Study Of The Law And Influencing Factors Of Ground Settlements Caused By Mining Method In Water-enriched And Complex Soil Layers In Shenzhen

Shenzhen Metro Line 5 mainly underground crosses the water-enriched and complex soil layers, because the engineering property of the soil layers are very complex, the construction procedured with the Mining Method is also complicated, the adjacent buildings and buried pipelines are densely covered, these factors make the ground settlement caused by the tunneling excavation is extremely complicated. Based on the scientific project of Shenzhen Metro Line 5, a analysis and study is carried out on the field monitoring data of ground settlements in the whole Mining Method areas of Metro Line 5. The study contents and results of this paper are as follows:(1) With having investigated lots of literature about ground settlement caused by the metro Mining Method home and abroad, this paper emphatically analyzes the mechanism of ground settlements with Mining Method in the Water-enriched soil layers, making clear the influence factors of ground settlement with tunnel excavation.(2) Based on the condition of water-enriched and complex soil layers in Shenzhen, it analyzes the main factors which influence the ground settlements caused by Mining Method in this paper, the factors are as follow:the formation property(soil mass characteristic and groundwater), tunnel structural design(tunnel shape, thickness of covering soil, the superposition effect of adjacent tunnels, the stiffness of initial support), the construction method(excavation velocity, excavation method, dewatering and non-dewatering construction), ground loss, construction management and so on.(3) On the basis of processing and analyzing the ground settlement data which from the 19 mining excavated areas of Metro Line 5, it summarizes and obtains the law of longitudinal and transverse ground settlement in the whole Metro Line as follows:①As for the longitudinal ground settlements, the accumulative ground settlement in Xingdong-Liuxiandong section is maximum, the magnitude is 688.7mm; The settlement in the Buji-Baigelong section is minimum, the magnitude is 7.8mm; the average value is 82.3mm. We can see that the distribution of accumulative ground settlements in all mining excavated section is much non-uniform. If sets the 70mm as the control criteria, also only 53.37% monitoring-point's accumulative settlement value less than 70mm, so it shows that the ground settlement of Metro Line 5 is general large.②As for the transverse ground settlements, the influence area by tunnel excavation in Yangmei-Shangshuijing section is largest, the value reaches to 140.47m, it's average value is also biggest; the influence area in Fanshen-Lingzhi section is the smallest, the value is 28.63m; the average value in whole line is 46.78m. So the distribution of influence area in every Mining Method section is very non-uniform. By using statistical regression analysis, the number of monitoring section whose influence area between 20m to 40m is the most, which is 41.89% in total; the number of influence area lager than 60m is 18.93% in total. It is noteworthy that there are 5.41% sections'influence area larger than 100m.(4) With the in-depth analysis on the main factors of ground settlements in water-enriched and complex soil layers, this paper summarizes the law and influence of 5 main factors such as the soil categories and properties, the thickness of soil layers, the tunnel depth, the forms of tunnel cross section and the construction methods.①According to soil categories, the order of influence degree from big to small is as follows:plain fill, diluvial layer, hillside-deposit layer, eluvium layer, granite rocks.②The more thickness of plain fill, diluvial layer and hillside-deposit, the bigger ground settlement; The more thickness of eluvium layer and granite rocks, the smaller ground settlement.③The more deep of tunnel depth, the bigger of ground settlement, when the depth gets to about 12m, the ground settlement also reach to the maximum, then with the depth getting larger, the settlement change smaller.④Firstly, the larger of cross-section area, the bigger of ground settlement:the average settlement value with theⅡsection is bigger than theⅠsection, that is ground settlement caused by double-line tunnel bigger than the single-line tunnel; Secondly, the more stiffness of the supporting, the smaller of the ground settlement: as for the C, D and 2-2 type of cross-section, because they have used the bolt-shotcrete support, so their ground settlements are smaller in the same type section.⑤The order of the ability to control ground settlement from big to small is as follows:the both side heading method, the whole section excavation method, side drift method, CRD method, CD method, center drift method, ring-benching method, short-benching method, benching method. The ground settlement caused by ring-benching method is half or less than that of benching method, so it shows that the ring-benching method has obvious effect in controlling ground settlements.