Generally, under the same conditions, the cost of support form from low to high is:Slope excavation < soil nailing wall (composite soil nailing wall) < cement gravity retaining wall < steel cement mixing wall (composite soil nailing wall)SMW construction method) < row pile < floor wall。

一、Slope excavation

Figure 1 Real picture of slope excavation

1, the slope rate should be determined according to the nature of the soil layer and the digging depth, the digging depth is greater than 4m should use multi-stage slope, multi-stage slope should be set up a platform;In areas with good soil conditions, natural slope should be preferred.For foundation pit excavation in soft soil area, the slope surface of the slope should be equipped with steel mesh layer.

Figure 2 Multi-stage slope discharge diagram (Note: The excavation face needs to be set below the water table)

2、If the excavation face is below the water table,Well point dewatering measures should be taken at the top of the slope and the platformImprove slope stability;The top of the slope is equipped with a water barrier or drainage ditch to prevent the water outside the pit from flowing into the pit and eroding the slope;

3、If there is a local deep pit near the slope foot, the distance between the slope foot and the local deep pit should not be less thanA pit twice as deep falls deepIf it is not guaranteed, the slope stability should be checked according to the depth of the deep pit。

一、Soil nailing wall

If the site conditions can not meet the needs of large slope excavation, soil nailing wall support can be used to reduce the slope range。

图 3 Soil nailing wall real picture

1. There are steel pipe soil nails and reinforced soil nails in the form of soil nails, and the slope adopts the steel mesh jet concrete surface layer;

2. When there is water retention behind the soil nailing wall, drainage holes should be set up on the wall of the aquifer part or other drainage measures should be taken to reduce the water pressure behind the wall and improve the stability of the soil nailing wall;

3. When the prestressed bolt composite soil nailing wall is used, the prestressed bolt should adopt the steel strand bolt, and the bolt should be arranged in the upper part of the soil nailing wall;When it is used to strengthen the resistance of the surface layer to earth pressure, the anchor rod should be arranged in the place where the earth pressure is larger and the soil layer behind the wall is weak。

三、 Cement soil gravity retaining wall

Figure 4 Real picture of soil-cement gravity dam

1, gravity retaining wall form: generally choose a two-axis or three-axis cement mixing pile, mixing pile can be constructed by lap, lap length control in 150mm~200mm, the top surface of the retaining wall should be set up concrete panel;

2, under the condition of the general soil layer, the mixing depth of less than 16m should be preferentially selected with a lower cost of biaxial, more than 16m should be selected with three axes, encountered silt and other soft soil layer, the cement content is appropriately increased;

3, cement mixing pile should be arranged according to the grid, the recommended grid layout is shown in the figure (taking the two-axis as an example)。

FIG. 5 Plane layout diagram of gravity retaining wall of double-axis mixing pile

Four, steel cement mixing wall (SMW method)

FIG. 6 SMW construction method real picture

1, the general selection of two-axis or three-axis mixing pile, mixing pile as a water curtain should be built according to a set of holes to ensure the effect of water stop;

2, the general steel leasing period should be controlled within 6 months，When the lease is longer than 6 months，The rental cost of section steel is higher，At present, the main types of steel in the leasing market are H400×400, H500×200, H700×300, H800×300, etc，Can choose plug one jump one, plug two jump one, close plug form，Wall stiffness increases successively;

Figure 7 steel layout

3, the section steel should be reduced friction treatment, to facilitate the removal of the section steel after the completion of the basement structure。

五、 Cast-in-place piles surround the retaining wall

FIG. 8 Real picture of enclosure row pile

1. When the excavation surface of the foundation pit involves groundwater, a water curtain should be set outside the cast-in pile;

2, curtain selection: if the water curtain depth is less than 16m, it is recommended to use a lower cost of dual shaft;If the curtain depth exceeds 16m or there is a deep dense sand layer in the shallow layer, it is recommended to use a three-axis with better water sealing effect;

3, curtain depth: for the pit only need to dewatering the pit，In soft soil areas, the depth of the water curtain in the weak permeable layer of cohesive soil should be controlled at 6~7m below the foundation pit foundation (such as the project in Shanghai area);In case of silty, sandy soil and other (relatively) strong permeable layer，The aquifer thickness is moderate and the bottom depth is not deep，It may be considered to partition the aquifer with curtain (such as the areas along the Yangtze River in Nantong and Wuhan);If the stability of confined water of foundation pit does not meet the requirements, the confined water should be reduced，The thickness of the confined aquifer is not thick and the depth of the bottom layer is not deep，Water barrier curtains should also isolate confined aquifers as much as possible，In order to reduce the impact of depressurized precipitation on the surrounding environment;

FIG. 9 Profile of water seal curtain under different soil layers

4, in order to avoid the waste of supporting structure，The retaining pile, which is usually abandoned after the completion of foundation pit, can be used as a part of the main underground structure in the normal use stage，Form a "pile and wall"，The retaining pile can bear most of the earth pressure，Reduce the force on the exterior wall of basement，It can effectively reduce the thickness of basement wall and the number of side piles，Increase basement building area，Achieve energy saving and consumption reduction，It has better economic benefits。

Left figure 10 "pile wall integration" section diagram

Figure 11 on the right adopts the effect photo of "pile wall in one" basement

六Underground continuous wall

It is more economical to adopt "two walls in one" ground wall for foundation pit above three basement in soft soil area than pile row scheme。The so-called "two walls in one" means that in the construction stage of foundation pit engineering, the underground continuous wall is used as the retaining structure to play the purpose of retaining soil and stopping water;In the permanent use phase of the structure, the external wall of the main basement structure is no longer installed by setting an effective connection with the horizontal beam and plate members inside the main underground structure。The common thickness of the underground diaphragm wall is 600mm, 800mm, 1000mm, 1200mm。

Left figure 12 "Two walls in one" floor wall section schematic diagram 

Right picture 13 "Two walls in one" ground wall real picture

1. Reinforced concrete guide walls should be set on both sides of the ground wall;

2, when the shallow distribution of silty soil or sandy soil is thicker, both sides of the wall can be appropriately used cement soil mixing pile for pre-reinforcement of the trough wall。In principle, the depth of mixing pile only needs to be covered with silty soil or sandy soil layer，Avoid the troughing construction of the floor wall section without reinforcement (otherwise, the troughing construction should be carried out without reinforcement of the trough wall，It is easy to cause the filling coefficient of the floor wall to be too large and increase the amount of concrete pouring，The larger amount of chiselling of subsequent overfilling concrete will also increase the cost);

FIG. 14 Schematic diagram of reinforcement of the trough wall

3, the wall into the trough slurry wall: should be preferred natural slurry wall into the trough, if the natural slurry can not meet the requirements of the trough, then choose artificial slurry;

4. High pressure rotary jet pile can be set outside the trough section to strengthen the joint water sealing, and the anti-seepage structural measures of buttress post and drainage ditch combined with the construction of masonry wall should be set inside the wall。

FIG. 15 The anti-seepage scheme of the ground wall adopts buttress post combined with brick lining