Geotechnical Engineering Analysis And Evaluation On Building Foundation On Beihai Sandbank Sedimentary Strata

As the rapid growing of social economic and urban construction, more and more coastal cities in China have encroached further into the coastal zone.Beihai City is surround by sea in south, east and west, the limitation on land is obvious, so urban construction has to move towards the coastal zone(northern shore, southern shore and Waisha Island).Waisha Island, situated in the northern shore of Beihai City, surrounded by sea, looked like a ribbon, is a sandbank form by early Pleistocene Series Zhanjiang Formation marine deposit during the mid-holocene transgression. By the late holocene, the elevating coastal zone of this area formed a first marine deposition terrace, at the same time the sandbank and the coastal zone enclosed into a lagoon deposit.The bottom bedrock of sandbank of Waisha Island is sandstones of lower silurian Lingshan formation, with embedded depth around80～100m, from bottom up, covered by tertiary Nankang formation:clay on the top, sand gravel and coarse sand in the middle, mainly clay with some sand gravel, sand gravel of quaternary underpart of Pleistocene series, and loose soils of quaternary system of Holocene series at the bottomWaisha Island is located in the groundwater system’s discharge zone. Ground water goes from the south to the north and into the sea. There are two types of groundwater of a site:phreatic water and confined water. Phreatic water can be recharged by sea water and rainwater penetration. Confined water can be recharged through lithological skylight by the leakage of phreatic water. Hence confined water and phreatic water have some kind of connection. Hence its water level can be affected by tidal effect. Groundwater there is brackish water. Obviously, it is affected by sea water invasion and mixing.By exercising geotechnical engineering testing techniques such as drilling sampling,cone dynamic penetration test, standard penetration test, soil test, we obtain physical mechanical properties on every soil layers. Then we apply methods on soil mechanics, Rock and Soil Engineering, Eco-hydrogeology to do not only qualitative analysis but also quantitative analysis. Finally we evaluate geotechnical property of each soil layers. Next, we analysis foundation scheme. Taking account of the requirement of basement function and structure and seepage control, if we adopt pile foundation to improve bearing capacity, a recommended choice would be pile-raft foundation, of which pile point supporting layer is layer soil.As to the foundation pit engineering, after qualitative analysis we can decide the type of supporting structure and estimate embedded depth. By analyzing and calculating seepage deformation and water yield, we can ascertain the embedded depth of suspended cutoffs and pit dewatering as our dewatering method.In order to reduce the impact of ocean wave and sea water lateral pressure, we suggest to build a anti-billows wall and introduce waterproof curtain between pit and the sea.Hence the geotechnical engineering condition of building foundation of Beihai Waisha Island sandbank sedimentary strata has its inborn complicated characteristic:in the upper lever, the quaternary marine deposits sand stratum is relatively loose, which means bad mechanical property and may induce the liquefaction of saturated sands during an earthquake. What is more, the clay aquiclude on the top of Zhanjian formation’s suffering from denudation, results the water content of phreatic aquifer of the loose sand stratum of quaternary marine deposits communicate with first confined aquifer of gravel sand of Zhanjiang formation, which renders the upper water bearing layer exceed30meters and groundwater level close to surface of ground. As we know, groundwater and sea water have a deep hydraulic connection.Furthermore, the sandbank area of Waisha Island is widely affected by marine hydrology conditions like tides, ocean waves, storm surges, sea water and etc. The construction of the foundation of a building must take the lateral pressure of these conditions into consideration.In this paper, by investigating the geotechnical engineering analysis on one building site in that area, we study the foundation scheme, foundation pit engineering and seawater and groundwater’s prevention and control measure, and hence provide reference frame for ground foundation and foundation pit engineering of building on sandbank sedimentary strata site.