Study On The Engineering Properties And Stability Of Typical Ancient Outwash Congeries In Southwestern Valley, China

In southwest China, a set of outwash congeries forming at the the Pleistocene Epoch of Quaternary widely spread in the valley terrace with large rivers. Many of them are huge, often with the volume of millions m3, or even tens of millions m3. Influenced by the changes of sediment environment, the outwash congeries have formed into complicated sediment properties and structures during the long history of evolution. In this century, with the successive commencement of infrastructure projects in the valley areas, the outwash congeries, as a geological background of engineering construction, become an important condition for engineering construction. Therefore, it is rather necessary to make an intensive study of the engineering properties of the ancient outwash congeries and the prospective problems which might be encountered in the engineering practice.This paper, after intensive investigation and analysis on sediment properties, particle assocaiation,and structure properties of several outwash congeries in the southwestern valley, conducts an intensive analysis and study on seepage properties, strength properties, and deformation properties of the typical outwash congeries, on the basis of a large number of experiments and the collection of the relevant data. Besides, illustrated with three examples of the ancient outwash congeries, three main types of problems in the engineering geology and the treatment measures are deeply analyzed and discussed. The main results are as follows: (1) During the long history of evolution and as a result of climate changes, the sediment properties and structure properties of ancient outwash congeries are usually complicated, featuring with the macroscopic stratification of the sediment properties and the diversity of the sediment properties in single-layer structures, to be specific. The ancient outwash congeries mainly consist of giant particles, coarse particles, and partially concentrating fine-grained materials. As the difference existing in hydrodynamically sedimentary environments among sedimentary stratums, those who mainly consist of coarse particles are often featured with the rhythmicity of separation, roundness, and deposition; while those mainly consisting of giant particles are relatively inferior in separation and roundness. Structurally, the ancient outwash congeries are distinctly binary, with closely interlocking of giant particles and coarse particles. It is usually characterized with the structure properties of mainly argillaceous binding and partially calcareous binding. That is, the solid matrix is made by giant particles and coarse particles, congested by fine-grained materials. Thus, the giant particles and coarse particles are closely interlocked, often featured with argillaceous binding to some extent; when calcium is high, they usually show some superior calcareous binding characteristics.(2)The way of determining ancient outwash congeries has been established in terms of sediment sources, sediment properties, and structure properties. Firstly, we should estimate the feasibility that the congeries forming with outwash deposits by Material source and movement pathway, then, we can Confirm wethere it is an outwash congeries or not, by wethere it with the macroscopic stratification of the sediment properties, wethere it having the rhythmic sedimentation characters, and wethere it having binary and Cementation characters in structure.(3)According to a large amount of dating data and the contact relation of the outwash congeries with the fluvial cover and the palaeo-weathered residual clay, the relationship between the sediment properties and palaeoclimate features, glacier melt hydrodynamics and palaeotopography are discussed. It is deemed that in the last interglacial of Late Pleistocene and in the Interstadial of last second glacial stage of Middle Pleistocene there were at least two times of large-scaled geological events with temperature elevation and glaciers ablation. It is also revealed that the channel is often occupied and the fluvial cover is often buried by the outwash congeries forming at Late Pleistocene. Meanwhile, during the interglacial period, paleoclimate repeated many times, resulting in several times interruptions of glaciers ablation, weathering and degradation of congeries, and the formation of palaeo-weathered residual clay inside the congeries.(4)According to a large amount of experiments, it finds that the ancient outwash congeries in the southwestern valley are featured with inferior seepage abilities and strong anti-seepage abilities. Also, when hydraulic gradient is small, the seepage of outwash congeries basically meet Darcy criterion; and when hydraulic gradient is large, it shows a nonlinear relationship between permeability velocity and hydraulic gradient is indicated, thus the seepage constitutive can be described as v = KJ0.7, and the estimation methods and formulas of the leakage factor of ancient outwash congeries can be established asucK = 0.5e2CC.(5)It is shown from large physic experiments that the outwash congeries, characterized with compactness, high-density, and small gap, is a typical kind of soil with high capacity and low compression. Outwash congeries are often with preferable shearing strength. Naturally, the cohesion can reach to 150～400KPa, and friction angle can be about 35°～39°; when under the saturated condition, the cohesion can keep at 60～100Kpa, and internal friction angle can be about 30°～37°. Moreover, the value of C is much more sensitive to water than that ofφ. Generally, under the saturated condition, the cohesion may be reduced by half or so, but the change of the value ofφis almost within 1～3°. It is also shown that different stress conditions have distinct influence on the strength properties of outwash congeries. That means the strength envelope meets Mohr– Coulomb criterion under low stress; while the strength envelope is nonlinear under high stress.(6) The study on the three typical ancient outwash congeries'stability finds that one of the major distinctions between the ancient outwash congeries and the general loose deposits is that the ancient outwash congeries are featured with complicated sediment properties and structure properties. The materials forming outwash congeries are not only gravelly soils with high strength, but also weak zones with low strength. In most time, the stability of the congeries are good if the weak zones and the free faces of congeries are not combines favorably, but if the free face is changed by someway and combined favorably with weak zones, the congeries will slide along the weak zones possibly. Therefore, it is suggested that the sedimentary and structural features of the outwash congeries be studied carefully, and mistaken treatment of them as isotropic medium be avoided when it comes to the ancient outwash congeries in engineering.