Rock-Socketed Shafts for Highway Structure Foundations (2006) / Chapter Skim
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Pages 53-69
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From page 53... ...
Axial loading in both compression and uplift requires structural analysis and reinforced-concrete design for drilled shafts. When lateral loading is not significant, structural design is reasonably straightforward.
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From page 54... ...
for reinforced-concrete shafts. The elastic continuum approach for laterally loaded deep foundations was developed by Poulos (1971)
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From page 55... ...
. Very few lateral load tests on drilled shafts in rock, with the instrumentation necessary to back-calculate p-y curves, 56 have been conducted and published to date.
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From page 56... ...
Comparison of pile head deflections measured during the load test and from p-y analyses are shown in Figure 38. With a constant value of EI as given above, the analytical results show close agreement with the measured displacements up to a lateral load of about 350 kN.
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From page 57... ...
The authors recommend verification by load testing if deflections exceed 0.04% of the shaft diameter, which would exceed service limit state criteria in most practical situations. Brittle fracture of the rock is assumed if the resistance p becomes greater than the shaft diameter times one-half of the uniaxial compressive strength of the rock.
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From page 58... ...
The model was then used to make forward predictions of lateral load response for subsequent load tests on socketed shafts at two locations in weathered rock profiles different than those used to develop the model. The procedure for establishing values of subgrade modulus Kh involves determination of the rock mass modulus (EM)
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From page 59... ...
The limit normal stress is estimated on the basis of Hoek– Brown strength parameters as determined through correlations with RMR and GSI, and is given by (113) in which γ ' = effective unit weight of the rock mass, z = depth from the rock mass surface, and the coefficients mb, s, and a are the Hoek–Brown coefficients given by Eqs.
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From page 60... ...
Both methods incorporate the Hoek–Brown strength criterion for rock mass to evaluate the limit normal stress pL, and both rely on correlations with GSI to determine the required Hoek–Brown strength parameters. In the Liang and Yang (2006)
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From page 61... ...
were subjected to lateral loading. Strain gage measurements were used to back-calculate p-y curves, which are presented in normalized form, with p normalized by shaft diameter and rock compressive strength (p/Bqu)
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From page 62... ...
The analysis is approached by structural decomposition of the shaft and its loading, as shown in Figure 46b. It was assumed that the magnitude of applied lateral loading is sufficient to cause yielding within the soil from the ground surface to the top of the rock mass.
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From page 63... ...
For the applied lateral load H and moment M, the shaft is analyzed by assuming the soil and rock mass are elastic, and the lateral reaction force p of the soil and rock mass along the shaft is determined by solution of the governing differential equation and boundary conditions at the head of the shaft.
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From page 64... ...
Predictions using the proposed method are compared with results of field load tests reported by Frantzen and Stratten (1987) for shafts socketed into sandy shale and sandstone.
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From page 65... ...
All of the currently available methods suffer from a lack of field data for verification and are best applied in conjunction with local and agency experience, thorough knowledge of the geologic environment, and field load testing. STRUCTURAL ISSUES Twenty of the questionnaire responses indicated that structural design of drilled shaft foundations is carried out by engineers in the Bridge Design or Structures Division of their state DOTs.
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From page 66... ...
. Article 10.8.3.9.3 states the following: Where the potential for lateral loading is insignificant, drilled shafts may be reinforced for axial loads only.
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From page 67... ...
L EI VV V θ θ = ( ) ⎡ ⎣⎢ ⎤ ⎦⎥ 2 1 2 68 Moment Transfer Rock sockets subjected to high lateral and/or moment loading require a minimum depth of embedment to transfer moment to the rock mass and to satisfy minimum development length requirements for reinforcing steel.
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From page 68... ...
Each option has advantages and disadvantages. Two variables that can be adjusted to increase shear resistance are concrete 28-day compressive strength, f 'c, and shaft diameter, B
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From page 69... ...
SUMMARY Lateral loading is a major design consideration for transportation structures and in many cases governs the design of rock-socketed drilled shafts. Design for lateral loading must satisfy performance criteria with respect to (1)
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