P-y curve model of weak rock - Reese (1997)

P-Y curves for weak rock in the PileLAT program are calculated using the method proposed by Reese (1997) and shown in the figure below.

P-Y curve model of weak rock - Reese (1997)

The ultimate resistance of weak rock is determined according to the following equations:

where Pur is ultimate soil resistance per unit length; αr is the strength reduction factor; Qur is the unconfined compressive strength of rock; D = pile diameter and Xr = depth below rock surface.

The lateral resistance-deflection (P-Y) relationship for weak rock is represented by a three-segment curve. The relationship is described by:

YA can be found by solving the following equation:

The initial modulus / slope of the P-Y curve, Mir can be determined by:

where Em is the rock mass modulus and kir is a dimensionless factor calculated by:

The parameter Ym can be determined by:

where krm is a dimensionless constant and normally ranges from 0.0005 to 0.00005 in the analysis.

In the PileLAT program, the required input parameters for p-y curve of weak rock are shown in the figure below:

PileLAT Inputs for Weak Rock - Reese (1997)

Total Unit Weight - the total unit weight of the soil material

UCS - unconfined compressive strength (kPa)

UCS Strength – Rate of change - the increase rate of the unconfined compressive strength (UCS) with the depth from the top of the soil layer

Empirical Factor (krm) - Dimensionless constant ranging from 0.00005 to 0.0005 and the following dialog can help the users to select this parameter:

Typical values for emprical factor Krm

Mass Modulus, Em - The default value of Em is determined by the method of Rowe and Armitage (1984) as below:

where σc is the unconfined compressive strength of rock.

RQD - this is rock quality designation parameter and varies between 0 and 100%. The following dialog can help the users to select this parameter based on the defect spacing:

Typical value for rock RQD

Mass Modulus – Rate of Change, Em-inc - this is the rock mass modulus increment rate with the layer depth from the layer top.

A typical plot dialog for -y curves of weak rock from the PileLAT program is shown as below:

Typical P-Y curve plots from PileLAT program

Reference:

Reese, L. C. (1997). Analysis of Laterally Loaded Piles in Weak Rock. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 123(11), 1010–1017. https://doi.org/10.1061/(ASCE)1090-0241(1997)123:11(1010)