PILE FOUNDATION.docx

**PILE FOUNDATION** ###### Need for pile foundation - very large design loads, - a poor [soil](https://en.wikipedia.org/wiki/Soil) at shallow depth, - site constraints (like PROPERTY LINE) **Fig.1.pile foundation.** **Fig.2 Classification of piles** - Timber piles are made of-tree tru...

**PILE FOUNDATION** ###### Need for pile foundation - very large design loads, - a poor [soil](https://en.wikipedia.org/wiki/Soil) at shallow depth, - site constraints (like PROPERTY LINE) **Fig.1.pile foundation.** **Fig.2 Classification of piles** - Timber piles are made of-tree trunks driven with small end as a point - Maximum length: 35 m; optimum length: 9 - 20m - Max load for usual conditions: 450 kN; optimum load range = 80 - 240 kN Disadvantages of using timber piles: Advantages: ###### Steel piles - Maximum length practically unlimited, optimum length: 12-50m - Load for usual conditions = maximum allowable stress x cross-sectional area - The members are usually rolled HP shapes/pipe piles. Wide flange beams & I beams proportioned to withstand the hard driving stress to which the pile may be subjected. In HP pile the flange thickness = web thickness, piles are either welded or seamless steel pipes, which may be driven either open ended or closed end. Closed end piles are usually filled with concrete after driving. - Open end piles may be filled but this is not often necessary., dm Advantages of steel piles: Disadvantages: - Vulnerable to corrosion. - HP section may be damaged/deflected by major obstruction ###### Concrete Piles - Concrete piles may be precast, prestressed, cast in place, or of composite construction - Precast concrete piles may be made using ordinary reinforcement or they may be prestressed. - Precast piles using ordinary reinforcement are designed to resist bending stresses during picking up & transport to the site & bending moments from lateral loads and to provide sufficient resistance to vertical loads and any tension forces developed during driving. - Prestressed piles are formed by tensioning high strength steel prestress cables, and - Max length: 10 - 15 m for precast, 20 - 30 m for prestressed - Optimum length 10 - 12 m for precast. 18 - 25m prestressed - Loads for usual conditions 900 for precast. 8500 kN for prestressed - Optimum load range: 350 - 3500 kN Advantages: 1. High load capacities, corrosion resistance can be attained, hard driving possible 2. Cylinder piles in particular are suited for bending resistance. 3. Cast in place concrete piles are formed by drilling a hole in the ground & filling it with concrete. The hole may be drilled or formed by driving a shell or casing into the ground. Disadvantages: 1. Concrete piles are considered permanent, however certain soils (usually organic) contain materials that may form acids that can damage the concrete. 2. Salt water may also adversely react with the concrete unless special precautions are taken when the mix proportions are designed. Additionally, concrete piles used for marine structures may undergo abrasion from wave action and floating debris in the water. 3. Difficult to handle unless prestressed, high initial cost, considerable displacement, prestressed piles are difficult to splice. 4. Alternate freezing thawing can cause concrete damage in any exposed situation. ###### Composite piles  **Composite pile Concrete pile** ###### Cast-in-situ piles ###### 2. Types of Piles based on installation type. 1. **Displacement** piling technique 2. **Replacement**. piling technique  1. **Types of Piles based on load transfer.** 2. **Classification based on method of installation:** i. **Bored piles:** ###### Board piles are of following types: ###### Driven piles: ###### Driven and cast-in-situ piles: a. **Classification based on the function:** i. **End bearing piles:** ###### Friction piles: ###### Tension pile: ###### Compaction piles: ###### Anchor piles: ###### Fender piles and dolphins: ###### Piles based on uses ###### Factors affecting the selection of piles. ###### Load carrying capacity of pile 1. Static Analysis 2. Dynamic Analysis 3. Pile Load Testing 4. Correlation with field tests (SPT, CPT etc)(Penetration tests) ###### Static method ###### Qup = m c' As +9 CpAp ###### Dynamic formulae. 1. Drop hammer **Q~a~ =** [ 𝑾𝑯 ] 𝟔(𝑺+𝟐.𝟓) 2. Single acting stream hammer **Qa** [ 𝑾𝑯 ] 3. Double acting hammer **Qa** ###### Hiley's Formula (IS: 2911 part-I) 1964 -- -- -- -- Table 1 Values of Hammer Coefficient K ###### PILE LOAD TEST - To determine settlement under working load - To determine ultimate bearing capacity - To ascertain as a proof of acceptability ###### The test can be initial or routine test - The load is applied in increments of 20% of the estimated safe load. Hence the failure load is reached in 8-10 increments. - Settlement is recorded for each Settlement is recorded for each increment until the rate of - The ultimate load is said to have reached when the final settlement is more than 10% of the diameter is more than 10% of the diameter of pile or the settlement keeps on increasing at constant load. 45 - After reaching ultimate load the after reaching ultimate load, the load is released in decrements of 1/6th of the total load and recovery is measured until full recovery is measured until full rebounds is established and next unload is done. - After final unload the settlement is measured for 24 hrs to estimate full elastic recovery. - Load settlement curve depends on the type of pile  - Load applied in increment at the rate of 25 % of working load till working load is reached - For each load increment maintain the load constant till settlement is 0.1 mm for 5 min as per IS Code, 0.1 mm for 20 min as per BS Code - Go for next loading - When working load is reached hold the load for 24 hr and unload - Reload from working load to higher loads - Hold load constant till settlement is 0.1 mm for 5 min as per IS Code, 0.1 mm for 20 min as per BS Code - Repeat the process for subsequent load increments - Go either up to 5/2 times the working load for initial or routine test or to a settlement equal to 10 % of pile diameter for straight piles and 7.5 % of base diameter for belled pile  ###### Pile group. ###### EFFICIENCY OF PILE GROUP ###### Efficiency of pile group 1. When closely spaced piles are grouped together it is reasonable to expect that the soil as resistance will overlap. 2. The bearing capacity of pile group may or may not be the sum of the bearing capacity of individual piles constituting the group. 3. Theory and tests have shown the total bearing capacity Qug of a group of friction piles particularly in clay may be less than the product of the friction bearing value Qup of individual pile multiplied by the number of piles in a group. 4. There is no reduction in the case of end bearing piles. 5. For combined end bearing and friction piles only the load carrying capacity of the frictional portion is reduced. 6. A method of estimating the bearing capacity of a pile group of friction piles is to multiply the quantity **nQup** by a reduction factor called the efficiency of pile group. Qug=n.Qup. ηg ηg== efficiency of pile group. The efficiency of the pile group depends upon the following factors - Characteristics of pile - Spacing of pile - Total number of piles - No of formulae are available for finding the efficiency of pile. ###### Pile Spacing.  1. Overlapping of stresses of adjacent piles, 2. Cost of foundation, 3. Efficiency of the pile group. a. Self-weight of unconsolidated recent fill, b. Surcharge-induced consolidation settlement, c. Consolidation settlement after dissipation of excess pore pressure induced by pile driving, d. Lowering of groundwater level, e. Collapse settlements due to wetting of unsaturated fill, and f. Crushing of crushable subsoil under sustained loading, causing subsoil settlement  1. A group of 9 piles arranged in a square pattern with diameter and length of each pile as 25cm and 10m respectively, is used as a foundation in soft clay deposit. The unconfined compressive strength of clay as 120kN/m^2^ and the pile spacing as 100cm c/c. Find the load capacity of the group. Assume the bearing capacity factor as (Nc) 9 and adhesion factor (m) =0.75. Factor of safety of 3.5 may be taken. ###### Solution ###### Pile acting individually, C=120/2 =60kN/m2 2. **Pile acting on a group,** **B= 2s +d = 2.25m** **Q =** 𝐐𝐮 𝐦𝐢[𝐧] 𝑭 ###### = 3419 /23.5 = 1367kN ###### Solution: Neglecting the bearing resistance**, Qup= As rf** ###### (1) Pile acting individually, **B = 2 X 1.2 + 0.6 = 4.2 m.**