Photovoltaic array support - cast-in-place pile foundation

Structure and Features

1.1 Basic form
The structure of the cast-in-place pile is shown in Figure 1.

Figure 1 - Cast-in-place pile structure — Figure 1 – Cast-in-place pile structure

1) Reinforcement ratio: when the diameter of the pile body is 300~2000mm, the reinforcement ratio of the normal section can be taken as 0.65%~0.2% (the high value for the small diameter pile); The reinforcement ratio of piles and end-bearing piles shall be determined according to the calculation, and shall not be less than the above specified values.
The pile body of the photovoltaic support foundation is cylindrical, and the diameter of the pile core is mostly 200~400mm. The length of the pile body is determined according to the load of the upper support, the distribution of rock layers, the characteristic value of friction resistance of the rock (soil) layer on the side of the pile, and the thickness of the frozen soil layer. The pile body is composed of pressure poured cement mortar or small stone concrete and reinforcement materials. Depending on the application, the reinforcing material used for micro piles can be steel bars, steel pipes or other shaped steels. The micro piles can be arranged vertically or inclined, arranged in rows, or crossed into a net-like arrangement.

2) Reinforcing length
a) End-bearing piles and foundation piles located on slopes and banks should be reinforced along the pile body with equal cross-section or variable cross-section.
b) The length of reinforcement for friction type cast-in-place piles should not be less than 2/3 of the pile length; when subjected to horizontal load, the length of reinforcement should not be less than 4.0/. (a is the horizontal deformation coefficient of the pile).
c) For foundation piles subjected to earthquakes, the length of the reinforcement of the pile body should pass through the liquefiable soil layer and the soft soil layer, and the depth into the stable soil layer should meet the relevant specifications.

d) For piles with negative frictional resistance and piles that rebound with the foundation soil due to the excavation of the foundation pit after the pile is formed, the length of the reinforcement shall pass through the soft soil layer and enter the stable soil layer, and the depth of entry shall not be less than ( 2~3) d.
e) Pull-resistant piles and piles subjected to pull-out force due to earthquake action, frost heave or expansion force should be reinforced with equal cross-section or variable cross-section through-length reinforcement.
f) For piles under horizontal load, the main reinforcement should not be less than 8p12; for compression piles and uplift piles, the main reinforcement should not be smaller than 6p10; the longitudinal main reinforcement should be evenly arranged along the periphery of the pile body, and its clear distance should not be less than 60mm.

g) The stirrups should be spiral, the diameter should not be less than 6mm, and the spacing should be 200~300mm; pile foundations subjected to large horizontal loads, pile foundations subjected to horizontal earthquake action, and pile body compressive bearing capacity are calculated considering the action of main reinforcement. , the stirrups within 5d below the pile top should be densified, and the spacing should not be greater than 100mm; when the pile body is within the liquefied soil layer, the stirrups should be densified; when considering the stress of the stirrups, the stirrup configuration should conform to the current national The relevant provisions of the standard “Code for Design of Concrete Structures” GB50010; when the length of the reinforcement cage exceeds 4m, a welded stiffening stirrup with a diameter of not less than 12mm should be installed every 2m. In photovoltaic power stations, the longitudinal stress steel bars of cast-in-place piles are generally HRB400, and the stirrups are generally selected HPB300.

3) The thickness of the pile body concrete and the concrete protective layer shall meet the following requirements. The strength grade of the pile body concrete shall not be less than C25, and the strength grade of the concrete precast pile tip shall not be less than C30; The thickness of the concrete protective layer of the main reinforced concrete shall not be less than 50mm; the thickness of the concrete protective layer of the pile body in the fourth and fifth types of environments shall comply with the relevant provisions of the current national standards “Code for Design of Concrete Structures of Port Engineering” JTJ267 and “Code for Design of Industrial Building Anti-corrosion” GB50046 .
The concrete strength grade of cast-in-place piles for fixed photovoltaic supports generally adopts C30.

1.2 Features

It is more convenient to form holes, the elevation of the top surface of the foundation can be adjusted according to the terrain, the elevation of the top is easy to control, the amount of concrete reinforcement is small, the amount of excavation is small, the construction is fast, and the damage to the original vegetation is small. However, the formation of holes in the pile foundation has certain requirements on geological conditions. For example, it is difficult to form holes in sandy soil areas or pebble areas, which are not suitable for cast-in-place piles, but are suitable for plastic and cohesive soil areas. The diameter of the pile is large, and it is not easy to drill, so the cast-in-place pile foundation is generally not used in bedrock areas.

Construction method

Figure 2 - Process flow of ordinary drilling grouting, Figure 3 - Process flow of micro-hole grouting — Figure 2 – Process flow of ordinary drilling grouting, Figure 3 – Process flow of micro-hole grouting

The construction method is shown in Figure 2 and Figure 3.

Ordinary bored cast-in-place piles are generally directly poured with concrete with a grade greater than or equal to C30, while micro-porous cast-in-place piles are generally made of crushed stone through grouting solidification or small stone concrete pouring.