Study On Engineering Geological Properties Of The Unconsolidated Sandstone

Unconsolidated sandstone is a set of strata with the loose structure and worse self-stabilization, which has been exposed by large underground engineerings in recent years with frequent engineering accidents, such as gushing water, collapse and other problems. Aiming at the shortage of correlation study of engineering geological properties of this sandstone, this thesis focuses on unconsolidated sandstone as research subjects. Relying on many under construction projects and completed projects, the fundamental engineering geological properties of the unconsolidated sandstone are analyzed and summarized through field investigation and systematic indoor test. Based on these researches, its frost effect was studied and its genesis problem was discussed. The results of this paper provide not only basic date on the properties of unconsolidated sandstone, especially its physical and mechanical fundamental properties and structural characteristics, but also theoretical and experimental date to support relevant research and geotechnical engineering projects involving unconsolidated sandstone strata. The study results demonstrated:(1) The analysis result of distribution and material composition characteristics shows that unconsolidated sandstone is widely distributed and mainly located in Longdong (eastern Gansu) basins and Lanzhou basins. This sandstone, which is one layer or several layers of the red-rock strata, is almost the sediment in Paleogene and late Neogene. They consist mainly of quartz and little feldspar with very little cementing material and impurity.(2) The analysis of its structure and physical and mechanical properties indicated that unconsolidated sandstone belongs to ranular structure and seem-continuum in macroscopic aspect, it belongs to skeletal structure with bad cemented, and has well-distributed and loose porosity texture in microscopic aspects. Unconsolidated sandstone is extremely soft rock with low mechanical strength and soft and weak lithology. It is argillaceous (fine) siltstone with loose structure and large pores. It has high psephicity and good sorting. Its framework grains mainly contact with their points. It also has low degrees of cementations with shale or very little calcareous cementations. The physical and mechanical properties of this sandstone, which are similar to those of general sands, are poor with weak permeability, easy disintegration, low shear strength and low compression.(3) Frost effect study of unconsolidated sandstone indicated that its physical properties, especially its structure and mechanical properties are influenced by freezing and thawing action. The physical properties of this sandstone, such as the rate of water content, density, permeability and so on, change with the frozen states. Its structural effects include the change of the original structure, forming of new micro-cracks, increasing of the pores, the change of clay minerals, and so on. The mechanical properties and performance of the frozen sandstone are greatly improved, but after thawing, the shear strength dropped sharply to a level lower than at its natural state. The change law of its physical and mechanical properties under the condition of freezing and thawing can be seriously considered and appropriately used.(4) All of those characteristics can be inferred that unconsolidated sandstone in Gansu is the wind-sand activities product, and part of the strata is influenced by the late effect of water environment. Characterized by weak and complex diagenesis environment, unconsolidated sandstone is the wind-sand activities product and suitable to the dry and hot climate. Part of the strata is influenced by the late effect of burial diagenesis and existing environment. Based on this genetic analysis result of unconsolidated sandstone, dry climate and hot environment in Paleogene and late Neogene was discussed.