CPT Guide 5 th Edition. CPT Applications - Deep Foundations. Gregg Drilling & Testing, Inc. Dr. Peter K. Robertson Webinar # /2/2013

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1 Gregg Drilling & Testing, Inc. Site Investigation Experts CPT Applications - Deep Foundations Dr. Peter K. Robertson Webinar # CPT Guide 5 th Edition Robertson & Cabal (Robertson) 5 th Edition 2012 Download FREE copy from:

2 Deep Foundations Deep Foundations Pile axial capacity earliest application of CPT data Complicated by large variety of pile types and installations procedures as well as soil type 2

3 Axial Capacity Capacity controlled by details of installation Few design methods account for installation Most design techniques based on empirical methods CPT is essentially a model pile Theoretical work supports most empirical methods Axial capacity can be difficult to define many methods to define Q ult Axial Capacity P D Q ult = Q s + Q b Q s = shaft resistance Q s = shaft resistance = f p A s Q b = base resistance = q p A p f p & q p unit shaft & base resistance Q b = base resistance 3

4 CPT for Geotechnical Design CPT DATA DIRECT Empirical Based on past foundation performance Less general INDIRECT Less empirical Soil parameters More general Often more difficult Indirect Approach Psuedo-theoretical methods Soil parameters from CPT Sand friction angle Clays undrained shear strength Empirical methods Unit side friction f p = βσ v Unit end bearing q p = N t σ b 4

5 CPT Direct Method - LCPC Based on over 200 full scale pile load tests - after Bustamante and Gianeselli, 1982 Wide range of pile types and soil conditions Uses only CPT penetration resistance, q c Accounts for different pile types/installation methods Best when calibrated against pile load test results aid in correct selection of pile/installation category LCPC CPT Method Published evaluations Briaud & Tucker (1988) 78 pile load test results Robertson et al. (1988) 8 pile load test results Tand & Funegard (1989) 13 pile load test results plus several more recent publications LCPC CPT-method gave the best results in all studies 5

6 Scale effect for unit end resistance Pile or CPT Zone of influence LCPC Method after Bustamante and Gianeselli, 1982 Unit base resistance, q p q p = q ca k c where: q ca = equivalent CPT tip resistance at level of pile tip k c = empirical bearing capacity factor 6

7 Calculation of equivalent average cone resistance Average cone resistance (q ca ) calculated over a distance of 3 pile diameters (3D) - 1.5D above & 1.5D below. High and low points (<0.7q ca and >1.3q ca ) removed above High points (>1.3q ca ) removed below after Bustamante and Gianeselli, 1982 Bearing Capacity Factors, k c After Bustamante and Gianeselli, 1982 Factors k c Nature of soil q c (MPa) Group I Group II Soft clay and mud < Moderately compact clay 1 to Silt and loose sand Compact to stiff clay and compact silt > Soft chalk Moderately compact sand and gravel 5 to Weathered to fragmented chalk Compact to very compact sand and gravel > Group I: plain bored piles, mud bored piles, micro piles (grouted under low pressure), cased bored piles, hollow auger bored piles, piers, barrettes. i.e. low displacement piles Group II: cast screwed piles, driven pre-cast piles, pre-stressed tubular piles, driven cast piles, jacked metal piles, micropiles (small diameter piles grouted under high pressure with diameter <250mm), driven grouted piles (low pressure grouting), driven metal piles, driven rammed piles, jacket concrete piles, high pressure grouted piles of large diameter, i.e. high displacement piles 7

8 LCPC Method Unit shaft resistance, f p f p = Σ q c /α where: q c = CPT tip resistance α = empirical friction coefficient Note: f p held to maximum values Friction Coefficient, α After Bustamante and Gianeselli,

9 CPT q c PILE Q DEPTH z DEPTH z Q ULT Q s Q b Example pile capacity profile from CPT Factor of Safety (FS) - Piles Depends on many factors Reliability and sufficiency of field data Confidence in method Previous experience with similar piles in similar soils Pile load test results are available Generally FS is around 2.0 LCPC suggest 2.0 for Q s (shaft resistance mobilized with small displacement) 3.0 for Q b (base resistance mobilized with large displacement) 9

10 Load Settlement Response Single Pile Controlled by combined behavior of Q s and Q b Side resistance mobilized at small movement (0.5% of D or 5 to 10mm) Base resistance mobilized at large movement (10 to 20% of D depending on pile type and ground) Load Settlement Response Friction Pile (Q s >> Q b ) Plunging failure at about 0.5% of diameter End Bearing Pile (Q b >> Q s ) No clear failure until very large settlements 10 to 20% of diameter (D) to failure Settlement criteria usually controls 10

11 P Single Pile Load Settlement Response D Q s = Shaft Resistance Q s Q sf 1.0 SHAFT Q b = Base Resistance ~ 0.5% (5mm ~ 10mm) S/D s D s = Diameter of Shaft 1.0 D b = Diameter of Base Q b Q bf BASE 10% S/D b P Friction Pile (Q s >> Q b ) Qs δ E = PL/AE Q Total P Q s Shaft Qb P WORKING Q b Base δ v 11

12 P Q s End Bearing Pile (Q b >> Q s ) P δ E = PL/AE Q Q b Total Base Q b P WORKING Q s Shaft % of Q s and Q b change with P δ v Amherst Site 12

13 Amherst Pile Test 0.95m dia. drilled shaft, 14.3m long 1200kN capacity Amherst Pile Test Measured for 14.3m long pile 0.95m dia. drilled shaft, 14.3m long 13

14 Drilled Shaft, Piedmont residuum Coweta Site Coweta Site 0.91m dia. drilled shaft, 19.2m long 7000kN capacity 14

Load-settlement elastic solution Poulos and Davis, 1990 (see Mayne, 2000) Soil modulus either constant or linearly increasing with depth Axial pile settlement, s (both shaft and base) s = Q I p / E

15 Load-settlement elastic solution Poulos and Davis, 1990 (see Mayne, 2000) Soil modulus either constant or linearly increasing with depth Axial pile settlement, s (both shaft and base) s = Q I p / E sl D p where: E sl = E o (1 Q/Q ult ) 0.3 ) and E o = 2.5 G o and G o =r V s 2 Case History - Drilled Shaft Opelika NGES, Alabama (Brown, ASCE JGGE, Dec. 2002) Eight Drilled Shafts: d = 3 feet L = 36 feet Construction Methods Dry (Cased) Bentonite Dry Polymer Slurry Liquid Polymer Slurry After Mayne,

16 SCPT at Opelika NGES, Alabama Piedmont Residual fine sandy silts q t (MPa) f s (kpa) u 2 (kpa) V s (m/s) Depth BGS (m) u o After Mayne, 2000 SCPTu SDMT Crosshole SASW Axial Drilled Shaft Load Test Opelika, AL Axial Load, Q (MN) Q (total) Drilled Shaft 01 (cased) Top Deflection (mm) Qtotal = Qs + Qb Pred. Qs Pred. Qb Meas. Total d = 0.91 m L = 11.0 m Q shaft 25 Meas. Shaft Meas. Base Q base 30 After Mayne,

17 Summary CPT provides reliable profiles of ground conditions Fast Cost effective Continuous Reliable LCPC CPT method accounts for method of installation Good track record Simple Best when calibrated against pile load test results CPeT-IT incorporates LCPC and simple load-settlement method Bearing capacity CPT Direct method Coarse-grained soils (sands): Ultimate bearing capacity, q ult = K φ q c(av) where: q c(av) = average CPT below depth of footing, z = B typically take K φ = 0.16 (Depends on B/D, footing shape and soil density) 17

18 Bearing Capacity on Sands - CPT DEEP SHALLOW Eslaamizaad and Robertson (1996) q ult = K q c Bearing Capacity Fine-grained soils (clays, etc.): Ultimate bearing capacity, q ult = K su q c(av) where: q c(av) = average CPT below depth of footing, z = B typically take K su = 0.30 (Depends on B/D, footing shape, OCR, and sensitivity) 18

19 5/2/2013 Worked Examples CPeT-IT (see petit) John Th. Ioannides Located in 5-star Mandarin Oriental Hotel Submit an abstract to CPT

USE OF CPT/CPTU FOR SOLUTION OF

USE OF CPT/CPTU FOR SOLUTION OF PRACTICAL PROBLEMS Tom Lunne, NGI USE OF CPT/CPTU FOR SULUTION OF PRACTICAL PROBLEMS Indirect design method: Interprete CPT/CPTU results to arrive at soil design parameters