Fundamental Drawbacks And Disastrous Consequences Of Current Geotechnical Safety Design Theories For Slopes

Abstract

The fundamental drawbacks and their disastrous consequences of the current geotechnical safety design theories for slopes are pointed out, analyzed and evaluated. The drawbacks are as follows: (1) The shear strength parameters of soils (effective cohesion and angle of internal friction) are assumed to have constant values in the design theories. However, they are changeable and can be reduced to zero due to the progressive tensile deformation and increase in void ratios and pore-water contents in the slope soils. (2) The classical theories of soils (such as the effective stress principle and soil consolidation theory) are valid only under the condition of compressive-shearing loading with reduction of soil voids and pore-water contents. They are not suitable to the condition of tensile-shearing loading with increase of soil voids and pore-water contents. This condition is actually what happened during the process of slope failure and landslide. (3) The factor of safety is a ratio of the soil shear strength to the downward sliding shear stress. As the shear strength of soils is small, its increase according to the ratio is very small, which cannot make substantial improvements to the soil quality and resistance to failure. Accordingly, failures and landslides in engineered slopes can commonly occur around the world although geotechnical engineers put tremendous efforts in preventing and controlling their occurrences. The classical soil mechanics is only applicable to the soils subjected to complete compression and shear loading condition, and is a completely compression-shear soil mechanics theory. A new soil mechanics and new geotechnical design methods applicable to soils subjected to either the compression-shear or tension-shear loading conditions shall be developed.

滑坡及其災難在世界各地、經現代巖土工程設計施工的斜坡中不斷地發生。論文提出、分析和論證了造成工程斜坡滑坡的一個本質原因。它就是現代斜坡工程安全設計理論存在根本缺陷。這個缺陷表現在3個方面。第一,土體抗剪強度參數(有效黏聚力和有效內摩擦角)在斜坡安全設計中假定為常數,不隨土體孔隙和含水率的增大而減低到零,孔隙水對土體剪切強度的影響僅體現在孔隙水壓強對有效應力的影響。第二,經典巖土力學理論(特別是有效應力原理和排水固結理論)僅能預測外部加載產生土體壓剪、孔隙率減小、土體強度增大的巖土穩定工況過程。它們不適用于外部卸載造成土體拉張、孔隙率增大、土體強度減少的巖土破垮工況過程。第三,斜坡安全系數是土體抗剪強度與施加剪切應力的比值。由于土體強度本值(品質)很低,加固工程增加這個比值安全系數達到設計最小值時所能夠提供的強度本質(品質)增加量也就不高。斜坡工程巖土體的強度和品質沒有得到能夠防治巖土體拉張變形和滑垮的根本改善和增強。從而,工程人員必須精心全力地確保低品質斜坡工程巖土不破垮和滑塌。可是,又由于工程斜坡眾多和它們的降雨、場地和環境變化較大,上述3個方面現代斜坡工程安全設計理論的根本缺陷可以導致:達到安全設計標準的工程斜坡,在施工和營運過程中,能夠發生滑垮、產生災難后果。本論文特別認為,經典巖土力學理論是僅適用于完全壓剪的,應稱為壓剪巖土力學理論。提出了應該建立既適用壓剪又適用拉剪的、新的土力學理論和防抗巖土災害設計方法。