“RECTIFICATION OF BUILDING TILT – AN UNCONVENTIONAL APPROACH”Ms. Jayshree S. Lad IV Civil SYNOPSIS 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 But this did not succeed. The 450mm with a differential of 410mm were measured Emergency cement grouting was done below/ the foundation to arrest the settlement. 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. Ms. Geeta P. Vairagade IV Civil 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 Sc Predominant in coarse grained soils Predominant in fine grained soils and organic soils. . in (b). θ. 4. S2. These conditions cannot be ideally fuldilled for any base. in (c). Tilting of a rigid bast. S1 – S 2 = L Δ L Θ = . If the differential Δ is divided by the distance between the points under consideration. In fig. 3. the deformation in dish-shaped showing settlement S1 at the edge and S2 > S1 at the centre. the rigid base tilts giving settlements S1 at the edgs. Thus. two footings settle by different magnitude. Uneven support partly on firm ground and partly on loose ground. 2. in each of these cases S1 – S2 = Δ. (a). Is rigid. This may result from: 1. The difference between the magnitudes of settlement at any two points is known as differential settlement.DIFFERNTIAL SETTLEMENT Uniform settlement of a base is possible if the base (1) (2) (3) Rests on a homogeneous soil. S1. and settlement tends to be non-uniform. Thus. and Is loaded uniformly. Dishing of flexible base. it is known as angular distortion. is the differential settlement. Unequal translation of adjacent footings dule to different compressibility to underlying soils. 5. Providing construction joints.Distortion in a structural member produces moments and bending stresses e. Subsidence of soil caused by collapse of underground structure like tunnel or conduit. a differential movement between two supports produces a moment of magnitude 6 EI Δ / L2 at the joint. 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) 2) 3) 4) 5) 6) Providing flexible components. Large bases are flexible and involve dish-shaped deformations. 3. Laying the foundations at great depths. is case of non-uniform loading. Preloading or precompaction of subsoil to reduce subsequent Grouting of subsoil in the parts of greater compressibility. : 1) 2) 3) 4) 5) 6) Provision of rigid rafts. Deeper foundations for heavier loads. Using thick rigid components to resist the effects of differential movement. Hence the magnitude of Δ must be restricted. Non-uniformity of subsoil : pockets or lenses or boulders. Staged construction or extensions to the existing structures. partly supported on hard layer and partly supported on compresible layer. Providing rigid framed structures as foundations. excessive differential settlement would produce large moments. Non-uniformity of loading: foundation of an oil tank is uniformly loaded while a multistoreyed tower flanked by lightly loaded wings. Transfer of loads to deep-seated firm layers piles.g. That means. The following techniques may be used to reduce the differential movement. Bases. . wherever possible. piers etc. leading to structural damage. 4. 6. Avoiding eccentric loading. 2. The following are the sources of differential settlements : 1. No subsoil investigation was done at the site prior to construction The buildings were provided with RCC raft foundation placed 1.. It is found that differential settlement seldom exceeds 75 p.Settlement observations of existing buildings have shown that is some dependence of differential settlement on the total settlement. as 25 mm or 40 mm. it is less than 50 p. SUBSOIL CONDITION Fig. in order that differential settlement may not exceed 20 mm a limit on the total settlement can be specified. The effect of unconsolidated fill / rice mill waste and other disturbed soil which existed below the foundation were not considered in the design. 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. Thus. The area is known to consist of low-lying ditches which had been filled up with all kinds of wastes. Hence differential settlement can indirectly restricted by putting a limit on the total settlement. it was found that the top soil consisted of 2m of soft cohesive soil followed by another 3m of rice mill waste. 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. of total settlement and in most cases. . c. of the total settlement. 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.2m below GL During subsequent investigation for working out remedial measures. c. These measures proved effective. The maximum settlement had reached 810mm and the building went out of plumb by 850mm showing a tilt of 1/15. The building stood like this for two years. the rigidity of the RCC foundation ensured a somewhat rigid body tilt of the building no major cracks were noticed in walls and floors.000 bricks on the rear half of the building on the ground floor and first floor. a somewhat unconventional method of rectifying the damage was thought of. Also. fig. This failed to arrest the settlement presumably because no effective stabilization of the soil within influence zone of foundation for Block 2 was done .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. During the period thoughts were given to determine further course of action. Further settlement of the building was soon arrested. : Four Storey Residential Building: Front Elevation 1. grouting was done with cement slurry and bentonite on the rear side of the building to stabilize the soil. 4. In spite of the grouting the building continued to settle and the settlement reached 800mm by September 1993. The grouted soil mass remained essentially outside the influence zone. Fig. Even demolishing the top floor was contemplated to reduce the bearing pressure of the soil. Finally. However. This was done by stacking 40. This was too high for a conventional civil engineering structure. In fact a slight reduction of settlement (25mm) was noticed during grouting and counter loading. Block 2 having tilted towards Block Fig. The matter was referred to the author at the stage. The grout was injected into the soil to depths of 8 to 10m below G L both vertically and inclined towards Block 2. 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. Visit to the site in September 1993 revealed perceptible tilt in Block. . This would loosen the soil to facilitate manual scooping. about 1 am below the building .. Even some Soil would be washed out under the force of water.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. from east to west.000 bricks already there on the ground and first floors. This removal of earth would make room for the building to tilt back under the action of the counter balancing weight of 40. Who were the constructors entrushed with the rectification work. Ltd.. 50mm dia perforated pipes were inserted in to the soil along the length of the building in the rear half. This proposal in envisazed pushing perforated pipes below the rear side of the building and pumping water under pressure through the perforation. With this in view a scheme was propose by the engineers of super construction Pvt. 1. The detailed scheme worked out as follows : Four nos. 1. The perforated pipes were then pulled out of the soil for removal of muck from inside and then reinserted.. season. 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. Also soil was scoped out from underneath the foundation manually.foundation. 2. 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. were inserted into the soil 600 mm below the raft from the rear side put across the building for this. By March 1997 the settlement of the building reduced to 585mm. the pipes could not be removed as they got stuck under the high pressure of the building when it tilted back. The work was resumed in October 1997 This time 50mm perforated pipes. The process of pumping water under pressure \\as resumed lowly and carefully and in another three months i. Fig. 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. Some water was allowed to pass through the perforation to soften the soil and allow easy pushing of the pipes during insertion. This was also pulled out and a No work was done between June 1997 and September 1997 due to the rainy . June 1997 the settlement reduced further to 420mm After this.e. 3. Water under pressure was then pumped into the perforated pipes for removal of the loose earth under the force of water. 4. R. 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 . 5m was inserted. The removal of soil was done by pumping water ider pressure through perforated pipes inserted below the building foundation. SOM 2. Arora . The process was continued until a length of about 2. 5. A lam Singh 5. The rectification work was completed in March 1998 But the building was kept ir observation for another six months. The building was cleared for occupation in December 1998. Kasmalkar 3. B. Dr. The Millennium conference IGC 2000 -By N. C.5m long pile inserted through the hole. J. No further settlement or tilt was noticed. Dr. B. Punmia 4. below the ounding ground. 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. Steps were provided to facilitate access to the ground floor and ngements were made for proper drainage of the water out of building area.1. REFERENCES 1.K.tion of counter balancing weight. These pipes were placed at an average pacing of 0. further movement ilt has been noticed since then.5m along the entire length of the building. No. Dr. The ground around the building was then made up for appropriate finishing ground level of the building had finally stabilized at 800mm.
Report "Rectification of Building tilt An unconventional Approach"