| China is an earthquake-prone country, there are23seismic zones on which earthquakes (Ms≥6) occurred in history, five of which are very active and entering a period of earthquake. These seismic zones distribute in northeast china, north china, southwest china and south of the Qinghai-Tibet plateau. A large number of macro-earthquake damage surveys show that:in liquefied zones, destruction of the upper structure was mainly due to the soil liquefaction caused by earthquake; few or nothing is directly caused by earthquake. Therefore, the severity of liquefaction hazards has long been concerned.The traffic channel between Beijing and Shanghai is an important artery to maintain china’s economic development and it is also a transportation hub connecting Beijing-Tianjin region and Yangtze River Delta region, the two major economic zones of China. Beijing-Shanghai region’s GDP accounts for40%of the national total and it’s population accounts for over25%of the total, therefore, it is of great significance to carry out construction of Beijing-Shanghai High-Speed Railway, which can ease a serious shortage of long-term transportation capacity burdened by the existed Beijing-Shanghai railway, form national railway passenger line network, and promote social and economic development.The results of geologic survey of Beijing-Shanghai High-Speed Railway area show that, a vast zone with liquefied saturated silt existed in this area which lies in highly seismic region of Ⅶ,Ⅷor Ⅸ. According to the principle of no break under light seismicity, repairable under intermediate seismicity and no collapse under intense seismicity, preventing long time interruption of trains resulted from foundation soil liquefaction, subgrade destroy and settlement during the earthquake, it is necessary to carry out a deep research on seismic reinforcement technology of liquefied foundation for High-Speed Railway.Combined with the construction of Beijing-Shanghai High-Speed Railway, researches were carried out to study dynamic characteristics of saturated silt foundation under earthquake. A new seismic reinforced foundation-subgrade was proposed for liquefied region. And two types of foundation treatment corresponded to the new subgrade, gravel piles-net and CFG piles-net, were preliminary designed. Through a series of experiments including dynamic triaxial and large scale model shaking table tests, and numerical simulation, indepth research was conducted to study the dynamic characteristics and integral seismic stability of saturated silt foundation before and after being reinforced, and the following conclusions were obtained:1. The results of triaxial shear tests studying on the static strength and stress-strain behavior of saturated silt before and after reinforcement are shown that:1.1The stress-strain relationship of saturated silt before reinforcement(dry density1.46g/cm3, the same thereafter)shows a typical strain hardening curve, on the other hand it(dry density1.586g/cm3, the same thereafter) shows a strain softening curve after reinforcement;1.2The shear strength of saturated silt increases with the increase of confining pressure, and after reinforcement all strength indexes of saturated silt are higher than those before reinforcement. The results indicate that consolidation confining pressure and density of saturated silt are important to improve resistance strength of silt to shear deformation.2. The results of cyclic triaxial tests studying on the dynamic characteristics of saturated silt of High-Speed Railway before and after reinforcement for the first time are shown that:2.1Both liquefaction strength and dynamic strength of saturated silt increase with the increase of confining pressure, and decrease with times of cyclic loading. Both liquefaction strength and dynamic strength of saturated silt after reinforcement are higher than those before reinforcement, and the lower confining pressure is, the greater increase rate of liquefaction strength and dynamic strength of reinforced saturated silt is, for instance, specimens under confining pressure of50kPa,100kPa and150kPa, the liquefaction strength and dynamic strength of reinforced saturated silt increase by about5times,3times and2times respectively, comparing with unreinforced specimens;2.2Dynamic shear stress ratio of saturated silt decreases with times of cyclic loading, and dynamic shear stress ratio of reinforced saturated silt is higher than that of unreinforced silt. Dynamic shear stress ratio of unreinforced saturated silt increases with the increase of confining pressure, but reinforced silt decreases, and the lower confining pressure is, the greater increase rate of dynamic shear stress ratio is;2.3The pore-water pressure development of saturated silt has nothing to do with the initial consolidation confining pressure:The development pattern of pore-water pressure of unreinforced saturated silt against time is the same as that of sand, namely, during pre-liquefaction, which accounts for about80%of the whole liquefaction period, the development of pore-water pressure is relatively flat, while the rest about20%of the liquefaction period, pore-water pressure increases rapidly and eventually gets full liquefaction; But for reinforced saturated silt, the development pattern of pore-water pressure is completely different from that of above, during of pre-liquefaction, which accounts for less than1/20of the liquefaction period, pore-water pressure increases rapidly to60%of its peak value, and then during the subsequent time, it slowly reaches the peak, the pore-water pressure development process of reinforced saturated silt is in line with the typical exponential law;2.4The dynamic shear modulus of saturated silt decreases with the increase of its dynamic shear strain, and the curve of its dynamic shear modulus and dynamic shear strain shows a hyperbolic relationship; The dynamic shear modulus of reinforced silt is greater than that of unreinforced, and the smaller dynamic shear strain is, the greater increase rate of dynamic shear modulus is, when the magnitude of dynamic shear strain is greater than10-2, both of dynamic shear modulus of unreinforced and reinforced silt become more and more consistent;2.5The damping ratio of saturated silt increases with the increase of its dynamic shear strain, and the curve of its damping ratio against dynamic shear strain is in line with the hyperbolic model. The damping ratio of reinforced saturated silt is less than that of unreinforced, and the larger dynamic shear strain is, the greater decrease rate of its damping ratio is.3. For the saturated silt foundation which may be liquefied, a brand new patented foundation-subgrade structure was proposed. This structure included three parts-piles, cushion reinforced by cement, graded broken stone and geogrids and subgrade reinforced by geogrids. In the structure, subgrade, foundation reinforced by piles and cushion were connected into a whole interaction, which increased the deformation compatibility of foundation-cushion-subgrade during earthquake, and greatly improved the integral seismic performance of the foundation-subgrade; With reference to above reinforcement structure, on the principle of settlement deformation control, preliminary reinforcement designs were carried out for liquefied saturated soil foundation of Beijing-Shanghai High-Speed Railway by using two kinds reinforcement forms, gravel piles-net structure and CFG piles-net structure.4. By lager-scale model shaking table tests, for the first time comprehensively analyzed and verified the characteristics of saturated soil liquefied foundation of High-Speed Railway before and after reinforcement, which included response acceleration, pore-water pressure, shear deformation, geogrids strain within cushion and subgrade, foundation-subgrade deformation and liquefaction flow within foundation etc, the results show that:4.1For the same measured point, when table acceleration increases, response acceleration of the structure also increases. When table acceleration increases to a certain value, the response acceleration increases along the vertical profile of foundation from top to bottom. In the clay layer, the response acceleration is almost equal to table acceleration, however, in silt layer, the response acceleration increases quickly. It shows that silt soil liquefaction play a role in amplifying response acceleration but clay soil don’t.4.2For unreinforced foundation, the amplification coefficient contours of response acceleration show convexity in the foundation-subgrade under both sides of the shoulder(zone I), and the greater this convex curvature is, the greater table acceleration is, moreover, the contours even show sharp convex in the subgrade. However, the contours show horizontal in the foundation-subgrade under both sides of the slope (zone II). For reinforced foundation, the amplification coefficient contours of response acceleration show horizontal or slightly convex in zone I, and decaying trend in zone II. That show these reinforcement measures improve the consistency of the foundation-subgrade stiffness, significantly reduce the response acceleration difference between zone I and zone II, smooth dynamic response of foundation-subgrade and increase seismic performance of structure;4.3With the increase of table acceleration, amplification coefficients of response acceleration of unreinforced foundation and CFG piles-net structure foundation first increase and then decrease to a stable value, but that of gravel piles-net structure foundation always increase; When table acceleration is less than a certain value, the amplification coefficients of response acceleration of reinforced foundation significantly reduce comparing with that of unreinforced foundation, and the coefficients decrease more apparent in the gravel piles-net foundation. The results show that:gravel piles-net structure is more effective on inhibiting the response acceleration from being amplified and improving the seismic performance of foundation than CFG piles-net structure in saturated silt foundation; However, when table acceleration exceeds this value, the seismic performance of foundation reinforced by gravel piles-net is worse than that of unreinforced, but that of reinforced by CFG piles-net is still better than that of unreinforced.4.4Pore-water pressure increases vertically with the increase of its depth and decreases horizontally with the increase of its position from the center of subgrade; with the increase of table acceleration, pore-water pressures in foundation first increases to its peak, then decreases and at last tends to stabilize. When pore-water pressures of unreinforced foundation, gravel piles-net structure foundation and CFG piles-net structure foundation reach their peak, their corresponding table acceleration are0.283g,0.252g and0.161g, and in the same position, the pore-water pressure is the largest in unreinforced foundation, next is that in gravel piles-net structure foundation, and the smallest is that in CFG piles-net structure foundation.4.5At each table acceleration level, the clay layer lateral shear displacement is small around zero. When table acceleration is too small to cause foundation liquefaction, the accumulative lateral displacement of silt layer is almost zero too, but when table acceleration is large enough to cause foundation liquefaction, lateral shear displacement increases with the increase of its vertical distance from the table; The maximum shear displacement increases with the increase of table acceleration; When table acceleration is less than about0.36g, the maximum shear displacement of gravel piles-net structure foundation is less than that of the other two types of foundations, it has the strongest resistance to shear deformation, followed by CFG piles-net structure foundation, the worst is unreinforced foundation; However, when table acceleration is more than about0.36g, the maximum shear displacement of reinforced foundation is still less than that of unreinforced, but the resistance to shear deformation of gravel piles-net structure foundation is less than that of CFG piles-net structure foundation;4.6With the increase of table acceleration, the maximum geogrids strain of cushion first increases and then gradually stabilizes. The maximum geogrids strain distribution in unreinforced foundation and CFG piles-net structure foundation shows decreasing charact-eristics from the middle to both sides, but that in gravel piles-net structure foundation shows increasing characteristics from the middle to both sides and the most uniform, comparing with the largest geogrids strain difference between both sides and the middle in unreinforced foundation.4.7In unreinforced foundation and gravel piles-net structure foundation, the geogrids strain in bottom layer of subgrade changes in line with that of the cushion geogrids with the increase of table acceleration, but its absolute value is very small comparing to the latter, in addition, the other layers geogrids have no deformation; For CFG piles-net structure foundation, the geogrids strain development in each layer of subgrade is similar to that in the cushion with the increase of table acceleration, its the maximum strain almost equals to the latter’s, but the geogrids strain in each layer has no rule to follow.4.8With the increase of table acceleration, the accumulated deformation of subgrade increases. At each table acceleration, the accumulated subgrade deformation of reinforced foundation, including subgrade settlement, horizontal and vertical displacement at the base of subgrade, significantly reduces comparing with that of unreinforced foundation, and that of gravel piles-net structure foundation is the smallest.4.9Comparing the final accumulated deformation of the three types of foundations after loading, that of unreinforced foundation is the largest, followed by CFG piles-net structure foundation, the smallest is gravel piles-net structure foundation.4.10In unreinforced foundation, in addition to showing uplift at the toe of slope, the others show different degrees of subsidence; In reinforced foundation, the upper foundation shows different degrees of subsidence, the subsoil shows uplift, and the vertical displace-ment of each float ball at the same horizontal level is almost the same; Comparing the floating balls displacement in reinforced foundation, the shallow settlement of gravel piles-net structure foundation is smaller and more even, the upward displacement difference is also smaller at different foundation depths;5. Based on Terzaghi’s consolidation theory and Barran’s analytic solution of sand-drained ground consolidation, for the first time derived the semi-analytic solution of buildup and dissipation of pore-water pressures in gravel piles-net structure foundation subjected to earthquake, combined with the dynamic characteristics and the development pattern of pore-water pressures obtained by cyclic triaxial tests, and its results were in good agreement with the results of shaking table tests.6. Using finite element software, analyzed liquefaction zone expansion and calculated integral seismic stability of the foundation-subgrade before and after reinforcement, then the results show that:6.1At the same input acceleration, the liquefaction zone area of reinforced foundation reduces significantly than that of unreinforced, furthermore, the smallest liquefaction zone area is in gravel piles-net structure foundation and the next is in CFG piles-net structure foundation.6.2The minimum safety factors of seismic stability of three types of foundations both decrease with the increase of input acceleration; At each input acceleration, the minimum safety factor of seismic stability of unreinforced foundation is less than1.0, and the integral seismic stability of reinforced foundation substantial increases and the minimum safety factor is greater than1.5, in particular, the seismic stability of CFG piles-net structure foundation improves the most significantly. |
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