“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