Experimental Study On Anisotropic Seepage Erosion Characteristics Of Gap-graded Gravelly Soils For Core Wall

Embankment dams have been widely built around the world,but the accidents of dam breaking still occur from time to time.According to the statistical results,the internal erosion caused by seepage(called as seepage erosion)is an important reason.At present,although the seepage erosion has been paid attention to,the actual seepage direction in core wall forms a certain angle with the vertical direction,and compacting in layers makes the core wall materials anisotropic,the vertical seepage test could not fully study the seepage erosion characteristics.Moreover,the gravelly soils used as core wall materials often has the characteristics of gap grade,large size of particles and high content of silt and clay.The conventional experimental apparatuses are difficult to meet those requirements.So the researchs on seepage erosion of gravelly soils are less.Therefore,it is urgent to develop a new experimental apparatus to carry out the anisotropic seepage erosion tests of gap-graded gravelly soils for core wall.In order to carry out the anisotropic seepage erosion tests of gap-graded gravelly soils for core wall and reduce the influence of the difference in the size and shape of soil samples,an anisotropic seepage erosion experimental apparatus has been developed.The experimental apparatus realizes horizontal and vertical seepage erosion tests in the same cavity.And the experimental apparatus can hold a horizontal seepage soil sample with the size of 540 mm×500 mm×470 mm or a vertical seepage soil sample with the size of 540 mm×540 mm×440 mm.Moreover,the experimental apparatus can provide a vertical stress on soil samples.It has been verified that the dominant flow on side wall was not obvious,and the tests have good repeatability.The horizontal seepage erosion tests and the vertical seepage erosion tests of gap-graded gravelly soils for core wall with different gravel content have been carried out by using the anisotropic seepage erosion experimental apparatus.The anisotropic seepage erosion characteristics of gap-graded gravelly soils for core wall and the influence of gravel content on anisotropic seepage erosion characteristics have been studied mainly from permeability coefficient,characteristic hydraulic gradients of seepage erosion and failure and local permeability coefficient.The results show that the anisotropy is mainly manifested in permeability coefficient and characteristic hydraulic gradient.The horizontal permeability coefficient is 9.62~25.82 times larger than the vertical permeability coefficient,while the vertical characteristic hydraulic gradient is 1.64~2.41 times larger than the horizontal characteristic hydraulic gradient.The "staggered wall" seepage model has been used to explain the reason for the anisotropy of permeability coefficient that the orientation of coarse particles lead to a shorter seepage path of horizontal seepage and a longer seepage path of vertical seepage.The curves of local permeability coefficient both in horizontal seepage erosion test and vertical seepage erosion test show a "tree"-like development trend that maintain close at first and then separate quickly,reflecting the development process of internal fine particles that being stable-starting erosion-migration and clogging-loss.Moreover,the effect of gravel content on vertical permeability is more pronounced than its effect on horizontal permeability.With the increase of gravel content,the difference between horizontal permeability coefficient and vertical permeability coefficient of soil samples gradually decreases,while the difference between horizontal characteristic hydraulic gradient and vertical characteristic hydraul ic gradient gradually increases.Finally,the engineering suggestions of the strict control of gravel content and the consideration of the influence of anisotropic seepage erosion characteristics in core wall design have been proposed.