Study Of Seepage Deformation Of Foundation Soils At The Plain Of Haihe River Basin

Several hundreds meters loose Quaternary deposits of various geological origin and complex compositions distribute at the plain of Haihe Basin due to flooding of Yongding River and course changes of Yellow River, which mainly still consist of fine soils, having medium to thinly laminated bedding or microstructure. Soils are saturated in general as the water level is fairly high. This is the general geological settings for the hydroelectric projects on the Haihe Plain.Since the foundation soil mass on the Haihe Plain were deposited under various settings or different geological origins, the nature and state of the soil may be changed widely both in short distance and depth, which results in variability of soil behaviour. When the hydraulic head between the upstream and downstream of the embankment is higher than the critical hydraulic gradient of the foundation soils, seepage failure would be occurred. Seepage deformation thus is one of the main engineering geological problems.As shown by project experiences both at home and abroad, strength reduction and permeability increase of foundation soils, even foundation failure may be arose from seepage deformation. Therefore, critical hydraulic gradient of soil mass is an important parameter in analysis of foundation stability against seepage and selection of engineering measures, it has practical meanings for project safety and cost.In view of study area morphogenic characteristics, soil compositions and geotechnical development situations, especially the limitations of the critical hydraulic gradient calculation equations that widely applied at home and abroad, factors that obviously influenced the critical hydraulic gradient are discussed in this paper based upon data from laboratory tests, including age, origin type, nature and state of the foundation soils. A new equation for calculating critical hydraulic gradient, based on numeric analysis, is proposed for each soil type in different age. Engineering measures and relevant environmental geological problems against seepage deformation of foundation soils are also included in this paper.