Geotechnical engineering problems of the proposed Hangzhou Metro 1 Line?

What are the geotechnical problems of the proposed Hangzhou Metro 1 Line? The following Zhong Da consulting bidding teacher will answer it for your reference.

I. Introduction

Metro 1 Line starts from Linping Town, Yuhang District in the north, passes through Qiaosi, Pengbu, East Railway Station, East Bus Station, Wulin Square, Yan 'an Road, Chengzhan Road, Chengzhan Road, Tao Qiu Road, Qianjiang Road and Fuxing District, then crosses Qiantang River to Xiaoshan District via the Fourth Qianjiang Bridge, turns to the urban road along Beitang Road, and finally reaches Shushan Depot, with a total length of about 52 kilometers. Among them, the sections from xinglong village Station to Fenghuang Station and Xiaoshan City are underground lines, and the buried depth is about 10 ~ 20m below the present surface. The rest are overhead lines or ground wires. Among them, some underground lines are downtown areas with dense crowds, dense buildings and heavy traffic.

Second, the composition and characteristics of foundation soil along the line

Hangzhou is located in the northern part of Zhejiang, at the intersection of Hangjiahu Plain and the mountainous area in western Zhejiang, downstream of Qiantang River and at the southern end of Beijing-Hangzhou Canal. Its geographical location is 3015' north latitude and12010' east longitude. Due to the influence of many transgressions and regressions in geological history, and the mountains and Qiantang River passing through the city, Hangzhou has diverse landforms and complex soil layers.

As far as the areas passing along the subway are concerned, there are two main landforms. One is the alluvial plain near Qiantang River, which belongs to the alluvial silt and sandy land area at the mouth of Qiantang River. Due to the different accumulation age and consolidation conditions and different properties, the longitudinal density changes from loose to medium density, and the thickness is generally about 20m; Under it are muddy soft soil and cohesive soil deposited on land and sea; The underground depth is about 40 ~ 50m, and it is a pebble bed accumulated on the riverbed of the ancient Qiantang River, which is in a medium-dense ~ dense state. The buried depth of bedrock at the bottom is generally about 50 ~ 65m below the ground. The other is the cohesive soil area where land and sea cross, mainly concentrated in the area from Genshanmen Station to Lu Zhonghe Station and Xiaoshan downtown. The stratum is soft and hard alternately, generally, the upper part is mainly soft cohesive soil of about 20m, and the bedrock buried in the lower part is about 40 ~ 45m away from the ground.

According to a large number of drilling data, in-situ test and indoor geotechnical test results, the natural water content of Hangzhou silty soft clay is generally about 30 ~ 45%, the natural void ratio is generally about 0.85 ~ 1.50, the cone tip resistance of double-bridge cone penetration test is about 500 ~ 800 kPa, the compression modulus is about 1.5 ~ 3.0 MPa, and the foundation bearing capacity fk. Mud part is mixed with mud, or interbedded. The properties of Shanghai soft soil are similar to those of muddy cohesive soil. However, the properties of silt and sand (mainly distributed in the east of the city) formed in the estuaries on both sides of Qiantang River have changed greatly due to the differences in accumulation age, sedimentary environment and consolidation conditions. The data show that the density is generally loose to medium density, the water content is generally about 23 ~ 35%, the pore ratio is about 0.8 ~ 1. 1, the resistance at the cone penetration end of double bridge is generally 2000 ~ 9000 kPa, and the SPT blow count is generally 8 ~ 20 /30cm. The particle composition is mainly silt, generally clay silt and sandy silt, with fine upper part and coarse lower part, which conforms to the general sedimentary law. Its compression modulus is 6 ~ 20 MPa, and the bearing capacity of foundation fk is about 80 ~ 220 kpa.

To sum up, the overall characteristics of each soil layer within the excavation range of underground lines are: high water content, large void ratio, high compressibility and low strength; Silty soft clay has high sensitivity and weak water permeability; Silt and sandy soil have good water permeability, which is easy to cause sand flow and piping.

Three, the geotechnical engineering problems that may be encountered in the process of underground line excavation

(A) the strength of the foundation soil

The foundation soil in the excavation range is mainly saturated silt, sandy soil and soft clay, which generally has the characteristics of low strength, so shield tunneling is easier. Due to the strength difference of silt, sandy soil and soft clay, and the plastic cohesive soil in some sections (such as Yan 'an section) at a depth of about 15 ~ 20m, there are two kinds of strata with different strengths in the heading face, which is easy to cause the soft layer to discharge soil too quickly during the heading process, leading to stratum sinking or shield machine shifting towards the line direction. At the same time, due to the low strength, the tunnel should be lined in time and corresponding water-stop measures should be taken to prevent the stress release and settlement of the strata in the heading face. Hangzhou soft clay still has high sensitivity, so it has obvious thixotropy and rheology. Under the dynamic action, it is easy to destroy the soil structure and reduce its strength. In addition, it takes a long time for soil to be consolidated by drainage. If the construction is improper, it will easily lead to large uneven settlement after construction. Therefore, it is necessary to strictly control the offset during construction and try to avoid hunting.

(2) Deformation of foundation soil layer

Tunnel foundation soil is mainly silt and muddy soft soil, which has the characteristics of low strength and high compressibility, so it is necessary to check the strength and deformation of foundation soil. At the same time, silt, sandy soil and soft clay have different deformation characteristics, and their compression settlement is also different. When the tunnel passes through two strata, it is easy to cause settlement difference near the interface. In addition, the consolidation characteristics of the two foundation soils are obviously different. The excess pore water pressure of silt and sandy soil dissipates quickly, the consolidation time is short, and the consolidation period of soft clay is long. Different post-construction settlements caused by construction lead to different settlements.

In addition, soft clay also has creep characteristics, with large late settlement and long time, which will produce soft and large deformation during construction and operation. Some subway lines built in soft soil strata in China also have similar engineering problems. Therefore, we should pay full attention to deformation in design and construction.

(3) Groundwater problem

There are three types of groundwater in the area: upper stagnant water, shallow phreatic water and deep confined water, and the groundwater level is generally about 1 ~ 4m below the surface. The confined water aquifer is a deep gravel layer, and the water level generally changes periodically every year. The confined water head is generally about 6 ~ 7m below the surface. Soft clay is a stratum with weak water permeability within the scope of tunnel excavation, while silt and sand have good water permeability, and its permeability coefficient is generally10-5 ~10-4 cm/s. In the process of tunnel excavation, lining must be done in time, and grouting should be carried out to stop water, so as to prevent silt and sand from piping under the action of head difference. The problem of groundwater is particularly prominent in the foundation pit excavation of underground stations, which must be paid enough attention to. Because of the deep excavation of the foundation pit, the influence of the lower confined water must be considered.

(D) Underground station foundation pit excavation problems

Because underground stations are mostly concentrated in downtown areas, the surrounding buildings are dense, there are many underground pipelines, and the environmental conditions are complex. The underground stations are deeply buried and the foundation pit is deep, generally around 10~20m ~ 20m; Poor soil conditions and high groundwater level. When the foundation pit is excavated, the soil on the pit wall cannot stand on its own feet under the action of soil and water pressure, so effective supporting measures must be taken to avoid collapse and affect the safety of the project and the surrounding environment. According to local experience, for this deep and large foundation pit, diaphragm wall or row piles are generally used for support, combined with internal support or anchor pulling, and at the same time, water curtain and drainage must be done well. The surrounding environment must be effectively monitored during construction.

Because there are many underground stations, the amount of foundation pit works is large, and the general conventional methods have high cost and long cycle, we should try our best to develop and utilize new technologies and new processes, such as new pile types, new water sealing and dewatering measures.

(five) the impact of engineering construction on the environment and its prevention and control

The construction of underground line will cause the change of stress field in the surrounding soil, the foundation soil of tunnel will rebound, the thixotropy of soft clay will change the structural strength of soil, and the precipitation will cause the re-consolidation of soil weight, which will have an impact on the surrounding environment. When the tunnel construction is close to the ground buildings, it will cause adverse effects such as collapse and settlement.

Shield construction can be widely used in urban underground engineering, mainly because it can control the influence of construction on the surrounding environment in a small range, but it cannot be completely eliminated. With the advance of shield, foundation deformation will generally occur, such as unbalanced water and soil pressure on the excavation surface, excessive discharge of mud, disturbance of surrounding strata caused by shield advance, groundwater level drop and water leakage. All these actions will create a loose zone in a certain range above the tunnel, which will cause ground subsidence and even collapse.

Hangzhou subway will be built in saturated silt, sandy soil and soft clay. In order to ensure the safety of the surrounding environment and tunnel construction, it is necessary to adopt appropriate construction technology, control the advancing route and speed, and try to avoid disturbing the surrounding soil. Before construction, the use of buildings and structures along the line should be investigated in detail, especially the pile foundation and underground pipelines, and the whole process of dynamic monitoring should be carried out for the adjacent buildings and structures, ground roads and underground pipelines within the influence range, especially for Yan 'an Road, downtown road and other old cities. This work is particularly important. Buildings (structures) that may be affected but cannot be demolished should be reinforced and protected in advance.

(VI) Geotechnical engineering investigation problems

"Engineering construction, investigation first", investigation is prediction and prediction. Detailed, comprehensive, accurate and reliable geological survey data is extremely important for subway construction. On this basis, the unfavorable factors that may be encountered in the construction direction of shield tunneling can be predicted in advance, such as strata, obstacles, groundwater, etc., so as to take corresponding effective measures in advance to ensure the smooth and safe construction.

The geotechnical investigation of Hangzhou subway construction should be mainly soft clay, silt and sandy soil, and the depth of investigation should generally be less than 30m, but it should be deepened for underground stations. Focus on finding out the distribution and law of two types of strata, their strength and deformation characteristics, dynamic characteristics under cyclic dynamic load, particle composition and permeability of silt and sand, creep of soft clay, seismic liquefaction characteristics of saturated silt, etc. Groundwater should also be determined.

Due to the limitations of indoor geotechnical tests, in-situ testing methods, such as pressuremeter test, flat dilatometer test, vane shear test, pore water pressure measurement, static cone penetration test and dynamic cone penetration test, should be widely used in subway survey to obtain accurate and reliable test data.

Because there are a large number of engineering projects built or under construction along the line, these engineering data should be fully collected, analyzed, screened and utilized, which can save engineering investment and maximize the measurement accuracy; At the same time, we can also analyze and take effective measures to protect these adjacent buildings.

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