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Rectification of Building tilt An unconventional ApproachRectification of Building tilt An unconventional Approach
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“RECTIFICATION OF BUILDING TILT – AN UNCONVENTIONAL
APPROACH”
Ms. Jayshree S. Lad Ms. Geeta P. Vairagade
IV Civil IV CivilSYNOPSIS
The paper describes the rectification work done to correct the hit of a four-storey
building in Calcutta. The building was founded on filled up and within two years after
construction settled as much as 800mm and went out of plumb by 850mm. Initially a
counter balancing weight of 40,000 bricks were placed on the ground and first floors and
extensive grouting was done. Then soil was removed from beneath the upper side of the
foundation by inserting perforated pipes and pumping water under pressure. This Hushed
away the soil by dilution under water pressure. Soil was also scooped out manually. The
removal of earth and the action of counter balancing weight gradually straightened the
building until it attained verticality.
INTRODUCTION
A four storey residential building was built in 1990-92 at 60, Belgachia Road,
Calcutta about half-a-kilometre to the east of Balgachia Station for staff housing of
Calcutta Metro Railway. It was a small building covering a plan area of 6.8m x 16.2m
with two flats in each floor. The construction was over in July 1992. Soon after, the
building started to settle, By November 1992 a vertical settlement of 1 50mm with a
differential settlement of 100mm was observed. The monitoring of the settlement
thereafter showed increasing tilt of the building. In March 1993. total settlement of
450mm with a differential of 410mm were measured Emergency cement grouting was
done below/ the foundation to arrest the settlement. But this did not succeed. The
building continued to settle differentially. By March 1994, the total settlement increased
to 800mm and the building went out of plumb by 850mm. This resulted in severe tilt of
the building and the same was declared unfit for occupation.
LITERATURE REVIEW
Settlement
Settlement indicates the sinking of a structure due to the compression and deformation of
underlying soil.
Total settlement = immediate settlement + time – dependant settlement
Time dependant settlement = settlement due to consolidation + settlement due to
secondary compression (creep)
S = Si + Sc + Ss
Si = immediate settlement
Sc = consolidation settlement (Primary settlement)
Ss = Secondary settlement
Si Predominant in coarse grained soils
Sc Predominant in fine grained soils and organic soils.
DIFFERNTIAL SETTLEMENT
Uniform settlement of a base is possible if the base
(1) Rests on a homogeneous soil,
(2) Is rigid, and
(3) Is loaded uniformly.
These conditions cannot be ideally fuldilled for any base, and settlement tends to
be non-uniform. The difference between the magnitudes of settlement at any two points
is known as differential settlement. This may result from:
1. Tilting of a rigid bast,
2. Dishing of flexible base,
3. Unequal translation of adjacent footings dule to different compressibility to
underlying soils,
4. Uneven support partly on firm ground and partly on loose ground.
In fig. (a), the rigid base tilts giving settlements S1 at the edgs; in (b), the
deformation in dish-shaped showing settlement S1 at the edge and S2 > S1 at the centre; in
(c), two footings settle by different magnitude, S1, S2. Thus, in each of these cases S1 – S2
= Δ, is the differential settlement. If the differential Δ is divided by the distance between
the points under consideration, it is known as angular distortion, θ. Thus,
S1 – S2 ΔΘ = =
L L
Distortion in a structural member produces moments and bending stresses e.g. a
differential movement between two supports produces a moment of magnitude 6 EI Δ /
L2 at the joint. That means, excessive differential settlement would produce large
moments, leading to structural damage. Hence the magnitude of Δ must be restricted.
The following are the sources of differential settlements :
1. Non-uniformity of subsoil : pockets or lenses or boulders.
2. Non-uniformity of loading: foundation of an oil tank is uniformly loaded while
a multistoreyed tower flanked by lightly loaded wings, is case of non-uniform
loading.
3. Large bases are flexible and involve dish-shaped deformations.
4. Bases, partly supported on hard layer and partly supported on compresible layer.
5. Staged construction or extensions to the existing structures.
6. Subsidence of soil caused by collapse of underground structure like tunnel or
conduit.
The following techniques may be used to reduce the differential movement. :
1) Provision of rigid rafts,
2) Laying the foundations at great depths,
3) Deeper foundations for heavier loads,
4) Transfer of loads to deep-seated firm layers piles, piers etc.
5) Avoiding eccentric loading, wherever possible,
6) Increasing the size of heaviest footing on clay and decreasing the size of lightest
footing on sand
The adverse effect of differential settlement can be minimized by :
1) Providing flexible components,
2) Providing construction joints,
3) Using thick rigid components to resist the effects of differential movement,
4) Preloading or precompaction of subsoil to reduce subsequent
5) Grouting of subsoil in the parts of greater compressibility.
6) Providing rigid framed structures as foundations.
Settlement observations of existing buildings have shown that is some
dependence of differential settlement on the total settlement. It is found that differential
settlement seldom exceeds 75 p. c. of total settlement and in most cases, it is less than 50
p. c. of the total settlement. Hence differential settlement can indirectly restricted by
putting a limit on the total settlement. Thus, in order that differential settlement may not
exceed 20 mm a limit on the total settlement can be specified, as 25 mm or 40 mm.
The foundation settlement is either due to the compression of the supporting layer
within the pressure bulb or due to compression of one or more deep-seated compressible
layers or both..
SUBSOIL CONDITION
Fig. shows the layout plan of two buildings at the site Out of the two identical
buildings Block I did not show major tilt or distress after construction and was cleared for
occupation Block 2 is the building which suffered major settlement and tilt. No subsoil
investigation was done at the site prior to construction The buildings were provided with
RCC raft foundation placed 1.2m below GL During subsequent investigation for working
out remedial measures, it was found that the top soil consisted of 2m of soft cohesive soil
followed by another 3m of rice mill waste. The area is known to consist of low-lying
ditches which had been filled up with all kinds of wastes. As no soil tests were done the
existence of such wastes and disturbed soil went undetected Shallow open foundation
was provided for the building with a net soil bearing pressure of 5t/nr. The effect of
unconsolidated fill / rice mill waste and other disturbed soil which existed below the
foundation were not considered in the design.
INITIAL REMEDIAL MEASURES
Emergency remedial measures were undertaken in March 1993 after the building
went out of plumb by 400mm and the maximum settlement readied 450mm Cement -
bentonite (20.1) grouting was done in between Blocks 1 and 2, Block 2 having tilted
towards Block
Fig. : Four Storey Residential Building: Front Elevation
1, Fig. The grout was injected into the soil to depths of 8 to 10m below G L both
vertically and inclined towards Block 2. This failed to arrest the settlement presumably
because no effective stabilization of the soil within influence zone of foundation for
Block 2 was done . The grouted soil mass remained essentially outside the influence
zone. In spite of the grouting the building continued to settle and the settlement reached
800mm by September 1993. The matter was referred to the author at the stage.
Visit to the site in September 1993 revealed perceptible tilt in Block. The
maximum settlement had reached 810mm and the building went out of plumb by 850mm
showing a tilt of 1/15. This was too high for a conventional civil engineering structure.
However, the rigidity of the RCC foundation ensured a somewhat rigid body tilt of the
building no major cracks were noticed in walls and floors.
In a order to stabilized the building and the prevent it from thew further tilt and
ultimate collapse it was decided to urgently provide counter loading on the rear side of
the building to the extent possible. This was done by stacking 40,000 bricks on the rear
half of the building on the ground floor and first floor, fig. 4. Also, grouting was done
with cement slurry and bentonite on the rear side of the building to stabilize the soil.
These measures proved effective. Further settlement of the building was soon arrested.
In fact a slight reduction of settlement (25mm) was noticed during grouting and counter
loading.
The building stood like this for two years. During the period thoughts were given
to determine further course of action. Even demolishing the top floor was contemplated
to reduce the bearing pressure of the soil. Finally, a somewhat unconventional method of
rectifying the damage was thought of.
RECTIFICATION WORK :
It was felt that the only way to correct the tilt of the building would be to loosen
and remove earth carefully from below the rear side of the building and allow it to tilt
back under the counter balancing weight. With this in view a scheme was propose by the
engineers of super construction Pvt., Ltd., Who were the constructors entrushed with the
rectification work. This proposal in envisazed pushing perforated pipes below the rear
side of the building and pumping water under pressure through the perforation. This
would loosen the soil to facilitate manual scooping. Even some Soil would be washed
out under the force of water. This removal of earth would make room for the building to
tilt back under the action of the counter balancing weight of 40,000 bricks already there
on the ground and first floors.
1. The detailed scheme worked out as follows :
1. Four nos. 50mm dia perforated pipes were inserted in to the soil along the length
of the building in the rear half, from east to west, about 1 am below the building
foundation. For this a trench was made along the northern side of the building and the
pipes were pushed into the soil horizontally with the help of a rotating cutter,
Fig. Some water was allowed to pass through the perforation to soften the soil and
allow easy pushing of the pipes during insertion. Water under pressure was then pumped
into the perforated pipes for removal of the loose earth under the force of water. Also soil
was scoped out from underneath the foundation manually. The removal of earth from the
rear side of the building by the above processes and the effect of counter balancing
weight allowed the building to tilt back. By March 1997 the settlement of the building
reduced to 585mm.
2. The perforated pipes were then pulled out of the soil for removal of muck from
inside and then reinserted. The process of pumping water under pressure \\as resumed
lowly and carefully and in another three months i.e., June 1997 the settlement reduced
further to 420mm After this, the pipes could not be removed as they got stuck under the
high pressure of the building when it tilted back.
3. No work was done between June 1997 and September 1997 due to the rainy
season.
4. The work was resumed in October 1997 This time 50mm perforated pipes, were
inserted into the soil 600 mm below the raft from the rear side put across the building for
this, a pipe was inserted in small length of 500mm to make a hole and then pulled back
Next, a pipe of I am length was pushed through the hole. This was also pulled out and a
1.5m long pile inserted through the hole. The process was continued until a length of
about 2. 5m was inserted. These pipes were placed at an average pacing of 0.5m along
the entire length of the building.
5. When this was achieved earth was backfilled into the trenches made around the
ling on the rear side and extensive cement bentonite grouting was done all around
building for fun her stabilization of the soil The counter balancing weight of )00 bricks
was then removed.
The rectification work was completed in March 1998 But the building was kept ir
observation for another six months. No further settlement or tilt was noticed. The ground
around the building was then made up for appropriate finishing ground level of the
building had finally stabilized at 800mm. below the ounding ground. Steps were provided
to facilitate access to the ground floor and ngements were made for proper drainage of
the water out of building area.
The building was cleared for occupation in December 1998. No. further movement ilt has
been noticed since then.
CONCLUSION
A badly tilted building had been rectified to achieve the original verticality by i
unconventional method of removal of earth from underneath the building and the ;tion of
counter balancing weight. The removal of soil was done by pumping water ider pressure
through perforated pipes inserted below the building foundation.
REFERENCES
1. The Millennium conference IGC 2000
-By N. SOM
2. Dr. B. J. Kasmalkar
3. Dr. B. C. Punmia
4. Dr. A lam Singh
5.K. R. Arora