Pile Foundations Essay Example For Students

Pile Foundations Essay CHAPTER ONE INTRODUCTION 1. OVERVIEW OF FOUNDATIONS The word foundation is derived from a latin word fondare meaning to set or ground on something solid. A foundation is that part of a structure which transmits the weight of the structure to the ground in a manner that the soil below does not fail in shear and the settlement is within the safe limits. Foundations are broadly classified into two categories: †¢ Shallow Foundation †¢ Deep Foundation A shallow foundation, according to Terzaghi is one whose width is greater than its depth ie. D/ B or = 1. The main types of shallow foundations are footings are footings and raft or mat foundation. Footings can be further subdivided as shown below: †¢ Strip Footing †¢ Spread or Isolated footing †¢ Combined Footing †¢ Strap or Cantilever Footing. The loads must be spread to the soil in a manner such that its limiting strength is not exceeded and resulting deformations are tolerable. Shallow foundations accomplish this by spreading the loads laterally, hence the term spread footing. Whereas spread footing (or simply footing) supports a single column, a mat is a special footing used to support several randomly spaced columns or to support several rows of parallel columns and may underlie a portion of or the entire building. The mat may also be supported, in turn, by piles or drilled piers. Foundations supporting machinery and such are sometimes termed bases. Machinery and the like can produce substantial load intensity over a small area, so the base is used as a load-spreading device similar to the footing. Deep foundations are analogous to spread footings but distribute the load vertically rather than horizontally. The terms drilled pier and drilled caisson are for the pile type member that is constructed by drilling a 0. 76m diameter hole in the soil, adding reinforcing as necessary, and backfilling. On the other hand, a deep foundation is that which transmits the load at considerable depth below the ground surface. The main distinction between a deep foundation and a shallow foundation is generally made according to Terzaghi’s criterion which as discussed earlier termed shallow foundation as that which its depth equals or is less than its width. A very prominent example of deep foundation that will be discussed extensively in this work is the pile foundation. 1. 2WHAT IS A PILE? A pile is a slender structural member made of steel, concrete or wood which transfer the load to a deeper soil or rock of larger bearing capacity. Piles are generally driven, drilled or jacked into the ground. Depending on the type of soil, pile material, load transmitting characteristics etc. , piles are classified accordingly as will be discussed later. Alongside piles, pile cap is a vital component of a pile foundation. Pile foundations have been used as load carrying and load transferring systems for many years. In the early days of civilization, from the communication, defense or strategic point of view villages and towns were situated near to rivers and lakes. It was therefore important to strengthen the bearing ground with some form of piling. Timber piles were driven in to the ground by hand or holes were dug and filled with sand and stones. In 1740 Christoffoer Polhem invented pile driving equipment which resembled to days pile driving mechanism. Steel piles have been used since 1800 and concrete piles since about 1900. The industrial revolution brought about important changes to pile driving system through the invention of steam and diesel driven machines. More recently, the growing need for housing and construction has forced authorities and development agencies to exploit lands with poor soil characteristics. This has led to the development and improved piles and pile driving systems. Today there are many advanced techniques of pile installation. 1. FUNCTION OF PILES Just like other types of foundations, the purpose of pile foundations is: 1. To carry vertical compression load to a solid ground. 2. To resist uplift load. 3. To resist horizontal or inclined loads A structure can be founded on piles if the soil immediately beneath its base does not have adequate bearing capacity. If the results of site investigation show that the shallow soil is unstable and weak or if the magnitude of the estimated settlement is not accepta ble a pile foundation may become considered. Considering the cost estimatation, pile foundation may pose to be cheaper compared to any other ground improvement and development costs. In the cases of heavy constructions, it is likely that the bearing capacity of the shallow soil will not be satisfactory, and the construction should be built on pile foundations. Piles can also be used in normal ground conditions to resist horizontal loads. Piles are a convenient method of foundation for works over water, such as jetties or bridge piers. 1. 4OTHER TYPES OF DEEP FOUNDATIONS 1. 4. 1Piers A pier is a vertical column of relatively larger cross-section than a pile. A pier is installed in a dry area by excavating a cylindrical hole of larger diameter to the desired depth and then backfilling it with concrete. In foundations for large buildings, piers are usually cylindrical concrete shafts, cast in prepared holes, while in bridges they take the form of caissons, which are sunk into position. Piers serve the same purpose as piles but are not installed by hammers and, if based on a stable substrate, will support a greater load than a pile. In massive construction jobs, pier shafts having widths of more than 1. m (6 feet) have been excavated to depths greater than 30 m. The lower portion of a pier may be widened to better distribute the downward pressure of a massive overlying structure. Formerly hand-dug shafts were widely used for piers where groundwater presented no serious problem, but hand excavation has been largely superseded by the use of rotary or percussion drilling. The massive augers used to drill shafts for the piers of modern skyscrap ers are mounted vertically on derricks, and the piers themselves are sufficiently long and wide to support the tremendous weight of even the tallest building. Piers for bridges are often installed by the caisson method. The caisson is a hollow boxlike structure that is sunk down through the water and then through the ground to the bearing stratum by excavating from its interior; it ultimately becomes a permanent part of the completed pier. The methods of drilled pier construction can be classified in three categories as 1. The dry method 2. The casing method 3. The slurry method ADVANTAGES AND DISADVANTAGES OF DRILLED PIER FOUNDATIONS Advantages 1. Pier of any length and size can be constructed at the site 2. Construction equipment is normally mobile and construction can proceed rapidly 3. Inspection of drilled holes is possible because of the larger diameter of the shafts 4. Very large loads can be carried by a single drilled pier foundation thus eliminating the necessity of a pile cap 5. The drilled pier is applicable to a wide variety of soil conditions 6. Changes can be made in the design criteria during the progress of a job 7. Ground vibration that is normally associated with driven piles is absent in drilled pier construction . Bearing capacity can be increased by underreaming the bottom (in non-caving materials) Disadvantages 1. Installation of drilled piers needs a careful supervision and quality control of all the Materials used in the construction 2. The method is cumbersome. It needs sufficient storage space for all the materials used in the construction 3. The advantage of increased bearing capacity due to compaction in granular soil that could be obtained in driven piles is not there in drilled pier construction 4. Construction of drilled piers at places where there is a heavy current of ground water flow due to artesian pressure is very difficult 1. 4. 3 Caissons Caissons are boxlike structure used in construction work underwater or as a foundation. It is usually rectangular or circular in plan and may be tens of metres in diameter. A box caisson, open at the top and closed at the bottom, is usually constructed on land, then launched, floated to position, and sunk onto a previously prepared foundation, leaving its upper edge above water level. It serves as a suitable shell for a pier, seawall, breakwater, jetty, or similar work, remaining permanently in place on the sea bottom. An open caisson, open at both the bottom and the top, is fitted with a cutting bottom edge, which facilitates sinking through soft material while excavation is carried out inside through a honeycomb of large pipes, or dredging wells. As excavating proceeds and the caisson sinks, additional sections are added to the shaft above. This process is continued until the caisson has sunk to the required depth. A floor, usually of concrete, is laid to provide a bottom seal. The dredging wells can then be filled with concrete to complete the structure. Pneumatic caissons are similar to open caissons except that they are provided with airtight bulkheads above the cutting edge. The space between the bulkhead and cutting edge, called the working chamber, is pressurized to the extent necessary to control the inflow of soil and water; thus the excavating can be performed by workmen operating in the working chamber at the bottom of the caisson. 1. 4. 3 Drilled Shafts The mining industry has been the primary constructor of shafts, because at many locations these are essential for access to ore, for ventilation, and for material transport. Depths of several thousand feet are common. In public-works projects, such as sewer tunnels, shafts are usually only a few hundred feet deep and because of their high cost are avoided in the design stage wherever practical. Shallower shafts find many uses, however, for penstocks and access to underground hydro plants, for dropping aqueduct tunnels beneath rivers, for missile silos, and for oil and liquefied-gas storage. Being essentially vertical tunnels, shafts involve the same problems of different types of ground and water conditions but on an aggravated scale, since vertical transport makes the operation slower, more costly, and even more congested than with horizontal tunneling. Except when there is a high horizontal geostress in rock, the loading on a shaft support is generally less than for a tunnel. Inflowing water, however, is far more dangerous during construction and generally intolerable during operation. Hence, most shafts are concrete-lined and waterproofed, and the lining installation usually follows only a short distance behind excavation. The shape is usually circular, although, before current mechanized excavation methods, mining shafts were frequently rectangular. Shafts may be sunk from the surface (or drilled in smaller sizes), or, if an existing tunnel provides access, they may be raised from below. CHAPTER TWO 2. 1NECESSITY OF PILES Some conditions where pile foundations become a necessity compared to shallow foundations in engineering practice are: 1. In the case of irregularity of a structure’s plan relative to its load distribution and outline. Non-uniform settlement will set in if the structure is constructed on a shallow foundation and therefore requires a pile foundation to reduce the differential settlement. 2. Pile foundations are used to resist horizontal forces and at the same time, support the vertical loads in earth-retaining structures that are subjected to horizontal forces due to earthquake. 3. When there is a case of high compressibility and also weakness in the strata at or just below the ground surface so that the load transmitted by the structure cannot be supported. . They are required for the transmission of structural loads through deep water to a firm stratum. 5. When the soil conditions are such that a wash out, scour or erosion of the soil may occur from underneath a shallow foundation, pile foundation is required. 6. Piles are used to transfer the load beyond the zone of possible moisture changes in the soil. This is because compressible soils such as loess have breakdown of structures accompanied by a sudden decrease in void ratio when there is an increase in water content. 7. In case of expansive soils such as black cotton soil which swell and shrink as the water content changes, pile foundation is employed so as to transfer the load below the active zone 8. In some structures which are obviously subject to uplift, Piles are used in the foundation. Examples of such structures are transmission towers, off-shore platforms etc. 9. To stiffen the soil beneath machine foundations to control both amplitudes of vibration and the natural frequency of the system. 10. When the superstructure loads are very large and shallow foundations can’t bear them CHAPTER THREE 3. 0CLASSIFICATION OF PILES Piles may be classified as long or short in accordance with the L/d ratio of the pile (where L = length, d = diameter of pile). A short pile behaves as a rigid body and rotates as a unit under Lateral loads. The load transferred to the tip of the pile bears a significant proportion of the total vertical load on the top. In the case of a long pile, the length beyond a particular depth loses its significance under lateral loads, but when subjected to vertical load, the frictional load on the sides of the pile bears a significant part to the total load. Piles may further be classified as vertical piles or inclined piles. Vertical piles are normally used to carry mainly vertical loads and very little lateral load. When piles are inclined at an angle to the vertical, they are called batter piles or raker piles. Further classifications and those which will be reviewed in this term-paper are: a) Constitutive material b) Mode of load transfer c) Method of construction d) Their use e) The displacement of soil 3. 1 MATERIAL USED Timber piles Timber piles are made of tree trunks with the branches carefully trimmed off, usually treated with a preservative, and driven with the small end as a point. Occasionally the large end is driven for special purposes as in very soft soil where the soil will flow back against the shaft and with the butt resting on a firm stratum for increased bearing. The tip may be provided with a metal driving shoe when the pile is to penetrate hard or gravelly soils; otherwise it may be cut either square or with some point. Generally there are limitations on the size of the tip and butt end as well as on the misalignment that can be tolerated. Used from earliest record time and still used for permanent works in regions where timber is plentiful. Timber is most suitable for long cohesion piling and piling beneath embankments. The timber should be in a good condition and should not have been attacked by insects. For timber piles of length less than 14 meters, the diameter of the tip should be greater than 150 mm. If the length is greater than 18 meters a tip with a diameter of 125 mm is acceptable. It is essential that the timber is driven in the right direction and should not be driven into firm ground as this can easily damage the pile. Keeping the timber below the ground water level will protect the timber against decay and putrefaction. To protect and strengthen the tip of the pile, timber piles can be provided with toe cover. Pressure creosoting is the usual method of protecting timber piles. Driving of timber piles usually results in the crushing of the fibres on the head (or brooming) which can be somewhat controlled by using a driving cap, or ring around the butt. The usual maximum design load per pile does not exceed 250 KN. Timber piles are usually less expensive in places where timber is plentiful. Concrete piles a) Precast Concrete Piles Piles in this category are formed in a central casting yard to the specified length, cured, and then shipped to the construction site. If space is available and a sufficient quantity of piles needed, a casting yard may be provided at the site to reduce transportation costs. Precast piles may be made using ordinary reinforcement or they may be prestressed. Precast piles using ordinary reinforcement are designed to resist bending stresses during pickup and transport to the site and bending moments from lateral loads and to provide sufficient resistance to vertical loads and any tension forces developed during driving. The design procedures can be found in any text on reinforced-concrete design. However temporary stresses from handling and driving (tensile) may be used that are on the order of 50 percent larger than the allowable concrete design stresses. The minimum pile reinforcement should be 1 percent. Usually of square, triangle, circle or octagonal section, they are produced in short length in one metre intervals between 3 and 13 meters. They are pre-caste so that they can be easily connected together in order to reach to the required length. This will not decrease the design load capacity. Reinforcement is necessary within the pile to help withstand both handling and driving stresses. Maximum load on a prestressed concrete pile is approximately 2000 KN and on precast piles 1000 KN. The optimum load range is 400 to 600 kN. Pre stressed concrete piles are also used and are becoming more popular than the ordinary pre cast as less reinforcement is required. b) Driven and Cast-in-Place Concrete Piles A cast-in-place pile is formed by drilling a hole in the ground and filling it with concrete. The hole may be drilled (as in caissons), or formed by driving a shell or casing into the ground. The casing may be driven using a mandrel, after which withdrawal of the mandrel empties the casing. The casing may also be driven with a driving tip on the point, providing a shell that is ready for filling with concrete immediately, or the casing may be driven open-end, the soil entrapped in the casing being jetted1 out after the driving is completed. Various methods with slightly different end results are available and patented. Note that they are basically of three types: (1) shell or cased, (2) shell-less (uncased), or (3) pedestal types. Two of the main types used in the UK are: West’s shell pile : Pre cast, reinforced concrete tubes, about 1 m long, are threaded on to a steel mandrel and driven into the ground after a concrete shoe has been placed at the front of the shells. Once the shells have been driven to specified depth the mandrel is withdrawn and reinforced concrete inserted in the core. Diameters vary from 325 to 600 mm. Franki Pile: A steel tube is erected vertically over the place where the pile is to be driven, and about a metre depth of gravel is placed at the end of the tube. A drop hammer, 1500 to 4000kg mass, compacts the aggregate into a solid plug which then penetrates the soil and takes the steel tube down with it. When the required depth has been achieved the tube is raised slightly and the aggregate broken out. Dry concrete is now added and hammered until a bulb is formed. Reinforcement is placed in position and more dry concrete is placed and rammed until the pile top comes up to ground level. Steel piles These members are usually rolled HP shapes or pipe piles. Wide-flange beams or I beams may also be used; however, the H shape is especially proportioned to withstand the hard driving stress to which the pile may be subjected. In the HP pile the flanges and web are of equal thickness; the standard W and I shapes usually have a thinner web than flange. Pipe piles are either welded or seamless steel pipes, which may be driven either open-end or closed-end. Closed-end pipe piles are usually filled with concrete after driving. Open-end piles may be filled, but this is often not necessary, because there will be a dense soil plug at some depth below the top (and visible). Here it may only be necessary to jet out some of the upper soil plug to the necessary depth for any reinforcing bars required for bending (and to pump out the water used for jetting), before filling the remainder of the pile cavity with concrete. Concrete in only this shaft depth may be necessary for dowel bars. The HP pile is a small-volume displacement pile since the cross-sectional area is not very large. A plug tends to form between the flanges at greater depths, however, so the bottom several meters may remold the soil on the order of the volume of the plug. An open-end pipe is also considered a small-volume displacement pile; however, a plug also forms inside with a depth one or more meters below the outside ground level probably from a combination of inside perimeter friction and driving vibrations. From the depth at which the plug stabilizes (not visible during driving because of the pile cap and hammer interference) to the final driving depth, the lower soil may be remolded based on the volume of the plug and not the actual area of the pipe section. Steel Iron piles are suitable for handling and driving in long lengths. Their relatively small cross-sectional area combined with their high strength makes penetration easier in firm soil. They can be easily cut off or joined by welding. If the pile is driven into a soil with low pH value, then there is a risk of corrosion, but risk of corrosion is not as great as one might think. Although tar coating or cathodic protection can be employed in permanent works. It is common to allow for an amount of corrosion in design by simply over dimensioning the cross-sectional area of the steel pile. In this way the corrosion process can be prolonged up to 50 years. Normally the speed of corrosion is 0. 2-0. 5 mm/year and, in design, this value can be taken as 1mm/year Composite piles A composite pile comprises the combination of different materials in the same of pile. As indicated earlier, part of a timber pile which is installed above ground water could be vulnerable to insect attack and decay. To avoid this, concrete or steel pile is used above the ground water level, whilst wood pile is installed under the ground water level. This type is rarely used used in practice due to the difficulty encountered in the provision of a proper joint. . 2 Classification of pile with respect to load transmission and functional behavior a) End bearing piles (point bearing piles) b) Friction piles (floating piles) c) Combination of friction and end-bearing piles End bearing piles (Point bearing piles) These piles transfer their load through their bottom tip on to a firm stratum located at a considerable depth below the base of the structure and they derive mo st of their carrying capacity from the penetration resistance of the soil at the toe of the pile. The pile behaves as an ordinary column and should be designed as such. artificial intelligence EssayOnce the unit skin friction has been estimated, the shaft resistance is determined from equation For cohesive soils, the ultimate load can be determined by adding the point resistance and the shaft resistance equation (5. 1) Thus; As the clay gets remoulded when the pile is driven, this factor must be taken into account when estimating the load carrying capacity. The remoulded strength is always less than the undisturbed strength, but because of thixotropy, the strength depends upon the consolidation characteristics of the soil and the rate of dissipation of excess water pressure. When using equation , the value of c and c should be judiciously evaluated. 5. 2 NEGATIVE SKIN FRICTION AND ITS EFFECT ON PILES When the fill starts consolidating under its own overburden pressure, it develops a drag on the surface of the pile. This drag on the surface of the pile is called negative friction. Negative friction may develop if the fill material is loose cohesionless soil. Negative friction can also occur when fill is placed over peat or a soft clay stratum. The superimposed loading on such compressible stratum causes heavy settlement of the fill with consequent drag on piles. Negative friction may develop by lowering the ground water which increases the effective stress causing consolidation of the soil with resultant settlement and friction forces being developed on the pile. Negative friction must be allowed when considering the factor of safety on the ultimate carrying capacity of a pile. The factor of safety, Fs, where negative friction is likely to occur may be written as Computation of Negative Friction on a Single Pile The magnitude of negative friction Fn for a single pile in a fill may be taken as (a) For cohesive soils . b) For cohesionless soils Where Ln = length of piles in the compressible material, s = shear strength of cohesive soils in the fill, P = perimeter of pile, K = earth pressure coefficient normally lies between the active and the passive earth pressure coefficients, ? = angle of wall friction which may vary from ? /2 to ?. Negative Friction on Pile Groups When a group of pils passes through a compressible fill, the negative fri ction, Fn , on the group may be found by any of the following methods (a) †¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦.. i (b) †¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦ii where n = number of piles in the group, = unit weight of soil within the pile group to a depth Ln, P = perimeter of pile group, Ag sectional area of pile group within the perimeter Pg , s = shear strength of soil along the perimeter of the group. Equation i gives the negative friction forces of the group as equal to the sum of the friction forces of all the single piles. Eq ii assumes the possibility of block shear failure along the perimeter of the group which includes the volume of the soil yLnA enclosed in the group. The maximum value obtained from Eqs i or ii should be used in the design. OTHER METHODS OF DETERMINING ULTIMATE LOAD BEARING CAPACITY OF A SINGLE VERTICAL PILE The ultimate bearing capacity, Qu, of a single vertical pile may also be determined by any of the following methods: 1. By the use of SPT and CPT values. 2. By field load tests. 3. By dynamic method. 5. 3DYNAMIC FORMULAE The load-carrying capacity of a driven pile can be estimated from the resistance against penetration developed during the driving operation. The methods give fairly good results only in the case of free-draining sands and hard clays in which high pore pressure does not develop during the driving of the piles. In saturated fine-grained soils, high pore water pressure develops during the driving operation and the strength of the soil is considerably changed and the methods do not give reliable results. The methods cannot be used for submerged, uniform fine sands which may become loose enough to become quick temporarily and show a much less resistance. The dynamic formulae are based on the assumption that the kinetic energy delivered by the hammer during driving operation is equal to the work done on the pile. Thus; Where W – weight of the hammer h – Height of the ram drop – Efficiency of the pile hammer R – Pile resistance taken to equal Qu S – Pile penetration per blow In equation above, no allowance has been made for the loss of energy during driving operation, loss caused by elastic contraction of the pile, soil, pile cap, cushion and due to the inertia of the pile. Some energy is also lost due to generation of heat. Various formulae have been proposed, which basically differ only in the methods for accounting of the energy losses, as described below: 5. 3. 1Engineering News Record Formulae According to Engineering News Record (ENR) Formulae (1888), the ultimate load is given by Where S – penetration of the pile per hammer blow C – constant ( for drop hammer, C = 2. 54cm and for steam hammer, C = 0. 245) In equation (5. 17), the product can be replaced by the rated energy of hammer (En) in KN-cm. Thus, The efficiency ? h of the drop hammer is generally 0. 7 and 0. 9, and that for a single-acting and a double-acting hammer is between 0. 75 and 0. 85. For diesel hammer, it usually lies between 0. 80 and 0. 90. A factor of safety of 6 is usually recommended. However, the pile load tests reveal that the actual factor varies between 23 and 30. The formula is, therefore, not dependable. Modified formula The Engineering News Record Formulae has been modified recently. In the modified formula, the energy losses in the hammer system and that due to impact are considered. According to this formula; Where P – weight of pile e Coefficient of restitution ?h -hammer efficiency The hammer efficiency (? h) depends upon various factors, such as pile driving equipment, driving procedure, type of pile and the ground conditions. For drop hammers, it is usually taken between 0. 75 and 1. 0; for single acting hammers between 0. 75 and 0. 85; for double-acting or differential hammer, ? 0. 85 and for diesel hammer, ? = 0. 85 to 1. 0. The representative values of the coefficient of restitution (e) are as follows: Broomed timber pile=0. 0 Good timber pile =0. 25 Driving cap with timber dolly on steel pile =0. 3 Driving cap with plastic dolly on steel pile=0. 5 Helmet with composite plastic dolly and packing on R. C. C. pile = 0. 4 5. 3. 2. Hiley Formula Hiley (1925, 1930) gave a formula which takes into account various losses. Where ? h efficiency of hammer blow H height of free fall of the room or hammer S final set or penetration per blow C Sum of temporary elastic composition of the pile, dolly, packings and ground (= C1 + C2 + C3), C1 = temporary compression of dolly and packings (= 1. 77 R/A), when the driving is without dolly, = 9. 05 R/A when the driving is with short dolly), C2 = temporary compression of pile (= 0. 657 RD/A), C3 = temporary compression of ground (= 3. 55 R/A). D Length of the pile A Cross-sectional area of pile P Pile resistance The efficiency of the hammer blow (? h) depends upon the weight of hammer (w), weight of pile, anvil and helmet follower (P) and the coefficient of restitution (e). ) For , b) For , The coefficient of restitution (e) varies from zero for a deteriorated condition of the head of pile to 0. 5 for a steel ram of double acting hammer striking on steel anvil and driving a reinforced concrete pile. For a C. I. ram of a single – acting or drop hammer striking on the head of R. C. C. pile, e = 0. 4 and that striking on a well-conditioned driving cap and helmet with h ard wood on R. C. C. pile, e = 0. 25 (IS:2911 – 1979). 5. 3. 3Danish Formula According to Danish Formula (1929) Where In which So=elastic compression of pile D=Length of pile A=Cross-sectional area E=Modules of elasticity of pile material The allowable load is found by taking of factor of safety of 3 or 4. Equation (5. 24) can also be used to determine the final set (S) per blow Taking . Where 5. 4Comments On The Use Of Dynamic Formulae: 1. Detailed investigations carried out by Vesic (1967) on deep foundations in granular soils indicate that the Engineering News Record Formula applicable to drop hammers. In order to obtain better agreement between the one computed and observed loads, Vesic suggests the following values for the coefficient C. For steel pipe piles, C = 1 cm. For precast concrete piles C = 1. 5 cm. 2. The tests carried out by Vesic in granular soils indicate that Hileys formula does not give consistent results. The values computed from are sometimes higher and sometimes lower than the observed values. 3. Dynamic formulae in general have limited value in pile foundation work mainly because the dynamic resistance of soil does not represent the static resistance, and because often the results obtained from the use of dynamic equations are of questionable dependability. 4. Dynamic formulae could be used with more confidence in freely draining materials such as coarse sand. If the pile is driven to saturated loose fine sand and silt, there is every possibility of development of liquefaction which reduces the bearing capacity of the pile. 5. Dynamic formulae are not recommended for computing allowable loads of piles driven into cohesive soils. In cohesive soils, the resistance to driving increases through the sudden increase in stress in pore water and decreases because of the decreased value of the internal friction between soil and pile because of pore water. These two oppositely directed forces do not lend themselves to analytical treatment and as such the dynamic penetration resistance to pile driving has no relationship to static bearing capacity. There is another effect of pile driving in cohesive soils. During driving the soil becomes remolded and the shear strength of the soil is reduced considerably. Though there will be a regaining of shear strength after a lapse of some days after the driving operation, this will not be reflected in the resistance value obtained from the dynamic formulae. 5. IN-SITU PENETRATION TEST FOR PILE CAPACITY 5. 5. Standard penetration Test. The load carrying capacity of a pile can be estimated from the standard penetration test value (N). i. For driven piles in sand, the unit tip resistance (qp) is related to the uncorrected blow count (N) near the pile point (Meyerhof 1976). Q p = 40N (D/B) ; 400N Where qp = point resistance (Kn/m2), D = Length of pile, B = width (diameter) of pile. The value of qp is u sually limited to 400N. The average unit frictional resistance (fs) is related to the average value of the blow count (N). For high displacement piles, fs = 2. 0N kNm2 For low displacement piles, fs = 1. N kNm2 Where N is average of uncorrected N-value along the length of the piles. ii. For bored piles in sand, qp = 14 N (Db/B) kN/m2 Where Db = actual penetration into the granular soil. For bored piles in sand, the unit frictional resistance (fs) is given by fs = 0. 67 N kNm2 5. 5. 2Dutch cone test: Meyerhof (1965) relates the unit point resistance (qp) and unit skin traction (fs) of driven piles to the cone point resistance. (qc) Point resistance Unit skin friction (a) fs (dense sand) = qc / 200 (b) fs (loose sand) = qc / 400 (c) fc (silt) = qc / 150 . 5. 3 PILE LOAD TEST The load test may be carried out either on a driven pile or a cast-in-situ pile. Load tests may be made either on a single pile or a group of piles. Load tests on a pile group are very costly and may be undertake n only in very important projects. Pile load tests on a single pile or a group of piles are conducted for the determination of 1. Vertical load bearing capacity, 2. Uplift load capacity, 3. Lateral load capacity. Generally load tests are made to determine the bearing capacity and to establish the load settlement relationship under a compressive load. The other two types of tests may be carried out only when piles are required to resist large uplift or lateral forces. Usually pile foundations are designed with an estimated capacity which is determined from a thorough study of the site conditions. At the beginning of construction, load tests are made for the purpose of verifying the adequacy of the design capacity. If the test results show an inadequate factor of safety or excessive settlement, the design must be revised before construction is under way. Load tests may be carried out either on 1. A working pile or 2. A test pile. A working pile is a pile driven or cast-in-situ along with the other piles to carry the loads from the superstructure. The maximum test load on such piles should not exceed one and a half times the design load. A test pile is a pile which does not carry the loads coming from the structure. The maximum load that can be put on such piles may be about 2J/2 times the design load or the load imposed must be such as to give a total settlement not less than one-tenth the pile diameter. Method of Carrying Out Vertical Pile Load Test A vertical pile load test assembly is shown in Fig. 15. 19(a). It consists of 1. An arrangement to take the reaction of the load applied on the pile head, 2. A hydraulic jack of sufficient capacity to apply load on the pile head, and 3. A set of three dial gauges to measure settlement of the pile head. Load Application A load test may be of two types: 1. Continuous load test. 2. Cyclic load test. In the case of a continuous load test, continuous increments of load are applied to the pile head. Settlement of the pile head is recorded at each load level. In the case of the cyclic load test, the load is raised to a particular level, then reduced to zero, again raised to a higher level and reduced to zero. Settlements are recorded at each increment or decrement of load. Cyclic load tests help to separate frictional load from point load. The total elastic recovery or settlement Se, is due to 1. The total plastic recovery of the pile material, 2. Elastic recovery of the soil at the tip of the pile, Sg The total settlement S due to any load can be separated into elastic and plastic settlements by carrying out cyclic load tests as shown in Fig. 15. 19(b). A pile loaded to Ql gives a total settlement Sr When this load is reduced to zero, there is an elastic recovery which is equal to Sel. This elastic recovery is due to the elastic compression of the pile material and the soil. The net settlement or plastic compression is S r The pile is loaded again from zero to the next higher load Q2 and reduced to zero thereafter. The corresponding settlements may be found as before. The method of loading and unloading may be repeated as before. Allowable Load from Single Pile Load Test Data There are many methods by which allowable loads on a single pile may be etermined by making use of load test data. If the ultimate load can be determined from load-settlement curves, allowable loads are found by dividing the ultimate load by a suitable factor of safety which varies from 2 to 3. A factor of safety of 2. 5 is normally recommended. A few of the methods that are useful for the determination of ultimate or allowable loads on a single pile are given below: 1. The ultimate load, Qu, can be determined as the abscissa of the point where the curved part of the load-settlement curve changes to a falling straight line. . Qu is the abscissa of the point of intersection of the initial and final tangents of the load-settlement curve 3. The allowable load Q is 50 percent of the ultimate load at which the total settlement amounts to one-tenth of the diameter of the pile for uniform diameter piles. the manner in which the foundation piles are to be installed. The test piles should be atleast 3 B or 2. 5 m clear from the anchor piles. 4. The allowable load Qa is sometimes taken as equal to two-thirds of the load which causes a total settlement of 12 mm. 5. The allowable load Qa is sometimes taken as equal to two-thirds of the load which causes a net (plastic) settlement of 6 mm. DETERMINATION OF ULTIMATE LOAD FROM LOAD SETTLEMENT CURVES If pile groups are loaded to failure, the ultimate load of the group, Q u, may be found by any one of the first two methods mentioned above for single piles. However, if the groups are subjected to only one and a half-times the design load of the group, the allowable load on the group cannot be found on the basis of 12 or 6 mm settlement criteria applicable to single piles. In the case of a group with piles spaced at less than 6 to 8 times the pile diameter, the stress interaction of the adjacent piles affects the settlement considerably. The settlement criteria applicable to pile groups should be the same as that applicable to shallow foundations at design loads. OTHER TYPES OF PILE LOAD TESTS 1. Constant rate of penetration test. In a constant-rate of penetration tests, the load on the pile is continuously increased to maintain a constant rate of penetration (from 0. 25 to 5 mm per minutes). The force reqiur3ed to achieve that rate of penetration is recorded, and a load settlement curve is drawn. The ultimate curve can be determined from the curve. 1) Routine Load test. This test is carried out on a working pile with a view to determine the settlement corresponding to the allowable load. As the working pile would ultimately form a part of the foundation, the maximum load is limited to one and half times the safe load or upto the load which gives a total settlement of 12 mm. 2) Cyclic Load test. The load is carried out for separation of skin friction and point resistance of a pile. In the test, an incremental load is repeatedly applied and removed. 3) Lateral Load test. The test is conducted to determine the safe lateral load on a pile. A hydraulic jack is generally introduced between two pile to apply a lateral load. The reaction may also be suitably obtained from some other support. The test may also be carried out by applying a lateral pull by a suitable set-up. 4) Pull out test. The test is carried out to determine the safe tension of a pile. In the set-up, the hydraulic jack rests against a frame attached to the top of the test pile such that the pile gets pulled up. CHAPTER SIX GROUP ACTION OF PILES. A pile is not used singularly beneath a column or a wall, because it is extremely difficult to drive the pile absolutely vertical and to place the foundation exactly over its centre over its centre line. If eccentric loading result, the connection between the pile and the column may break or the pile may fail structurally because of bending stresses. In actual practice, structural load are supported by several piles acting as a group. For columns, a minimum of three piles in a triangular pattern are used. For walls, piles are installed in a staggered arrangement on both sides of its center line. The loads are usually transferred to the pile group through a reinforced concrete slab, structurally tied to the pile tops such as that the piles act as one unit. The slab is known as a pile cap. The load acts on the pile cap which distributes the load tom the piles. The load carrying capacity of a pile group is not necessarily equal to the sum of the capacity of the individual piles. Estimation of the load-carrying capacity of a pile group is a complicated problem. When the piles are spaced a sufficient distance apart, the group capacity may approach the sum of the individual capacities. On the other hand, if the piles are closely spaced, the stresses transmitted by the piles to the soil may overlap, and this may reduce the load-carrying capacity is limited by the group action. The efficiency () of a group of pile is defined as the ratio of the ultimate load of the group to the sum of individual ultimate loads. Or Where Qg(u) = ultimate load of the group, Q(u) = ultimate load of the individual pile, N = Number of piles in the group. Thus the group efficiency is equal to the ratio of the average load per pile in the group at which the failure occurs to the ultimate load of a comparable single pile. The group efficiency depends upon the spacing of the piles. Ideally, the spacing should be sure that the efficiency is 100%. Generally, the centre to centre spacing is kept between 2. 5 B and 3. 5 B, where B is the diameter of the pile. The methods for the determi