| In recent years, the foundation pit tends to ultra deep and large scale development, safety and economy of foundation pit have become increasingly demanding. This paper depends on the project, the foundation pit of anchorage for Longjiang River super large bridge,which is a deep pit with30-38meters for the depth in ihe Tertiary basalt. Because of the special engineering geological and hydrogeological conditions, the study on stability of deep pit in this geostrome has a certain theoretical significance and practical value.Apply analysis principles of engineering geology, this paper firstly analyze the engineering geological environment of foundation pit,from the geographic and geomorphic, geological structure, interface of rock and soil, structure characteristics, parameters of rock and soil,and qualitatively analyze the stability and possible instability mode of anchorage foundation pit. On the basis of this, quantitatively evaluate the stability of pit by modeling and numerical simulating.The main research work and contents are as follows:1. Characteristics of the Tertiary basalt around the foundation pit of anchorage for Longjiang River super large bridge is summarized in that:1) the multicycle prosodic structure characteristics, forming a plurality of deposition-eruption cycles, bed stratum orientation flat, undulating;2)the clastic bed stratum of the cycle structure formate weak structural surface, easily produce bedding landslide and plastic deformation,and is adverse to the stability of artificial slope;3)the rock mass consist of multiple structural plane, such as weak rock and weak intercalated layer, tectonic joint and weathering interface, due to difference of the rock mass and cutting effect by the joint, coupled with the difference of weathering degree, the rock mass present different structure types and multiple instability modes.2.The qualitative analysis on the instability patterns of the foundation pit of anchorage for Longjiang River super large bridge. Due to strong weathering, the Tertiary basalt is reformed seriously by structural plane,present the granular structure or a layered structure, failure of rock mass is not subject to the main structural plane;Also, rock mass mainly present soil,the engineering properties is poor. Therefore comprehensively judge that the foundation pit slope is a homogeneous body consist of the soil and semi rock, the main failure mode of slope is creep-sliding and fracturing failure. Another probable failure mode is plastic-flowing and fracturing failure under structural plane or weak intercalated layer control.3. On the basis of the qualitative analysis, carry on mechanical quantitative analysis using Janbu method and numerical simulation method, considering the natural state and watering state of the pit slope, conclude:1) There’s two kinds of instability modes of pit slope:creep-sliding and fracturing, plastic-flowing and fracturing. The former is a typical failure mode of homogeneous rock and soil slope, the latter is relevant to the weak intercalated layer of the Tertiary basalt, a variety of structural surface of the rock body can easily be sliding surface. Therefore, it has resulted in a variety of instability modes due to the geological structure and interface of the Tertiary basalt, which is consistent with the results of qualitative analysis.2) FLAC/SLOP simulation and Janbu method results show a consistent variation, the coefficient of stability are quite close in two ways overall, the coefficient of stability using FLAC/SLOP simulation is slightly larger; The coefficients of stability under watering condition reduce greatly,which show the groundwater seepage is an important factor affecting slope stability.It should take intercepting water and drainage measures,avoid the rainy season in the construction.3)The coefficient of stability reduce with the height of slope increasing under the same failure modes,which indicating that the slope height is also an important factor affecting slope stability. |
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