Hoyoung Seo
Purdue University, Civil Engineering Building, 550 Stadium Mall Drive, West Lafayette, IN 47907-2051, USA
Rodrigo Salgado
Purdue University, Civil Engineering Building, 550 Stadium Mall Drive, West Lafayette, IN 47907-2051, USA
Monica Prezzi
Purdue University, Civil Engineering Building, 550 Stadium Mall Drive, West Lafayette, IN 47907-2051, USA. Tel.: +1 765 494 5034; fax: +1 765 496 1364. E-mail addresses: mprezzi@ecn.purdue.edu (Corresponding author)
Pile foundations have been used in construction for thousands of years but only in the last few decades has there been significant progress in the technology of pile installation. This progress has not been matched by progress in the analysis and design of these foundation elements. This is in large part due to considerable difficulties in analyzing both pile installation and the response of piles to various types of loadings rigorously. Given these difficulties, the profession has in general used relatively crude design approaches. This is likely to change due to pressures from different directions, particularly the progress in code design (the push towards load and resistance factor design in geotechnical engineering requires a much better grip on all the factors that need to be considered in calculating pile resistances and what the uncertainties in quantities and analyses are, requiring sounder analytical frameworks) and economics (materials costs have started to rise, a trend that, if continued, would make it more economically interesting to have optimal designs).
If pile resistance is determined using sound mechanics from a soil model that captures the essence of soil load response without reliance on empiricism or direct correlations, the fundamental basis for rigorous analysis of pile load response is in place. We have shown this to be true for non-displacement piles.
For non-displacement piles installed in sand, the limit base resistance can be calculated with good accuracy using a cavity expansion-based analysis. We have shown using finite element analysis that the ratio of ultimate to limit base resistance is in the 0.13 (for dense sand) to 0.2 (for loose sand) range. The shaft resistance of non-displacement piles in sand is the product of a lateral stress ratio K by the vertical effective stress and the tangent of the critical-state friction angle. We have shown that K lies between just under K0 for loose sands to about twice K0 for dense sands.
For non-displacement piles installed in clay, numerical analyses (both finite element and limit analysis) have established accurate bounds on the values of limit unit base resistance. The ratio of net limit unit base resistance to undrained shear strength has been shown convincingly to be in the 10 to 13.5 range. The same ratio for ultimate instead of limit resistance would be less than that; some authors suggest numbers just under 10. The shaft resistance is most frequently calculated using the α method. We have shown that the value of α can be determined analytically if the critical-state and residual friction angles, as well as the relationship between the residual friction angle and the normal effective stress on the shearing plane, are known. The analysis can also be used to determine α for soils containing clay contents greater than 25%.
In part because of the absence of realistic analysis allowing calculation of settlement given an axial load on the pile, design has relied on calculations of ultimate resistances reduced by factors of safety that would indirectly prevent settlement-based limit states. However, recent work suggests realistic analyses of pile settlement will soon be available, facilitating cost-effective pile design. Explicit analytical solutions for a vertically loaded pile subjected to compressive or tensile loadings and embedded in a multi-layered elastic soil were presented. The solutions satisfy the boundary conditions of the problem. The governing differential equations are derived based on the principle of minimum potential energy and calculus of variations. The integration constants are determined using Cramer’s rule and a recurrence formula. The solutions allow calculation of the vertical pile displacement as a function of depth, the load transferred to the pile shaft at any depth, and the vertical soil displacement as a function of the radial direction at any depth if the following is known: radius, length and Young’s modulus of the pile, Poisson’s ratio and Young’s modulus of the soil in each layer, thickness of each soil layer, number of soil layers, and applied load. The use of the analysis was illustrated by obtaining load-transfer and load-settlement curves for a case discussed in the literature for which pile load test results are available.
Read more: Monica Prezzi
Purdue University, Civil Engineering Building, 550 Stadium Mall Drive, West Lafayette, IN 47907-2051, USA
Rodrigo Salgado
Purdue University, Civil Engineering Building, 550 Stadium Mall Drive, West Lafayette, IN 47907-2051, USA
Monica Prezzi
Purdue University, Civil Engineering Building, 550 Stadium Mall Drive, West Lafayette, IN 47907-2051, USA. Tel.: +1 765 494 5034; fax: +1 765 496 1364. E-mail addresses: mprezzi@ecn.purdue.edu (Corresponding author)
Pile foundations have been used in construction for thousands of years but only in the last few decades has there been significant progress in the technology of pile installation. This progress has not been matched by progress in the analysis and design of these foundation elements. This is in large part due to considerable difficulties in analyzing both pile installation and the response of piles to various types of loadings rigorously. Given these difficulties, the profession has in general used relatively crude design approaches. This is likely to change due to pressures from different directions, particularly the progress in code design (the push towards load and resistance factor design in geotechnical engineering requires a much better grip on all the factors that need to be considered in calculating pile resistances and what the uncertainties in quantities and analyses are, requiring sounder analytical frameworks) and economics (materials costs have started to rise, a trend that, if continued, would make it more economically interesting to have optimal designs).
If pile resistance is determined using sound mechanics from a soil model that captures the essence of soil load response without reliance on empiricism or direct correlations, the fundamental basis for rigorous analysis of pile load response is in place. We have shown this to be true for non-displacement piles.
For non-displacement piles installed in sand, the limit base resistance can be calculated with good accuracy using a cavity expansion-based analysis. We have shown using finite element analysis that the ratio of ultimate to limit base resistance is in the 0.13 (for dense sand) to 0.2 (for loose sand) range. The shaft resistance of non-displacement piles in sand is the product of a lateral stress ratio K by the vertical effective stress and the tangent of the critical-state friction angle. We have shown that K lies between just under K0 for loose sands to about twice K0 for dense sands.
For non-displacement piles installed in clay, numerical analyses (both finite element and limit analysis) have established accurate bounds on the values of limit unit base resistance. The ratio of net limit unit base resistance to undrained shear strength has been shown convincingly to be in the 10 to 13.5 range. The same ratio for ultimate instead of limit resistance would be less than that; some authors suggest numbers just under 10. The shaft resistance is most frequently calculated using the α method. We have shown that the value of α can be determined analytically if the critical-state and residual friction angles, as well as the relationship between the residual friction angle and the normal effective stress on the shearing plane, are known. The analysis can also be used to determine α for soils containing clay contents greater than 25%.
In part because of the absence of realistic analysis allowing calculation of settlement given an axial load on the pile, design has relied on calculations of ultimate resistances reduced by factors of safety that would indirectly prevent settlement-based limit states. However, recent work suggests realistic analyses of pile settlement will soon be available, facilitating cost-effective pile design. Explicit analytical solutions for a vertically loaded pile subjected to compressive or tensile loadings and embedded in a multi-layered elastic soil were presented. The solutions satisfy the boundary conditions of the problem. The governing differential equations are derived based on the principle of minimum potential energy and calculus of variations. The integration constants are determined using Cramer’s rule and a recurrence formula. The solutions allow calculation of the vertical pile displacement as a function of depth, the load transferred to the pile shaft at any depth, and the vertical soil displacement as a function of the radial direction at any depth if the following is known: radius, length and Young’s modulus of the pile, Poisson’s ratio and Young’s modulus of the soil in each layer, thickness of each soil layer, number of soil layers, and applied load. The use of the analysis was illustrated by obtaining load-transfer and load-settlement curves for a case discussed in the literature for which pile load test results are available.
Read more: Monica Prezzi
No comments:
Post a Comment