{"id":473,"date":"2025-03-07T05:18:16","date_gmt":"2025-03-07T05:18:16","guid":{"rendered":"https:\/\/libraryresources.nse.org.ng\/fundamentalsoffoundationengineering\/chapter\/6-12-ultimate-geotechnical-strength-of-piles-subjected-to-axial-compressive-load-from-cpt-test-results\/"},"modified":"2026-03-16T14:08:22","modified_gmt":"2026-03-16T14:08:22","slug":"6-12-ultimate-geotechnical-strength-of-piles-subjected-to-axial-compressive-load-from-cpt-test-results","status":"publish","type":"chapter","link":"https:\/\/libraryresources.nse.org.ng\/fundamentalsoffoundationengineering\/chapter\/6-12-ultimate-geotechnical-strength-of-piles-subjected-to-axial-compressive-load-from-cpt-test-results\/","title":{"raw":"6.12 Ultimate geotechnical strength of piles subjected to axial compressive load from CPT test results","rendered":"6.12 Ultimate geotechnical strength of piles subjected to axial compressive load from CPT test results"},"content":{"raw":"Initially, the cone penetration test was developed exactly for the estimation of the collapse load of driven piles, and several methodologies have been proposed for the estimating pile capacity on the basis of measurements obtained from CPT tests performed at the proposed pile location. Most of these methods are based on determining the collapse load as the sum of the skin friction resistance along the pile shaft and the end-bearing resistance at the pile toe, similar to the \u03b1<\/em>-<\/em> and \u03b2<\/em>-Method. As consequence the two key quantities that the Geotechnical Engineer is called to calculate is the (depth-dependent) skin friction stress along the pile, and the end-bearing resistance in terms of stress. It is not the aim of this section to provide a critical analysis of different methods, and endorse one over the other. The interested reader is referred to Schneider et al.<\/em> (2008), Lehane et al.<\/em> (2005a) and various other publications for a discussion on the advantages and prediction capabilities of the methods. Here the most widely used methods are presented, in chronological order, starting with Schmertmann\u2019s method.\n\n
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6.12.1 Schmertmann (1978) method for calculating the skin friction resistance<\/h2>\nDespite being proposed almost 50 years ago, and the publication of several more refined methods since, Schmertmann\u2019s (1978) method for estimating the skin friction resistance is still applied in practice, perhaps owing to its simplicity as well as its consideration of different pile materials. According to Schmertmann, the skin friction resistance \u0394Qsf<\/sub><\/em> (units: force\/unit pile length) of a pile segment with length dL<\/em> can be correlated to the sleeve penetration resistance from a CPT test fs<\/sub><\/em> measured at the same depth as:\n\n(6.23)<\/strong> [latex]\\Delta {Q_{sf}} = {f_{sf}}\\left( {\\pi D} \\right)\\Delta L = \\left( {\\alpha '{f_s}} \\right)\\left( {\\pi D} \\right)dL[\/latex]\n\nwhere \u03b1\u2032 <\/em>is a correction factor that depends on the material of the pile and the embedment ratio z\/D <\/em>for piles driven in coarse-grained soils, or the ratio of the CPT sleeve friction resistance over the atmospheric pressure fs<\/sub>\/pa<\/sub><\/em> in fine-grained soils (Figure 6.23<\/a>).\n\nInstead of using the charts provided in Figure 6.23<\/a>, one can use the following curve-fitting formulas for the estimation of the factor \u03b1<\/em>\u2032<\/em>:\n\nFor concrete piles in coarse-grained soils (Figure 6.23a<\/a>):\n\n(6.24)\u00a0<\/strong>[latex]\\alpha ' = 1.85 - 0.1\\left( {\\dfrac{z}{D}} \\right) + 0.0025{\\left( {\\dfrac{z}{D}} \\right)^2} - 0.259 \\times {10^{ - 4}}{\\left( {\\dfrac{z}{D}} \\right)^3}[\/latex]\n\nFor steel piles in coarse-grained soils (Figure 6.23a<\/a>):\n\n(6.25)\u00a0<\/strong>[latex]\\alpha ' = 3.88 - 0.482\\left( {\\dfrac{z}{D}} \\right) + 0.0282{\\left( {\\dfrac{z}{D}} \\right)^2} - 0.718 \\times {10^{ - 3}}{\\left( {\\dfrac{z}{D}} \\right)^3} + 0.668 \\times {10^{ - 5}}{\\left( {\\dfrac{z}{D}} \\right)^4}[\/latex]\n\nFor concrete\/timber piles in fine-grained soils (Figure 6.23b<\/a>):\n\n(6.26)\u00a0<\/strong>[latex]\\alpha ' = 1.28 - 1.473\\left( {\\dfrac{{{f_s}}}{{{p_a}}}} \\right) + 0.839{\\left( {\\dfrac{{{f_s}}}{{{p_a}}}} \\right)^2} - 0.1634{\\left( {\\dfrac{{{f_s}}}{{{p_a}}}} \\right)^3}[\/latex]\n\nFor steel piles in fine-grained soils (Figure 6.23b<\/a>):\n\n(6.27)\u00a0<\/strong>[latex]\\alpha ' = 1.29 - 1.606\\left( {\\dfrac{{{f_s}}}{{{p_a}}}} \\right) + 0.846{\\left( {\\dfrac{{{f_s}}}{{{p_a}}}} \\right)^2} - 0.159{\\left( {\\dfrac{{{f_s}}}{{{p_a}}}} \\right)^3}[\/latex]\n\n[caption id=\"attachment_471\" align=\"aligncenter\" width=\"900\"]\"Figure Figure 6.23. Correction factor \u03b1\u2032<\/em> for the estimation of the skin friction resistance from CPT test measurements for (a) coarse-grained soils, and (b) fine-grained soils, under short-term undrained loading conditions.[\/caption]\n\nAccordingly, the total pile skin friction resistance is calculated as the sum of the unit friction resistance for all pile segments dL<\/em>:\n\n(6.28)<\/strong> [latex]{Q_{sf}} = \\int\\limits_0^L {\\alpha '{f_s}\\left( z \\right)} \\pi Ddz[\/latex]\n\n
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6.12.2 LCPC method (Bustamante and Gianeselli 1982)<\/h2>\nAnother widely used method is the one proposed by Bustamante and Gianeselli (1982), also know as the LCPC method. It is based on the statistical analysis of 197 pile load tests covering a wide range of soil conditions, and is applicable to both drilled shafts and driven piles.\n\nThe end-bearing resistance is estimated with the LCPC method as:\n\n(6.29)<\/strong> [latex]{Q_b} = {q_{c\\left( {eq} \\right)}}{k_c}{A_b}[\/latex]\n\nwhere qc(eq) <\/sub><\/em>is an equivalent average cone resistance in the vicinity of the pile toe from the CPT test, calculated as shown below; kc <\/sub><\/em>is a bearing capacity factor from Table 6.5 or, according to Briaud and Miran (1992), can be assumed equal to kc <\/sub><\/em>= 0.6 for clays and silts and kc <\/sub><\/em>= 0.375 for sands; Ab<\/sub><\/em> is the area of the pile base.\n\nThe equivalent average cone resistance qc(eq) <\/sub><\/em>is estimated as illustrated in Figure 6.24<\/a> (Das 2007):\n