A foundation pit is a closed planar shape formed by combining several straight or curved lines. Due to the diversity of foundation pit plan shapes and the differences in excavation depth, each foundation pit project has its own characteristics. The foundation pit boundary described in this section refers to the side profile of the foundation pit and its adjacent area, and the foundation pit boundary form refers to the specific retaining or support method adopted to ensure the stability of the pit wall. The same foundation pit may have only one boundary form, or it may be a combination of multiple boundary forms. Common boundary forms are shown in Figures 23-17a, 23-17b, 23-17c, 23-17d, 23-17e, and 23-17f.

Through long-term engineering practice, there are currently a variety of economical and reasonable foundation pit boundary forms suitable for different geological conditions and foundation pit depths. Different boundary forms require different earthwork excavation methods. This section mainly describes the principles of layered earthwork excavation at the foundation pit boundary and the excavation methods according to the foundation pit boundary form. The construction conditions of layered excavation at the foundation pit boundary correspond to the conditions set in the support design, and it can be judged based on actual conditions whether the construction conditions are consistent with the design conditions.

I. Earthwork excavation methods for sloped foundation pits

1. Earthwork excavation for a foundation pit with a single-stage slope over the full depth range

When the site permits and the stability of the soil slope can be ensured, slope excavation may be adopted. Due to regional differences, the requirements for slope excavation vary considerably; for example, the Shanghai region stipulates that the excavation depth of a single-stage sloped foundation pit shall not exceed 4.0 m. The slope gradient for slope excavation shall be determined by calculation based on geological and hydrogeological data, the duration for which the slope is left standing, surcharge loading at the slope crest, and other conditions, and each region shall determine the allowable depth and gradient for slope excavation in accordance with relevant regulations. When the soil conditions are good and the groundwater level is low, slope protection treatment may be omitted on the excavated slope surface of the foundation pit; when the foundation pit slope is exposed for a long time and the groundwater level is high, necessary slope protection measures may be taken to prevent the slope from being eroded by rainwater and intruded by groundwater. Slope protection may adopt methods such as wire mesh cement mortar, wire mesh fine-stone concrete, wire mesh shotcrete, and soil improvement.

When the geological conditions are relatively good and the excavation depth is relatively shallow, a vertical one-time excavation method may be adopted, and its typical excavation method is shown in Figure 23-18a. When the geological conditions are poor, or the excavation depth is relatively large, or the performance of the excavator is limited, a layered excavation method may be adopted, and its typical excavation method is shown in Figure 23-18b. The earthwork excavation method for a foundation pit with a single-stage slope over the full depth range can be applied to open-cut construction projects.‍

2. Earthwork excavation for a foundation pit with multi-stage slopes over the full depth range

When the site permits and the stability of the soil slope can be ensured, multi-stage slope excavation may be adopted for relatively deep foundation pits. Due to regional differences, the requirements for multi-stage slope excavation vary considerably; for example, the Shanghai region stipulates that the excavation depth of a multi-stage sloped foundation pit shall not exceed 7.0 m. The stability of each stage of slope and the overall stability of the multi-stage slopes shall be determined by calculation based on geological and hydrogeological data, the duration for which the slopes are left standing, loads at the slope crest, and other conditions. Foundation pits using multi-stage slopes shall generally adopt slope protection measures, and slope protection may adopt methods such as wire mesh cement mortar, wire mesh fine-stone concrete, wire mesh shotcrete, and soil improvement.

When the geological conditions are relatively good and the depth of each stage of slope is relatively shallow, layered excavation may be carried out using the height of each stage of slope as the layer thickness, and its typical excavation method is shown in Figure 23-19a. When the geological conditions are poor, or the depth of each stage of slope is relatively large, or the performance of the excavator is limited, each stage of slope may also adopt a layered excavation method, and its typical excavation method is shown in Figure 23-19b. The earthwork excavation method for a foundation pit with multi-stage slopes over the full depth range can be applied to open-cut construction projects.‍

II. Earthwork excavation methods for foundation pits with retaining structures

1. Earthwork excavation for a foundation pit with retaining structures but without internal bracing over the full depth range

Foundation pits with retaining structures but without internal bracing generally include foundation pits using soil nail support, composite soil nail support, soil anchor support, cantilever sheet-pile retaining walls, cement-soil gravity retaining walls, and steel sheet pile anchored support. The earthwork excavation method for a foundation pit with retaining structures but without internal bracing over the full depth range can be applied to open-cut construction projects.

For the excavation of the foundation pit boundary using soil nail support, composite soil nail support, or soil anchor support, a layered excavation method shall be adopted and carried out alternately with the support construction. The excavation depth of each layer of earthwork is generally the vertical spacing of the soil nails or anchors, and construction shall proceed according to the principle of excavating one layer of earthwork and installing one row of soil nails or anchors. If the vertical spacing of the soil anchors is relatively large, the earthwork between the upper and lower anchors shall be excavated in layers. Earthwork excavation shall closely coordinate with the support construction, and the next layer of earthwork may only be excavated after the soil nail or anchor support is completed and cured to meet the design requirements. The layered excavation method for foundation pits with soil nail support, composite soil nail support, and soil anchor support is shown in Figure 23-20.

For the excavation of the foundation pit boundary with cantilever sheet-pile retaining walls and cement-soil gravity retaining walls, the layered excavation method shall be determined based on geological conditions, excavation depth, surrounding environment, control requirements for surcharge loading at the pit edge, excavator performance, and other factors. If the excavation depth of the foundation pit is relatively shallow and the surrounding environmental conditions are relatively good, a vertical one-time excavation method may be adopted; taking a cantilever sheet-pile retaining wall as an example, its typical excavation method is shown in Figure 23-21a. For foundation pits in the Shanghai region that adopt vertical one-time excavation, the excavation depth generally does not exceed 4.0 m. If the excavation depth of the foundation pit is relatively deep, or the environmental protection requirements of the surrounding area are relatively high, the excavation of the foundation pit boundary may adopt a vertical layered excavation method; taking a cement-soil gravity retaining wall as an example, its typical excavation method is shown in Figure 23-21b.

For the excavation of the foundation pit boundary with steel sheet pile anchored support, a layered excavation method shall be adopted. The first layer of earthwork shall first be excavated to 200 mm to 300 mm below the bottom of the anchor wale, and only after the anchored support is formed and prestress is applied in accordance with the design requirements may the lower layer of earthwork be excavated; its typical excavation method is shown in Figure 23-22.

For some foundation pit projects with retaining structures but without internal bracing, due to site condition constraints or the special needs of the support works, a combined support method may be adopted in the vertical direction. The vertically combined support method may be selected from forms such as soil nail support, composite soil nail support, soil anchor support, cantilever sheet-pile retaining walls, cement-soil gravity retaining walls, and steel sheet pile anchored support, and the layered earthwork excavation method may be determined by reference to each support form.‍

2. Earthwork excavation for a foundation pit with retaining structures and internal bracing over the full depth range

Internal bracing systems can be divided into bracing systems with wales and bracing systems without wales. Bracing systems with wales may adopt steel pipe struts, section steel struts, or reinforced concrete struts; bracing systems without wales may adopt steel pipe struts or section steel struts; circular wales are a special type of internal bracing system. Foundation pits that use horizontal structures in place of temporary internal bracing also belong to a form of foundation pit with internal bracing over the full depth range, including forms that use horizontal structural beams or horizontal structural beam-slabs in place of temporary bracing. The earthwork excavation method for a foundation pit with internal bracing over the full depth range can be applied to open-cut or covered (subsurface) excavation construction projects.

For foundation pits with internal bracing constructed by the top-down method, the boundary shall adopt a layered excavation method, and the layering principle is to excavate the next layer of earthwork after each strut is constructed. The excavation depth of the first layer of earthwork is generally from the ground surface to the bottom of the first strut, the excavation depth of each intermediate layer of earthwork is generally the vertical spacing between two adjacent struts, and the excavation depth of the last layer of earthwork shall be from the bottom of the lowest strut to the pit bottom. The layered excavation method for the boundary of a foundation pit with internal bracing constructed by the top-down method is shown in Figure 23-23a.

For foundation pits constructed by the bottom-up method, the boundary also adopts a layered excavation method. The layering principle is similar to that of the top-down method, and the layered excavation method is shown in Figure 23-23b. Because the horizontal structures replacing temporary bracing are permanent structures, corresponding formwork construction schemes shall be adopted according to the structural construction requirements, and forms such as plywood timber formwork, composite steel formwork, and mud-bottom formwork may generally be used. The plywood timber formwork method ensures the construction quality of the structure, while the mud-bottom formwork method makes structural quality difficult to control, and mud-bottom formwork is generally used only under special circumstances. When the plywood timber formwork method is adopted, the commonly used support form is the short bent-frame formwork support method, so the layered thickness of earthwork shall also take into account the space requirements for short bent-frame formwork support, and the depth of layered excavation shall be kept at a certain distance from the bottom elevation of the structure.

For some foundation pit projects with internal bracing, due to site condition constraints or the special needs of the support works, a vertical combination of the top-down method and the bottom-up method may be adopted, or a vertical combination of the support method with retaining structures but without internal bracing and the support method with retaining structures and internal bracing may also be adopted, and the layered earthwork excavation method may be carried out by reference to the earthwork excavation methods under each retaining and support form.‍

III. Earthwork excavation methods for foundation pits combining sloping and retaining structures

1. Earthwork excavation for a foundation pit with a single-stage slope in the upper section and retaining structures without internal bracing in the lower section

To save construction costs and shorten the construction period, for foundation pits with relatively good geological conditions and surrounding environmental conditions, a relatively shallow excavation depth, and available space for sloping, the boundary form of a single-stage slope in the upper section and retaining structures without internal bracing in the lower section may be adopted. A foundation pit with a single-stage slope in the upper section and retaining structures without internal bracing in the lower section is a vertical combination of single-stage sloping and the support form of retaining structures without internal bracing. The lower-section support with retaining structures but without internal bracing generally includes forms such as soil nail support, soil anchor support, cement-soil gravity retaining walls, and cantilever sheet-pile retaining walls. The earthwork excavation method for a foundation pit with a single-stage slope in the upper section and retaining structures without internal bracing in the lower section can be applied to open-cut construction projects.

The boundary excavation of this type of foundation pit shall adopt a layered method. Taking as an example the upper section with a single-stage slope excavated without layering and the lower section with soil nail support excavated in layers, its typical excavation method is shown in Figure 23-24a; taking as an example the upper section with a single-stage slope excavated without layering and the lower section with a cement-soil gravity retaining wall excavated without layering, its typical excavation method is shown in Figure 23-24b.‍

2. Earthwork excavation for a foundation pit with a single-stage slope in the upper section and retaining structures with internal bracing in the lower section

A foundation pit with a single-stage slope in the upper section and retaining structures with internal bracing in the lower section, or with horizontal structures replacing internal bracing, is a vertical combination of single-stage sloping and the support form of retaining structures with internal bracing, and the boundary excavation of this type of foundation pit shall adopt a layered method. The earthwork excavation method for a foundation pit with a single-stage slope in the upper section and internal bracing in the lower section can be applied to open-cut or covered (subsurface) excavation construction projects.

Taking as an example the upper section with a single-stage slope excavated without layering and the lower section with internal bracing constructed by the top-down method and excavated in layers, its typical excavation method is shown in Figure 23-25a; taking as an example the upper section with a single-stage slope excavated without layering and the lower section with horizontal structures replacing internal bracing constructed by the bottom-up method and excavated in layers, its typical excavation method is shown in Figure 23-25b.‍

3. Earthwork excavation for a foundation pit with multi-stage slopes in the upper section and retaining structures without bracing in the lower section

The excavation of a foundation pit with multi-stage slopes in the upper section and retaining structures without bracing in the lower section shall adopt a layered excavation method. The earthwork excavation method for a foundation pit with multi-stage slopes in the upper section and retaining structures without internal bracing in the lower section can be applied to open-cut construction projects.

Taking as an example the upper section with a two-stage slope excavated in layers and the lower section with soil nail support excavated in layers, its typical excavation method is shown in Figure 23-26a; taking as an example the upper section with a two-stage slope excavated in layers and the lower section with a cement-soil gravity retaining wall excavated without layering, its typical excavation method is shown in Figure 23-26b.‍

4. Earthwork excavation for a foundation pit with multi-stage slopes in the upper section and retaining structures with internal bracing in the lower section

A foundation pit with multi-stage slopes in the upper section and retaining structures with internal bracing in the lower section, or with horizontal structures replacing internal bracing, is a vertical combination of multi-stage sloping and the support form of retaining structures with internal bracing, and the boundary excavation of this type of foundation pit shall adopt a layered method. The earthwork excavation method for a foundation pit with multi-stage slopes in the upper section and internal bracing in the lower section can be applied to open-cut or covered (subsurface) excavation construction projects.

Taking as an example the upper section with a two-stage slope excavated in layers and the lower section with internal bracing constructed by the top-down method and excavated in layers, its typical excavation method is shown in Figure 23-27a; taking as an example the upper section with a two-stage slope excavated in layers and the lower section with horizontal structures replacing internal bracing constructed by the bottom-up method and excavated in layers, its typical excavation method is shown in Figure 23-27b.‍