The National Academies Press

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Pages 32-52

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From page 32... ... Base capacity, analyzed as a bearing capacity problem, may require uniaxial compressive strength of intact rock, shear strength of discontinuities, or the Hoek–Brown strength parameters of fractured rock mass, depending upon the occurrence, orientation, and condition of joint surfaces in the rock mass below the base. For analysis of axial load-displacement response, the rock mass modulus is required. Read the entire page →
From page 33... ... where EM and ν are the rock mass modulus and Poisson's ratio, respectively; Δr is the dilation, and r is the original shaft radius. A normal stiffness K can be defined as the ratio of normal stress increase to dilation, as follows: (20) Read the entire page →
From page 34... ... Fundamentally, this can be explained by the higher load transfer in side shear reducing the proportion of load transferred to the base. Because side resistance increases with interface roughness, rock sockets with higher interface roughness will transfer a higher proportion of load in side resistance than smooth sockets. Read the entire page →
From page 35... ... reported a case in which the bearing stress at the base of an instrumented rock socket increased, at a steadily decreasing rate, over a period of 4 years; although the total applied head FIGURE 22 Effect of rock mass modulus at base on axial load transfer (Wyllie 1999, based on Osterberg and Gill 1973) Read the entire page →
From page 36... ... to obtain an average value of unit side resistance at failure: fsu = Qs/As (23) Q f BLs su= × π Q f dA B fdzs su L= =∫ ∫surface π 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 1 2 3 4 5 6 7 8 9 10 L/B Q b /Q t O-cell, good base O-cell, base disturbed top load test FIGURE 23 Base load transfer interpreted from load tests (data from Crapps and Schmertmann 2002) Read the entire page →
From page 37... ... (1992) consisting of 47 load tests to failure from 23 different Florida limestone sites. Read the entire page →
From page 38... ... 30 with C = 1 for predicting side resistance of normal rock sockets for drilled shafts. The authors also note the importance of using compressive strength values (qu) Read the entire page →
From page 39... ... qu (32) SRC = + η ν c s n r d1 Δ 40 in which ηc = construction method reduction factor, as defined in Table 19; n = ratio of rock mass modulus to uniaxial compressive strength of intact rock (EM/qu) Read the entire page →
From page 40... ... Based on a parametric finite-element study and a database of 14 case histories consisting of fullscale load tests and field pullout tests, the following expression was found to provide a reasonable estimate of ultimate unit side resistance: (33) In which qu = uniaxial compressive strength and qt = split tensile strength. Read the entire page →
From page 41... ... Base Resistance Load transmitted to the base of a rock-socketed shaft, expressed as a percentage of the axial compression load applied α α φ = 26 30 tan tan rc o 0.61 m grout plug w/ #9 wire cage drilled hole, 165 mm dia top of rock centerhole jack nut plate timber 200-mm dia casing 0.76 m cored hole, 140 mm dia steel bearing plate 35 mm dia threaded bar FIGURE 26 Small-scale pullout test used in Florida limestone (after Crapps 1986) Read the entire page →
From page 42... ... Load tests, described in chapter five, provide a means to determine the effects of construction on base load transfer. The ultimate base resistance of a rock-socketed drilled shaft, Qb, is the product of the limiting normal stress, or bearing capacity, qult, at the base and the cross-sectional area of the shaft base (Ab) Read the entire page →
From page 43... ... suggest that rock mass cohesion in Eq. 53 can be approximated as 0.1qu, where qu = uniaxial compressive strength of intact rock. Read the entire page →
From page 44... ... From chapter two, the strength criterion is given by (55) where σ1' and σ3' = major and minor principal effective stresses, respectively; qu = uniaxial compressive strength of intact rock; and mb, s, and a are empirically determined strength parameters for the rock mass. Read the entire page →
From page 45... ... 44 through 59 and depicted in Figure 28 have been evaluated and verified against results of full-scale field load tests on rock-socketed drilled shafts. The primary reason for this is a lack of load test data accompanied by sufficient information on rock mass properties needed to apply the models. Read the entire page →
From page 46... ... Design of most sockets is governed by the requirement to limit settlement to a specified allowable value. The problem of predicting vertical displacement at the top of a rock socket has been studied through theoretical and numerical analyses along with limited results from full-scale field load testing. Read the entire page →
From page 47... ... When clean base conditions during construction can be verified and instrumentation is provided for measuring base load, a complete socket is assumed. Frequently, however, base resistance is eliminated by casting the socket above the base of the drilled socket, in which case the test shaft is modeled as a shear socket. Read the entire page →
From page 48... ... (73) where Gr = elastic shear modulus of rock mass. Read the entire page →
From page 49... ... In this case, the ultimate side resistance is recommended for design at the strength limit state, unless "progressive side shear failure could occur," in which case the resistance should be reduced "according to the judgment of the geotechnical engineer." Several states indicate in their response to Question 20 that load testing, especially using the Osterberg load cell, is one of the ways in which the issue is addressed. Load tests that provide independent measurements of side and base resistance as a function of displacement and that are carried to large displacements provide the best available data for establishing resistance values. Read the entire page →
From page 50... ... . The equations Method/Condition Resistance Factor Nominal Axial Compressive Resistance of Single-Drilled Shafts Side resistance in rock Tip resistance in rock Side resistance, IGMs Tip resistance, IGMs Static load test Compression, all materials 1. Read the entire page →
From page 51... ... AASHTO also allows higher resistance factors on both side and base resistances when they are determined from a field load test. The cost benefits achieved by using a load test as the basis for design can help to offset the costs of conducting load tests. Read the entire page →
From page 52... ... Methods for calculating nominal (ultimate) unit side and base resistances and associated resistance factors according to the Interim 2006 AASHTO LRFD Bridge Design Specifications are summarized in Table 20. Read the entire page →

From page 32...

... Base capacity, analyzed as a bearing capacity problem, may require uniaxial compressive strength of intact rock, shear strength of discontinuities, or the Hoek–Brown strength parameters of fractured rock mass, depending upon the occurrence, orientation, and condition of joint surfaces in the rock mass below the base. For analysis of axial load-displacement response, the rock mass modulus is required.