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.

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

where **Pur** is ultimate soil resistance per unit length; **Alpha-r** 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 **k-rm** 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:

**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 (k-rm) **- Dimensionless constant ranging from 0.00005 to 0.0005 and the following dialog can help the users to select this parameter:

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

where Sigma-c 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:

**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:

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