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442 FOUNDATIONScomputed on the assumption that no piles are present.The ultimate settlement of the pile foundations shown onthe right side of the diagrams can be estimated roughlyon the basis of the following simplifying assumption.Above the level of the lower third-point of the length ofthe piles the water content of the clay remains unchanged,and below this level consolidation proceeds as if thebuilding were supported on a flexible raft located at thatlevel. The presence of the piles is disregarded. Accordingto this assumption, the benefit derived from the pilesis equivalent to replacing the subsoil by a practicallyincompressible material that extends from the base of thefoundation to a depth equal to two thirds of the lengthof the piles. If this depth is several times greater than thewidth of the footings and the footings are widely spaced,the settlement of the pile foundation will be small, nomatter how com pressible the subsoil may be. On the otherhand, if this depth is considerably sm aller than the widthof the loaded are a and the loaded area is large, the ultimatesettlement may be excessive even under a moderate load.These conclusions have been confirmed consistently byexperience. Both experience and theory have also shownthat raft foundations supported by uniformly loaded andequally spaced friction piles, like sim ple raft foundations,always tend to assume the shape of a shallow bowl.If the structure contains a basemen t, the load that pro-duces consolidation is equal to the difference betweenthe effective weight of the building and the effectiveweight of the soil that was excavated to for m the basement(Article 5 1.3).52.5.8 Heave and Lateral Movement due to PileIkiv ingIf a pile is driven through silt or clay, the neighboringpiles may rise. If the piles were driven to en d-bearing onrock or a stratum of hard soil, their points may losecontact with the point-supporting material. Composite orspliced piles may even separate at the joint. Subsequentapplication of load causes a settlement equal to the pre-ceding rise. Hence, if the rise is excessive, the piles shouldbe redriven. On the other hand, if the piles were driventhrough weak strata for som e distance into a stiffer stratumfrom which the piles will receive their support by sideresistance, displacements may cause the piles to heave,but the heave m ay not diminish their capacity. Redrivingis then unnecessary (Klohn 1963, Koutsoftas 1982).Heave may be reduced by predrilling to remove partof the soil that would otherwise be displaced, or by usinga pile, such as an H-pile, that has a small cross-section.However, even H-piles may cause enoug h displacement toproduce large heave under some conditions (Olko 1963),particularly if a plug of soil forms between the flanges.Whe rever conditions conducive to heave exist, the ele-vations of the tops of the piles should be monitored. If

the type of pile permits, tell-tales should be used to detectheave of the points. If it is not obvious that contact witha hard bearing stratum has been lost and that redrivingwill therefore be needed, load tests should be performedto determine whether the capacity of the heaved pileshas been impaired. Criteria for redriving can then beestablished to control the job.The displacements may cause not only heave but alsolateral moveme nts, especially if there are adjacent excava-tions extending below the level from which the piles aredriven (Hagerty and Peck 1971). Wh ere the movementsare likely to be objectionable, they can be reduced byremoving part of the soil in the space to be occupied byeach pile. This is usually done by predrilling with anauger or by coring with a rotary cutting tool combinedwith water jets that transform the clay into a slurry wherethe pile is to be located.

52.5.9 Efficiency EquationsThe preceding discussions have demonstrated that thesettlement of a pile foundation exceeds that of a singlepile under a load equal to the load per pile in the founda-tion. The realization of this fact led to various attemptsto express the influence of the number and spacing ofthe piles on the settlement of the foundation by so-calledefficiency equations (Seiler and Keeney 1944, Masters1943, Feld 1943). However, the extraordinary variety ofsoils encountered in piling practice excludes the possibil-ity of establishing a limited number of sufficiently accu-rate efficiency equations of general validity. The effectof the number and spacing of the piles on the ratio betweenthe settlement of a single pile under a given load and thatof a group under the same load per pile depends to alarge extent on the sequence and properties of the soilstrata. Furthermore, at a given length and spacing of thepiles the ratio changes to a considerable extent with theload per pile. Nevertheless, in none of the existing theoriesare these vital facts given adequate consideration.Because of the great number and diversity of the factorsinvolved, it seems very doub tful, to say the least, whetherthe efficiency e quations represent a step in the right direc-tion. Estim ates of the ratio based on the theory of elasticitymay be logically more defensible but, as discussed inArticle 52.5.1, they have inherent shortcomings.

At the present state of knowledge it is preferable toconsider eve ry case individually and to evaluate the proba-ble settlement of a proposed pile foundation on the basisof the physical properties of the soils onto which the loadis transmitted by the piles. Examples of the use of thisprocedure have been given under the preceding subhead-ings. If the probable se ttlement exceeds the tolerable maxi-mum, the design must be modified. The maximumtolerable settlement of pile foundations is determined by

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ARTICLE 52 PILE FOUNDATIONS 443the same factors as those that govern the perm issible settle-ment of footing and raft foundations (Articles 50 and 51).If the distribution of the loads over the area to beoccupied by a structure is very uneven, the secondarystresses in the structure due to unequal settlement can b eappreciably relieved by dividing the structure into blocksseparated from each other by continuous vertical joints.52.5.10 Selection of Type of PileThe designer of a pile foundation can choose amongseveral different types of piles, any one of which mayprovide adequate support for the proposed foundation.The final choice is governed by eco nomic considerationsand by conditions imposed by the character of the job.Until the early 190 0's untreated timber piles w ere usedalmost exclusively. This type of pile is relatively cheap,but it has two major disadvantages. First, an untreatedwood pile must be cut off below the lowest water table;if the water table is subsequently lowered on accountof a permanent change in groundwater conditions, theuppermost parts of the pile disintegrate within a relativelyshort time. Second, a wood pile may break if it is driventoo hard, although the foreman may not detect anythingunusual. The risk of deterioration may be reduced byimpregnation with wood preservatives, but the risk ofbreakage can be reduced only by stopping the driving ofthe pile while its bearing capacity is still relatively low.Because concrete or steel piles can be driven harder than.vood p iles without risk of damag e, the safe design loadfor such piles is considerably greater than that for woodpiles. Recognition of this fact in practice is exemplifiedby the values that represent the loads comm only assignedto piles of various types. Such values are given in Table52.2. However, under many circumstances the designloads differ widely from those in the table.Although the safe design loads for piles of differenttypes vary, the spacing betw een piles of all types is practi-cally the same. Therefore, the footings required to transfera given load onto wood piles are considerably larger andmore expensive than footings supported by concrete orsteel piles. Furthermore, the bases of footings resting onconcrete or steel piles can be established at any c onvenientelevation, whereas those of footings on untreated woodpiles must be located below the lowest position of thewater table. These advantages in many instances compe n-sate for the additional cost of concrete or steel piles.Before the beginning of the 20th century all concretepiles were the precast, reinforced type. During the nextdecade cast-in-place piles became widely used, and themanufacture of concrete piles developed into a highlyspecialized industry. Later, prestressed concrete piles alsoentered the field. Structural steel sections and steel pipehave similarly become commonplace. The piles fromwhich the designer may choose differ in their method of

Table 52.2on Driven Piles"Customary Range of Working Loads

TYPeLoad(MN)

Timber (200 mm tip diam.)Concrete, precast, or prestressed250 mm diameter450 mm squareSteel pipe or shell, concrete-filled, notmandrel-driven27 3 X 4.8 mm pipe273 X 6.4 mm pipe32 4 X 6.4 mm pipe35 0 X 7.8 mm pipe40 0 X 9.5 mm pipeMonotube, 7-gagemandrel-drivenb

Steel pipe or shell, concrete-filled,Raymond Step-taper with 260 m m pointRaymond Step-taper with 308 m m point305 mm corrugated, 16 gage25 4 X 3.2 mm pipeSteel H-sectioncHP 10 X 42"HP 12 X 53"HP 14 X 89 "HP 14 X 117"

0.1-0.30.2-0.60.7-2 .O

0.3-0.50.4-0.70.5-0.80.6-0.91.0-1.20.3-0.5

0.3-0.50.4-0.70.3-0.60.3-0.5

0.5-0.80.5-1 .O1 O-1.61.5-2.0"Indicated maximum loads can be exceeded if freezeor setup (Article 52.3.4) oc curs after pile has been driven

to resistance corresponding to tabulated value.bUse of the mandrel perm its driving these piles to aresistance great enough to warrant working loads basedon the full structural capacity of the pile."When driven with adequate hamm er to resistance indi-cated by wave equation H-piles may be stressed to asmuch as 90MPa under working loads; in soils likely todeform the tips, the same stress may be allowed if thepiles are equipped with drive points."Nominal width in inches; weights in pounds per lin-eal foot.

installation, their shape, the texture of their surface, andseveral other aspects. Almost every type of pile has fea-tures that make it exceptionally suitable under certain soilconditions and less suitable or inapplicable under others.If vibrations due to pile driving cannot be tolerated forsome reason, a pile must be adopted that can be jackedor augered down or else installed in a drill hole. Thesefactors must be considered by the des igner in connectionwith every pile job. Proper choice of a pile type requires