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.
![](https://static.wixstatic.com/media/306b4f_776d6bbe6e244a05bb85517e73f670ea~mv2.png/v1/fill/w_55,h_38,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/306b4f_776d6bbe6e244a05bb85517e73f670ea~mv2.png)
The ultimate resistance of weak rock is determined according to the following equations:
![](https://static.wixstatic.com/media/306b4f_bc8595aaf96940d9ba6b3671b77023b1~mv2.png/v1/fill/w_195,h_145,al_c,q_85,blur_3,enc_auto/306b4f_bc8595aaf96940d9ba6b3671b77023b1~mv2.png)
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:
![](https://static.wixstatic.com/media/306b4f_b76326af07584d538940135b49687991~mv2.png/v1/fill/w_83,h_33,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/306b4f_b76326af07584d538940135b49687991~mv2.png)
YA can be found by solving the following equation:
![](https://static.wixstatic.com/media/306b4f_f2b7a9b5b4ba4f54a830eac91717d94d~mv2.png/v1/fill/w_188,h_59,al_c,q_85,blur_3,enc_auto/306b4f_f2b7a9b5b4ba4f54a830eac91717d94d~mv2.png)
The initial modulus / slope of the P-Y curve, Mir can be determined by:
![](https://static.wixstatic.com/media/306b4f_ff7e70aebed946bcb000fe6def155d54~mv2.png/v1/fill/w_103,h_28,al_c,q_85,blur_3,enc_auto/306b4f_ff7e70aebed946bcb000fe6def155d54~mv2.png)
where Em is the rock mass modulus and kir is a dimensionless factor calculated by:
![](https://static.wixstatic.com/media/306b4f_712279b2d4ec4b9ea7ab5067e447990d~mv2.png/v1/fill/w_80,h_23,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/306b4f_712279b2d4ec4b9ea7ab5067e447990d~mv2.png)
The parameter Ym can be determined by:
![](https://static.wixstatic.com/media/306b4f_208501468a5545dc89a3f5fea2a42283~mv2.png/v1/fill/w_100,h_41,al_c,q_85,blur_3,enc_auto/306b4f_208501468a5545dc89a3f5fea2a42283~mv2.png)
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:
![](https://static.wixstatic.com/media/306b4f_1196dca7b21f43caa306a1c5838d1f77~mv2.png/v1/fill/w_105,h_134,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/306b4f_1196dca7b21f43caa306a1c5838d1f77~mv2.png)
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:
![](https://static.wixstatic.com/media/306b4f_92a415356fc64b5a973a7bb2c6953d53~mv2.png/v1/fill/w_60,h_32,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/306b4f_92a415356fc64b5a973a7bb2c6953d53~mv2.png)
Mass Modulus, Em - The default value of Em is determined by the method of Rowe and Armitage (1984) as below:
![](https://static.wixstatic.com/media/306b4f_30128e57864b48579e173418197049a7~mv2.png/v1/fill/w_146,h_29,al_c,q_85,blur_3,enc_auto/306b4f_30128e57864b48579e173418197049a7~mv2.png)
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:
![](https://static.wixstatic.com/media/306b4f_714d7a46758a404f99bdfd0447be70ce~mv2.png/v1/fill/w_60,h_32,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/306b4f_714d7a46758a404f99bdfd0447be70ce~mv2.png)
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:
![](https://static.wixstatic.com/media/306b4f_beb0e24820064cb1992b154990cbd556~mv2.png/v1/fill/w_49,h_32,al_c,q_85,usm_0.66_1.00_0.01,blur_2,enc_auto/306b4f_beb0e24820064cb1992b154990cbd556~mv2.png)