## 1 Introduction  
The Luang Prabang Suture Zone is located in the north-central part of Laos territory, which is the connection area of the Lamping-Simao terrane and the Indonesian terrane; it is named for the Luang Prabang Province of Laos [1, 2, 3], as shown in Figure 1. The China-Laos Railway crosses Laos Luang Prabang Suture Zone, and many tunnels have been built within the influence of the Luang Prabang Suture Zone. Affected by Luang Prabang Suture Zone, the tunnels' surrounding rock is an almost carbonaceous slate with low strength. Coupled with the serious influence of tectonic stress, the tunnels have caused the problem of squeezing deformation. The uniaxial compressive strength (UCS) of rock is an important rock mechanical parameter in rock engineering. It is crucial information to infer and determine the type and grade of tunnel squeezing deformation. However, UCS testing requires high-quality core samples of proper geometry. However, preparing high-quality cores, particularly from weak, highly fractured, thinly bedded, foliated, or weathered rocks is a difficult task. In addition, this test is time-consuming and expensive. By comparison, the point load strength (PLS) test operation is simple, the irregular rock can be used, it is convenient for field implementation, and the feasibility is relatively strong.  
The PLS test method was first proposed by [PERSON] and [PERSON] [4]. Because of the correlation between PLS and UCS, PLS was also considered the optimum method for indirectly estimating the rock UCS. Based on related research, the International Society of Rock Mechanics (ISRM) proposed that the UCS of rock was equivalent to 20-25 times the PLS (ISRM, 1985). Many relevant scholars had performed studies on the relationship between rock PLS and UCS based on different rocks in different countries and regions [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. At present, the linear correlation was most widely used in two types of strength conversion, and some researchers had also presented various expressions, such as power equations, exponential equations, logarithmic equations, and quadratic equations. In addition, relevant scholars had conducted relevant research on the failure mechanism and calculation method of PLS [19; 20; 21; 22]. The failure mechanism of the point load test was based on the tensile stress failure concept at the loading point position [23; 24]. The calculation method of the equivalent core diameter was widely adopted in the current PLS test. However, some relevant scholars still considered that the influence of the sample failure mechanism and the influence of the sample shape coefficient were much more significant [25; 26].  
According to recent research, the investigation of the correlation expressions between PLS and UCS is largely based on several types of rocks limited to a specific area, which has obvious regional applicability and rock type applicability. In addition, the correlation expressions of soft rocks (UCS \(\leq\) 30 MPa) are rarely presented. Carbonaceous slate, as a common layered soft rock, and the relationship between PLS and UCS are rarely researched, and the traditional correlation expressions are inapplicable. The main objective of this study was to clarify the relationship between the PLS and UCS of carbonaceous slate, which is a typical soft rock found in the Luang Prabang Suture Zone, Laos. Based on a series of PLS tests and UCS tests of carbonaceous slate with different structures and different loading directions, the relationship between PLS and UCS is established by considering irregular  
Figure 1: China–Laos Railway location and study area.  
samples and the size correction. The results could improve the accuracy of soft rock UCS estimating by PLS testing, and it will provide a design reference for soft rock engineering. It also will be beneficial to the prediction of the level of tunnel squeezing deformation.