S S SUBMITTED BY:- CHARU BHARDWAJ civil engineering 0804300010 PILE FOUNDATION S S SUBMITTED BY:- CHARU BHARDWAJ civil engineering 0804300010

DEFINITION Pile foundation is a deep foundation used to transfer the loading to a deeper, more competent strata at depth if unsuitable soils are present near the surface. This is usually done when depth >3 m below finished ground level. In pile foundation, piles of different material in different manner are used.

Continued Piles are relatively long , slender members that transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata of high bearing capacity. In addition to supporting structures , piles are used to anchor structures against uplift forces and to assist structures in resisting lateral and overturning forces.

NECESSITY OF PILE FOUNDATION   When the strata at or just below the ground surface is highly compressible and very weak to support the load transmitted by the structure. Pile foundations are required for the transmission of structure loads through  deep water to a firm stratum. In case of expansive soil, such as black cotton soil, which swell or shrink as the water content changes, piles are used to transfer the load below the  active zone.

CLASSIFICATION OF PILE FOUNDATION Pile foundation is classified into 3 categories as given below: According to type of pile:- End bearing piles Friction piles Settlement reducing piles Tension piles Laterally loaded piles Piles in fill

According to type of construction: Continued According to type of construction: Precast driven piles Driven cast-in-situ piles Bored cast-in-situ piles Bored pre-cast piles Driven steel piles Driven timber piles

According to type of material used:- Continued According to type of material used:- Timber piles Steel piles Pre-stressed concrete piles Composite piles

FACTORS INFLUENCING CHOICE OF PILE Location and type of structure Ground conditions Durability Cost

LOADS APPLIED TO PILES V M H Combinations of vertical, horizontal and moment loading may be applied at the soil surface from the overlying structure. For the majority of foundations the loads applied to the piles are primarily vertical. For piles in jetties, foundations for bridge piers, tall chimneys, and offshore piled foundations the lateral resistance is an important consideration.

continued The analysis of piles subjected to lateral and moment loading is more complex than simple vertical loading because of the soil-structure interaction. Pile installation will always cause change of adjacent soil properties, sometimes good, sometimes bad.

Modes of failure The soil failure is always caused by punching shear. The failure mode of pile is always in buckling failure mode.

Ultimate capacity of axially loaded single pile in soil Like a shallow foundation, a pile foundation should be safe against shear failure and also the settlement should be within the permissible limits. The methods for load carrying capacity are grouped in 4 categories given below: Static Methods Dynamic Methods In-situ penetration tests Pile load test

Static methods Qu These methods give ultimate capacity of individual pile, depending upon characteristics of the soil. Qu = Qp + Qs where, Qu = ultimate failure load Qp = point resistance of pile Qs = shaft resistance developed by friction b/w soil and pile shaft. Qs Qp

Static methods for driven piles in sand Qu = Qp + Qs where Qp = qpAp and Qs = fsAs qp is ultimate bearing capacity of soil Ap is area of pile tip fs is average unit friction b/w sand and pile surface As is effective surface area of pile in contact with soil

Static methods for driven piles in saturated clay Qp = qpAp qp = unit point resistance equal to ultimate bearing capacity qu = cNc + qNq Nq = 1 for cohesive soil Nc depends upon D/B ratio and it varies from 6 to 9. Qs = caAs unit adhesion b/w clay and pile shaft (Ca )= αc’ α is adhesion factor c’ is average cohesion along shaft length

Static methods for bored piles Bored piles in sand:- Qu = (q’Nq)Ap + Ʃ(K ’v tan (As)i where ’v = effective vertical pressure lateral earth pressure coefficient (K)= 1-sin tan = coefficient of friction b/w sand  concrete Bored piles in clay:- Qu = c.Nc .Ap + c’As where As = area of shaft  depends upon pile type and method of drilling

DYNAMIC METHODS Engineering news record formula:- Qu = (Whh)/(S+C) where, S = penetration of pile per blow C = constant(for drop hammer,C= 2.54cm  for steam hammer,C = 0.254cm) h = efficiency of drop hammer (b/w 0.70.9 for single acting hammer and b/w 0.750.85 for double acting hammer b/w 0.80.9 for diesel hammer) Note:-A factor of safety of 6 is usually recommened.

continued Danish formula:- Qu = (W.h.h)/(S+0.5So) where So = [(2hWhD)/(AE)]^0.5 So = elastic compression of pile D = length of pile A = cross–sectional area E = modulus of elasticity of pile material Note:- The allowable load is found by taking factor of safety of 3 to 4.

continued Hiley formula:- Qu = (W.h.b.h)/(S+ 0.5C) where, h = efficiency of drop hammer h = height of free fall of ram or hammer S = final penetration per blow cm C = sum of temporary elastic compression of pile, dolly and ground b = efficiency of hammer blow

IN- SITU PENETRATION TEST Standard penetration test Dutch cone test

PILE LOAD TEST Static pile load test is the most reliable means of determining the load capacity of a pile. The test procedure consists of applying static load to the pile in increments up to a designated level of load and recording the vertical deflection of the pile. The load is usually transmitted by means of a hydraulic jack placed between the top of the pile and a beam supported by two or more reaction piles. The vertical deflection of the top of the pile is usually measured by mechanical gauges attached to a beam, which span over the test pile.

Soil Mechanics and foundation engineering by K.R.Arora References Soil Mechanics and foundation engineering by K.R.Arora www.seminarprojects.com www.google.com

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