What are the methods of soil wall support in earthwork construction?

Earthwork classified soil is a natural substance, and there are many kinds and classification methods. At present, in the foundation of building foundation engineering, according to the hardness of soil and the difficulty of excavation, the division of soil is shown in Table 4-3. Earthwork can be divided into manual earthwork and mechanized earthwork according to construction methods. It is the main component of building construction, including site leveling, foundation pit (trench) excavation, ground filling and subgrade filling.

Balance allocation of earthwork is an important content of earthwork planning and design. It mainly deals with where the excavated soil needs to be transported (used or dumped) and where the soil needed for filling needs to be taken for comprehensive coordination. Its purpose is to determine the allocation direction and quantity of earthwork in the excavation and fill area under the condition of minimum earthwork transportation volume or earthwork transportation cost, so as to shorten the construction period and improve economic benefits. The principle of earthwork balanced distribution is as follows: 1. Digging and filling are basically balanced, while digging and filling, reducing repeated transshipment; 2. The sum of the product of excavation (filling) volume and transportation distance should be as small as possible, so as to minimize the total earthwork transportation volume or transportation cost; 3. Partition allocation should be coordinated with the whole field allocation, and the overall situation should not be hindered by local balance; 4. Earthwork allocation should be combined with the construction of underground buildings or structures as far as possible; 5. Choose the appropriate deployment direction and transportation route, so that there is no convection and turbulence in earthwork transportation, which is convenient for mechanized construction; 6. When the project is constructed by stages and batches, the earthwork balance of the preliminary project should be combined with the needs of the later project, and the quantity and stacking position used should be considered so as to be deployed nearby. The earthwork allocation method is: 1. Divide the earthwork allocation area. That is to say, draw the dividing line between excavation area and fill area on the general plan, that is, the zero line, and draw several deployment areas in the excavation area and fill area. 2. Calculate the earthwork volume of each allocation area and indicate it on the allocation diagram. 3. Calculate the average transportation distance of each allocation area, that is, the distance between the earthwork center of gravity in the excavation allocation area and the earthwork center of gravity in the fill allocation area. 4. Draw the earthwork allocation diagram, and indicate the allocation direction, earthwork volume and average transportation distance in the diagram. 5. List the earthwork balance sheet.

Site leveling refers to the work of digging, filling and leveling the whole construction site before the foundation pit (groove) of the building is excavated. Before the site is leveled, the design elevation of the site should be determined first, the excavation and filling quantities should be calculated, the balanced allocation of excavation and filling should be determined, and the earthwork construction scheme should be determined according to the project scale, construction period and existing earthwork machinery conditions. When the site is leveled, the engineering quantity is usually calculated by grid method, and the specific steps are as follows: 1. On the topographic map, the whole construction site is divided into grids with side length of 10 ~ 40m; 2. Calculate the natural ground elevation of each corner; 3. Determine the design elevation of the site, and calculate the design elevation of each corner according to the requirements of drainage slope; 4. Determine the digging and filling height of the grid corners, that is, the difference between the natural elevation of the ground and the design elevation; 5. Determine the zero line, that is, dig and fill the dividing line; 6. Calculate the earthwork volume of each square; 7. Calculate the earthwork volume of the site slope, and finally get the total amount of excavation and filling of the whole site.

In order to prevent landslide and ensure construction safety, when the excavation depth of foundation pit (groove) exceeds a certain limit, the earth wall is often made into a slope, that is, an earthwork slope. The earthwork slope is expressed by the ratio of its excavation depth h to its slope bottom width b, namely:

Slope gradient = h/b =1/(b/h) =1/m.

Where m = b/h is called the slope coefficient.

According to the Code for Construction and Acceptance of Earthwork and Blasting Engineering (GBJ20 1-83), when the soil is uniform and the groundwater level is lower than the elevation of foundation pit (trench) or pipe trench bottom, unsupported straight walls can be used for slope excavation, and the excavation depth should not exceed the provisions in Table 4-4; When the soil quality is good and uniform, and the groundwater level is lower than the elevation of the foundation pit (trench) or the bottom of the pipe trench, the maximum slope of the unsupported slope with excavation depth within 5m shall comply with the provisions in Table 4-5.

During the excavation of foundation pit (trough), due to some factors, it is sometimes not allowed to put the slope according to the required slope width, or there is a requirement to prevent groundwater from infiltrating into the foundation pit, and when excavating the deep foundation pit (trough), the amount of earthwork increased by putting the slope is too large. At this time, it is necessary to adopt the method of soil wall support and set up supports for excavation to prevent landslides. When the foundation pit (trench) or pipe trench needs to be supported by earth wall, it should be selected and designed according to the excavation depth, soil quality, groundwater level, construction method and adjacent buildings and structures. The pit wall is supported by steel (wood), steel (wood) sheet piles, reinforced concrete slope protection piles and reinforced concrete diaphragm wall. Steel (wood) support can be divided into intermittent support (sparse support) and continuous support (dense support) according to the different placement methods of retaining plates. According to the form of support, it can be divided into horizontal support, anchor support and inclined column support. As shown in Table 4-6.

Construction Drainage During earthwork construction, construction drainage must be done well. Construction drainage can be divided into two categories: eliminating surface water and reducing groundwater level. The discharge of surface water can be carried out by setting ditches, intercepting ditches or building earth dikes; Collecting well dewatering method and well point dewatering method can reduce groundwater level. 1, sump dewatering method. Collecting well dewatering method is to set a collecting well at the bottom of the foundation pit, and dig a drainage ditch around or in the center of the bottom of the foundation pit to make water flow into the collecting well, and then pump it out, as shown in Figure 4- 1. Collecting wells should be set below the foundation range and upstream of groundwater, with one set every 20 ~ 40 meters. The diameter or width of the collecting well is 0.6 ~ 0.8m, and the depth should always be 0.7 ~ 1m lower than the excavation surface as the excavation deepens. The water collecting shaft wall can be simply reinforced with bamboo and wood. When digging to the design elevation, the collecting well should be 1 ~ 2 meters below the foundation pit bottom, and a gravel filter layer should be laid to avoid pumping away mud and sand and stirring the soil at the bottom of the pit. The water pump can be centrifugal pump, submersible pump and soft water pump.

2. Well point dewatering method. Well point drainage is often used in aquifer construction below groundwater level. Well-point dewatering method is a method of burying a certain number of filter tubes (wells) around the foundation pit before excavation, and pumping water with pumping equipment to keep the excavated soil dry all the time. The well point types used in well point dewatering method are: light well point, jet well point, electro-osmotic well point, tube well point, deep well point and so on. According to the permeability coefficient of soil, it is required to reduce the water level depth and equipment conditions during construction. Please refer to Table 4-7 for selection.

Bulldozer construction Bulldozer is a kind of earth-moving machinery equipped with working devices such as bulldozer (shovel) on tractor. It can cut, push and unload soil independently, and can also be used as an auxiliary machine in cooperation with other earth-moving machinery. It is suitable for excavating 1 ~ 3 soil, with the economic transportation distance within 100 meters and the highest efficiency of 50 ~ 60 meters. It is mainly used for site leveling, foundation pit (trench) with excavation depth of 1.5m, trench backfilling, dam with pile height of 1.5m, etc. During construction, in order to improve production efficiency, the following methods can be adopted: 1. Bulldozers repeatedly cut and bulldoze the soil on each operation line, so that shallow trenches are gradually formed on the ground, with a depth of about 1 m and a ridge width of about 50 cm. When a plurality of grooves are pushed out, the soil ridges are pushed into the grooves from behind and then carried out. This method can reduce the soil leakage on both sides of the blade, thus increasing the amount of bulldozing 10% ~ 20%. 2. Downhill bulldozing method. That is, the bulldozer cuts the slope and pushes it down, but the slope shall not exceed 15, so as to avoid the difficulty of climbing when retreating. When there is no natural slope, the front soil can be pushed first, thus gradually forming the bulldozer terrain of downhill. 3. Multi-knife soil delivery method. Refers to the method that in hard soil, when the cutting depth is not large, the soil can be piled up in one or several intermediate positions with bulldozers first, and then pushed to the unloading area in batches. However, it should be noted that the stacking distance should not be greater than 30 meters, and the stacking height should be 2 meters. 4. Side-by-side bulldozing. When the site is leveled in a large area, 2 ~ 3 bulldozers work in parallel, that is, the amount of bulldozed soil can be increased by 15% ~ 40%. However, the blade spacing is required to be 15 ~ 30 cm, and the average transportation distance is not more than 50 ~ 75 meters and not less than 20 meters.

LHD construction LHD can independently shovel, move, unload and level the soil, which is suitable for large-scale site leveling, large-scale foundation pit excavation and subgrade filling with small terrain fluctuation and small slope within the range of 15. It is most suitable for excavating loose soil and ordinary soil with water content less than 27%. For hard soil, pre-loosening is needed before excavation, but it is not suitable for construction in gravel layer, frozen soil layer and swamp area. According to the different walking mechanisms, it can be divided into traction type and self-propelled type. Among them, the haul distance of the traction scraper is 800 meters, and the economical haul distance of the self-propelled scraper is 800 ~ 1500 meters. In construction, the following methods are often used to improve production efficiency: 1. Downhill shovel method. That is, the scraper shovels down the slope. The slope is generally 3 ~ 9, and the shovel thickness is about 20 cm. During construction, be careful not to turn sharply on the slope to avoid overturning. 2. Cross shovel method. When digging in hard soil, the horizontal shovel method can be used to shovel soil at intervals by reserving soil ridges. 3. Staggered shovel method. Suitable for digging hard soil. The specific method is shown in Figure 4-3. 4. Shovel-assisted method. It refers to the method that the self-propelled scraper is equipped with a bulldozer to push the scraper and then drag the rod to assist the shovel in hard soil. However, the width and length of auxiliary earth borrowing by the shovel blade should not be less than 20m and 40m respectively. 5. Wave shovel method. It means that when the bucket begins to shovel, the shovel depth can be larger. With the increase of resistance, the shovel depth gradually decreases, and then the shovel thickness deepens or decreases, forming a wavy shovel surface. As shown in Figure 4-4.