Expandable independent foundation and strip foundation for photovoltaic array support

Expandable independent foundation

1.1 Structure and Features
1.1.1 Basic form
The structure of the independent foundation shall meet the following requirements:

1) The height of the edge of the foundation should not be less than 200mm. In the case of a tapered foundation, the slope in both directions should not be greater than 1:3; the height of each step of the stepped foundation should be 300~500mm.
2) The thickness of the cushion should not be less than 70mm, and the concrete strength grade of the cushion should not be lower than C10.
3) The minimum reinforcement ratio of the stressed steel bars of the independent foundation should not be less than 0.15%, the minimum diameter of the stressed steel bars of the bottom plate should not be less than 10mm, and the spacing should not be greater than 200mm, nor less than 100mm. When there is a cushion, the thickness of the steel bar protection layer should not be less than 40mm; if there is no cushion, it should not be less than 70mm.
4) The concrete strength grade should not be lower than C20.
5) When the length of the side of the reinforced concrete independent foundation under the column is greater than or equal to 2.5m, the length of the reinforcing steel bars on the bottom plate may be 0.9 times the length of the side or the width, and should be staggered (Figure 1 and Figure 2).
6) The pre-embedded anchor bolts on the upper part of the independent foundation, the thread length is not less than 70mm, and the anchoring length is not less than 360mm. The photovoltaic support column is connected with the pre-embedded anchor bolts on the upper part of the independent foundation.

Figure 1 - Independent foundation diagram, Figure 2 - Sectional view of independent foundation 1-1 — Figure 1 – Independent foundation diagram, Figure 2 – Sectional view of independent foundation 1-1

1.1.2 Features
In the design and practice of photovoltaic power generation module bracket foundation, the extended independent foundation has the following characteristics:
1) The requirements for topographic conditions and geological conditions are relatively high, and the construction method is relatively cumbersome.
2) The foundation type is reliable, which is more beneficial to the bearing layer with smaller foundation bearing capacity.
3) The use of formwork during construction is large, the labor cost is high, and the construction period is long. At present, there is basically no such type of foundation for photovoltaic power stations.

1.2 Scope of application
Due to the large amount of earthwork excavation, concrete pouring, steel bar consumption and mechanical labor costs during the construction process, the economic benefit is poor, and it is only suitable for the foundation of dual-axis tracking photovoltaic support.

1.3 Construction method
The flow chart of the independent foundation construction process is shown in Figure 3. The independent foundation is deeply buried, requiring a large amount of reinforced concrete engineering, a lot of labor, a large amount of earthwork excavation and backfilling, a long construction period, and a large destructive force to the environment. Therefore, this kind of foundation has great limitations. of photovoltaic power plants have been rarely used.

Figure 3 - Process flow chart of independent foundation construction — Figure 3 – Process flow chart of independent foundation construction

Bar base

2.1 Structure and Features
2.1.1 Basic form
The schematic diagram of the strip foundation is shown in Figure 4.

Figure 4 - Schematic diagram of the strip foundation — Figure 4 – Schematic diagram of the strip foundation

The structure of the strip foundation under the column shall not only meet the structural requirements of Section 8.2 for the expanded independent foundation, but also meet the following requirements:
1) The height of the strip foundation beam under the column should be 1/4~1/8 of the column spacing. The thickness of the wing plate should not be less than 200mm. When the thickness of the wing plate is greater than 250mm, the variable-thickness wing plate should be used, and the slope should be less than or equal to 1:3.
2) The end of the strip foundation should protrude outward, and its length should be 0.25 times the first span.
3) In addition to meeting the calculation requirements, the longitudinal reinforcement bars at the top and bottom of the strip foundation beam should be fully connected according to the calculated reinforcement, and the bottom through-length reinforcement bars should not be less than 1/3 of the total cross-sectional area of the bottom stressed reinforcement bars.
4) The concrete strength grade of the strip foundation under the column should not be lower than C20.
5) At the junction of T-shaped and cross-shaped base plates of reinforced concrete strip foundations, the transverse stress bars of the floor are only arranged along one main stress direction, and the transverse stress bars in the other direction can be arranged to the width of the floor in the main stress direction. 1/4; at the corners, the transverse stress reinforcement of the bottom plate should be arranged in two directions.

2.1.2 Calculation
The calculation of the strip foundation under the column shall meet the following requirements:
1) On a relatively uniform foundation, the rigidity of the upper structure is good, the load distribution is relatively uniform, and the height of the strip foundation beam is not less than 1/6 of the column spacing, the foundation reaction force can be distributed in a straight line, and the internal force of the strip foundation beam It can be calculated as a continuous beam. At this time, the bending moment of the side span and the bending moment of the first inner support should be multiplied by a factor of 1.2.
2) When the requirements of Article 1 are not met, it should be calculated according to the elastic foundation beam.
3) For the cross-strip foundation, the column load at the intersection can be distributed according to the requirements of the rigidity or deformation coordination of the cross beam; the internal force can be calculated separately according to the above provisions of this article.
4) Check the shear bearing capacity of the foundation beam at the edge of the column.
5) When there is torque, the torsion resistance should be calculated.
6) When the concrete strength grade of the strip foundation is lower than that of the column, the local compressive bearing capacity of the top surface of the strip foundation beam under the column should be checked.

2.1.3 Features
1) The basic form is suitable for the design requirements of most large-scale photovoltaic grid-connected power stations in deserts. There is no need for large-scale site leveling, and the site terrain is arranged to ensure that the distance between the front and rear rows meets the requirements of the specification.
2) The excavation depth of the strip foundation trench is 200~300mm, and there is no need to carry out a lot of earthwork excavation and backfilling work on site, which can effectively reduce the progress of the foundation pit excavation process due to obstacles such as rocks, etc., and can be used in the foundation construction process. Save a lot of time.
3) The processing technology of the formwork and steel bar of the strip foundation is simple. The concrete is poured and formed at one time, and the work can be started after the on-site bottoming. However, compared with the prefabricated hole ground anchor pile and the screw pile foundation, the construction is still cumbersome.
4) The strip-shaped foundation has a simple structure and is set on the ground, which can gather multiple construction teams on a large working surface at the same time.
5) The strip foundation is conducive to ensuring the overall stability of the upper support structure, and the economy is moderate.
By setting the foundation beam between the front and rear columns of the photovoltaic support, the center of gravity of the foundation is transferred between the front and rear columns, and the anti-overturning force arm of the foundation is increased, which can only resist the overturning moment of the photovoltaic support caused by the wind load through its own weight; The contact area of the foundation soil is large, and it is suitable for areas with flat sites and low groundwater levels. Due to the relatively large surface area of the foundation, the buried depth is generally 200~300mm. Suitable for soil areas with slightly uneven settlement.

2.2 Construction method

Place the strip foundation directly on the natural ground on a flat site, or place it on the natural ground after a little leveling of the site, which can effectively reduce earthwork excavation and filling and reduce vegetation damage. However, the requirements for the flatness of the site are relatively high, and the anti-overturning resistance is basically provided by the weight of the foundation. Compared to cast-in-place freestanding foundations, the amount of concrete work has increased.

If a prefabricated strip foundation is used, which is prefabricated in the factory or on-site, the pouring quality is easy to guarantee, and the construction speed on site is faster. However, compared with the cast-in-place foundation, the work of handling and hoisting and on-site secondary adjustment and positioning is increased. The overall cost of prefabrication is higher than that of cast-in-place foundation. . In view of the complex situation of the photovoltaic site, it is recommended to give priority to the use of cast-in-place foundations and one-time construction in place without on-site water use constraints.