SETTLEMENT OF A STRUCTURE -A CASE STUDY

Abstract

A (G+2) Building intended for public use as an institutional building showed signs of unequal settlement and cracks when the superstructure was under construction. The construction of plinth beams was complete when the cracks were noticed. Visual observations at site showed inclined cracks developed in the plinth beams and were localized near a corner of the building Structure. Preliminary Site investigation was carried out to reconfirm the soil parameters assumed in the design. Further deep soil investigation was carried out to confirm the preliminary findings. Based on the detailed soil investigation, the cause of the distress was ascertained and remedial measures were taken and the building was restored and further construction was resumed.

Full text

Proceedings of Indian Geotechnical Conference
December 15-17,2011, Kochi (Paper No.Q-285)
SETTLEMENT OF A STRUCTURE – A CASE STUDY
Shruti Naik, Design Engineer, M/s Alphacon Consultants Pvt. Ltd., Panjim – Goa, shruti0811@yahoo.in
Nisha P. Naik, Lecturer in Civil Engineering, GEC Farmagudi - Goa, nisha@gec.ac.in
Sumitra S. Kandolkar,.Lecturer in Civil Engg. Govt Polytechnic Bicholim, – Goa, sumitra_tari@yahoo.com
R. L. Mandrekar, Retd. Superitending Engineer, PWD Goa, rlmandrekar@yahoo.co.in
ABSTRACT: A (G+2) Building intended for public use as an institutional building showed signs of unequal settlement
and cracks when the superstructure was under construction. The construction of plinth beams was complete when the
cracks were noticed. Visual observations at site showed inclined cracks developed in the plinth beams and were localized
near a corner of the building Structure. Preliminary Site investigation was carried out to re-confirm the soil parameters
assumed in the design. Further deep soil investigation was carried out to confirm the preliminary findings. Based on the
detailed soil investigation, the cause of the distress was ascertained and remedial measures were taken and the building was
restored and further construction was resumed.
INTRODUCTION
Settlement in a Structure can occur due to various factors
such as underground erosion, structural collapse of soil,
thermal changes, frost heave, vibration & shocks,
Landslides, creep and mining subsidence, besides the
inherent variable soil conditions. Differential Settlement or
unequal settlement can occur due to variable soil
conditions and can be detrimental to the Structure,
depending on the type of the structure. Cracks in a RCC
framed structure occur due to various factors, being
inherent in the design of RC Section. When the cracks are
unexpectedly noticed in a structure at a stage when the load
has not exceeded even 10% of its full capacity, it becomes a
matter of concern and a warning sign of an impending
failure. In this paper a case study is presented of a (G+2)
Institutional Building located in South Goa which
developed cracks when the construction had reached till the
plinth beam level.
CASE STUDY
The Building under study was a (G+2) RC Framed
Structure then under construction. The Building is intended
for use as an institutional building. The foundation work
was complete and the Superstructure construction was in
progress, when cracks were noticed in the plinth Beams.
Fig 1 is a photograph taken on the Site, which shows cracks
in plinth beams. The cracks were noticed in a portion of the
Building near the front left corner, which is marked in Fig
2. The working drawings were studied.
Observations on the Site
It was observed that –
(i) The cracks in the plinth beams were inclined.
(ii) The safe bearing capacity assumed in footing
design was not stated on the foundation drawings
(iii) The nature of the founding strata was not described
on the working drawings.
STAGES OF INVESTIGATION
The investigation was carried out in the following stages-
(i) Reconnaissance
(ii) Preliminary soil investigation
(iii) Detailed soil Investigation
(i) Reconnaissance
The Site was re-inspected and the available maps were
studied. Laterite outcrops were found all over the site as is
typically in the State of Goa. The study indicated the
existence of an abandoned laterite quarry at the location of
the site. Subsequently, it was backfilled before the present
construction had started.
(ii) Preliminary Soil Investigation
Preliminary soil Exploration was carried out to ascertain the
necessity and the extent of the detailed soil exploration.
This was done using a crude method of driving a 20 mm
diameter steel rods at various points as shown Fig 3. It was
found that the rods penetrated upto a depth of 5.4 m without
Fig 1 - Cracks in plinth beams
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Shruti Naik, Nisha P. Naik & R. L. Mandrekar
any resistance. This indicated that the soil was very loose
and unconsolidated. It was also found necessary to carry
out detailed Soil Exploration. Accordingly, a detailed Soil
exploration programme was chalked out.
Fig 3 - 20mm diameter steel rods driven at various
points
(iii) Detailed Soil Investigation
Bore holes were driven at different locations to ascertain
the engineering properties and to investigate the variation if
any in the assumed design parameters. SPT (Standard
Penetration test) was conducted at different intervals of the
affected area, to define the size of abandoned quarry / filled
up ground, by collecting samples of undisturbed soil and to
determine the depth of natural ground. Fig 4 shows the
locations of boreholes. Fig 5 shows the setup for Soil
investigation .
Findings of Soil Investigation
Soil & Rock Strata
The soil & rock profile as revealed by 9 boreholes drilled
showed the following strata existing at site:
a) Layer 1 - TOP LOOSE FILL of red,brown,soft
lateritic clayey silt with gravels,fragment. Thickness of
this strata was found to vary from 1.85m(BH2) to
4.75m(BH3) below existing GL and SPT-N varied from 01
to 14.
b) Layer 2 - STRATA of
red,pink,yellow,brown,grey,very stiff lateritic sandy
clayey silt with gravels and fragments and occasional
lateritic boulders.Thickness of this strata below the top
loose fill was found to vary from 2.25m(BH3) to 7.40m
(BH6) whereas the thickness of this strata in
BH1,BH2,BH5,BH8&BH9 was not known due to
termination of borehole in this strata. SPT-N was found to
vary from 16 to 50 and above.
c) Layer 3 - Pink, yellow brown, gray, very stiff phyllitic
clayey silt. This strata was met with in BH3, BH4, BH6
and BH7 only. Thickness of this strata was found to vary
from 0.45m (BH6) to 2.60m (BH4) and exact thickness of
this strata in BH7 only was not known due to termination of
borehole in this strata. SPT-N was found to vary from 25 to
50 and above.
d) Layer 4 -Underlying strata of red, yellow, brown,
gray, very stiff lateritic clayey silt with gravels and
fragments. This strata was met with in BH3 and BH4 only.
Exact thickness of this strata was not known. The SPT-N
was found to be greater than 50.
e) No hard rock was met with in any of the boreholes upto
about 10m to 12m depth below GL.
Causes of failure
Some foundations were located above the natural ground at
a depth of 2m in unconsolidated filled up ground of an
abandoned laterite stone quarry, where SPT was found to
be less than12,which resulted in differential settlement.
This differential settlement was observed towards the front
left corner of the Building which was lying on the filled up
ground. The differential settlement led to cracks in the
Plinth beam and the Foundation Concrete.
REMEDIAL MEASURES
In order to avoid the transfer of stresses in the connecting
components, the affected beams were separated from
sinking columns as shown in Fig 6. Four footings of the
affected portion were separated. The depth of these footings
was increased to 5m so as to be founded in the natural
ground bypassing the filled up portion of the Soil.
Expansion joints were provided between this portion and
the rest of the intact part of the structure. The Part of the
building that was separated by providing expansion joint is
shown in Fig 7.
Fig 5 - Soil Investigation in progress
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Settlement of a Structure – a Case Study
Fig 6 - Beams separated from sinking columns
Fig 2 - Layout of the Building showing affected portion
Fig 4 - Layout of the Building showing location of Bore-holes
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Shruti Naik, Nisha P. Naik & R. L. Mandrekar
CONCLUSIONS
The site of construction was basically a laterite stone quarry
which was abandoned and later backfilled for doing
construction. Since the lateritic fill was very loose and
unconsolidated, it was not suitable for founding any
Structure It was very essential to look into the history of the
location of the site. Detailed Soil Investigation was very
essential to reveal the variation of the Soil Strata across the
Site and investigate the extent of such quarries.
In the building under study, it would have been appropriate
to place all the foundations into the original firm lateritic
Strata. Careful Supervision and attention at the Excavation
stage and subsequent reporting of the variation in the strata
was crucial to make changes in the design at the execution
stage itself.
Isolating the distressed portion of the structure with
Expansion joints was the best feasible solution considering
that the Building was entirely constructed till the plinth
level.
REFERENCES
1. Punmia B. C, Soil Mechanics and Foundations, 16th
Ed., Prentice Hall, New Delhi, India.
2. Garg S. K., Soil Mechanics and Foundation
Engineering, 5
th
revised edition, Khanna Publishers,
Delhi, India.
3. Boone SJ (2001) “Assessing Construction and
Settlement induced building damage” A return to
fundamental principles, Institution of Mining and
Metallurgy, London, 559-570.
4. Gould R.A, “Construction over organic Soils in an
urban Environment”: Four Case histories.
5. Keneth E. T (2001), “ Settlement of Foundations on
Expansive clays due to moisture demand of clays”.
6. Rebecca Grant, John T. Christian (1974) ” Differential
Settlement of Building “ Journal of Geotechnical Engg.
Division, Vol 100, No. 9.
7. Sahara Mamoru, “ A Case history of reducing
differential settlement with building foundation
controlled length of friction piles, Journal of
Architecture and Building Science.
8. http:eprints.iisc.ernet.in/275/1/microfinal.pdf,
Sivakumar Babu, Srinivas Murthy, D.S.N
Murthy,“Bearing Capacity improvement using
micropiles – A Case study”.
9. Timothy B., “Differential Settlements in Steel Tanks”,
ASCE Journal of Geotechnical & Geo-Environmental
Engineering, Vol 113, No9.
Fig 7 - Portion of the Structure separated by an Expansion joint
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