Research Of Permeability And Anti-Floating Water Level About Sandstone Stratum In Lanzhou City

In recent years, with the development and utilization of urban underground space, the buoyancy problem of the underground garage has gradually caused the attention of designers, the weight of the underground garage is not big, the anti-floating becomes the important factor about restricting safety and economy of engineering. If the buoyancy is highly designed, anti-floating measures will be complex and expensive, low will make the underground structures instability with floating, influencing structure safety. It is not specific for the anti-floating design of joint development rock and sticky soil in the engineering specification.,the most projects are calculated at 100% of the fortification level of buoyancy, increasing the cost of construction project.Sandstone is used to be bearing layer of underground garage in parts of lanzhou,pebble layer containing water lies on the sandstone layer,the underground garage anti-floating design is needed.There is a consider of anti-uplift water level reduction for dry, hard and medium weathering sandstone.Studying the permeability of sandstone and anti-uplift water level reduction of sandstone foundation to get the safe and economic anti-floating design scheme.The paper mainly studied groundwater level for subsurface structure buoyancy resistance through permeability test and the indoor model test of sandstone foundation,measuring permeability coefficient of sandstone under different confining pressure and osmotic pressure respectively,the experimental results showed that: the magnitude of the permeability coefficient of sandstone is 10-9 ~ 10-8 under low confining pressure and low osmotic pressure,belonging to the impermeable rock.,and the sandstone can be used as impervious material in designing anti-floating water level; With the increase of osmotic pressure, the permeability coefficient tend to increase gradually.The sample produce tiny cracks with 120 k Pa osmotic pressure,but there is no passageway in cracks and have no influence on the permeability coefficient.Conducting indoor simulation test about water pressure reduction for different condition and the foundation soils,getting the conclusion:Although the permeability coefficient of sandstone is very small, there is relatively largebuoyancy acting the bottom of foundation.Groundwater seep into the basement through the interface with no dealing with between foundation and ground,anti-uplift water level is 82% ~ 91% of the pebble foundation’s;Groundwater permeates bottom of foundation through sandstone when using closed measures in interface between foundation and ground,the reduction coefficient of anti-uplift water level is 0.31~0.47;Groundwater seep into the basement through cracks when using closed measures in interface between foundation and ground and there are cracks in sandstone, the reduction coefficient of anti-uplift water level is 0.63~0.73.Thus,it can be seen that the reduction coefficient of anti-uplift water level is half ofactual water level because of sandstone permeability,the anti-uplift water level can be reduced when there are cracks in sandstone.For every anti-uplift water level reduction,we must put forward reliable sealing water treatment measures between foundation and ground.