Study of the Limestone Strength Weakening Mechanism at the Eastern Margin of the KangdianPalaeouplife in Southwest China

Abstract

The limestone of the South China Lower Permian Qixia & Maokou Group usually features high purity, high strength, and poor weathering, while the limestone of the Qixia & Maokou Group at margin of the Chuan-Dian Paleouplife of southeast China is characterized by obvious strength weakening. The weakening of limestone strength leads not only to excessive errors in the estimation of useful material reserves in the limestone quarry, but also increases the engineering slope support cost in the limestone distribution area on the eastern margin of the Kangdian Paleouplife. In order to ascertain the cause mechanism of the strength weakening of the limestone in the stockyard, we take the stockyard in the dry valley as an anatomical point. The strength weakening of limestone is comprehensively analyzed by means of drilling, large-section excavation, site investigation, sampling, and physical mechanics tests. The main findings are as follows: (1) The limestone area of the Qixia & Maokou Formation near the Kangdian Paleouplife generally contains terrigenous fine-grained materials and mud iron; (2) The weakened rock masses are divided into two types of weakening: deep dissolution type and deep strong weathering type. The uniaxial compressive strength of the deep weakened rock mass is only 2.5-25 MPa, which is 70%-90% lower than that of the fresh rock mass. Compared with the natural state, the compressive strength of the saturated rock mass is reduced by 60%-90%. The deep weakened rock mass has the characteristics of weakened strength and is softened by water; and (3) The dissolution type rock mass is weakened as a result of the dissolution of the high-level exposed broken spring along the east-west fracture zone; the formation mechanism of the strong weathering rock mass weakening is that under the conditions of tectonic shear, terrigenous muddy interlayer and dry-heating condition. The buried depth of groundwater is large, the hydrodynamic force is weak, and the coupling of water and rock under long-term infiltration environments makes the limestone body softened and argilized in situ, leading to a significant decrease in the strength of the rock mass.