Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities

230
Figure A Values of coefficients B for soil resistance vs. depth (after Reese et al. 1974).
A.5 P-Y CURVES FOR WEAK ROCK (REESE 1997)
Reese (1997) proposed a p-y criterion for drilled shafts socketed in weak rock, defined as having an unconfined compressive strength 0.5 MPa ≤ q
u
≤ 5 MPa (5.2 tsf ≤ q
u
≤ 52 tsf). In this criterion, the ultimate resistance of the weak rock is a function of the uniaxial unconfined compressive strength of intact rock, the drilled shaft diameter, rock quality designation (RQD), and depth.

231
Figure A Basic shape of p-y curve for weak rock (Reese 1997).
The shape of the Reese (1997) p-y curve is shown in Figure A. The ultimate reaction, p
u
(F/L), of rock was given by for 0 ≤ z r
≤ D
𝑝𝑝
𝑢𝑢
= 𝛼𝛼
𝑐𝑐
𝜎𝜎
𝑐𝑐𝑖𝑖
𝐷𝐷 �1 + 1.4
𝑘𝑘
𝑐𝑐
𝐷𝐷� Equation A) for z r D
𝑝𝑝
𝑢𝑢
= 5.2𝛼𝛼
𝑐𝑐
𝜎𝜎
𝑐𝑐𝑖𝑖
𝐷𝐷 Equation A) where σ
ci
is the uniaxial unconfined compressive strength of intact rock α
r
is the strength reduction factor, which is used to account for fracturing of rock mass D is the diameter of the drilled shaft and z
r
is the depth below rock surface. α
r
is assumed to be 0.33 for RQD = 100 percent and to increase linearly up to 1.0 for RQD = 0. The slope of the initial portion of the p-y curve was given by
𝐾𝐾
𝑖𝑖𝑐𝑐
= Equation A) where K
ir
= initial tangent to p-y curve E
m
= deformation modulus of rock masses, which could be obtained from a pressuremeter or dilatometer test and k
ir
= dimensionless constant. The expressions for k
ir
, derived by correlation with experimental data, areas follows
𝑘𝑘
𝑖𝑖𝑐𝑐
= �100 +
400𝑘𝑘
𝑐𝑐
3𝐷𝐷
� for 0 ≤ z r
≤ D Equation A)
𝑘𝑘
𝑖𝑖𝑐𝑐
= 500 Equation AA complete description of the interim p-y criteria maybe summarized as follows First segment