Correction of Point Load Strength on Irregular Carbonaceous Slate in the Luang Prabang Suture Zone and the Prediction of Uniaxial Compressive Strength

Uniaxial compressive strength (UCS) testing requires high-quality core samples, which is a difficult task for weak, highly fractured, thinly bedded, foliated, and weathered rocks. In addition, it is time-consuming and expensive. Because of the good relationship between rock point load strength (PLS) and UCS, the PLS could be used to estimate rock UCS quickly. The lump structure and layer structure of carbonaceous slate are revealed in the tunnels of the China–Laos Railway in the Laos Luang Prabang Suture Zone as one of the important factors leading to tunnel squeezing deformation and support structures. To reveal the relationship between the PLS and UCS of carbonaceous slate in the Luang Prabang Suture Zone, PLS tests and UCS tests of lump-structure carbonaceous slate (lamina plane inconspicuous) and layer-structure carbonaceous slate (lamina plane conspicuous) were performed. Results show that the Is(50) of lump-structure carbonaceous slate ranged from 0.06 MPa to 0.30 Mpa, the Is(50) of layer-structure carbonaceous slates which were loaded perpendicular to the lamina plane ranged from 0.64 MPa to 1.25 MPa, the Is(50) of layer-structure carbonaceous slates which were loaded parallel to the lamina plane ranged from 0.49 MPa to 0.71 MPa, and the correction power index m ranged from 0.42 to 0.51 with an average value of 0.47. Four correlation expressions of carbonaceous slate relationships between PLS and UCS were fitted by zero-intercept linear expression, nonzero intercept linear expression, power expression, and logarithmic expression, and the calculation results were compared with results calculated by the International Society of Rock Mechanics (ISRM) correlation equation. It is concluded that the correlation equation between UCS and PLS recommended by ISRM specifications easily causes soft rock strength overestimation, which affects the correct evaluation of the surrounding rock property and the structural design safety of tunnels and underground projects. The zero-intercept linear equation UCS = 18.45Is(50) has better goodness of fit and higher accuracy in predicting the UCS of the carbonaceous slate in the Luang Prabang Suture Zone.