SOIL ENGINEERING: TESTING, DESIGN, AND REMEDIATION phần 6

10.5.1 PIER HOLE CLEANING To ensure that the bottom of the pier hole is clean and free of loose earth, the pier hole must be properly cleaned. This can usually be accomplished by adding a small amount of water into the pier hole and spinning the auger lightly so that the loose earth will adhere to the auger and be removed. If loose rocks and soft mud are present in the bottom of the hole, it may be necessary to send a helper down the hole to clean the hole by hand. Such an operation is possible only for large-diameter piers. Failure to clean the bottom of the pier hole can sometimes result in excessive settlement. Fortunately, most of the small-diameter piers are overdesigned, and the skin friction alone is sufficient to support the column load. The condition of the pier bottom is therefore not as critical. 10.5.2 DEWATERING Groundwater can seep into the pier hole through the upper overburden soils or through the lower bedrock. A pier-drilling operation usually seals the seams in the soil and stops seepage temporarily. Consequently, if concrete is available at the site and poured immediately after the completion of the drilling, dewatering can be avoided. On the other hand, if the pier hole is allowed to stand for a long period of time, water will seep into the hole and must be pumped out before pouring concrete. Pier holes left open for a long time can also result in costly hole remediation. An experienced driller, under such conditions, would rather fill up the hole and re-drill the hole when concrete is available for immediate pouring. If water enters into the pier hole rapidly through the upper granular soils, it will be necessary to case the hole above bedrock to control seepage. In most instances, the use of casing will seal all seepage through the overburden soils. However, casing above bedrock cannot stop the infiltration of water through the seams and fissures of the bedrock. Such water must be pumped out. If water is mixed with auger cuttings in a form of slurry, such a mixture can be bailed out by the use of a bailing bucket. 10.5.3 CONCRETE IN WATER Specifications generally call for the pouring of concrete in less than 6 in. of water in the pier hole. In fact, concrete can be poured successfully in less than 12 in. of water. Concrete displaces water and forces water to the top of the pier hole, where it drains away. If it is necessary to pour concrete in deep water, a tremie should be used. The bottom of the tremie should be kept below the surface of the concrete. Concrete is introduced into the hole by the use of an elephant trunk or by pressure pumping to avoid the effect of the segregation of concrete. If concrete is not allowed to hit the wall of the drill hole, high free fall of as much as 100 ft will not cause segregation. 10.5.4 CASING REMOVAL Steel casings are costly, and whenever possible the driller removes the casing after the completion of the pouring. Hasty removal of the casing can introduce air pockets ©2000 CRC Press LLC in the pier shaft that eventually will be filled with surrounding soils. This is especially serious where the pier is heavily reinforced. The John Hancock Building in Chicago suffered considerable construction delays due to poor piers that resulted from hasty casing removal. In an army project in Colorado, a 24-in. diameter pier reinforced with six 3/4-in. bars in a cage settled more than 3 ft even before the load was applied to the column. Under some difficult circumstances, it is prudent to leave the casing in rather than expend the effort to remove it. However, in such cases, the engineer should be aware of the loss of skin friction with a smooth casing surface. Direct observation of the elevation of the top of concrete is difficult. If the surface of the concrete rises even momentarily as the casing is being withdrawn, it is virtually certain that the pier hole will be invaded by the surrounding soils or foreign material. The appearance of a sinkhole or depression of the ground surface near the pier hole also offers a good indication of faulty installation. It is also necessary to compare the volume of concrete poured with the volume of the pier hole. Defects in the pier shaft due to casing removal can only be prevented by an experienced driller and a field engineer with powers of keen observation. 10.5.5 SPECIFICATION Construction specifications sometimes are prepared by engineers with no field expe-rience who copy from some previous project. Errors related to concrete slump, aggregate size, pier diameter, etc. are found in some hastily prepared specifications. An experienced contractor would point out these problems before the commence-ment of the project. They would rather stop the work than adhere blindly to the specification. 10.5.6 ANGLED DRILLING In some unusual cases, it is necessary to drill pier holes at an angle (Figure 10.10). Near-horizontal drilling has been attempted. Such piers can be used as a tie-back for retaining walls or sheeting for deep excavation. The mechanics of such piers is seldom reported. The design as well as the method of construction should be carefully studied by both structural and geotechnical engineers. 10.6 PIER INSPECTION For a geotechnical engineer, more important than any of the above theoretical approaches to pier analysis is the pier inspection. Excessive settlements of the piers resulting in building distress generally are not caused by faulty analysis or errors in design, but by defective pier construction. The more common problems resulting in defective piers are: 10.6.1 REGULATIONS Prior to 1960, there were no regulations on the safety requirements for pier inspec-tion. Engineers rode on the driller’s kelley bar descending into an uncased drill hole. ©2000 CRC Press LLC FIGURE 10.10 Pier drilled at an angle. FIGURE 10.11 Skyline of downtown Denver, buildings founded on drilled pier. It was quite an experience for those in a deep bore hole, looking up at the sky which appeared to be the size of a dime. Although the risk involved in entering an uncased hole is large, accidents are seldom reported. ©2000 CRC Press LLC Today, OSHA has strict regulations on pier inspection. The commonly accepted rules are: Never enter an uncased drill hole. Always wear a harness with a safety device. This is to guard against attack by noxious gas. An inspector should not enter holes with too much water. Water should be pumped out prior to entering. Inspection procedures should be completed as quickly as possible. The entire operation should be completed in about 5 min to avoid delaying the pour-ing of concrete and the deterioration of the condition of a clean hole. 10.6.2 PIER BOTTOM The cleaning of loose soil at the bottom of the pier hole after the pier drilling has reached the design depth is important to prevent undue pier settlement. For small-diameter or uncased piers, the holes can be inspected by shining a mirror or a strong light into the hole. If the hole is not too deep, a fair evaluation can be achieved. For deep holes, above-ground inspection is not adequate; it is necessary to enter the pier hole and visually evaluate the condition. The presence of as little as 1 in. of loose soil at the bottom of the shaft can cause unacceptable settlement. Geotechnical consultants in Pierre, South Dakota, specified that all deep pier holes must be inspected by the use of a hand penetrometer. Inspection of pier holes becomes difficult when water is present. The engineer should try to enter the pier hole immediately after the completion of the drilling and before any seepage has built up. In some cases, it will be necessary to pump the water out before entering. If the settlement due to loose soil at the bottom of the pier is not excessive, it can be corrected by shimming the pier top. However, care should be taken to ensure that all settlement has taken place. Oftentimes, total settlement will not take place until the structure is completed and occupied. 10.6.3 PIER SHAFT Concrete can adhere to the wall of the shaft, creating a large void along the wall and preventing the concrete from dropping to the bottom of the shaft. For large-diameter piers, such occurrences are usually caused by large aggregates logged between the shaft and the steel cage. For small-diameter piers, the adhesion between concrete and shaft also can prevent the concrete from reaching to the bottom. Such occurrences generally are caused by using either too large an aggregate or too stiff a concrete. Architects sometimes use the same specifications of concrete for piers as for slabs and beams. As a result, the use of low-slump concrete and oversize aggregates causes the problem. It is always desirable for the geotechnical engineer to review the foundation specification before entering the bid. Checking the volume of the drill hole with the amount of concrete actually used can sometimes reveal the error. However, most of the time the defective piers can ©2000 CRC Press LLC temporarily be held up by skin friction and are not detected until the building load is applied. Settlement of the pier by as much as several feet has been reported. This can be very serious and very difficult to correct. In one Corps of Engineers project, it was necessary to build platforms on top of each pier and drill holes through the piers to detect the condition of the pier bottom. The diameter of the piers in expansive soils should be as small as possible, in order to concentrate the dead load to prevent pier uplift. Experience indicates that piers smaller than 12 in. are difficult to clean. It is recommended that all piers drilled in expansive soils should have a diameter no less than 10 in. REFERENCES F.H. Chen, Foundations on Expansive Soils, Elsevier Science, New York, 1988. P.M. Goeke and P.A. Hustad. Instrumented Drilled Shafts in Clay-Shale, presented at the October ASCE Convention and Exposition, Atlanta, GA, 1979. W.R. and W.S. Greer, Drilled Pier Foundations, McGraw-Hill, 1972. R.G. Horvath and T.C. Kenney, Shaft Resistance of Rock-Socked Drilled Piers, presented at the ASCE Convention and Exposition, Atlanta, GA, 1979. D. Jubervilles and R. Hepworth, Drilled Pier Foundation in Shale, Denver, Colorado Area, Proceedings of the Session on Drilled Piers and Caissons, ASCE/St. Louis, MO, 1981. M.W. O’Neill and N. Poormoayed, Methodology for Foundations on Expansive Clays, Journal of the Geotechnical Engineering Division, ASCE, Vol. 106, No. GT 12, 1980. H.G. Poulos and E.H. Dais, Settlement Analysis of Single Piles, Pile Foundation Analysis and Design, John Wiley & Sons, New York, 1980. W.C. Teng, Foundation Design, Prentice-Hall, Englewood Cliffs, NJ, 1962. ©2000 CRC Press LLC ... - tailieumienphi.vn