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Geotechnical Design of Deep Foundations

Geotechnical Design of Deep Foundations. Selection of Pile. Selection of i ) type, ii) length and iii) capacity of pile is based on two parameters Soil conditions Magnitude of load

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Geotechnical Design of Deep Foundations

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  1. Geotechnical Design of Deep Foundations

  2. Selection of Pile Selection of i) type, ii) length and iii) capacity of pile is based on two parameters • Soil conditions • Magnitude of load Before the actual construction begins, pile load tests must be made to verify the design values and the foundation design must be revised according to the test results.

  3. Factors effecting the selection of pile • Length of pile in relation to the load and type of soil • Character of structure • Availability of materials • Type of loading • Factors causing deterioration • Ease of maintenance • Estimated costs of types of piles, taking into account the initial cost, life expectancy and cost of maintenance. • Availability of funds

  4. Geotechnical Design By Geotechnical Design, we mean; • Depth below the ground. • Dimensions in plan (dia etc.) These are dependant upon the pile capacity. Total ultimate load carried by and element of pile is called total ultimate capacity of pile Qu “Qu = Qs + Qb” Qs = Shaft resistance Qb = Bearing resistance And for Allowance capacity, there are two schools of thought I.e. Qa = Qu / FOS or Qa = Qs/ (FOS)1 + Qb/ (FOS)2 Qs is more uncertain than Qb, so apply higher factor of safety to Qs.

  5. METHOD FOR CALCULATING THE CAPACITY: Three methods are used for calculating the capacity of pile, 1. Static formula (applied for driven and in-situ piles) 2. Dynamic formula (applied for driven piles) 3. Load test (Testing is done for 2 times the design load , if it is stable then o.k. It is a full scale test and is the best method. But it is the most expensive and time consuming. Load application is very difficult.)

  6. STATIC FORMULA For finding out Qs; Qs = (Fs) (As) *Fs = c + q K tan  Where Fs = unit skin friction As = Effective Area of shaft The length of the pile Coming in contact with soil. Below the N.S. L 1.5 to 1.8 m is generally ignored (damage due to installation and cracking) q = Avg. Effective overburden pressure.  = Adhesion factor or Cohesion Reduction factor *This is called Alpha () Method proposed by Tomlinson (1971) Qs

  7. Overburden Pressure Avg. Pressure Line r Df/2 r Df STATIC FORMULA Qs

  8. Adhesion factor or Cohesion Reduction factor Before 1971, it was known that  depends on the consistency of soil (spt value).If the consistency is very poor, then 100% contact. But if consistency is very hard, less contact. • So for • V. loose to loose sand & v. soft to soft clay = 1 • Very hard (stiff to very stiff clay) = 0.4 After 1971, Tomlinson said that, the “” value depends upon the penetration, actual formation, sequence of layer. He related the “” value with the penetration Ratio (PR) and gave a table where PR = Effective Length of pile driven in cohesive soil / Dia. of pile = Le/ D STATIC FORMULA Qs

  9. Factors K = co-efficient of lateral earth pressure. Generally K value is from zero to 1.75 Where Ko = pressure at rest. = (1-sin ) √OCR • OCR = Over consolidation Ratio & for N.C. C , OCR = 1 • Safe to use the value close to K0. •  value of K are higher for driven piles as compared with cast insitu piles. • Usually for driven piles the values are  1 and for insitu piles it is  1 STATIC FORMULA Qs

  10. Factor Tan  ; Is the co-efficient of friction and  = fraction Angle, depending upon the material of construction, for timber pile Timber  = 2/3 ’ For steel pile =20Degree For concrete pile,  = ¾   Now fs can be calculated , As is also known . STATIC FORMULA Qs

  11. TO FIND Fs USING EMPERICAL METHOED For the empirical method, we do two tests i.e. SPT test and CPT test. I) SPT TEST ( MAYORHOFF 1976):  According to Mayorhoff   Fs = (€m) (N) in Kpa €m = Constant. Depending upon the method of installation. = 2 (for large volume displacement (Driven piles)) = 1(for No volume or displaced (Cast piles)) N = Average value of SPT Resistance (Average is taken for the SPT Values at depths ‘8B’ above the tip and ‘4B’ below the tip. Qs

  12. Depth for SPT B 8B 4B Qs

  13. CPT TEST (MAYERHOFF): This test is further of two types, (a) By Mentle Cone: In this only a cone is attached to the Rod. This type of cone is called Mentle Cone. Here only the cone comes in contact. The resistance measured is called the cone resistance “qC “ Fs = 0.0005 qc in Kpa (b) Friction Jacket Cone: In this case, there is a Sleeve. The Resistance offered by sleeve is qcs Fs = (€m) (qcs) €m = 1.5 for driven piles €m = 1.0 for cast Insitu piles qcs = Resistance offered by shaft qcs = total Resistance - qc Qs

  14. FOR CALCULATING Qb: Qb = (Ab) (qult) qult = Bearing capacity of Soil. Ab = X – Sectional area of the Pile So the problem lies only in finding out of qult. it can be founded by • Bearing capacity equations • Empirical methods Qb

  15. Bearing capacity equations qult = CNC + rDf Nq+ ½ r B Nr This equation was for shallow footing. Modifying it for deep footing i.e. D/B ≥ 4 qult = CNC’ + rDf Nq’ + 0.5rBNr’ NC’, Nq’,Nr’, is constants different from NC and Nr. Nc’ = 9 (instead of 5.7) For pure clay) Φ=0) Nc’ = 9 , Nq’= 1, Nr’ = 0 Thus (qult)Net’ = CNC’ = 9c For pure sand: qult = 0+ q’Nq’+0 For driven piles, B is very small so the 3rd term may be negative. qult = q’ Nq’ q’is different from q because q = Avg. & q’ = Maximum Qb

  16. EMPERICAL METHODS(SPT and CPT tests) • SPT TEST (Mayerhoff) qult = 40N Lb/ B ≤ 400N in Kpa Lb = Length of pile in the stream of end bearing. (b) CPT TEST qult = qc qc = Cone Resistance QU = Qs+Qb Note: We know that Fs= c + K Tan δ qult = CN’C + q’ Nq’ + 0.5rBN’r q’ = Effective Earth pressure depending on the length of the pile q’ = rD Stratum # 01 Stratum # 02 Lb End Bearing

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