
for design of retaining wall.
(b) Detailing of reinforcement in wall and column
(c) Reinforcement detailing in beam and base slab
Fig. 1 Design of retaining wall as framed structure
The wall was founded in the SDR layer and was
constructed using M 25 grade concrete and Fe 415
grade steel with a clear cover of 40 mm and 25 mm
to the reinforcement on earth side and other faces,
respectively. Columns of size 450 mm x 300 mm
were proposed to be constructed at intervals of
3.2 m for proper bearing of floor beams onto the
retaining wall. The bottom beam of 300 mm width
and 600 mm depth was aimed at providing stiffness
to the columns and ensuring uniform distribution
of load onto the base slab. The retaining wall was
proposed to be connected to the main building at
the cellar roof slab level and again at the ground
floor level. Unfortunately, it failed soon after
construction, before the columns and beams could
be built.
Redesign of Retaining Wall
To verify the design given by the structural
engineer, the retaining wall is redesigned as a
reinforced cement concrete (R.C.C.) cantilever
wall (Fig. 2) based on limit state by incorporating
the shear parameters and density of backfill given
in Table 1. Since the retaining wall is founded in
SDR, a corrected standard penetration resistance of
50 is considered. Considering possible rise of
ground water table upto ground surface and
adopting a factor of safety of 3.0 against risk of
Fig. 2 Retaining wall redesigned as an R.C.C.
cantilever wall
The computed maximum bending moment and
shear force in the stem, toe slab and heel slab are
138 kNm, 67 kNm, 65 kNm and 89 kN, 94 kN,
100 kN respectively. The area of reinforcement and
development length are calculated as per
IS:456 [4]. The clear cover provided to all
reinforcement in the stem and base slab are 40 mm
and 50 mm respectively [4]. To satisfy the
development length criterion, the main and
distribution reinforcement of the stem are to be
anchored into the base slab over a distance of
840 mm and 340 mm respectively.
DISCUSSION
Table 2 compares the two retaining wall designs
illustrated earlier. It can be observed that by
designing the retaining wall as a conventional
R.C.C. cantilever wall, the section and percentage
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4. S.V. Abhishek, V. Tarachand & C.N.V. Satyanarayana Reddy
of reinforcement required are much higher when
compared to the integrated frame design. Due to
unforeseen delay in construction of beams and
columns caused by various reasons and due to
saturation of backfill owing to heavy rains of storm
“Neelam”, the retaining wall yielded in. This is
reflected by the very low factor of safety (F.S.) of
0.37 with respect to overturning.
Table 2 Comparison of retaining wall design
Description
Design of
Redesign
Retaining as R.C.C.
Wall as
Cantilever
Base Pressure at Heel -593.9 kPa 19.0 kPa
F.S. (Overturning)
0.37
2.21
F.S. (Sliding)
0.71
1.84
Main Reinforcement
(a) Stem
754 mm2 2244 mm2
(b) Toe Slab
754 mm2 1436 mm2
(c) Heel Slab
524 mm2 1745 mm2
Distribution Steel
(a) Stem
524 mm2
457 mm2
2
(b) Base Slab
393 mm
457 mm2
Although the factor of safety with respect to sliding
is also quite low, the mobilization of passive
resistance of soil possibly averted sliding of the
retaining wall. The formation of cracks at the
corners of the boundary is attributed to deficient
wall section, underprovided reinforcement and
separation of heel slab from foundation soil due to
overturning. Absence of proper weep holes also
resulted in additional lateral thrust being exerted on
the wall by the saturated backfill. Lack of
provision of a temporary supporting system during
setback in progress of work is furthermore
considered to be one of the reasons behind the
failure of the retaining wall.
Foundation Depth
Base Slab
(a) Toe Slab Width
(b) Heel Slab Width
(c) Total Width
(b) Thickness
Stem
(a) Height
(b) Thickness
Resultant Eccentricity
Base Pressure at Toe
Framed
Structure
0.60 m
Retaining
Wall
1.00 m
0.67 m
0.53 m
1.20 m
230 mm
1.00 m
1.50 m
2.50 m
350 mm
5.87 m
230 mm
2.12 m
717.5 kPa
6.15 m
350 mm
0.33 m
161.4 kPa
grouping it with the design of beams and columns
(unlike a framed structure). Otherwise, suitable
temporary supporting systems must be assembled
to support the wall in the eventuality of any
unanticipated delay in construction of the cellar
and sub-cellar structural components.
REFERENCES
1. IS:2720, Methods of tests for soils – relevant
parts, BIS, New Delhi.
2. Teng, W.C. (1962), Foundation Design, Wiley,
New York.
3. IS:8009 (Part 1)-1976, Code of practice for
calculation of settlement of foundations
(shallow foundations subjected to symmetrical
static vertical loads), BIS, New Delhi.
4. IS:456-2000, Code of practice for plain and
reinforced concrete, BIS, New Delhi.
CONCLUSIONS
A combination of various factors such as improper
interpretation of backfill properties, absence of
proper weep holes and alteration in the behaviour
of the wall due to delay in progress of work, are
considered to be responsible for failure of the
retaining wall. It is desirable to design and
construct a basement retaining wall as a
conventional, distinct retaining wall rather than
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