DEEP FOUNDATIONS – Pile Foundations

1 CHAPTER 7. DEEP FOUNDATIONS pile FOUNDATIONS ULTIMATE pile CAPACITY. Beacause of the non-homogeneity of soil and the unlimited variables that affecting pile behaviour, different methods of approach are, therefore, existed. Among these methods are:- a. The static methods, b. The load-transfer methods, c. The empirical methods d. The dynamic methods, and e. Field tests methods. A. STATIC APPROACH. Piles in Compression: L. Q ult = P( .C u + K s . v . tan ).dz + A b ( c + vb .N q + . ) WP .. (1). 0. Q ult = Qs + .Q b Wp ..(2). Qall = Qs / + .Q b / Wp ..(3). Qall = Qult / .. (4). where: SF = Safety factor = for driven piles, and = or = and = for bored piles. FACTOR OF SAFETY OF SINGLE pile IN CLAY. Type of pile Factor of Safety Qall. Driven piles = Qall. = QP / Bored piles (i) = Qall. = QP / (Take the smaller value) (ii) F1 = , F2 = Q all.

2 = Q s / + Q b / Piles in Tension: Tult = Qs + W .. (5). Tall = Qs / + W .. (6). where: Qult =Ultimate pile capacity, Tult = Ultimate tension or pullout capacity, Qb = End bearing resistance, Qs = Frictional resistance, Qall = Allowable bearing capacity, WP = Weight of pile Weight of removed soil, d = (Diameter) or least dimension of pile , L = Length of pile . End bearing 10%B (for driven piles) and 30%B (for bored piles and caissons), and Friction =<< 10%B . Foundation for Civil Engineers Bearing Capacity of pile FOUNDATIONS 1 Dr. Farouk Majeed Muhauwiss o For Piles in Clay; Equation (1) will be: Since for piles in normally consolidated clay the undrained condition is control, u = 0 and = 0. ks . v . tan = 0 and c = c u = q u / 2 . Also, for u = 0 : Nc = 9 (from Skempton chart for [. circular or square), N q = and N =.]

3 In addition to, the difference between A b .q Wp is ]. only in ( conc. soil ) which is very small, that can be neglected, therefore: - L. QP = .d( .cu ).dL + A b (cu .N c ) .. (1-a). 0. where, L. Qs = .d( .cu ).dL = .cu .As = Ca .As ;. 0. Qb = A b .(cu .Nc ) , = adhesion factor, and Ca = adhesion between pile and soil; obtained by one of the following methods:- (1) Tomlinson (1971) method, (2) Meyerhof (1976) method, (3) Vijayvergia and Focht (1972) method. o For Piles in Sand; Equation (1) will be: [ ]. Since c = 0, then = 0 and due to the term A b .( .N ) Wp is very small, that can be neglected, therefore:- L. QP = .d(k s . v . tan ).dL + A b (q .N q ) .. (1-b). 0. where, L. Qs = .d(k s . v(avg.) . tan ).dL = s .As , 0. Q b = A b (q .N q ) , q = v = .L = Overburden pressure at the base of pile , N q = Meyerhof's bearing capacity factor for deep FOUNDATIONS , s = Interaction between sand and pile = k s.

4 V avg.. tan which should be 100 .kN / m 2 , and L. As = Surface area of pile = .. 0. 2. Meyerhof 's (1976) N i Bearing Capacity Factors (from penetration test data). For u = 0 : Q b = A b .q .N q Ab ( ). For > 0 : 1. Use R1 = L / B ; obtain R 2 = Lc / B for the given angle from Fig.(1), 2. Enter the curves with : o If R1 > 2 and 30 ; obtain factors from the upper N i curves, and o If R1 < 2 and 30 ; use a linear ratio between the lower and upper N i curves;. from R1. N q = N q + ( N q N q ) , and Q b = A b .q .N q ..(7). 2. o If > 30 and depending on L / B ; project to the reduced curves shown in the upper right part of Fig.(1) and interpolate as necessary. ((loose or dense sand, soils with varying degess of compressibility and for overconsolidated ( ) clays)). Fig.(1): Bearing capacity factors for deep FOUNDATIONS (after Meyerhof, 1976).

5 End Bearing ( Q b ) For Driven Piles For Clay Soils (in undrained condition, u = 0 ): In this case; c = Su , N c = 9 (from Skempton chart for circular or square) and N q = , then the end bearing becomes: For constant cross-sectional piles: Qb = A b ( ) .. (8-a). 3. For tapered piles (from Skempton, 1966): Qb = A b ( ).. (8-b). where, = for B 1m , and for B > 1m. Using Cone Penetration Test (CPT) Results: Qb = Ab .q c ..( ..qc ) .. (8-c). where, qc = average CPT value in a zone of 8B above to 3B below the pile point. For Sand Soils ( > 0 ): If L/B < Lc / B Q b = A b .q .N q .. (9-a). Provided that q .N q 10 .7 MN / m 2. or 10700 .kN / m 2 . If L/B Lc / B Q b = A b .q .N q A b ( q . tan ) (kN) .. (9-b). B. Provided that q .N q MN / m or 10700 .kN / m 2 . 2. where, N q = Meyerhof's bearing capacity factor for deep FOUNDATIONS obtained from Fig.

6 (1). L. Lc Bearing stratum Using Standard Penetration Test (SPT) Results (Meyerhof's, 1956, 1976): Qb = A b ( ).Lb / B.. b ..(kN) .. (9-c). where, A b = Cross sectional area of pile , N= N55 = Corrected average SPT value in a zone of 8B above to 3B below the pile point, Lb = Length of the pile embedded in sand, B = Width or diameter of pile , and Lb / B = Average depth ratio of the corresponding point into point bearing stratum. For c Soils: Q b = A b ( c + .q .N q ) .. (10). where, A b = Cross sectional area of pile , c = Cohesion, q = Effective vertical stress at pile point, = 1. for all bearing capacity factors except the Vesic (1975) N i factors since = (1 + 2k o ) / 3 , k o = At rest earth pressure coefficient = 1 sin OCR ; where OCR = Overconsolidation ratio, N c =. Author 's bearing capacity factor for cohesion adjusted for shape and depth, N q = Author 's bearing capacity factor adjusted for L/B > 1 and depends on initial (undisturbed soil) angle of internal friction , L = Length of pile , and B = Diameter or least dimension of pile .

7 Foundation for Civil Engineers Bearing Capacity of pile FOUNDATIONS Dr. Farouk Majeed Muhauwiss 4. End Bearing ( Qb ) For Bored Piles Q b = 1 / 3.(Q b )driven A b ( q . tan ) .. (11). (a) Frictional Resistance ( Qs ) For Piles In Clay For driven piles, the side friction C a was obtained using one of the following methods: Ca = .C u (avg.) or Ca = .. v (avg.) or [. Ca = v(avg.) + u (avg.) ]. Provided that (C a / m 2 ) due to several factors which affect the adhesion; such as: (i) Smear effect that occurs due to drag down of pile during installation, (ii) The presence of soft layer overlying a stiff layer, and (iii) Shrinkage that occurs in case of stiff clay and leads to separation between pile and soil. Annular cracking around the top of pile occurs and B. therefore (Ca to a depth = 20 B) provided that (C a / m 2 ).

8 Soft clay Stiff clay Ca (to a depth = 20 B). For bored piles, Qs can be calculated using either Tomlinson or Meyerhof methods that used for driven piles provided that (C a / m 2 ) . For expanded bored piles, the following should be taken into consideration:- (i) If a fissured clay is present at base of pile , then C u ( used ) = u ( triaxial). (ii) If expansion of bored pile exists, neglect the d surface area of the shaft side above the expansion to a distance of (2 x diameter of the shaft); , Qs (L ). L. As = ..d.(L ) . Ca = 0 for this zone. Qb 5. (1) Tomlinson (1971) method: For c Soils: ( ). Qs = .c u + k s . v (avg.) . tan .A s ..(12-a). where, = adhesion factor ; obtained as before from [Table (2) or Fig.(2-a) ]. -Values of Some Typical Soils (Skempton, 1966). - Value Type of Clay Driven Piles Bored Piles Londen clay - Sensitive clay Highly expansive clay Soft clay Table (2): Values of adhesion factors for piles driven into stiff to very stiff cohesive soils for design (after Tomlinson, 1971).

9 Penetration Adhesion factor Case Soil conditions ratio+ . Sands or sandy gravels overlying stiff to < 20 1. very stiff cohesive soil > 20 Figure (2-a). Soft clays or silts overlying stiff to 8< PR 20 2. very stiff cohesive soil > 20 Figure (2-a). Stiff to very stiff cohesive soils without 8< PR 20 3. overlying strata > 20 Figure (2-a)..Pentration .int .Soil + .( PR ) =. Diameter .of . pile The Method. Fig.(2-a): Relationship between soil and adhesion factor (after Tomlinson, 1971, and API, 1984). For Clayey Soils ( u = 0 in undrained condition): Qs = ( .cu ).As ..(12-b). Provided that [ Ca = ( .c u ).. / m 2 ]. 6. (2) Meyerhof (1976) method: This method is widely used for soft and medium clays. ( ). Qs = k s . v(avg.) . tan .A s = . v(avg.) .As ..(13). where, k s = lateral earth pressure coefficient obtained as follows:- For Driven piles: soft to medium clays where Cu 100 kN/m2 ks = (1 sin ).

10 Stiff clays where Cu > 100 kN/m2 k s = (1 sin ) OCR. For Bored piles: soft to medium clays where Cu 100 kN/m2 ks = (1 sin ). stiff clays where Cu > 100 kN/m 2. k s = .L. avg. = average effective overburden pressure = , 2. = soil to pile friction angle, and = skin friction factor = ks . tan ; obtained from Fig.(2-b). 0. Depth of penetration , z (m). 5. 10. 15. 20. 25. 30. The Method. 35. 40. 45. 50. 55. 60. 0 Skin friction factor, Fig.(2-b): Skin friction factor, , for piles driven into soft and medium clays (after Meyerhof, 1976). (3) Vijayvergia and Focht (1972) method: This method is essentialy used for steel piles as well as driven piles in stiff clay. ( ). Qs = v(avg.) + u (avg.) .As ..(14). where, = dimensionless coefficient; obtained from Fig.(2-c), .L. avg. = average effective overburden pressure = , and 2.