In Construction, What is a Raised Foundation?

Deep foundation construction means that when the requirements of the building on the deformation and strength of the foundation cannot be met, the foundation treatment can be carried out according to the bad quality of the natural foundation; the harder soil layer below the foundation can also be used as the foundation support The force layer is designed as a deep foundation. Caisson and underground diaphragm wall. ^[1]

Deep foundation is a large burial depth. The lower solid soil or rock layer is used as the foundation of the bearing layer. Its role is to transmit the load relatively concentrated to the deep layer of the foundation. Distribute the load to the shallow layer of the foundation. Therefore, when the shallow soil of the construction site cannot meet the requirements of the building for the bearing capacity and deformation of the foundation, and the foundation treatment measures are not suitable, the deep foundation should be considered. There are several types of deep foundations, such as pile foundation, pier foundation, diaphragm wall, caisson and caisson.

Compared to shallow foundations, deep foundations are buried deeper. The structural form and construction method are relatively shallow and the foundation is complicated. The influence of the soil on the side of the foundation needs to be considered in the design calculation. ^[2]

There are several reasons for adopting deep foundation:

(1) Lack of necessary soil conditions for supporting foundation. When the soil conditions near the bottom of the building mostly do not allow the use of conventional

In terms of foundation burial depth, foundations with a depth greater than 5 m or greater than the width of the foundation are generally referred to as deep foundations in the project. Such as pile foundation, underground continuous wall, caisson foundation and caisson foundation.

1. Pile foundation

When the shallow foundation cannot meet the building's requirements for the bearing capacity and deformation of the foundation, and it is not appropriate to take foundation treatment measures, it is necessary to consider the following solid soil or rock layer as the deep foundation of the bearing layer. Pile foundations are the most widely used. Pile foundation refers to square, round or other shapes of thin and long piles made of various materials and buried underground. A pile foundation is usually composed of a pile and a cap on the top of the pile, and the upper load is transmitted to the deeper and harder foundation through the cap. The role of the pile foundation is to transfer the load through the pile to the deeper buried ground. Hard soil layers, or the frictional forces surrounding the piles, are transmitted to the foundation and are mostly used for high-rise buildings.

According to the force of the piles, the piles are divided into friction piles and end bearing piles. When the pile sinks into a weak soil layer to a certain depth, the upper load is transmitted and diffused into the soil around the pile through the friction of the soil on the side of the pile. The soil at the end of the pile also plays a certain supporting role. The soil supported by the pile tip is not very dense. When there is a certain relative displacement with respect to the soil. That is the friction pile. When the pile passes through the weak soil layer and transmits the load of the building through the pile to the hard soil layer or rock layer at the end of the pile, it is the end bearing pile. The weaker soil on the side of the pile has little friction on the pile body, and its friction is negligible.

According to the construction method, piles are divided into two types: precast piles and cast-in-situ piles. Prefabricated piles are piles of various materials and forms made at the factory or construction site, and then the piles are driven, pressed, screwed or vibrated (sometimes combined with high-pressure water) into the soil using pile sinking equipment. The cast-in-place pile is made by mechanically or manually forming a hole under the pile on the construction site, and then the concrete or reinforced concrete is poured into the hole. Compared with the precast pile, steel can be saved.

2. Diaphragm Wall

The underground continuous wall is formed by a special groove forming machine in the underground. Reinforced cages (nets) are placed in the groove and the underwater concrete is poured by the duct method to form a unit wall section. The wall sections that are sequentially completed are then used in a specific way. Connected to form a complete cast-in-place underground continuous wall. The underground diaphragm wall has a variety of functions such as soil retaining, anti-seepage and the main load-bearing structure; it can perform noiseless and vibration-free construction in difficult environments such as sinking wells, sheet pile support, etc .; it can enter the bedrock through various strata , The depth can reach more than 50m without taking measures to reduce groundwater, so it can be constructed in dense building groups. Especially for the basement building above two floors, it can cooperate with the "inverse construction method" construction to show its unique role.

3 Sunken Foundation

A sunken foundation is a deep foundation that uses a sunken well as the base structure to transfer the upper load to the foundation. A sunken well is a bottomless and coverless wellbore, which generally consists of cutting edges, well walls, and partition walls. Digging the soil in the caisson to sink it, after reaching the design elevation, concrete bottom sealing, core filling, and construction of the top cover constitute the foundation of the caisson.

The characteristics of caisson foundations are large burial depth, good integrity, good stability, large bearing area, and ability to withstand large vertical and horizontal loads. In addition, the caisson is the foundation. It is also a retaining and cofferdam structure during construction. Its construction technology is simple, the technology is stable and reliable, no special specialties are needed, and it can be used as a compensatory foundation to avoid excessive settlement. It is widely used in bridge pier and abutment foundations, underground pump rooms, pools, oil depots, mine shafts, large equipment foundations, high-rise and super-high-rise building foundations. However, the construction period of the foundation of the sinking well is long, and the quicksand phenomenon is easy to occur when pumping silt and fine sand in the well. Caused the sinking of the caisson; the surface of the large solitary rock, the trunk or the bottom of the well encountered during the sinking of the caisson was inclined too much, which would also cause some difficulties to the construction.

4 Caisson foundation

The caisson foundation is also called a pneumatic caisson foundation, which is the foundation of a bridge pier or other structure built with a pneumatic caisson. Pneumatic caissons are a bottomless box-shaped structure. Because compressed air is required to provide working conditions, they are called pneumatic caissons or caissons for short. ^[2]