Study On Technology And Environmental Effect Of Pressure-relief Of Confined Water In Hangzhou Deep Pit

A large number of super deep foundation pit appeared in the projects like tall buildings, subway project during the development of city construction, therefore the efffect of confined water to pit engineering was highlighted. This paper focused on the confined water encountered in Hangzhou Qingchun river tunnel project, we did some study on the characteristics of this ancient channel confine aquifer, confined water seepage calculation, effects of tide on pit Inrushing, treatment measures of confined water pressure, environmental effect during the process of dewatering and other aspects. In this paper, the author made the following work:1. Through a large number of geological investigation and historical data collection, confined water features of Qiantang ancient river in hangzhou city and the formation of which was Illustrated; based on study of the historical changes of the Qiantang River, plane distribution of Qiantang River paleochannel in Hangzhou region was analysed. Through the study of hydrogeological data, hydraulic connection between the paleochannel confined aquifer and the Qiantang River was illustrated; through the summary analysis of the geological survey of the Hangzhou Qingchun Road and pumping test of Qiantang river tunnel, the vertical distribution of pressure aquifer, soil characteristics and pore confined water level dynamics was studied. According to a combination of domestic engineering instance, the problem of confined water in four other large and medium-sized cities, comprehensive measures of Hangzhou and four other cities in the confined aquifer characteristics and the corresponding treatment works were compared and summaried.2. According to relations between water quantity and head change revealed by the Darcy law and mass conservation principle,non-deformable porous medium seepage continuity equation and pressure dense medium seepage continuity equation were established,also based on that confined water of two-dimensional and three-dimensional seepage partial differential equation were derived. On the qinchun road crossing river tunnel project Jiangnan working well was done for hydrogeologic pumping test, conmbined with the seepage equation above,the hydrogeological parameters of confined aquifer was deducted,meanwhile hydraulic connection between Phreatic aquifer and confined aquifer was determined.3. The advantages and disadvantages of existing Anti-heave piping calculation method of confined aquifer foundation pit was summarized; the factor which affect water head variation value in certain tidal variation was analysed, also confined water tidal effect on the calculation of heave piping was considered, through the description of tidal characteristics and cause of Zhejiang coast and the Qian Tang tide,the conclusion that the tide effect to confined aquifer should be considered in Strong tidal reaches was made.4. Taken the Qinchun crossing river tunnel pit excavation as an example, the necessity of reducing precipitation was analysed; also the adaptability of pit foundation reinforcement,pit foundation excavation,the precipitation water injection mode of processing in the engineering were analyzed,based on that,the applicable method was used in this project and field test was done to inspect the effect of it,also the key construction technology about super deep underground continuous wall was described in detail.5. The settlement caused by precipitation of confined aquifer was divided into initial settlement and consolidation settlement,Combined the complete well theory,additional stress formula which reflected the confined water of hypotensive effect was derived,the settlement formula was deducted with the introduction of Mindlin solution. Meanwhile,soil parameters effected on the overburden settlement was analysed with the help of example calculation. The influence of overburden thickness,modulus of elasticity,water pressure drawdown and water conduction coefficient to ground settlement were discussed. Also the consolidation settlement of overlying aquitard layer caused by cross flow was Derivated,which did not consider the change of potential water level.