In coastal and inland alluvial plain areas, the soil is often a very thick (50-60m deep) soft soil layer. When the upper structure load is large, this type of foundation often cannot be directly used as a bearing layer, and the low-com pressibility bearing layer is very deep. When using general pile foundations, a pile hammer with a large impact force must be used when sinking piles. It will be difficult to adapt to conventional reinforced concrete and prestressed concrete piles. For this reason, piling pipes are often used to reinforce the foundation. Therefore, pile pipes have been widely used in various countries.
The characteristics of pile pipe are:
(1) Light weight, good rigidity, convenient loading, unloading, transportation, and stacking, and not easy to damage;
(2) High bearing capacity. Due to the high strength of steel, it can be effectively driven into hard soil layers, the body is not easy to be damaged, and a large single pile bearing capacity can be obtained;
(3) The pile length is easy to adjust. The pile length can be adjusted by lengthening or cutting as needed;
(4) The amount of soil discharge is small and the impact on adjacent buildings is small. The lower end is open. As the pile is driven in, the amount of soil squeezed into the pile pipe is greatly reduced compared with the solid pile, and the disturbance to the surrounding foundation is also small, which can avoid soil uplift; the vertical displacement of the first pile and the horizontal displacement of the pile top can also be greatly reduced;
(5) The joint connection is simple. Electric welding is used, which is easy to operate, high in strength, and safe to use:
(6) The quality of the project can be fast in construction. However, pile pipes also have the disadvantages of large steel consumption and high project cost; the pile driving machinery and equipment are complex and have large vibrations and noises; the pile materials are poorly protected and prone to corrosion, etc., so sufficient technical and economic analysis and comparison should be made when selecting.
Piling pipe structure, type and specifications
The steel pipe is generally made of ordinary carbon steel with a tensile strength of 402MPa and a yield strength of 235.2MPa, or selected according to design requirements. According to the processing technology, there are two types of SSAW steel pipes and LSAW steel pipes. Due to the high rigidity of SSAW steel pipes, they are more commonly used in engineering. In order to facilitate transportation and limited by the height of the pile frame, the steel pipe is often composed of an upper section pile, a lower section pile and a few middle section piles. The length of each section is generally 13m or 15m.
Specifications and performance parameters of pile pipes
The diameter of pile pipes ranges from medium 406.4 to ф2032.0 mm, and the wall thickness ranges from 6 to 25 mm. The specifications and performance of commonly used pile pipes are shown in Table 7-92. They should be selected after comprehensive consideration of engineering geology, load, foundation plane, upper load and construction conditions. Commonly used domestic pipes include medium 406.4 mm, medium 609.6 mm and medium 914.4 mm, with wall thicknesses of 10, 11, 12.7, 13 mm, etc. Generally, the upper, middle and lower piles often use the same wall thickness. Sometimes, in order to enable the pile top to withstand huge hammer stress and prevent radial instability, the wall thickness of the upper pile can be appropriately increased, or a flat steel reinforcement hoop with a width of 200~300 mm and a thickness of 6~12 mm can be welded on the outer ring of the pile pipe . In order to reduce the frictional resistance of the pile pipe sinking and prevent the end from being damaged due to deformation when penetrating into the hard soil layer, a reinforcing hoop is also set at the lower end of the piling pipe, centered on the 406.4~Ф914.4mm steel pipe, with a height of 200~300mm.
In general, the selection of pile pipes needs to be considered in combination with the above factors, including engineering geological conditions, expected loads, foundation plane conditions, superstructure loads and specific construction conditions, to ensure the safety, stability and bearing capacity of the foundation project.
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