Advanced Foundation Engineering (57011)

Full text

(1)Advanced Foundation Engineering (57011) IV Year B.Tech, I Semester Dr. PVSN Pavan Kumar Associate Professor Associate Professor Guru Nanak Institutions Technical Campus Dr. PVSN Pavan Kumar.

(2) Syllabus • Unit – 1 Bearing Capacity theories, eccentric and inclined loading layered soil inclined loading, layered soil • Unit – 2 Settlement of foundations • Unit – Unit 3 Pile foundations – 3 Pile foundations Load carrying capacity Load carrying capacity • Unit – 4 Pile foundations – Settlement of piles • Unit – 5 Lateral earth pressure theories – Retaining walls • Unit – 6 sheet pile walls • Unit – 7 Caissons & well foundations • Unit – 8 Expansive soil and treatment methods Dr. PVSN Pavan Kumar.

(3) TEXT BOOKS • 1. 1 Das, Das B.M. B M (1999) Principles of Foundation Engineering – 4th edition, edition PWS Publishing, Singapore • 2. Bowles, J.E., (1988) Foundation Analysis and Design – 4th edition, McGraw Hill Interntaional. McGraw-Hill Interntaional • 3. Geotechnical Engineering : Principles and practices of soil mechanics and foundation engineering by VNS Murthy, Taylor & Francis Group REFERENCE BOOKS • 1.. Geo Geotechnical ec c Engineering g ee g by C. Ve Venkataramah,, NewAge ew ge International e o Pvt. Ltd, Publishers (2002). • 2. Analysis and Design of structures – Swami Saran, Oxford & IBH Publishing Company Pvt. Pvt Ltd. Ltd (1998) • 3. Basic and Applied Soil Mechanics by Gopal Ranjan & ASR Rao, New Age International Pvt. Ltd, Publishers (2002).. Dr. PVSN Pavan Kumar.

(4) Unit – I Bearing capacity theories. Dr. PVSN Pavan Kumar.

(5) Contents • Introduction – Terminology, Terzaghi and Meyerhof bearing capacity theories. • • • • •. Hansen bearing capacity theory Vesic bearing capacity theory Footings subjected to eccentric and inclined loading Footings subjected to eccentric and inclined loading Foundations on layered soil Tutorials and assignments. Dr. PVSN Pavan Kumar.

(6) Introduction Foundation • All the designed structures resting on the earth All th d i d t t ti th th must be carried by some kind interfacing element called fo ndation called foundation. • Foundation transmits the load into the supporting soil or rock. • Structure will consist of three parts – Super structure, sub structure and foundation.. Dr. PVSN Pavan Kumar.

(7) Introduction • Foundations are classified as shallow foundations and deep foundations. • Shallow foundation have D/B ≤ 1. Footings, combined footings, strap footings or mat/raft foundations. p /B ≥ 4. Examples piles, p p , • Deep foundation have L P/ drilled piers or drilled caissons. • Shallow foundations distribute the load by Shallow foundations distribute the load by spreading the loads laterally and support column. Dr. PVSN Pavan Kumar.

(8) Introduction • Mat is a special footing used to support several y p pp randomly spaced columns or to support several rows of parallel columns. • Deep foundations distribute the load vertically Deep foundations distribute the load vertically rather than horizontally. • Foundations are designed such that stress level due Foundations are designed such that stress level due to structure is less than bearing capacity of soil and settlements are within permissible limits settlements are within permissible limits.  . Dr. PVSN Pavan Kumar.

(9) Introduction • Few buildings collapse from excessive settlements, partial collapse or localized failure in a structural partial collapse or localized failure in a structural member. Unsightly wall and floor cracks uneven floors sticking doors and windows floors, sticking doors and windows. • Variability of soil in combination with unanticipated loads or subsequent soil movements (earthquakes) loads or subsequent soil movements (earthquakes) can result in settlement problems. . Dr. PVSN Pavan Kumar.

(10) Bearing capacity • Soil must be capable of carrying the loads from the p structure placed on it without shear failure and with resulting settlements within tolerable limits. • Ultimate bearing capacity, q Ultimate bearing capacity, qultlt is the maximum is the maximum pressure the footing is subjected for shear failure of soil below footing. soil below footing. • Footing punches into the ground with a simultaneous rotation simultaneous rotation. . Dr. PVSN Pavan Kumar.

(11) Bearing capacity Bearing capacity • Allowable Allowable bearing capacity is the safe pressure bearing capacity is the safe pressure the footing is subjected to avoid a base shear failure. failure q qa = ult FS. Dr. PVSN Pavan Kumar.

(12) Bearing capacity theory (ϕ = 0). S = c + σ tan φ φ Unit width strip footing, L L      ∞ Element 2. Element 1. Rotation of footing about point o g p Dr. PVSN Pavan Kumar.

(13) Bearing capacity theory Element 2. Element 1. σ1 and σ3 = Major and minor principal stress. Dr. PVSN Pavan Kumar.

(14) Bearing capacity equation Element 2 σ32 = q =q σ12 = q + 2c (for ϕ = 0°) Element 1 σ31 = σ12 = q + 2c σ11 = q + 2c +2c = q + 4c (for ϕ = 0°) Ultimate bearing capacity = qult = q + 4c Ultimate bearing capacity = q = q + 4c. Dr. PVSN Pavan Kumar.

(15) Bearing capacity (c‐ϕ soil) Element 2. Soil wedge agb moves down Element 1 l. Lateral pressures develop along line p g ag and translates block agf horizontally against wedge afe wedge afe.. Dr. PVSN Pavan Kumar.

(16) Bearing capacity (c‐ϕ soil) • Determine the passive pressure PP and consider the vertical equilibrium of forces to determine ultimate vertical equilibrium of forces to determine ultimate bearing capacity, qult as follows qult = CN = CNc + q + q’N Nq + γBN + γBNγ • Some limitations of the above procedure is – Zone agf is neglected. – Footing interface is rough and contributes to roughness effect ff t. Dr. PVSN Pavan Kumar.

(17) Bearing capacity (c‐ϕ soil) –Shape of block agfe poorly defines the zone resisting the wedge movement into the soil. A logarithmic spiral better defines the slip surface from g to f and partly along f to e. –Solution is for unit width strip across a very long footing, so it has to be adjusted for round, square, or finite‐length footings (it needs shape factors). –Shear resistance from plane ae to the ground surface has been neglected, it requires some kind of adjustment (i.e. depth factor) depth factor) –If load is inclined from vertical, inclination factors are required. required Dr. PVSN Pavan Kumar.

(18) Bearing capacity theories Bearing capacity theories • Terzaghi bearing capacity theory, 1943 bearing capacity theory 1943 • Meyerhof bearing capacity theory, 1963 • Hansen bearing capacity theory, 1970 b i i h 9 0. Dr. PVSN Pavan Kumar.

(19) Terzaghi bearing capacity equation (1943) • A comprehensive theory for the evaluation of the ultimate bearing capacity of rough shallow foundations g p y g (Df ≤ B). • Applicable for a continuous, or strip foundation (i.e., pp , p ( , one whose width to length ratio approaches zero). • The effect of soil above the bottom of the foundation The effect of soil above the bottom of the foundation may also be assumed to be replaced by an equivalent surcharge, q = γD g , q γ f ((where γγ is a unit weight of soil). g ) • General shear failure is assumed.. Dr. PVSN Pavan Kumar.

(20) Terzaghi bearing capacity theory. α = ϕ =ϕ. • ADC triangular zone below base of footing makes an angle ϕ with horizontal • Radial shear zones ADF and CDE, with the curves DE and DF being arcs of a logarithmic g spiral. p • Two triangular Rankine passive zones AFH and CEG Shear resistance of soil above the base of footing is neglected i.e. along the failure surfaces GI and HJ was neglected. Dr. PVSN Pavan Kumar.

(21) Terzaghi bearing capacity equation Considering vertical equilibrium of forces on footing . Rectangular footing Sc = 1+0.3 . B L. Sq = 1 Sγ = 1‐0.2 . B L. cc = cohesion of soil, B = Width, L = Length = cohesion of soil B = Width L = Length ϕ= angle of internal friction, q’ = effective over burden pressure at base of footing f f ti kp = Coefficient of passive pressure Dr. PVSN Pavan Kumar.

(22) Terzaghi bearing capacity theory • Terzaghi developed bearing‐capacity equations g g considering a general shear failure in a dense soil and a local shear failure for a loose soil. • For the local shear failure he proposed reducing the For the local shear failure he proposed reducing the cohesion and ϕ as cc" = 0.67c = 0 67c • For local shear failure modified bearing capacity f t are determined factors d t i d from f ϕ " = tan t ‐11 (0.67 (0 67 tan t ϕ)). Dr. PVSN Pavan Kumar.

(23) Tutorial 1 Compute the allowable bearing capacity using Terzaghi equation for a square footing and soil properties shown in Figure below. B = 3m Use factor of safety = 3.0 calculate l l t qa ϕ = 20° Nc = 17.7, N = 17 7 Nq = 7.4, N = 7 4 Nγ = 5.0 =50. Dr. PVSN Pavan Kumar.

(24) Meyerhof bearing capacity theory Terzaghi bearing capacity theory has following short comings – Shear resistance along failure surface in soil above the base foundation is neglected (along GI and HJ). – Load on the foundation may be inclined. . Dr. PVSN Pavan Kumar.

(25) Meyerhof bearing capacity theory • Meyerhof Meyerhof (1963) suggested the following general equation for (1963) suggested the following general equation for bearing capacity. Dr. PVSN Pavan Kumar.

(26) Meyerhof bearing capacity equation qult = cNcScdcic + q’NqSqdqiq + 0.5γBNγS γ d γ i γ Nc, N , Nq, Nγ ‐ bearing capacity factors bearing capacity factors Sc, Sq, Sγ ‐ Shape factors dc, d dq, dγ ‐ depth factors d th f t ic, iq, iγ ‐ inclination factors. Dr. PVSN Pavan Kumar.

(27) Meyerhof bearing capacity equation (. ). N c = N q − 1 cot ϕ. Any ϕ. ϕ⎞ ⎛ Nq = tan2 ⎜ 45+ ⎟eπ tanϕ 2⎠ ⎝. (. ). Nγ = Nq −1 tan(1.4ϕ). Any ϕ. S c = 1+ 0.2 K p. B L. d c = 1+ 0.2 K p. ϕ > 10°. ϕ > 10°. S q = S γ = 1+ 0.1K p. B L. ϕ = 0°. D B. d q = d γ = 1+ 0.1 K p ϕ = 0°. D B. d q = dγ = 1. S q = Sγ = 1 ⎛ θ ⎞ ⎟ Any ϕ i c = i q = ⎜⎜1 − ⎟ ⎝ 90° ⎠ 2 ° ⎛ θ ⎞ ϕ > 10° ⎟ i γ = ⎜1 − ⎜ ϕ° ⎟ ⎠ ⎝ °. 2. ϕ = 0° iγ = 0 for θ > 0 Dr. PVSN Pavan Kumar. • For depth D = B Meyerhof qult is same as Terzaghi theory. Difference is more pronounced at larger D/B ratios. • Inclination factors reduce the bearing capacity when the load is inclined from vertical..

(28) Tutorial A foundation column has to carryy a ggross allowable total mass Q of 15,290 kg. The depth of foundation is 0.7m. The load is inclined at angle 20° to the vertical as shown in Fig.1 below. Determine the width id h off the h foundation, f d i B Use B. U f factor off safety f off 3. 3 Use U Meyerhof’s method. For ϕ = 30°, Nc = 30.14, Nq = 18.4, Nγ = 22.4 (Dec 2012). Dr. PVSN Pavan Kumar.

(29) Tutorial qult = cNcScdcic + q’NqSqdqiq + 0.5γBNγS γ d γ i γ C = 0; ϕ = 30°; Nc = 30.14, Nq = 18.4, Nγ = 22.4 q‘=18*0.7 = 12.6 kN/m2 Considering strip footing Sc = 1.0; S Considering strip footing S 1.0; Sq = 1.0; S 1.0; Sγ = 1.0 1.0 kp = 3 dc = 1+(0.2√3*0.7/B) d = 1+(0 2√3*0 7/B) dq = d = dγ = 1+(0.2√3*0.7/B) = 1+(0 2√3*0 7/B) ic = iq = (1‐(20/90))2= 0.60 iγ = (1‐(20/30))2 = 0.11 qult =15290/B / B = 0.75m          Dr. PVSN Pavan Kumar.

(30) Tutorial A footing of size 2m x 4m is placed at a depth of 1.5m below the ground surface. Estimate the net safe load that can be supported by the footing. Take factor of safety = 2.5, c = 22 kN/m2. ϕ = 30° Nc = 30.1, Nq = 18.4, Nγ = 16.7. Use Meyerhof f recommendation (June/July 2014)  . Dr. PVSN Pavan Kumar.

(31) Hansen bearing capacity theory (1970). Hansen theory extends the bearing capacity equation for a footing tilted from horizontal and possibility of slope of ground supporting the footing. Dr. PVSN Pavan Kumar.

(32) Hansen bearing capacity theory (1970) qult = cNcScdcic gc bc + q’NqSqdqiq gq bq + 0.5γB’NγS γ d γ i γ gγ bγ B.C. Factors. (. ). N c = N q − 1 cot ϕ ϕ⎞ ⎛ Nq = tan2 ⎜ 45+ ⎟eπ tanϕ 2⎠ ⎝. (. ). N γ = 1.5 N q − 1 tan ϕ. Shape Factors Nq B Sc = 1+ Nc L B tan ϕ Sq = 1+ L Sγ. B = 1 − 0 .4 ≥ 0 .6 L. In case of eccentric loading B and L are replaced by B’ and L’, the effective dimensions of footing (B’=B‐2e, effective dimensions of footing (B B 2e, LL’=L‐2e) L 2e) Dr. PVSN Pavan Kumar.

(33) Hansen bearing capacity theory (1970) Depth Factors. Inclination Factors. d c = 1.0 + 0.4 K. ic = i q −. D K = D / B for ≤1 B D K = tan − 1 ( D / B ) for >1 B. K in radians. 1− 1 − iq N q −1. ⎡ ⎤ 0.5 H i q = ⎢1 − ⎥ V + Ac cot ϕ a ⎦ ⎣. 5. ⎡ ⎤ 0.7 H iγ = ⎢1 − ⎥ V + A Ac cot t ϕ a ⎣ ⎦. 5. d q = 1 + 2 tan ϕ (1 − sin ϕ ) 2 k d γ = 1.0 for all ϕ. H is horizontal load on footing, V vertical load on footing, A = B’L’ (effective area), Ca = Adhesion = 0.6 to 1.0c = Adhesion = 0 6 to 1 0c Dr. PVSN Pavan Kumar.

(34) Hansen bearing capacity theory (1970) Ground factors (base on slope) ( p ). Base factors (tilted base). Dr. PVSN Pavan Kumar.

(35) Hansen bearing capacity theory (1970) For ϕ = 0. qult = 5.14 su (1 + sc’ + dc’ ‐ ic’ ‐ bc’ ‐ gc’) + q’. K is defined above. Dr. PVSN Pavan Kumar.

(36) Vesic bearing capacity theory (1973) • Vesic conformed the basic nature of failure surface similar to Terzaghi. g • Inclined surface AC and BC make an angle 45°+ ϕ with 2 horizontal instead of ϕ ϕ.. Dr. PVSN Pavan Kumar.

(37) Vesic bearing capacity theory (1973) Same as Hansen theory except Nγ qult = cNcScdcic gc bc + q’NqSqdqiq gq bq + 0.5γB’NγS γ d γ i γ gγ bγ B.C. Factors. (. ). N c = N q − 1 cot ϕ. ϕ⎞ ⎛ Nq = tan2 ⎜ 45+ ⎟eπ tanϕ 2⎠ ⎝. (. ). N γ = 2 N q + 1 tan ϕ. Shape Factors Sc. Nq B = 1+ Nc L. Sq = 1+. Sγ. B tan ϕ L. B = 1 − 0 .4 ≥ 0 .6 L. Depth Factors d c = 1.0 + 0.4 K D K = D / B for ≤1 B D −1 K = tan ( D / B ) for >1 B. K in radians d q = 1 + 2 tan ϕ (1 − sin ϕ ) 2 k d γ = 1.0 for all ϕ. Dr. PVSN Pavan Kumar.

(38) Vesic bearing capacity theory (1973) Inclination Factors. Ground  factors. β is in radians Base factors (tilted base) ( ). Dr. PVSN Pavan Kumar.

(39) Vesic bearing capacity theory (1973) ϕu = 0. Dr. PVSN Pavan Kumar.

(40) Tutorial Compare the ultimate bearing capacity of a strip footing 1.5m wide with its base at a depth of 1m resting on a dry sand stratum with c’ = 0, ϕ’ = 38° and γd = 17 kN/m3. Use Meyerhof, Hansen and Vesic theory (June 2010). Dr. PVSN Pavan Kumar.

(41) Inclined loaded footing. Analysis of horizontal load y Dr. PVSN Pavan Kumar. Eccentric load.

(42) Bearing capacity of footings subjected to eccentric loading eccentric loading • Foundation subjected to lateral loads and moments result in eccentric loading. g • If point of application of resultant of all loads is away from centriod results in eccentric loading. • Eccentricity, e is distance between the point of application of resultant load and centre of footing. This should h ld be b < B/6. B/6 • Foundation subjected to an eccentric vertical load tilts towards the side of the eccentricity and the contact towards the side of the eccentricity and the contact pressure increases on the side of tilt and decreases on the opposite side. the opposite side. Dr. PVSN Pavan Kumar.

(43) Bearing capacity of footings subjected to eccentric loadingg. Eccentricity about x ‐ axis Eccentricity about x Eccentrically loaded footing. Dr. PVSN Pavan Kumar.

(44) Bearing capacity of footings subjected to eccentric loadingg. Eccentricity about yy ‐ axis. Eccentricity about x and y ‐ axis. Dr. PVSN Pavan Kumar.

(45) Bearing capacity of footings subjected to eccentric loadingg Meyerhof indicate the effective footing dimensions are L' = L‐2ex and B' = B‐2ey Effective area of footing, A’ = L' B‘ Ultimate load bearing capacity of a footing subjected to eccentric loads = Q'ult=qu A’ qu = ultimate bearing capacity of the footing of g p y g dimension L’ x B’ Dr. PVSN Pavan Kumar.

(46) Maximum and minimum base pressures. Maximum pressure develops at C and minimum at D given as follows. Dr. PVSN Pavan Kumar.

(47) Eccentric loading • IIncrease of eccentricity of load increases the f t i it f l d i th maximum pressure at one edge of footing and d decreases the pressure at other end (tension). th t th d (t i ) • Soil is poor in carrying tensile stress and the eccentricity is limited to an area known as Kern. ex < L/6 ey < b/6. Dr. PVSN Pavan Kumar.

(48) Tutorial A square footing 2.0 m wide at a depth of 2.0 m is subjected A square footing 2 0 m wide at a depth of 2 0 m is subjected to an axial load of 2000 kN and a moment of 360 kN.m. The soil below is granular with an angle of shearing resistance of g g g 36°. Is the footing safe? (Nov/Dec 2009). Dr. PVSN Pavan Kumar.

(49) Tutorial An eccentrically loaded foundation is shown in Figure A t i ll l d d f d ti i h i Fi below. Determine the ultimate load that the f foundation can carry (Nov 13). d ti (N 13). e = 0.1m 0 8m 0.8m Eccentricity in one direction only. Qall. 1.5 m x 1.5 m Centerline Dr. PVSN Pavan Kumar. γ= 17 kN/m3 c = 0 ϕ = 32°.

(50) Foundations on layered soil • Footings Footings are laid on stratified deposits where thickness of soil are laid on stratified deposits where thickness of soil top stratum below base of footing, d1 is less than, H = B tan⎛⎜ 45° + ϕ ⎞⎟ 2 2⎠ ⎝ • qult shall be modified B. Case 1: Footing on layered clays a. Top layer weaker than lower layer ( 1 < c2 ) (c b. Top layer stronger than bottom layer (c y ( 1 > c2 )) Case 2: Footing on layered c ‐ ϕ soil Case 3: Footing on layered sand and Case 3: Footing on layered sand and clay soils a. Sand overlying clay b. Clay overlying sand Dr. PVSN Pavan Kumar. 45+. ϕ 2. H. d1.

(51) Foundations on layered soil. Circular arc method where strength ratio, c2/ c1 between 0.6 to  1.3. Shear strength between top two layers is largely different following methods are used.  . Dr. PVSN Pavan Kumar.

(52) Foundations on layered soil CR ≤ 1 1.5d 1 + 5.14c R ≤ 5.14 Strip footing N cs = Strip footing B 3d 1 + 6.05c R ≤ 6.05 Circular footing N cr = B. CR > 1 Strip footing Strip footing N 1s. 0.5 B 1.1B = 4.14 + N 2 s = 4.14 + d1 d1. Circular footing N 1r. 0.33B 0.66 B = 5.05 + N 2 r = 5.05 + d1 d1 Dr. PVSN Pavan Kumar. 2 N1 N 2 Nc = N1 + N 2.

(53) Foundation on layered soil Case 2: Footing on layered c ϕ soil Case 2: Footing on layered c ‐ Modified f ϕ=. d 1ϕ1 + (H − d 1 )ϕ 2 H. Modified c =. d 1 c1 + (H − d 1 ) c 2 H. Ultimate bearing capacity, qult is determined from modified c, ϕ. Dr. PVSN Pavan Kumar.

(54) UNIT II Settlement of foundations. Dr. PVSN Pavan Kumar.

(55) Syllabus • Introduction • Elastic settlement of footings in sands and clays infinite thickness infinite thickness • Footings on soils of Finite thickness • Schmertamann's method • Janbu method. Dr. PVSN Pavan Kumar.

(56) Allowable bearing capacity • In many cases, the allowable settlement of a shallow foundation may control the allowable shallow foundation may control the allowable bearing capacity. • Settlements are large when the width of footing is l l h h d h ff large. Dr. PVSN Pavan Kumar.

(57) Causes of settlement • • • • • •. Due to building or super structure column load Due to building or super structure column load Due to weight of recently placed fill Fall of ground water level or pumping ll f d l l i Under ground mining/tunneling Formation of sinkholes Lateral movements from nearby excavations Lateral movements from nearby excavations  . Dr. PVSN Pavan Kumar.

(58) What is settlement What is settlement • Statistical Statistical accumulation of ground movements accumulation of ground movements due to the application of load is known as settlement. settlement. Dr. PVSN Pavan Kumar.

(59) Introduction • Foundation settlements must be estimated with great care for buildings, bridges, towers, power plants, and g, g , ,p p , similar high‐cost structures. • For structures such as fills, earth dams, braced , , sheeting, and retaining walls a greater margin of error in the settlements. • What is the consequence of Under and over prediction of settlements? Under prediction – Unsafe design and failure of structure. Over prediction leads to deep foundation such as pile or Over prediction leads to deep foundation such as pile or caisson foundation or improvement of soil. Dr. PVSN Pavan Kumar.

(60) Introduction. γ. D γD. qult. • Additional stress due to the footing produces a time‐ dependent accumulation of particle rolling, rolling sliding, sliding crushing, crushing and elastic distortions in a limited influence zone beneath the loaded area. • The statistical accumulation of movements in the direction of interest is the settlement. Dr. PVSN Pavan Kumar.

(61) Introduction • Particle sliding and rolling produce a decrease in the void ratio and grain crushing. void ratio and grain crushing. • Only a small fraction of settlement is elastic and removal of applied stress results in a little recovery removal of applied stress results in a little recovery of sample. • In spite of above soil is treated as a elastic material I it f b il i t t d l ti t i l with parameters Es, G', µ, and ks to estimate settlements. ttl t. Dr. PVSN Pavan Kumar.

(62) Classification of settlements • Immediate, or those that take place as the load is applied or within a time period of about 7 days. – Applicable for all fine-grained soils including silts and clays with a degree of saturation, s ≤ 90 % and for all coarse-grained soils with a large coefficient of permeability above 10-3 m/s.. • C Consolidation lid ti settlements ttl t are those th that th t are timeti dependent and take months to years to develop. – Leaning Tower of Pisa in Italy has been undergoing consolidation settlement for over 700 years. The lean is caused byy the consolidation settlement beingg ggreater on one side. Dr. PVSN Pavan Kumar.

(63) Classification of settlements Classification of settlements • Magnitude Magnitude and time required for settlement are and time required for settlement are important. • Pore water pressures are high if the soil is saturated Pore water pressures are high if the soil is saturated (S=100%) and negligible for S = 0. Dr. PVSN Pavan Kumar.

(64) Leaning tower of Pisa • Construction of the tower began in the year 1173 • By 1178 ring‐shaped footing of By 1178 ring shaped footing of 19.6 m diameter and 3.5 stories of the tower completed. Bearing pressure = 330 kPa. Tower started tilting. Construction stopped at this Construction stopped at this stage due to political and economical problems. • From 1271 construction resumed by tapering the successive stories and adding successive stories and adding extra weight to the high side. Dr. PVSN Pavan Kumar Work stopped in 1278..

(65) Leaning tower of Pisa • Tower completed between 1360 to 1370. Angle of tilt at that time is 3° from rest of tower. • It took 200 years to complete project. • Early nineteenth century, the tower had settled Early nineteenth century the tower had settled about 2.5 meters into the ground. • By end of the 20 B d f th 20th century the total tilt was about t th t t l tilt b t 5.5 degrees.   • Tower is clearly on the brink of collapse. • Pressure on soil 62 to 930 kPa. • A minor earthquake could cause it to topple. Dr. PVSN Pavan Kumar.

(66) Classification of settlement Secondary Compression: D l d progressive Delayed i slippage li off grain i as the th particles ti l adjust themselves to a medium dense condition. Settlement due to secondary compression cα t cα Secondary compression index Secondary compression index Ss = H log l 1 + e0 t p e0 initial void ratio H thickness of layer t is any time tp time for completion of primary consolidation lid i Dr. PVSN Pavan Kumar.

(67) h1. qult E1,µ1, ε1, ∆q 1. h2 H. • Depth of influence, H is taken as 4B to 5B or hard layer with bottom layer having E ten times higher than top layer • Obtaining a reliable stress profile. profile • Theory of elasticity assumes that soil is homogenous g and isotropic p n. E2, q2 2 µ2, ε2, ∆q h3 E3,µ3, ε3, ∆q ∆ 3. Total settlement ΔH = ∑ ε i h i i =1 1. Δq ε = Strain Strain = Es. Elastic settlement of Elastic settlement of foundation on layered soil Dr. PVSN Pavan Kumar.

(68) Problems in settlement analysis Problems in settlement analysis • Obtaining Obtaining reliable values of elastic parameters reliable values of elastic parameters (Es G', µ, and ks) • Recovering undisturbed soil samples Recovering undisturbed soil samples • Due to above problems greater tendency to use i i in situ tests such as SPT, DCPT, SCPT, Plate load h SPT DCPT SCPT Pl l d test etc. • These tests give horizontal values instead of vertical values actually needed (Anisotropy) 9. Dr. PVSN Pavan Kumar.

(69) Unconfined Compression test. Triaxial Compression test. Dr. PVSN Pavan Kumar.

(70) Stress – strain modulus, E Stress strain modulus, Es • Unconfined Unconfined compression test compression test • Triaxial test • In situ tests such as SPT, CPT, pressuremeter i h S C test, flat dilatometer. – The above tests give modulus of elasticity in horizontal direction but the modulus of elasticity in ertical direction is req ired in vertical direction is required. . Dr. PVSN Pavan Kumar.

(71) Modulus of elasticity from field tests y. Dr. PVSN Pavan Kumar.

(72) Stress increase in soil due to footing pressure. Other methods Other methods. 2 V : 1H Method Dr. PVSN Pavan Kumar.

(73) Stress increase in soil due to footing pressure. Dr. PVSN Pavan Kumar.

(74) Stress increase in soil due to footing pressure • Boussinesq theory (1885) give the stress at different p points below the ground surface due to g concentrated load, line and strip loads, rectangular and circular loaded areas. • Westergaard (1938) equation is used estimate of the stress qv when the soil mass consists of layered the stress q when the soil mass consists of layered strata of fine and coarse materials, as beneath a road pavement. road pavement.. Dr. PVSN Pavan Kumar.

(75) Immediate settlement Settlement of the corner of a rectangular base of dimensions B B' X LL' on the surface of an elastic half‐space can be computed from an equation from the Theory of Elasticity [e.g., Timoshenko and Goodier (1951)] as follows. Dr. PVSN Pavan Kumar.

(76) Immediate settlement q0 = intensity of contact pressure in units of Es B’ least B’= l t lateral l t l dimension di i off contributing t ib ti base b area Es, µ = elastic soil parameters (Avg. mod of different layers). (tan‐1 in radians) in radians). Dr. PVSN Pavan Kumar.

(77) Depth factor, If. • Settlement Settlement is reduced when it is placed at some depth in the is reduced when it is placed at some depth in the ground, depending on Poisson's ratio and L/B. Dr. PVSN Pavan Kumar.

(78) Immediate settlement • Above equation is applicable for flexible footings. • Thick slabs behave as rigid footings and the settlement Thick slabs behave as rigid footings and the settlement decreases by 7% of flexible footings. • Settlement of rigid footing = 0.93 x Settlement of flexible footing • Settlement of rigid footing = 0.8 x Settlement of flexible footing • Obtain the weighted average modulus of elasticity of soil. Dr. PVSN Pavan Kumar.

(79) • Estimate the settlement of the raft or mat g foundation for the following data. q0 = 134 kPa B x L = 33.5 x 39.5 m Soil is layered clay with one sand layer from ground Soil is layered clay with one sand layer from ground surface to sandstone bed rock at ‐14m. Raft is at ‐ 3m. 3m Es of clay layer from 3 to 6m = 42.5 Mpa Es of clay layer from 6 to 14m = 60 Mpa Es for sand stone > 500 Mpa μ = 0.35 Dr. PVSN Pavan Kumar.

(80) Elastic settlement of foundations • Net elastic settlement for a flexible surface footing is . Se = Elastic settlement = Elastic settlement B = width of foundation µ = Poisson ratio Es = Modulus of elasticity of soil If = Influence factor Dr. PVSN Pavan Kumar.

(81) Elastic settlement of foundation. Influence factor If (Bowles, 1988). Dr. PVSN Pavan Kumar.

(82) Elastic settlement of foundations Elastic settlement of foundations • Final elastic settlement Final elastic settlement Sef = Cr d f Se. Cr = Rigidity factor taken as 0.8 for highly rigid foundation df = depth factor p Se = Settlement of a surface flexible footing Dr. PVSN Pavan Kumar.

(83) Depth factor, df. Correction curves for elastic settlement for rectangular footings at different depths Dr. PVSN Pavan Kumar.

(84) • A rectangular footing of 1.5 m x 1.0 m size exerts a pressure of 150 kN/m2 on a cohesive soil having Es = 3 x 104 kN/m2 and m = 0.5. Determine the elastic settlement at the center of footing assuming the footing is flexible. Take the value of influence factor If as 1.36. (Nov 2008, Set No.1) influence factor, I as 1 36 (Nov 2008 Set No 1) • A square footing of 1.2 m size is subjected to a pressure of 200 kN/m2 in a cohesive soil. Determine the elastic 200 kN/m in a cohesive soil Determine the elastic settlement at the corner of the footing assuming the footing is rigid. Take the average influence factor I = 0.82 g g g and Es = 4 x 104 kN/m2. . Dr. PVSN Pavan Kumar.

(85) Schmertmann method • Schmertmann (1970) observed that variation of strain under the footing over sand is similar to the distribution of vertical stress due to footing pressure. • Pressure bulb changes more rapidly from a depth of about 0.4B to 0.6B and this depth is interpreted to have the largest strains for square and circular footings. • Method proposes to use a triangular relative‐strain relative strain diagram to model the strain distribution with ordinates of 0.1, 0 1 0.6, 0 6 and 0 at 0B, 0B 0.5B, 0 5B and 2B, 2B respectively for square and circular footing. Dr. PVSN Pavan Kumar.

(86) Schmertmann method • For strip footing of L/B > 10, maximum strain will occur at a depth B. Strain at a depth 4B below occur at a depth B. Strain at a depth 4B below the base is zero and immediately below the base of footing strain influence factor, Iz is 0.2 of footing strain influence factor, I is 0.2. Dr. PVSN Pavan Kumar.

(87) Schmertmann method Square and circular footing d l f Strip footing. Δq I z = 0.5 + 0.1 p o′ ∆q = Net foundation pressure = q0 ‐ q q0 footing contact pressure footing contact pressure q effective overburden pressure at base . p‘0 = effective overburden pressure at depths B/2 and B for square and strip foundations respectively. Dr. PVSN Pavan Kumar.

(88) Schmertmann method Settlement = Area of the strain influence factor diagram x strain influence factor diagram x strain Two correction factors for Two correction factors for embedment depth and time shall be adopted as follows: . q0 0.1 0.5B 0.6 Variation . For embedment For time 2B. t in years Dr. PVSN Pavan Kumar. of strain influence factor, Iz for square and and circular footing.

(89) Schmertmann method For square footing Es = 2.5 qc For strip footing, L/B ≥ 10 Es = 3.5 qc qc = Static cone penetration resistance ∆q = Net foundation pressure = q0 ‐ q. Dr. PVSN Pavan Kumar.

(90) Schmertmann method • Static cone penetration test is conducted in sub soil •Cone Cone penetration resistance diagram is divided into layers of approximately constant values of qc. •The strain influence factor,, Iz diagram g is p placed alongside cone penetration diagram beneath the foundation to the same scale. • Settlement of each layer resulting from the net contact pressure ∆q is then calculated using the values of Es and Iz appropriate to each layer. • Sum of the settlements in each layer is then corrected d for f the h depth d h and d creep factors f Dr. PVSN Pavan Kumar.

(91) Static Cone penetration test Dr. PVSN Pavan Kumar.

(92) • Estimate the elastic settlement by Schmertmann's method by making use of the relationship qc = 4 Ncor kg/cm2 where qc = static cone penetration value in kg/cm2. Assume settlement is required at the end of a period of 3 years. Depth of foundation = 2m. Dr. PVSN Pavan Kumar.

(93) Dr. PVSN Pavan Kumar.

(94) A continuous footing on a layer of sand is shown in fi figure below along with the variation of the b l l ih h i i f h modulus of elasticity of the soil, Es. Assuming that γ = 18 kN/m 18 kN/ 3 and assuming a creep time of 10 d i i f 10 years for the correction factor C2. Calculate the elastic settlement of the foundation, using the l ti ttl t f th f d ti i th strain influence factor (Nov 2012). . Dr. PVSN Pavan Kumar.


(96) Janbu method • Janbu et al. (1956) proposed an equation for evaluating the average settlement of flexible strip, rectangular, square or circular foundations on saturated clay soils (Poisson’s ratio, µ 0.5). q0 footing contact pressure B Width of footing B Width of footing Dr. PVSN Pavan Kumar.

(97) Janbu method. Dr. PVSN Pavan Kumar.

(98) Janbu method. Dr. PVSN Pavan Kumar.

(99) Dr. PVSN Pavan Kumar. Consolidation test set up.

(100) Consolidation test Time Ti (Min). Dial gauge reading Dial gauge reading 0.5 kg/cm2 Load ingg. Unload ingg. 1 kg/cm2 Loadin g. Unload ingg. 2 kg/cm2 Loadin g. Unload ingg. 4 kg/cm2 Loadin g. 0 0.5 1 2 4 8 16 25 1 2 4 8 16. Dr. PVSN Pavan Kumar. Unload ingg. 8 kg/cm2. 16 kg/cm2. Loadin g. Loadin g. Unload ingg. Unload ingg.

(101) Consolidation settlements Virgin Compression or normal consolidation Recompression or Over Consolidation Vi i C Virgin Compression i or normal consolidation Swelling. Result of consolidation test Dr. PVSN Pavan Kumar.

(102) Consolidation settlements • Settlements of fine‐grained, saturated cohesive soils will be time‐dependent, and consolidation th theory is usually used. i ll d • In case of normally consolidated clays . • where cc = compression index from the e versus log p plot = 0.009 (LL – 10), LL = Liquid limit (%) eo = in situ void ratio at the middle of clay stratum H = Stratum thickness,, for a thick stratum divide into several layers Dr. PVSN Pavan Kumar.

(103) Consolidation settlements p’o = effective overburden pressure at mid height of H ∆p = average increase in pressure at middle of clay layer from the foundation loads in layer H Overconsolidated clays y. Dr. PVSN Pavan Kumar.

(104) Preconsolidated clay Recompression. Virgin g Compression Normal consolidation. Dr. PVSN Pavan Kumar.

(105) Consolidation settlements If soil is preconsolidated Cr recompression index Cc compression index p. Dr. PVSN Pavan Kumar.

(106) Consolidation settlement Other equation to determine the settlement of foundation is   mv = Coefficient of volume compressibility ∆p = increase of pressure in middle of clay layer H = thickness of clay layer. Dr. PVSN Pavan Kumar.

(107) A square footing 1.2m×1.2m rests at a depth of 1m in a saturated clay layer 4m deep. deep The clay is normally consolidated, having an unconfined compressive strength of 40 kN/m2. The soil has a liquid limit of 30%, γsat= 17.8 kN/m3, w=28% and G = 2.68. Determine the load which the footingg can carry safely with a factor of safety of 3 against g shear. Also determine the settlement if the footing is loaded with this safe load (May 2010). Square q footingg qu = 1.3cNc + σ’Nq + 0.4γBNγ ϕ=0 0°, Nq = 1, 1 Nγ = 0 Dr. PVSN Pavan Kumar.

(108) Allowable settlement Allowable settlement. Maximum settlement. Dr. PVSN Pavan Kumar.

(109) Allowable settlement • Settlements can be computed for various points such as corner, center, or beneath the lightest and heaviest loaded footings to obtain the total settlement and the differential settlement between adjacent points. • If the entire structure moves vertically some amount or rotates as a plane rigid body, this movement will not generally cause structural or architectural distress. • If a structure t t settles ttl 20 mm on one side id and d 100 mm on the other with a linear settlement variation between the two p points,, structural damage g is not likely to develop but there are aesthetic and public confidence considerations. Settlement = 20mm Differential settlement = 80 mm Tilt = (100‐20)/L Dr. PVSN Pavan Kumar.

(110) Allowable settlement • Local settlements below tilt line will cause the structural distress of building structural distress of building. • Initial settlements that occur during construction can s all be hidden d rin completion of can usually be hidden during completion of building. A cracked wall or warped roof is much more difficult to conceal more difficult to conceal.. Dr. PVSN Pavan Kumar.

(111) Allowable settlement. Long time spans allow the structure to adjust and better   resist differential movement Dr. PVSN Pavan Kumar.

(112) Assignment • Write Write a short notes on elastic settlement of a short notes on elastic settlement of foundations • Explain Schmertmann Explain Schmertmann method with neat sketch method with neat sketch • What are the types of settlements and how consolidation settlement is estimated? lid i l i i d?. Dr. PVSN Pavan Kumar.

(113) UNIT III Pile Foundations. Dr. PVSN Pavan Kumar.

(114) Syllabus • • • • •. Static methods for load carrying capacity Static methods for load carrying capacity Dynamic methods Pile groups il Negative skin friction Under reamed piles.. Dr. PVSN Pavan Kumar.

(115) Necessity • Shallow foundations are normally used where the g p soil close to the ground surface and up to the influence zone possess sufficient bearing strength to carry the superstructure load without causing y p g distress to the superstructure due to settlement. • If top soil is either loose or soft or of a swelling type If top soil is either loose or soft or of a swelling type the load from the structure is to be transferred to deeper firm strata. deeper firm strata. • The structural loads may be transferred to deeper firm strata by means of piles firm strata by means of piles. Dr. PVSN Pavan Kumar.


(117) End bearing and frictional piles. Frictional pile. End bearing pile Dr. PVSN Pavan Kumar.

(118) End bearing and frictional piles End bearing and frictional piles • End bearing piles are used to transfer load through water or soft soil to a suitable bearing stratum They carry heavy loads safely to hard stratum. strata and settlements are less. • Frictional piles transfer the loads to the surrounding granular soil along their length by skin friction. friction • Piles carry super imposed load through end b i and bearing d skin ki friction. fi i Dr. PVSN Pavan Kumar.

(119) End bearing and frictional piles End bearing and frictional piles. End bearing cum frictional pile Dr. PVSN Pavan Kumar.

(120) Pile foundations • Piles are long slender columns either driven, bored or cast‐in‐situ. cast in situ • Driven piles are made of a variety of materials such h as – concrete, steel, timber. • Cast‐in‐situ piles are concrete piles. • If the diameter of a bored‐cast‐in‐situ p pile is greater than about 0.75 m, it is referred as a drilled p pier,, caisson or shaft. Dr. PVSN Pavan Kumar.

(121) Classification of Pile foundations • Piles may be subjected to vertical compression, tension lateral or inclined loads tension, lateral or inclined loads. • Piles are classified as short or long based on L/d ratio. • Piles are constructed as vertical or inclined piles. Inclined or batter piles are used to carry large lateral loads. . Dr. PVSN Pavan Kumar.

(122) Uses of piles. Uplift/tension /anchor Piles. Compression Piles. Dr. PVSN Pavan Kumar.

(123) W. Piles subjected to lateral load. Dr. PVSN Pavan Kumar.

(124) Timber piles. Protecting shoe. Splicing Dr. PVSN Pavan Kumar.

(125) Timber piles • Materials: Timber piles are made of tree trunks with the branches trimmed off. Such piles shall be of sound quality and free of defects. • Length of piles: 15 m or more. For larger lengths the ends are spliced. • Diameter of the piles at the butt end vary from 30 to 40 cm and at tip end more than 15 cm. • Life: Piles entirely submerged in water last long if marine borers are not present. The life of piles subjected to alternate wetting and drying is less. Piles shall be treated with a wood preservative, usually creosote at 250 kg/m ith d ti ll t t 250 k / 3 3 in sea water. for piles in fresh water and 350 kg/m Dr. PVSN Pavan Kumar.

(126) Timber piles • Driving: Crushing of the fibers on the head (or brooming) is controlled by using a driving cap or brooming) is controlled by using a driving cap, or ring around the butt (top). • Maximum design load Ma im m desi n load per pile is less than 250 per pile is less than 250 kN. • Timber piles are less expensive in places where timber is plentiful. • After being driven to final depth, all pile heads, treated or untreated, should be sawed square to sound undamaged wood to receive the pile cap. Dr. PVSN Pavan Kumar.

(127) Timber piles. Dr. PVSN Pavan Kumar.

(128) Concrete Piles • Either precast or cast‐in‐situ piles. • Precast concrete piles are cast and cured in a Precast concrete piles are cast and cured in a casting yard and then transported to the site of work for driving. work for driving. • Precast piles are made of uniform sections with pointed tips pointed tips. • Tapered piles are manufactured when greater b i bearing resistance is required. it i i d • Normally piles of square or octagonal sections are manufactured. These shapes are easy to cast in horizontal position. Dr. PVSN Pavan Kumar.

(129) Concrete Piles • Necessary reinforcement is provided to take care g of handling stresses. • Piles are also prestressed. • Maximum load on a prestressed Maximum load on a prestressed concrete pile is concrete pile is approximately 2000 kN and for precast piles 1000 kN Optimum load range is 400 to 600 kN. 1000 kN. Optimum load range is 400 to 600 kN. Dr. PVSN Pavan Kumar.

(130) Cast in place concrete piles Dr. PVSN Pavan Kumar.

(131) Precast Driven piles • Pil Piles may be b off timber, ti b steel t l or precastt concrete. t • They are driven either vertical or inclined. • Piles are driven using a pile hammers as follows – Drop, Drop single acting, double acting and differential acting steam hammer, diesel, hydraulic and vibratory hammers. • Compaction piles: Pile is driven into granular soil displaces the surrounding soil equal to the volume of the driven pile. pile • Compacts the soil around the sides of pile. • The displaced soil particles enter the soil spaces of the adjacent mass which leads to densification of the mass. • compaction of the soil mass around a pile increases its bearing capacity. Dr. PVSN Pavan Kumar.

(132) Compaction piles ¾ Compacts sandy soil. ϕ2 = ϕ = ϕ1/2 + 20 /2 + 20° ¾Decreases strength of clay soil which gradually regains with time hi h d ll i ih i Dr. PVSN Pavan Kumar.

(133) Precast Driven piles • Pile Pile is driven into saturated silty is dri en into sat rated silt or cohesive soil will or cohesi e soil ill not densify the soil around the pile because of its poor drainage qualities. • Displaced soil particles cannot enter the void space unless the water in the pores is pushed out. • Stress developed in soil due to pile driving have to S d l di il d il d i i h be borne by pore water. Results in the development of pore water pressure • Results in the development of pore water pressure and a consequent decrease in the bearing capacity of the soil. • Immediate effect of pile driving is to decrease in bearing capacity of soil. Remolded soil regains part of its lost Strength due to the reorientation of the of its lost Strength due to the reorientation of the disturbed particles with time (thixotrophy). Dr. PVSN Pavan Kumar.

(134) Precast Driven piles Advantages: • Can be precasted to the required specifications, any size length and shape. size, shape • Progress of the work is rapid. • Pile driven in granular soil increases bearing capacity. • Construction work is neat and clean,, • Supervision of work at the site is reduced • Storage space required is very much less. • Used in sites where a fear of meeting ground water under pressure due to drill holes. • Preferred for piles in wharf structures or jetties. P f df il i h f t t j tti Dr. PVSN Pavan Kumar.

(135) Precast Driven piles Disadvantgaes: Must be properly reinforced to with stand handling stresses during transportation and di i driving. • Requires heavy equipment for handling and driving. • Method involves cutting off extra lengths or adding more lengths thus increases the cost of project. • They are not suitable in soils of poor drainage qualities due to heaving of the soil or the lifting of the driven piles during the driving of a new pile. • Foundations of adjacent structures are likely to be affected due to the vibrations generated. Dr. PVSN Pavan Kumar.

(136) Driven cast in situ pile. Dr. PVSN Pavan Kumar.

(137) Driven cast in situ pile Driven cast in situ pile • Involves Involves driving of a steel tube to the required driving of a steel tube to the required depth with the end closed by a detachable conical tip conical tip. • Tube is next concreted and the shell is simultaneously withdrawn simultaneously withdrawn. • In some cases the shell will not be withdrawn.. Dr. PVSN Pavan Kumar.

(138) Bored Cast – in situ piles • Constructed by making holes in the ground to the required depth and then filling the hole with concrete. • Straight bored piles or piles with one or more bulbs at intervals may be cast at the site. The latter type are called under‐reamed piles. Advantages: Piles of any size and length are constructed, damage due to driving and handling is eliminated vibrations are avoided and is eliminated, vibrations are avoided and adjacent structures are safe. Suitable in soils of poor drainage qualities poor drainage qualities.   Dr. PVSN Pavan Kumar.

(139) Bored Cast in situ piles Disadvantages: • Careful supervision p and q qualityy control of all the materials is necessary for casting of piles. • Sufficient storage g space p is necessaryy for construction materials used in the construction. • No advantage g of increased bearingg capacity p y due to compaction in granular soil. ϕ decreases by 3°. piles in holes with a heavyy • Construction of these p ground water flow or artesian pressure is very difficult. Dr. PVSN Pavan Kumar.

(140) Steel piles • They are rolled H shapes or pipe piles. • Designed to withstand large impact stresses during hard driving. • Pipe p p piles are either welded or seamless steel pipes which may be driven either open‐end or closed‐end. • Pipe piles are often filled with concrete after driving. driving • Optimum load range on steel piles is 400 to 1200kN. 1200kN Dr. PVSN Pavan Kumar.

(141) Steel H piles Before driving. After driving. Dr. PVSN Pavan Kumar.

(142) Steel pipe piles. Dr. PVSN Pavan Kumar.

(143) Methods to determine load carrying capacity of single vertical pile f l l l 9Static bearing capacity equations 9St ti b i it ti 9Use of SPT and CPT values 9Field pile load tests 9Dynamic Dynamic methods methods. Dr. PVSN Pavan Kumar.

(144) Static capacity of single pile ¾Bearing capacity of pile depends • Type of pile, size and length of pile Type of pile, size and length of pile • Type of soil, position of water table • Method of installation M th d f i t ll ti ¾Design of pile should be safe against shear failure and settlements within limits.. Dr. PVSN Pavan Kumar.

(145) Static capacity of single pile Ultimate load, Qu = Qb + Qf Qb ,Base or point load ,Base or point load = q qbAb qb = Ultimate bearing capacity of the pile at base the pile at base Ab = bearing area of base of pile Qf = friction load or skin load = f = friction load or skin load = fsAs fs = unit skin friction As = Total surface area of pile T t l f f il embedded below ground surface Dr. PVSN Pavan Kumar.

(146) Static capacity of single pile Cohesionless pile. n. Net ultimate load capacity of pile, Qu = q0′ N q Ab + i∑=1 q0′ k s tan δAs qq'o = effective overburden pressure at the base level of the effective overburden pressure at the base level of the pile Nq = bearing capacity factor = bearing capacity factor q'0= average effective overburden pressure over the embedded depth of the pile embedded depth of the pile ks = average lateral earth pressure coefficient Pile Pile Values of ks Values of δ δ = angle of wall friction = angle of wall friction material Low Dr High Dr n = no. of layers Steel 0.5 1.0 20° Maximum skin friction < 110 kN/m2. C Concrete t. 3 /4 3ϕ/4. 10 1.0. 20 2.0. wood. 2ϕ/3. 1.5. 4.0. Dr. PVSN Pavan Kumar. Driven piles.

(147) Dr. PVSN Pavan Kumar. Variation of Nq with ϕ.

(148) Critical depth qb and fs increases with depth up to critical depth, Lc given as follows. Maximum base resistance qb is limited to 11000 k / 2 kN/m End bearing and frictional resistance of bored piles is less compared to driven pile Dr. PVSN Pavan Kumar.

(149) Static capacity of single pile Cohesive soils Net ultimate load capacity of pile N t lti t l d it f il Qu=cbNcAb+Asα cu cb = undrained d i d shear strength or cohesion of clay h t th h i f l at base level Nc = Bearing capacity factor = 9 B i it f t 9 cu = Average undrained shear strength of clay along the shaft l th h ft α = adhesion factor Dr. PVSN Pavan Kumar.

(150) Variation of α with cu Dr. PVSN Pavan Kumar.

No results found