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6 Piling, Nailing, and Mixing 6.1 PILING Driving piles is not generally thought of as a soil stabilization procedure, and indeed piles are driven primarily to support loads that cannot be safely carried at or near the surface. With few exceptions, piles are not driven for the major purpose of stabilizing soils. Nonetheless, driving piles into granular deposits will densify those deposits, and make them more capable of supporting pile loads. This is due to a combination of lateral displacement
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  6 Piling, Nailing, and Mixing 6.1 PILING Driving piles is not generally thought of as a soil stabilization procedure,and indeed piles are driven primarily to support loads that cannot be safelycarried at or near the surface. With few exceptions, piles are  not  driven forthe major purpose of stabilizing soils. Nonetheless, driving piles intogranular deposits will densify those deposits, and make them more capableof supporting pile loads. This is due to a combination of lateraldisplacement and vibration.If the pile is a hollow tube, fitted with a bottom plate that staysin place when the pile is withdrawn, sand can be added and compactedduring withdrawal, creating a  sand pile  capable of carrying a significantload. Well graded gravel or crushed stone may also be used in place of sand.Sand piles may also be created in cohesive soils, but do not densifythe formation as the piles are placed. Remolding can occur, which over ashort period of time may increase the adhesion to the pile above thecohesion value. The sand piles will reduce the drainage paths for consolida-tion to take place. In a loaded soil mass, the resulting settlement may resultin downdrag on the piles, decreasing their capacity to carry structuralloads. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  The actual load carrying capacity of a sand pile is much less than thatof a similar size concrete pile. The surest way to determine capacity is byfield load tests. Minipiles , those smaller in diameter than about 10 inches, function thesame as larger piles. They are particularly suited in places where operatingheadroom is low, and where conventional pile driving equipment will not fit.They, too, will densify granular deposits as they are driven. However, theirsmall cross sectional area makes them unsuitable for (load bearing) sandpiles. The smaller sizes, however, are filled with uncompacted, narrowlygraded sand in clay deposits, to create sand drains. 6.2 SOIL NAILING Soil nailing is similar to ground anchors or tiebacks in that a steel rod isgrouted into a pre-drilled hole. There are, however, several importantdifferences. Nails are considerably smaller and shorter than anchors, andwhile anchors are pre-stressed after placement, nails are not (with fewexceptions in which a very small pre-stress is applied), and do not pick upload until the soil mass deforms. Nails, like anchors, add shear resistance tothe soil mass.Soil nailing dates back to the early seventies, a process developed inEurope for stabilizing natural slopes. Originally, the nails (which were andstill are steel reinforcing bars) were driven directly into the soil without pre-drilling. This method is still in use, particularly when pre-drilled holes willnot stay open. In some instances augers have been drilled into place,avoiding the problem of caving drill holes. Recent experiments haveindicated that the effectiveness of a nail is directly related to its pull-outresistance. Therefore augers, while more costly than plain or deformed steelbars, are also more effective. Currently, the major use of nailing is tostabilize man-made slopes, which occur as excavation proceeds for below-ground structures.Typically, soil nailing is done as the excavation progresses, usually infive-foot vertical strips (or less, if soil conditions indicate), and in a lengthconsistent with a day’s work. Wire mesh is placed on the exposed soil faceand shot crete is applied. Nail holes are then drilled to form a square gridwith four or five foot spacing. The holes slant downward, up to 20 8  from thehorizontal. Nail lengths are designed to extend beyond the possible failureplane for unreinforced soil, usually 75 to 100 %  of the slope height.Reinforcing bars are placed in the holes, kept centered by plastic spacers.The final step is to grout the annulus with good quality cement. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  6.3 REINFORCED FILL Nailing is done to protect slopes as excavation proceeds—that is, it is aprocess that works from the top down. In contrast, when fill is placed toraise an area, the slope is created from the bottom up. For high fills, it maybe necessary to reinforce the soil in order to prevent a slope failure. Thismay be done with geotextile sheets, which are placed horizontally to coverthe entire fill surface at vertical intervals of several feet. The geotextile sheetsadd shear resistance to possible slip or failure planes. In order to beeffective, the sheets must extend a significant distance beyond the failureplanes for unreinforced soil. Rigorous design procedures are not yetavailable, and the parameters for field use are selected on the basis of pastfield experience. 6.4 SHALLOW SOIL MIXING The term  mixing  is applied to the addition of foreign materials intimatelyintermingled with the soil particles (as opposed to nails or sheets which areadded at wide intervals to the soil mass). The earliest mixing was probablydone in antiquity, when sand or stones were thrown into mud to make amore supportive medium. Modern practice uses specialized equipment toapportion and mix a wide variety of additives into existing soil formations.Most mixing procedures are done to stabilize and improve soils at shallowdepths. In the past several decades, however, effective equipment andprocedures have been developed to treat soils to substantial depths. Theseare discussed in the following section.Surface treatments are economically effective to depths up to 18inches. The most common additive is Portland cement. When added tosaturated sands, the cement hydrates and forms a strong material referred toas  soil-cement . If the soil is not saturated, it may be necessary to add waterto insure full hydration of the cement. After mixing, the soil cement iscompacted and graded and left to cure. The process works best withgranular materials. Seldom are concrete strengths approached. The presenceof silt and clay in the soil reduce the final strength, and the process obviouslycan’t be controlled as closely as making concrete. Nonetheless, soil cementgenerally provides a suitable wearing surface for light traffic and uses suchas warehouse floors, bike paths, etc.The desirable properties for the final pavement are determined bymoisture-density, freeze-thaw, wet-dry, and strength tests in the laboratory,using the unmodified in-situ soil. Generally, the freeze-thaw and wet-drytests are done first, to determine the amount of cement to be used. Thisvalue is then used for the moisture-density tests. Cement contents can vary Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.  between three and 15 % , with the lower values for coarser granular materials,the intermediate values for fine, granular materials, and the higher values forsoils containing organic materials and clays.Typically, the cement is spread on the soil surface, then thoroughlymixed to the design depth. Next, the required amount of water is added, andmixing continues until a homogeneous, uniform mixture is attained. Mixingcontinues until all of the mixture passes a 1/2 inch sieve. Many specs alsorequire 80 % of the mixture to pass a No. 4 sieve. Compaction to the valuesdetermined by the moisture-density tests and grading complete the work.Soil-cement has its initial set in a matter of hours, then cures to its finalstrength over a period of several weeks. It may be necessary to cover or wetthe surface periodically to promote proper curing. Soil-cement mixtures canattain significant strengths, as shown in Figure 6.1, where the road surfacehas bridged over a washout.Cohesive materials, particularly clays, are more effectively stabilizedby the addition of hydrated lime. Best results are obtained with clays of medium to high plasticity. The lime supplies calcium cations, which replacethe cations on the clay minerals, thus changing the mineralogy to a materialwith more desirable engineering characteristics. The chemical reactioncontinues for a long time, even years, as long as lime is present to keep thepH above 10.As with cement, mix design is determined by laboratory tests, and in-place mixing is done in the field to add the appropriate amount of lime, F IGURE  6.1  Bridging action of soil-cement road surface. Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.
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