Case history of axial pile capacity and load-settlement response by SCPTu

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1 Case history of axial pile capacity and load-settlement response by SCPTu F.S. Niazi & P.W. Mayne Georgia Institute of Technology, Atlanta, Georgia, USA D.J. Woeller ConeTec Investigations Ltd., Richmond, British Columbia, Canada ABSTRACT: The performance of pile foundations under axial compression loading can be rationally evaluated within an elastic continuum framework using field results from seismic piezocone tests (SCPTu). The SCPTu is an optimal means for collection of subsurface information because it captures up to 5 independent readings in one sounding: tip resistance (q t ), sleeve friction (f s ), tip or mid-face porewater pressure (u 1 ) and/or shoulder porewater pressure (u 2 ), and shear wave velocity (V s ). The measurements obtained are at opposite ends of the stress-strain-strength curves: the peak strength and stress state for capacity interpretations and the small-strain stiffness (G max ) for evaluating the initial deformations. Using a versatile Randolph-type elastic pile model, the approach can be applied to evaluate the axial response of pile load tests. The axial load distribution within the shaft is also evaluated. A case study is presented illustrating application of this approach on a bored cast in situ pile tested in glacial till near Grimsby, UK. 1 INTRODUCTION Deep foundations in the form of driven piles and drilled shafts are commonplace for large scale projects including high-rise buildings and multi-span bridges. Conventional investigation methods for subsurface characterization of deeper soil layers affecting the performance of these foundations (boring and sampling) are time consuming and tedious. The results are constrained by the effects of sample disturbance. Insitu tests using SCPTu provide an alternate, yet quick and reliable means of obtaining soil engineering parameters of the soil layers of interested depth. For specific sites the SCPTu provides up to 5 independent readings with depth in one sounding: q t, f s, u 1 and/or u 2, and V s. The recent utilization of the SCPTu offers improved interpretation procedures and correlations for the soil engineering parameters (Mayne 2005, Schnaid 2005). A multiple-readings based methodology has been devised for the analysis of pile foundations, as opposed to the older methods, many of which were based solely on q c alone. The measured q t is also corrected for u 2 acting behind the tip (Lunne et al. 1997). The penetrometer readings (q t, f s, u 1 and u 2 ) and downhole geophysics (V s ) are at the opposite ends of the stress-strain-strength curves: the peak strength for capacity evaluation and the fundamental stiffness (G max or E max ) for the initial soil de-

2 formations ( s < 10-6 %). These results have been successfully applied within the elastic theory (Poulos & Davis 1980, Randolph & Wroth 1978, 1979, Fleming et al. 1992) to evaluate the entire load-displacement-capacity behavior for axial loading performance of deep foundations (e.g., Mayne & Elhakim 2002, Mayne & Zavala 2004, Mayne & Woeller 2008, Mayne & Niazi 2009). Pile foundations analysis can be accomplished using the rational CPT assessments or the direct CPT approach, or a combination of both (Mayne 2007). 2 SOIL PARAMETER ASSESSMENTS BY CPT In rational approach, the SCPTu data are used to classify the soil types, calculate soil engineering parameters, and evaluate pile capacity. 2.1 Soil Classification For general use, the method based on the normalized CPTu parameters provides a soil classification index (I c ) (Jefferies & Been 2006): I c = {3 log [Q 1 B q + 1]} 2 + [ logf ] 2 (1) where Q = (q t vo )/ vo ', B q = (u 2 u o )/(q t vo ), F = f s 100/(q t vo ), and vo ' is the effective overburden stress = vo u o. As per Robertson (1990), the ranges of I c values for different soil types are as follows; organic clay soils (zone 2): I c > 3.22; clays (zone 3): 2.82 < I c < 3.22; silt mixtures (zone 4): 2.54 < I c < 2.82; sand mixtures (zone 5): 1.90 < I c < 2.54; sands (zone 6): 1.25 < I c < 1.90; and gravelly sands (zone 7): I c < Soil Engineering Parameters The unit weight ( t ) of soil layers needed to calculate the overburden stress ( vo ), frictional characteristics ( '), shear wave velocity (V s ) for calculating fundamental stiffness (G max ) and the equivalent Young s Modulus (E max ), stress history ( p '), geostatic stress state (K o = ho '/ vo '), and undrained shear strength s u of clays can be evaluated from the CPT data using the relationships already well documented (e.g., Mayne 2007; Mayne et al. 2009; and 2010). The (s u / vo ') NC for direct simple shear (DSS) mode has been shown suitable for direct use in the analysis of foundation (Ladd 1991). Values of (s u / vo ') OC for DSS mode can be obtained using appropriate equations. Fissured soils exhibit half the strengths associated with those of intact clays (Mayne 2007). G max represents the elastic region of soil behavior. The value of G must be reduced corresponding to strains for the applicable loads to utilize in the elastic continuum equations. Fahey & Carter (1993) proposed the following algorithm: G/G max = 1 (1 / FS) g (2) where 1 / FS (reciprocal of factor of safety) = Q / Q ult is the mobilized load level and g is a fitted exponent = 0.3 +/ 0.1 for well behaved soils (Mayne 2007).

3 2.3 Capacity Evaluation The pile unit side resistance (f p ) and end bearing (q b ) can be obtained using relationships presented by Kulhawy et al. (1983), Lee et al. (2003), and Vesić (1977). The axial compression capacity (Q t = Q ult ) of a circular pile foundation is calculated from: Q t = Q s + Q b = (f pi d z i ) + q b d 2 / 4 W p (3) where f pi is the unit side resistance at i th soil layer, d zi is the shaft area of the i th soil layer, and W p = the pile weight. 3 DIRECT CPT METHOD From the many available direct CPT methods, selected ones for the bored piles (relevant to the case study presented herein) are presented by Bustamante & Gianeselli (1982); Eslami & Fellenius (1997); Takesue et al. (1998); Jamiolkowski (2003) and Chen & Kulhawy (1994). 4 ELASTIC CONTINUUM SOLUTION The axial load-displacement response and the fraction of load transfer to the pile base (Q b ) for top-down loading can be evaluated using an analytical closed-form elastic continuum pile solution summarized in Figure 1 (after Randolph 2003, and Fleming et al. 1992). This solution can accommodate soil models with constant G (homogeneous soils) or soils having a linearly-varying G with depth (Gibson-type soils). The pile can be either a floating-type pile (G sl = G sb ) or end-bearing type where the pile base rests on a stiffer stratum (G sb > G sl ). Figure 1 also presents the stacked pile model for layered soils. 5 PILE LOAD TEST AT GRIMSBY RESEARCH SITE, UK The Grimsby research site was located near Waltham, Grimsby, UK, 900 m north of the nearest watercourse and 7.5 km southwest of the nearest coastline (Brown 2004). Brown et al. (2006) report the ground conditions at the site as matrix-dominant glacial till underlain by cretaceous chalk bedrock; till being cohesive, overconsolidated stiff to firm, grayish to dark brown, predominantly silty clay with cobbles, boulders and pebbles. Index properties include liquid limit: 20 36%, plastic limit: 12 18%, moisture content: 14 24%, specific gravity: 2.69, and clay fraction: 20 38%. Prior to the load test on a m deep and 0.6-m diameter bored pile, extensive site and laboratory investigations were conducted. Of interest is a 20 m deep SCPTu sounding (see Figure 2). The measured u 1 readings were converted to u 2 via the relationship: u 2 = u 1 (Chen & Mayne 1994). Brown (2004) reports of the loss of saturation of piezo-element in the upper layers for which probe was withdrawn and hole filled with water before resumption of the operation. The related effects on the u 2 profile can be observed in Figure 2. The u 2 profile has been reconstructed by averaging B q values between 9.7 and 20 m and applying it to depths: 4.4 to 9.7 m.

4 The pile testing program at Grimby research site was designed to compare the results from rapid and static load tests: rapid load test (RLT) being performed first, followed by constant rate of penetration (CRP) test at 0.01 mm/s and maintained load test (MLT) (Brown et al. 2006). For this study, only the measured results of CRP test from Brown (2004) are considered, as presented in the subsequent section. Figure 1. Elastic pile solution. Figure 2. SCPTu sounding for pile load test at Grimsby research site, UK (after Brown 2004).

5 6 PILE RESPONSE EVALUATION Figure 3 presents the soil classification and engineering parameters obtained by the post processing of SCPTu data using appropriate correlations. To validate the applicability of SCPTu based correlations, the measured values from Brown (2004) were also plotted. For most part, the values obtained from the correlations match closely with the measured values. Certain disagreements observed in the top layers may be attributed to the fact reported by Brown (2004) that the sounding was disrupted due to loss of saturation of the piezo-element and the final CPT data were formulated by a combination from the pre-drilled sounding (0 2.5 m) and the resumed operation after re-saturation. The B q values obtained for certain depths were outside the applicable range for use in appropriate correlation for estimating ' for clays. So the ' profile was also obtained using correlation developed for sands, and the mean ratio (1.3) of the ' from the two relationships over the remaining depths was applied to obtain applicable values. Figure 3. Soil parameters evaluation at Grimsby research site, UK.

6 Treating the stratum at the site as layered soil, the ultimate capacities were estimated for each layer. Figure 4 shows the individual load-settlement response based on the respective G max and the capacities thus calculated. f p and q b for each defined layer were estimated from the various CPTu based methods. Summarized forms of the results are presented in Table 1. The KTRI method tends to overestimate Q s, while the LCPC method yielded the least estimates for Q s and Q b in this case. Summation of the mean values of Q s and Q b found from different CPTu based methods (2386 kn) tends to match the measured Q t. Using hyperbolic fitting model proposed by Kondner (1963), the estimated value of Q ult ( kn) also goes with closely to the measured Q t. Since Q b values from all three methods shown in Table 1 yielded closer match, the maximum of these (307 kn) was selected. The difference of the estimated Q ult from the hyperbolic fitting model and the selected Q b was taken as the appropriate value for Q s ( kn). Cumulative load-settlement curves (Q t vs. w t, Q s vs. w t and Q b vs. w t ) in Figure 5 shows a fine match with the measured response. Table 1. Calculated pile capacities from direct and rational CPT methods. Method Tip capacity, Q b (kn) Shaft capacity, Q s (kn) f p1 ( m) Mean unit skin friction (kpa) f p2 ( m) f p3 ( m) f p4 ( m) f p5 ( m) LCPC Unicone KTRI Limit plasticity Beta Alpha Figure 4. Pile shaft resistance and load distribution (SCPTu based estimations vs. measured); and Load-settlement curves based on respective G max and Q t (elastic continuum solution).

7 Figure 5. Load-settlement response (elastic continuum solution). 7 CONCLUSION Elastic continuum solution provides a rational framework for evaluating field load test results on axially-loaded pile foundations. Various CPT based methods have been revisited, along with their application to load test on a m deep and 0.6-m diameter bored pile in clay till at the Grimsby research site, UK. Rational CPT methods with -analysis have shown closest agreement with the measured results. Geotechnical soil parameters for the evaluation of deep foundations are conveniently obtained from multi-independent readings (qt, fs, Vs, u 1, u2) taken during SCPTu sounding conducted at the site. 8 ACKNOWLEDGEMENT The authors are thankful to Dr. M. J. Brown at the Division of Civil Engineering, University of Dundee, UK for being forthcoming in providing the required data/information pertaining to the pile load test at Grimsby research site. PREFERENCES Brown, M.J The rapid load testing of piles in fine grained soils. Ph.D. Thesis, University of Sheffield, UK: Brown, M.J., Hyde, A.F.L. & Anderson, W.F Analysis of a rapid load test on an instrumented bored pile in clay. Geotechnique 56 (9): Bustamante, M., & Gianeselli, L Pile bearing capacity predictions by means of static penetrometer CPT. Proc., 2 nd European Symp. on Penetration Testing, Vol. 2, Amsterdam: Chen, B.S.Y. & Mayne, P.W Profiling the overconsolidation ratio of clays by piezocone tests. Report GIT-CEEGEO-94-1, Civil Engineering, Georgia Institute of Technology, Atlanta, 280 p. Chen, Y.J. & Kulhawy, F.H Case history evaluation of the behavior of drilled shafts under axial and lateral loading. EPRI TR , Electric Power Research Institute: Eslami, A. & Fellenius, B.H Pile capacity by direct CPT and CPTu methods applied to 102 case histories. Canadian Geotechnical J., 34 (6):

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