Bridge Manual 1998-86-108.docx

CHAPTER - IV ============ CONSTRUCTION OF FOUNDATIONS FOR BRIDGES 401. Setting the lay out of Bridges ------------------------------ 1. General a. It is necessary to accurately lay out the centre line of a bridge and the locations of its piers and abutments and to establish a s...

CHAPTER - IV ============ CONSTRUCTION OF FOUNDATIONS FOR BRIDGES 401. Setting the lay out of Bridges ------------------------------ 1. General a. It is necessary to accurately lay out the centre line of a bridge and the locations of its piers and abutments and to establish a system by which they can be checked with ease during the progress of the work. b. Position of the principal reference lines and level pegs should be so selected and laid that they are easily accessible for check during the execution of the work. c. The Principal reference lines to be established are the longitudinal centre line and the transverse centre lines of abutments and piers. If the bridge is on a curve, the tangent points of the curve and the directions of the tangents at either ends should also be established by pegs. 2. Setting out bridges without a base line : Where deep excavations are not involved and where there is no water flow in the river during the working season, setting out primarily involves fixing the alignment correctly using a theodolite. The distance between the abutment at either end and the nearest pier and the pier - to-pier distance can be set out by directly measuring and marking the centres using a good steel tape (See Annexure 4/1). The centre points of each structure (pier or abutment) should be punch marked on a flat or angle iron piece fixed flush with the top of a concrete block at the correct location. 3. Setting out bridges with the help of a base line : a. Where deep excavation, pile driving or well sinking is involved, and where there is standing water, base lines are set out at right angle to the centre line of the bridge, one on either end on the high banks, or on one side of the bridge or anywhere between the abutments where level ground is available. b. The actual position of the piers/abutments is determined by the intersection of three sight lines, one along the alignment sighted from stations located on either end, a second from a station on the base line on the down stream side and a third from a point on the base line on the upstream side. Theoretically all these three lines should intersect at one point. Normally a triangle of error gets formed and the correct centre is fixed by judgment within this triangle. (Refer Annexure 4/2). 4. Important points to be observed while setting out base lines: a. Linear measurement should be carried out with invar tape or Electronic distance measuring equipment. b. Concrete pillars with steel plates fixed over them should be located at tape lengths for accurate measurements. c. Spring balances should be used for giving specified tension to the tape. d. Tape readings should be corrected for tension, temperature and slope. Pipe and box Culverts with open foundation ------------------------------------------ Pipe and box culverts can be constructed after removing the top soil in bed to the required depth and replacing it with a layer of lean concrete after the bed is levelled and well consolidated by ramming or rolling. Reference may be made to Annexure 4/3 for a typical arrangement. Pipe and box culverts should not normally be provided where the bed is likely to be scoured. 403. Other Bridges with open foundation ---------------------------------- 5. Open foundations must rest on a stratum with adequate bearing capacity. In order to reduce the bearing pressure the base can be sufficiently widened by providing footings. The footings will rest on a lean concrete bed of adequate thickness. 6. The foundation should be taken to a depth not less that 1.75 metres below the lowest anticipated scoured bed level in ordinary soil. In rocky soil, it will be adequate if it is properly keyed into the rock for a minimum of 0.3 metre in case of hard rock and 1.5 metres in case of soft rock. Sloping rock may be suitably benched. Fissures and weathered rocks should be avoided. A typical arrangement is shown in Annexure 4/4. 7. In soft soils, rafts may be provided as foundation. Such rafts should be protected by means of suitable aprons and cut off walls or launching aprons, both on the upstream and downstream sides to prevent undermining of the foundations. 8. Excavation for open foundations with shoring : Excavation should be done in such a way that the surrounding soil can stand by itself by suitable sloping the sides. When excavations have to be deep or when the side slopes are not stable, suitable shoring may be provided from top, using timber planks, walling pieces and struts. Typical arrangements of shoring are shown in Annexure 4/ 9. In deep foundations and large size excavations, where the seepage is heavy, suitable pumps may be used for dewatering. A small sump on the side or corners should be provided for collection of the water to be pumped. 404. Excavation using coffer dam --------------------------- 1. Shallow foundations : Where excavation is required to be done under flowing or standing water, coffer dams of steel sheet pile, RCC or timber may be constructed. Driving is done from a floating platform. Annexure 4/6 shows a typical arrangement with steel sheet piling. 2. Deep foundations : PILE FOUNDATIONS ================ Choice of pile materials ------------------------ RCC piles both driven and bored may be used. Driven piles may be either precast or cast in-situ. Timber piles may, however, be used for temporary restoration of traffic. They should, be replaced with permanent structures. A typical arrangement of a temporary wooden pile bridge is shown in Annexure 4/8. Soil Exploration for design and construction of piles ----------------------------------------------------- For the satisfactory design and construction of piles, detailed soil exploration to a depth generally not less than 10 metres below the anticipated level of pile tip (unless bed rock or firm strata has been encountered earlier) should be carried out and the following particulars are collected. a. Ground water table and its tidal and seasonal fluctuations; b. Soil profile and bore hole log; c. In-situ bulk and dry density ; d. Index properties of soil ; e. Shear properties of soil. If required Standard Penetration Test (SPT) may be done; f. Consolidation properties, in case of clays ; g. Chemical analysis of soil and ground water to identify sulphate and chloride content or any other deleterious chemical content. Additional data such as high flood level, maximum scour depth , normal water level during working season, etc. should also be collected. Classification of pile foundations ---------------------------------- 3. Based on the manner of transfer of load. a. Friction piles : These piles transfer the load primarily by skin friction developed along their surface. They are used in soils not subjected to scour. b. Bearing piles : These piles transfer the load primarily by bearing resistance developed at the pile tip or base, without taking into account the frictional resistance. They are generally used in hard stratum. c. Bearing-cum-friction piles : 4. Based on construction methods : a. Driven Pre-cast piles ; b. Driven cast in-situ piles; c. Bored cast-in-situ piles. 5. Large diameter bored piles of more than one metre diameter are normally used for Railway bridge construction. Selection of type of piles -------------------------- The following factors are to be considered while selecting the type of piles : 6. Availability of space and head room : Driven piles require large area and headroom since they need larger and heavier driving rigs. Bored piles, however, require comparatively smaller space. 7. Proximity to the structure : 8. Reliability : 9. Limitation of length : 409. Spacing of piles ---------------- 10. The spacing of piles is determined based on the type of soil and empirical approach keeping in view the following aspects: a. Practical aspects of installing the piles. b. The nature of the load transfer to the soil and possible reduction in the bearing capacity of a group of piles thereby 11. Where piles are found on a very hard stratum and derive their capacity mainly from end bearing, the spacing will be governed by the competency of the end bearing stratum. The minimum spacing in such cases shall be 2.5 times the diameter of the pile shaft. 12. Piles deriving their bearing capacity mainly from friction shall be sufficiently apart to ensure that the zones of soil from which the piles derive support do not overlap to such an extent that their bearing values are reduced. Generally, the spacing in such cases shall not be less than 3 times the diameter of the pile shaft. 13. In the case of loose sand or filling, closer spacing than in dense sand may be possible since displacement during filling may be absorbed by vertical and horizontal compaction of the strata. Minimum spacing in such strata may be twice the diameter of the pile shaft. This is applicable for driven piles only. 14. Normally centre to centre spacing should not be more than 4 d, where d is the diameter of pile shaft. In the case of piles of non circular cross section, diameter of the circumscribing circle shall be adopted. Load carrying capacity of a pile / group of piles ------------------------------------------------- 1. Load carrying capacity of a single pile : a. The ultimate bearing capacity of a pile may be assessed by means of a dynamic pile formula, using the data obtained during driving of the piles or by a static formula on the basis of soil test results or by a load test. Reference may be made to IS : 2911-1979 Part - I, section -I (revised) for the details of dynamic and static formulae. b. For non-cohesive soils, Hiley's formula is more reliable than other formulae. (Appendix-B of IS 2911 Part -I, section -I ). Hiley's formula is not reliable in cohesive soils. c. Load test is most desirable. The load test on pile should be carried out four weeks after casting the pile d. Resistance due to skin friction will be available only below the scour line and this must be taken into account in all the three methods. 2. Factor of safety for Pile Foundations : a. The factor of safety shall be judiciously chosen after considering the following : i. Reliability of the ultimate bearing capacity of pile ; ii. Type of superstructure and type of loading ; iii. Allowable total/differential settlement of the structure ; iv. Experience of similar structures near the site. b. The minimum factor of safety with static or dynamic formula shall be 2.5. The value to be selected for the factor of safety shall, however, take into account, the allowable total settlement and differential settlement of the structure as a whole. The ultimate load capacity should be obtained, whenever practicable, from a load test (initial) (as per IS: 291 / (Part 4)-1985). Factor of safety for assessing safe load on piles from load test data should be increased in unfavorable conditions where : i. settlement is to be limited or unequal settlement avoided as in the case of accurately aligned machinery or a superstructure with fragile finishing. ii. large impact or vibrating loads are expected iii. the properties of the soil may be expected to deteriorate with time, and iv. the live load on a structure carried by friction piles is a considerable portion of the total load and approximates to the dead load in its duration. 3. Bearing capacity of a pile group : i. Equal to the bearing capacity of individual piles multiplied by the number of piles in the group, or ii. It may be less than the above. 411. Construction of pile foundation ------------------------------- 1. Driven precast piles : 2. Driven cast-in-situ piles : 3. Bored cast-in-situ piles : 412. Permissible tolerance while driving piles ----------------------------------------- 4. Control of alignment : 5. Piles should not deviate more than 75mm or D/10 in case of bored cast-in-Situ piles having diameter more than 600mm 6. Any deviation from the designed location, alignment or load capacity of any pile shall be noted and adequate measures taken well before the concreting of the pile cap and plinth beam. Sequence of piling ------------------ 1. In a pile group the sequence of installation of piles shall normally be from the centre to the periphery of the group or from one side to the other. 2. Consideration should be given to the possibility of doing harm to a pile recently formed by driving the tube nearby before the concrete has sufficiently set. The danger of doing harm is greater in compact soils than in loose soils. 3. Driving piles in loose sand tends to compact the sand which, in turn, increases the skin friction. Therefore, the order of installing of such a pile in a group should avoid creating a compacted block of ground into which further piles cannot be driven. 4. In case where stiff clay or compact sand layers have to be penetrated, similar precautions need be taken. This may be overcome by driving the piles from the centre to outward or by beginning at a selected edge and working across the group. However, in case of very soft soils, the driving may have to proceed from outside to inside so that the soil is restrained from flowing out during the operations. Defective piles --------------- 1. In case defective piles are formed, they shall be removed or left in place whichever is convenient without affecting the performance of the adjacent piles or the group as a whole. Additional piles shall be provided to replace them as necessary. 2. If there is a major variation between the depths at which adjacent piles in a group meet refusal, a boring shall be made nearby to ascertain the cause of this difference. If the boring shows that the soil contains pockets of highly compressive material below the level of the shorter pile, it may be necessary to take all the piles to a level below the bottom of the zone which shows such pockets. Tremie Concreting ----------------- 1. The concrete should be coherent, rich in cement (not less than 370 kg/m3 ) and of slump between 150 and 180 mm. 2. When concreting is carried out under water, a temporary casing should be installed to the full depth of the bore hole or 2m into non collapsible stratum so that fragments of ground cannot drop from sides of the hole into concrete as it is placed. The temporary casing may not be required except near the top when concreting under drilling mud (Bentonite slurry.) 3. The hopper and the tremie should be a closed system embedded in the placed concrete, through which water cannot pass. 4. The tremie should be large enough with due regard to size of the aggregates. For 20 mm sized aggregate the tremie pipe should be of diameter not less than 200 mm. Aggregates more than 20 mm in size shall not be used. 5. The first charge of concrete should be placed with a sliding plug pushed down the tube ahead of it or with a steel plate of adequate charge to prevent mixing of concrete and water. However plug should not be left in the concrete as a lump. 6. The tremie pipe should always penetrate well into the concrete with an adequate margin of safety against accidental withdrawal of the pipe surging to discharge the concrete. 7. The pile should be concreted wholly by tremie and the method of deposition should not be changed part way up the pile, to prevent laitance from being entrapped within the pile. 8. All tremie tubes should be scrupulously cleaned after use. 9. Normally concreting of the piles should be uninterrupted. In an exceptional case of interruption of concreting, but which can be resumed within 1 or 2 hours, the tremie shall not be taken out of the concrete. Instead, it shall be raised and lowered slowly, from time to tome to prevent the concrete around the tremie from setting. Concreting should be resumed by introducing a little richer concrete with a slump of about 200 mm for easy displacement of the partly set concrete. If the concreting cannot be resumed before final setting of concrete already placed the pile so cast may be rejected or accepted with modifications. 10. In case of withdrawal of tremie out of the concrete either accidentally or to remove a choke in the tremie, the tremie may be reintroduced in the following manner to prevent impregnation of laitance or scum lying on top of the concrete deposited in the bore. 11. In case of concreting through tremie or such tubes which are subsequently withdrawn, the concrete shall be placed in sufficient quantity to ensure that during withdrawal of the tube a sufficient head of concrete is maintained to prevent the in-flow of soil and water or bentonite slurry (Refer Annexure 4/10). 12. The top of the concrete in a pile shall be brought above the cut-off level to permit removal of all laitance and weak concrete before capping and to ensure good concrete at the cut off level for proper embedment into the pile cap. Acceptance of pile ------------------ 417. **Types of well** WELL FOUNDATIONS ================ 7. The types commonly used are : i. Circular ii. Double - D 8. The circular well is simple to construct, easy to sink and has uniform strength in all directions. It can be better controlled against tilt and tilt correction is also easier. The only disadvantage is the limitation in size which restricts its use to bridges with smaller piers. 9. The shape of Double-D well facilitates easy casting and sinking due to presence of two dredge holes. The overall length of the well generally is restricted to twice the width. 418. Components of wells ------------------- 10. Annexure 4/11 shows the cross section of typical well foundation with its components. 11. Well curb including cutting edge : 12. Well steining : a. General : b. Thickness of steining of cement concrete wells : i. sinking is possible without excessive kentledge ; ii. the steining is strong enough to resist damage during sinking; iii. tilt correction is possible without damage to the well ; iv. the steining is strong enough to resist earth pressure in conditions like sand blow or sudden drop of well during sinking; and v. stresses developed during sinking and in service conditions are within permissible limits. 13. Bottom plug : 14. Top plug : 15. Well cap : The bottom of the well cap shall, as far as possible, be located 300 mm above low water level. All the longitudinal bars from the well steining shall be anchored into the well cap. The well cap shall be designed as a slab resting on the well steining. Pitching of the cutting edge and well curb ------------------------------------------ The curb should be generally pitched at about 15 cm to 30 cm above the low water level. The pitching level may be kept higher if the water level in the river is subjected to greater fluctuations like in tidal areas. In case the site is dry, excavation should be carried out upto the level at which the well curb is proposed to be pitched and the centre of the well curb carefully marked. The well curb should then be assembled on wooden blocks or sand bags placed at intervals of about 1.5 metre. In case the well has to be sunk in water, an island is formed, and the top of the island is levelled and compacted lightly and marking for setting the cutting edge is done on the level surface. The concrete should be of mix not leaner than M-20 grade. After concreting the well curb the outer shuttering may be taken off after 24 to 48 hours depending on the temperature. The inside conical shuttering can be taken off after 72 hours. The wooden block supports can be taken out alternatively one by one, supporting the well curb on sand bags using a jack, for the purpose. Vertical gauges on four sides of the well from the centre of the cutting edge should be provided to monitor the verticality of the well during sinking. RCC well curb should be allowed to set for at least one week before sinking is started. A well is most unstable in the beginning when it has no grip in the sand or when the grip is very small. The chances of tilting increases considerably if the well is made top heavy by raising the masonry of the steining too high in the first instance. The best course is to sink the well curb alone after allowing it setting time without raising the steining above it. Concreting of steining ---------------------- The well steining should be built up in stages initially 1.20 to 1.50 metres at a time, as it is gradually sunk through the soil, keeping sufficient free board above the water level. Once well has acquired a grip of about 6 metres, the steining can be raised 3 metres at a time to obtain a better rate of progress. Inner shuttering and bracing should not be removed within 24 hours of casting. 421. Sinking of wells in water ------------------------- 16. The sinking of well is done by removing the soil with grabs or chiselling and drawing out the soil. Sinking of steining should not be done within 48 hours of casting. 17. In stagnant water and in water with velocity upto about 2.0 m/s and of depth upto about 5 metres, construction of island may be resorted to. In greater depths, the use of steel caissons would become unavoidable. 18. Construction of island : a. The island should provide sufficient working space of about 2 to 3 metres all round the well. A minimum free board of 0.6 metre should always be provided. b. For depths of water upto 1 metre, an island may be constructed by laying a few rings of sand bags enclosing the area of the island and filling with sand. Boulders should not be used in the construction of island as they may find a way inside the island and obstruct the sinking of the well. For greater depths, two rows of piles 1.5 metre apart at a spacing of 1 metre enclosing the area of the island may be provided. Upto 1.5 metre depth, bamboo piles may be used and timber ballies may be used upto 3 metre depth. The piles are lashed together with strap, wire ropes or coir ropes. Bamboo matting is then fixed along the inside faces of the piles and the space between the mattings is filled with puddle or sand bags. Beyond 13 metre depth, steel sheet piles should be used. The piles should have a grip of at least 3 metres. Sand bags should be dumped outside the island as a protection against scour. In cases, where velocity is high, wire netting (crates) filled with boulders may be used. (Refer Annexure 4/13). 422. Sinking of wells using caissons ------------------------------- 19. In deeper channels and swift rivers, caissons built of steel plates suitably strengthened by angle iron stiffeners and further strutted and tied together by MS angles may be used. Caissons are lowered through water and pitched in position before commencing the sinking. 20. Assembling and launching of caissons : Caissons can be assembled at site itself and launched straight away using barges or can be assembled when the river bed is dry and launched when water level in the river rises. In the first method, a temporary platform is constructed over two barges suitably anchored and a gantry is erected over the platform. The caisson is assembled on the temporary platform. After testing for leakages, the caisson is lifted from the platform and lowered in position after removing the temporary platform (Annexure 4/14) In the second method, the caissons are assembled, tested and kept ready duly filled with water on the river bed when it is dry. When the water level rises, the water in the caisson is pumped out and they are toed into position. The draught can be reduced by covering the dredge holes with steel plates and pumping compressed air into them. As an alternative method, caissons can be assembled on the river bank and brought into the water using the slip ways after which they can be toed into position for launching and grounding. This method of assembling the caissons in dry docks and launching them is very expensive and resorted to only under special circumstances. 21. Grounding of caissons: 423. Process of open sinking of wells -------------------------------- 22. Sinking of smaller wells may be done with the help of timber shear leg, derrick or timber scotch. For shallow depths, grabbing and removal of the earth can be done manually. For greater depths and where the sub soil water level is high, suitable dredger may be used. The grab can not be operated in the blind area below the curb. Hence, these are suitable for the wells with thinner steining. Steam or diesel winches of suitable capacity can be used for operating the grabs or dredgers. For faster sinking of wells of 6 metre dia and above, suitable cranes may be used. 23. Well Sinking through clay and hard strata : a. In stiff clay strata or in strata with compact sand, shingle and boulders, the use of rail chisel may be required. Use of chisels can be avoided if hammer grabs are used. A sketch of rail chisel commonly in use on Indian Railways is shown in Annexure 4/16. b. For sinking through stiff clay and other hard strata buoyancy effect of the soil may be reduced by dewatering the well to increase its effective weight. c. If due to heavy skin friction, the well is held in a floating condition, air or water jets may be used on the outer periphery of the well for reducing the friction. For this purpose pipes of 4 to 5 cm. dia. fitted with nozzles can be incorporated in the well close to the outside periphery, particularly in the curb portion and also for some height of the steining above. d. When any of the above methods is not effective, a few sticks of gelignite can be detonated under the water below the cutting edge. This results in shaking of the well and reducing the skin friction, which helps in its further sinking. Charging should be started with small quantities in each dredge hole at a time and gradually increased. When there is more than one dredge hole, such charging and detonation should be done in all the holes simultaneously. 24. Precautions to be taken during well sinking: a. Blowing of Sand: Great caution is necessary when dewatering of well is done at shallow depths or when the well has not gone into the soil by at least 1 metre. The difference in the hydraulic pressure inside and outside the well may create a passage for rush of sand from outside the well resulting In "blowing of the sand". Sand blowing can endanger the safety of men working inside the well and can also cause sudden tilting of the well. Seepage of water should be carefully watched during sinking and should be checked by putting sand bags over the area where such seepage is noticed. In severe cases of sand blowing, large quantity of the sand is sucked into the well and a funnel shaped depression is formed outside the well as shown in Annexure 4/17. Empty gunny bags and branches of the tree with green leaves may be thrown into the funnel and dredging continued till the sand blow gets arrested. The well can then be dewatered completely and excavation continued. b. Quick sand condition:- Even if it does develop, there is considerable margin of safety and the well does not sink below the bed level. c. While sinking wells in deep water, divers with their equipment should be present for emergencies. 424. Tilt and shift of the well -------------------------- 25. Limits of tilt and shift : a. As far as possible wells shall be sunk without any tilt and shift. A tilt of upto 1 in 100 (1%) and a shift of D/40 subject to a maximum of 150 mm can be permitted. b. Excessive tilt and shift, which cannot be corrected, should be taken into account for rechecking the design of the well and the resulting foundation pressure. c. The gauges marked at quarter points on the outer periphery of the steining starting from the bottom of the cutting edge mentioned earlier are used for checking the tilts. Water level readings on all the four gauges are observed frequently to get an idea of the direction and extent of tilt. Where the well is not sunk through water, plumb bobs are used on all four sides to judge the verticality of the well. It should be noted that plumb bobs are used only for checking for tilt during sinking and should never be used while building up the steining. 26. Tilt correction : Depending on the site conditions, any one of the following methods may be adopted either separately or in combination with others for the rectification of tilt which may occur due to the well encountering very soft material on one side and hard material on the other side or when there is a log of wood or a big boulder under the cutting edge on one side of the well. a. Eccentric dredging : The dredging is confined to the higher side and is done very close to the inside face of the steining and even a little under the steining. b. Eccentric loading : c. Applying pull to the well : Light pull can be applied to a well by taking a wire rope round the well and anchoring the tackle to a dead man anchorage or a large tree if available in the vicinity. The tackle is worked by a winch and tension is maintained as sinking progresses (Annexure 4/18). For applying heavy pull, wire ropes are taken round and fixed to a large dead man anchorage. Rails or other kentledges are then placed on the wire ropes to develop high tension and the tilt gets rectified as the well sinks (Annexure 4/18) d. Applying push to the well : e. Packing the low side of the well with sand bags under the splayed portion. Use of divers for sinking ------------------------- depths. Durations of ascends and stoppages at various depths and stay under water at various pressures as laid down in the relevant British standard Specifications should be strictly adhered to. 426. Pneumatic sinking of well ------------------------- 27. Pneumatic sinking is used when other methods are not found feasible, particularly when the wells have to pass through considerable depths of intervening layers of rock or when the bed is full of large boulders or interlocked small boulders. This method may be employed for depths varying from 12 metres to 33 metres. 28. The inside of the well is made into a closed air tight box chamber and all water from this chamber is expelled out by letting in compressed air. The pressure of air inside the chamber should be maintained at approximately 0.12 kg/cm2 above atmospheric pressure per metre depth below water level. The dredging operation may be carried out inside the chamber in near dry condition. The compressed air arrangements are then removed and sand filling etc. of the well are completed as for the open dredging. The working chamber must be practically air and water tight and yet there must be an opening for men to enter and leave the chamber, as well as an inlet and outlet for materials. These opening are provided with vertical shafts and air locks. A typical arrangement showing the chamber with the vertical shaft and air lock is at Annexure 4/19. 29. Precautions while working in air lock chamber : a. The lock usually becomes warm and water is required to be sprayed on its outside to keep the temperature down. b. Workmen should be medically examined before they are selected. c. Working time, rest intervals and time and rate of decompression must be carefully regulated when the pressure exceeds 0.7 kg/cm^2^ above atmospheric pressure. d. Medical arrangements under a medical officer specially trained in caisson disease must be ensured when the working pressure is more than 1.75 kg/cm2 above atmosphere and there must be a medical lock. e. The rate of decompression specified should be followed. Working above a pressure of 3.5 kg/cm2 above atmospheric pressure is not allowed except in emergency. f. The steining concrete for wells to be sunk using pneumatic caissons should be as dense as possible to reduce the pore pressure caused by air under pressure trying to escape through fine pores in concrete. g. The joints should be made as air tight as possible to prevent escape of air. Water proof cement plastering can be resorted to on the inside face of steining. 427. Founding the well ----------------- 30. Settling the well by blasting : 31. Dressing the bottom of the well : a. As far as possible, the well shall be evenly seated on sound rock devoid of fissures, cavities, etc. b. One or more cylindrical holes may be made in the base to give good anchorage to the well. (Annexure 4/20 a). Alternatively anchorage can be provided by the rock itself which is allowed to remain projecting at the centre. (Annexure 4/20 b). c. In shallow wells and wells resting on rock a few holes may be drilled and MS dowels fixed in them for providing a good bond between the base rock and the bottom plug. d. In case of sand overlaying steeply sloping rocky base below, it is preferable to bench the rock and take the well down to have a proper base. Where this is not feasible, a number of small wells or piles are sunk or driven through the sand till they penetrate into the base rock (Annexure 4/20 c & d) Bottom plugging of the well --------------------------- Sand hearting ------------- Construction of the top plug ---------------------------- Setting out of the piers on the top of well ------------------------------------------- Construction of the top plug ----------------------------