A load-transfer function for the side resistance of drilled shafts in soft rock

Pouyan Asem, P. Gardoni

Research output: Contribution to journalArticle

Abstract

The shear stress and shear displacement relationship for the rock socket sidewalls is required for the calculation of the drilled shaft butt settlement under the service loads. This paper first introduces a comprehensive database of in situ axial load tests on drilled shafts, anchors and plugs that are embedded in different soft rock formations. The database includes measurements of (i) the initial shear stiffness, (ii) the peak shear stress and (iii) the post-peak reduction in shear stresses for the socket sidewalls. In addition to the load test results, information on soft rock mass mechanical properties and rock socket geometry is also included. It is found that (i) the initial shear stiffness is directly related to the deformation modulus of the soft rock mass and inversely proportional to the rock socket diameter and length, (ii) the mobilized peak shear stress is related to the drained friction angle of the rock mass and normal stress on the socket sidewalls at failure. The rock mass friction angle is related to the rock type and the geological strength index, and the normal stress at failure is directly related to deformation modulus of rock mass and inversely to the product of rock socket length and diameter, and (iii) the post-peak brittleness is related to the soft rock type and the post-peak shear displacement. An empirical framework for the prediction of the load-transfer function for side resistance of sockets in soft sedimentary and fine-grained rock is developed using the load test database introduced herein. The proposed framework accounts for the socket geometrical characteristics, and the rock mass engineering properties. The pre-peak range in the load-transfer function is modeled using a hyperbolic function, and the post-peak range is modeled using a brittleness index to account for the reduction in shear stresses with post-peak displacement.

Original languageEnglish (US)
JournalSoils and Foundations
DOIs
StatePublished - Jan 1 2019

Fingerprint

soft rock
transfer function
shaft
Transfer functions
Rocks
rock
shear stress
Shear stress
stiffness
friction
Brittleness
geotechnical property
Stiffness
Friction
anchor
Hyperbolic functions
mechanical property
Axial loads
Anchors
geometry

Keywords

  • Load test database
  • Load-transfer function
  • Rock socket
  • Settlement calculation
  • Side resistance
  • Soft rock

ASJC Scopus subject areas

  • Civil and Structural Engineering
  • Geotechnical Engineering and Engineering Geology

Cite this

A load-transfer function for the side resistance of drilled shafts in soft rock. / Asem, Pouyan; Gardoni, P.

In: Soils and Foundations, 01.01.2019.

Research output: Contribution to journalArticle

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abstract = "The shear stress and shear displacement relationship for the rock socket sidewalls is required for the calculation of the drilled shaft butt settlement under the service loads. This paper first introduces a comprehensive database of in situ axial load tests on drilled shafts, anchors and plugs that are embedded in different soft rock formations. The database includes measurements of (i) the initial shear stiffness, (ii) the peak shear stress and (iii) the post-peak reduction in shear stresses for the socket sidewalls. In addition to the load test results, information on soft rock mass mechanical properties and rock socket geometry is also included. It is found that (i) the initial shear stiffness is directly related to the deformation modulus of the soft rock mass and inversely proportional to the rock socket diameter and length, (ii) the mobilized peak shear stress is related to the drained friction angle of the rock mass and normal stress on the socket sidewalls at failure. The rock mass friction angle is related to the rock type and the geological strength index, and the normal stress at failure is directly related to deformation modulus of rock mass and inversely to the product of rock socket length and diameter, and (iii) the post-peak brittleness is related to the soft rock type and the post-peak shear displacement. An empirical framework for the prediction of the load-transfer function for side resistance of sockets in soft sedimentary and fine-grained rock is developed using the load test database introduced herein. The proposed framework accounts for the socket geometrical characteristics, and the rock mass engineering properties. The pre-peak range in the load-transfer function is modeled using a hyperbolic function, and the post-peak range is modeled using a brittleness index to account for the reduction in shear stresses with post-peak displacement.",
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