差异风化型危岩多形成于由砂岩和泥岩等软、硬岩性组成的互层陡倾岩质边坡中，其崩塌破坏属地质灾害中的常见形态。以普洱渡危岩高边坡为例，推导出差异风化型危岩岩腔后壁泥岩压应力随岩腔深度增加而增大的计算公式。据普洱渡危岩高边坡地形地貌和地质构造特征建立了两类4种差异风化型危岩力学模型：第1类为主控结构面贯通率等于1时的泥岩基座压裂破坏型和转动破坏型危岩；第2类为主控结构面贯通率小于1时的坠落型和倾倒型危岩。利用岩石强度理论推导出研究区砂岩的二次抛物线型摩尔强度包络方程，以极限平衡理论和摩尔强度理论推导了4种危岩体在自重、地震力和裂隙水压力共同作用下的崩塌破坏判别表达式，并由此反演出危岩体的临界崩塌边界方程，得出危岩体崩塌边界（岩腔深度、危岩体厚度和高度、主控裂隙深度）之间的关系，为现场预测和判别危岩体的稳定性和崩落时序提供简便可靠的依据。

Differential weathering can cause unstable rock mass in rock slope with alternating strata. The alternating strata can be sandstone and mudstone and belong to the soft and hard rocks. The rock collapse damage is a common form of geological disasters. This paper uses unstable rock mass at Puerdu as case study. The calculation formula for the compressive stress of unstable hard rock block on the soft mudstone base is derived for the slope rock mass with cavity in the soft mudstone due to differential weathering. The stress increases as the cavity depth increase. According to the slope topography, geomorphology and characteristics of geological structure of unstable rock blocks at Puerdu, two kinds of four unstable rock mechanical models due to differential weathering are established. The first type is the type of mudstone base fracturing damage and rotational damage unstable rock block whose main control fissure connectivity rate is equal to 1. The second type is the type of falling and toppling of unstable rock block whose main control fissure connectivity rate is less than 1. The rock strength theory is used to derive the quadratic parabolic Mohr strength envelope equation for sandstone at the study area. The limit equilibrium theory and Mohr strength theory is used to deduce the collapsing damage and judgment expression of four kinds of unstable rock blocks under the gravity, earthquake force and fissure water pressure. The critical collapse boundary equation is derived for the unstable rock mass at the study area. The relationships among rock cavity depth, thickness, height and the main control fissure depth are also derived. Those results provide an intuitive and reliable basis for the prediction and judgment of stability and collapse of the unstable rock mass.