With the rapid development of the national economy, a large number of high-rise and super high-rise buildings have emerged in China. The functions of high-rise buildings are becoming increasingly complex, and underground parking garages, civil air defense facilities, and code provisions generally require a buried depth of no less than 1/18 to 1/15 of the total building height, among others. All of these have given rise to the issues of deep foundation pits and deep foundation pit support.

For Beijing, land is even more scarce, and underground space development is increasing (a large number of subway lines are under construction or about to be built, many sections of which require open-cut construction), so a large number of deep foundation pits will appear, and their scale will be relatively large. The foundation pit support method will directly affect project cost and safety. Therefore, the form of the support structure has become a very important issue.

1. Characteristics of foundation pit engineering in the Beijing area

(1) Foundation pits are being excavated deeper and deeper. Due to the emergence of high-rise and super high-rise buildings, or in order to use land more effectively, building investors have to develop underground space. In the past, even building 1 to 2 basement levels in large cities was uncommon, and it was even rarer in medium-sized cities. Now 3 or 4 basement levels are quite common in Beijing. Therefore, foundation pit depths are mostly greater than 10 m.

(2) The main high-rise and super high-rise buildings in the city are mostly concentrated in small sites with high building density, dense population, and congested traffic, so the construction conditions for foundation pit support works are very poor. There are often permanent buildings and municipal public facilities nearby that must be protected, and slope excavation is not possible, so the requirements for foundation pit stability and displacement control are quite strict.

(3) Foundation pit construction on adjacent sites, such as piling, dewatering, and earth excavation, will produce mutual influence and constraints in each construction step, increasing the factors that induce accidents.

(4) The stratum conditions are relatively good. The eastern part is generally dominated by fill and clay, while the western part is dominated by fill and sand, gravel, and pebble layers, with clay layers interbedded in between. The soil conditions are relatively good, which is conducive to foundation pit construction.

(5) The hydrogeological conditions are complex. Affected by many factors, the groundwater occurrence conditions in the Beijing area are complex, and the water level varies greatly. Among them, there are numerous underground pipelines in the urban area of Beijing, especially leakage from sewage, water supply, and heating pipelines, which often forms perched water, and its impact on foundation pit safety cannot be ignored.‍

2. Main support types for foundation pits in the Beijing area

As far as current deep foundation pit support methods are concerned, they can generally be divided into the following 6 major categories:

(1) Natural slope excavation;

(2) Cantilever (pile) support structures;

(3) Single (multiple) fulcrum combined support structures;

(4) Soil nail wall support structures;

(5) Gravity-type support structures;

(6) Other types of support structures. These mainly include portal-frame support structures; arch-type composite support structures; anchor-net support structures; caisson support structures; reinforced cement-soil wall support structures; freezing-method support, etc.

(7) Composite structures.
In the Beijing area, the first four categories and composite structures are the most commonly used, and this article will focus on introducing these five commonly used forms.

2.1  Slope excavation and its applicable scope

When the foundation soil conditions are good, the construction site is spacious, the excavation depth is not deep, and there are no buildings near the foundation pit, the natural slope method can be used to excavate the foundation pit. This method has low cost and fast construction speed, but it has great limitations (when the above conditions are not met and the slope ratio is large, it greatly increases the amount of earthwork, brings a lot of trouble to construction, and at the same time increases project costs, so it is not very suitable in such cases).

Figure 1: Natural slope excavation

The safety conditions required for natural slope excavation are:

K = anti-sliding force / sliding force = 1.1 to 1.2, where K is the stability safety factor of the slope.

According to the soil quality, the slope is cut at a certain slope ratio (single slope or stepped slope). Cement mortar or concrete (mortar) can be sprayed to protect the slope surface (if the soil is poor, a wire mesh can be hung first and then mortar sprayed), or the slope surface can be covered with geomembrane.

Because the soil stability in the Beijing area is relatively good, some foundation pits have been excavated with near-vertical slopes to a depth of 5 m to 6 m. At present, only a few foundation pits use slope excavation. In most cases, slope excavation is used in combination with other forms.

2.2  Cantilever (pile) support structures and their applicable scope

Cantilever (pile) support structures are generally suitable for foundation pit projects with relatively good soil and relatively shallow excavation depth. Because this structure relies on the embedded depth and the structure's own bending resistance to ensure the safety of the foundation pit, if the excavation depth is relatively deep, it will inevitably result in too great an embedded depth or too large a pile cross-section, making it uneconomical.

In addition, because horizontal outward displacement is a fifth-order function of the foundation pit excavation depth, the excavation depth will have a considerable impact on horizontal outward displacement. Therefore, it should be used with caution for deep foundation pit excavation or when there are strict requirements on foundation pit deformation.

The forms include: reinforced concrete soldier piles (sheet piles), steel sheet piles, double-row piles, diaphragm walls, etc.

Although the diaphragm wall belongs to the cantilever support form, its applicable scope is far greater than that of ordinary cantilever support. Because the diaphragm wall has high wall stiffness and good integrity, and its own structural deformation is small, it can be used for ultra-deep retaining structures, as well as various complex stratum conditions such as soft clay and sand below the groundwater level, and environments with strict requirements on deformation and displacement. However, due to its high cost, great construction difficulty, and the relatively good soil conditions in the Beijing area, it is generally rarely used.

(1) Bored cast-in-place pile support has the following characteristics: low noise during construction, no soil squeezing, and little impact on the surrounding environment; high pile strength, high stiffness, good support stability, and relatively small deformation; however, because the piles cannot be completely tight against each other, in areas with high water levels, corresponding dewatering and waterproofing measures should be adopted.

The soil between piles may be left untreated, or wire mesh may be hung and mortar sprayed.

Figure 2: Cantilever bored cast-in-place pile support

Figure 3: Cantilever bored cast-in-place pile support - wire mesh hung between piles and mortar sprayed

(2) Steel sheet piles are made of hot-rolled section steel with locks or clamps. By connecting such steel sheet piles to one another, a steel sheet pile wall is formed. The greatest feature is that after the foundation pit construction is completed, the supporting steel sheets can be pulled out and reused. In addition, construction is simple and the construction period is relatively short.

Steel sheet piles with locks can not only retain soil but also retain water.

Figure 4: Steel sheet piles with locks

Figure 5: Machinery for inserting and extracting steel sheet piles

(3) Double-row piles: a foundation pit support form that has emerged in recent years. In some special cases, when anchor rods, soil nails, and bracing are restricted by actual conditions and cannot be implemented, and single-row cantilever piles are difficult to meet bearing capacity requirements, the double-row pile support form can be adopted, forming a spatial portal-frame support structure system through vertical flexible piles, capping beams, and connecting beams.

Figure 6: Double-row piles - pile top connecting beams

Figure 7: Double-row pile support

(4) Diaphragm walls are mainly used in the following scenarios:

(a)  Projects adjacent to buildings and structures with high protection requirements, and with relatively high requirements for deformation and waterproofing of the foundation pit itself;

(b)  Projects where space within the site is limited, the basement exterior wall is extremely close to the red line, and other retaining forms cannot meet the requirements for leaving construction operation space;

(c)  Projects where the retaining structure also serves as part of the main structure and where there are relatively strict requirements for waterproofing and impermeability;

(d)  When the top-down construction method is adopted and above-ground and underground construction are carried out simultaneously, diaphragm walls are generally used as the retaining wall;

(e)  In ultra-deep foundation pits, such as 30 m to 50 m deep foundation pit projects, when other retaining bodies cannot meet the requirements, diaphragm walls are often used as the retaining body.

Figure 8: Diaphragm wall + anchor rod support

Figure 9: Diaphragm wall support

Figure 10: Diaphragm wall support - construction process

2.3  Single (multiple) fulcrum combined support structures and their applicable scope

This type of structure includes the common tieback anchor support and internal bracing support. When the soil of the foundation pit is relatively poor or the excavation depth is very deep, cantilever support can hardly meet strength and deformation requirements, or even if it does, the project cost is much higher than this form, so tieback anchors or internal bracing should be adopted.

The forms of internal bracing include horizontal cross bracing, corner bracing, diagonal bracing, etc. Internal bracing generally uses steel pipes or large-section composite section steel as struts.

Figure 11: Concrete internal bracing

Figure 12: Steel bracing

In the Beijing area, the most common deep foundation pit support is piles + anchor rods, and a large amount of engineering practice shows that this form is safe, reliable, and relatively economical. Anchor rod support is an active reinforcement and stabilization technology for rock and soil. As the main body, the anchor rod is anchored at one end into stable soil (rock) and connected at the other end to various forms of support structures. Through the tensile action of the rod body and the frictional anchoring effect of the deep soil layer on the anchor rod, the potential of the deep stratum is mobilized to achieve the purpose of stabilizing the foundation pit and buildings.

When piles alone cannot meet the requirements for limiting the outward displacement of the foundation pit, combining the two and making full use of their advantages will greatly limit the horizontal outward displacement of the foundation pit, thereby increasing the support depth of the foundation pit.

Figure 13: Soldier piles + anchor rods

2.4  Soil nail wall support structures and their application scope

Soil nail wall technology consists of reinforced soil, a group of soil nails installed in the soil, and a sprayed concrete facing, forming a retaining wall similar to a gravity retaining wall to resist the earth pressure and other loads behind the wall, thereby ensuring the stability of the excavation face. The shear strength of soil is relatively low, and its tensile strength is almost negligible, but the soil has a certain structural integrity. During excavation, there may be a critical height at which the excavation face remains vertical. Beyond this depth, or under the action of other factors, sudden overall failure will occur.

Soil nail wall technology involves installing soil nails of a certain length and density in the soil to work together with the soil, making up for the deficiency of the soil's own shear strength. Through the interaction between the soil and the soil nails, the potential structural strength of the soil itself is fully exerted, the deformation and failure behavior of the slope is changed, and overall stability is significantly improved.

Figure 14: Soil nail wall construction process

Figure 15: Soil nail wall support

Because soil nail support supports while excavating, occupies a relatively small site, can make full use of the self-stabilizing capacity of the in-situ soil, greatly saves project cost, and can shorten the construction period, it has been quite widely used in Beijing and is a relatively mature technology. Generally, slope excavation and soil nail support can be used in combination. Soil nail wall support is suitable for cohesive soil, silt, miscellaneous fill, non-loose sand, pebble soil, etc. above the groundwater level or after artificial dewatering, and is not suitable for foundation pit support in muddy soil or in soil layers below the groundwater level that have not been dewatered. In the Beijing area, the depth of foundation pits supported by soil nail walls generally does not exceed 15 m. In addition, if soil nail support is to be used as permanent support, the anti-corrosion issue of the steel bars must be considered.

3. Composite support structures

For the composite support commonly used in the Beijing area, two or more of the above slope protection methods are reasonably combined according to site conditions, making up for each other's shortcomings, giving full play to their respective advantages, and forming various combined support methods. For example, a commonly used composite support method for foundation pits in the Beijing area is: soil nail wall in the upper part and pile-anchor support in the lower part; composite soil nail support technology formed by soil nails and anchor rods or micro-piles. At present, this type of support method has been successfully applied in many projects and has achieved very good results. This will be a development trend for foundation pit support types in the future.

Figure 16: Composite soil nail wall support

Figure 17: Soil nail wall + pile-anchor support

Figure 18: SMW method pile support

4. Comparative analysis of support methods

In foundation pit support, choosing the foundation pit support method is the first problem to be solved, and it is directly related to the economic and safety issues of the project. It should be comprehensively considered in light of engineering geological conditions, the surrounding environment, the characteristics of various support methods, project cost, construction period, and other factors.

In the Beijing area, the soil conditions are relatively good, and the following principles should be followed:

Generally, when conditions permit, slope excavation should first be considered; when it cannot meet the conditions and the surrounding environment of the foundation pit is not very sensitive to deformation, flexible support such as soil nail walls should be given priority; when there are relatively strict requirements on outward displacement, cantilever support should first be considered (when the foundation pit excavation depth is not large, generally not greater than 6 m); otherwise, fulcrum support may be considered (when the surrounding environment is relatively favorable and there is space for anchor rods to extend, and they should not extend into the foundations of surrounding buildings, the anchorage section should extend into relatively stable strata; otherwise, internal bracing should be adopted); when the soil conditions are poor, the construction environment is poor, and the foundation pit is relatively deep, diaphragm walls should be considered.

If there is groundwater, the issue of water cutoff should be comprehensively considered. Dewatering or a waterproof curtain may be adopted, or a support method that can both retain soil and retain water may be used directly.‍