At present, the foundations of photovoltaic supports in-ground power stations are basically reinforced concrete cast-in-situ pile foundations. Spiral steel pile foundations are rarely used. The following editor will introduce this "familiar stranger" for everyone to do photovoltaic Refer to the selection of bracket foundation.
First, the type of spiral steel pile
The spiral steel pipe is a special-shaped pile with a complex geometric surface. It consists of a central steel shaft connected with several spiral blades. According to the blade arrangement form, it is generally divided into layered blade type spiral steel piles and continuous blade type spiral steel piles.
Second, the advantages of spiral steel piles
Aiming at the shortcomings of traditional reinforced concrete cast-in-situ pile foundations for photovoltaic supports, earthwork excavation, environmental damage, long construction period, high site conditions, and difficult removal, spiral piles have the following advantages:
(1) The construction is fast and economical. Spiral steel piles are generally constructed using special machinery. A spiral steel pile can be completed in a few minutes. The pile foundation can be used immediately after construction. There is no problem with concrete construction and maintenance, which saves the salary of a single basic staff. And the construction equipment has strong mobility, quick entry and exit costs are low, and can reach any difficult terrain or work in narrow spaces.
(2) Convenient quality control. Spiral steel piles are all produced in the factory, to avoid the uneven quality of the basic process due to the different technical levels of the construction workers during the construction of the reinforced concrete cast-in-situ piles.
(3) Environmentally friendly and recyclable. In the construction of steel piles, no earthwork development is required, and direct screwing is also performed, and there are almost no vibration and noise, and the ecological damage is the lowest. The steel pile can be pulled out in reverse after use without leaving any underground obstacles, and the site can be restored.
(4) The adjustability of mounting the photovoltaic module bracket is high. At present, the ground power station is basically a mountain photovoltaic, and the terrain is mostly uneven. In order to ensure the optimal inclination of the photovoltaic module, the bracket needs to be easy to install and adjustable. Because the spiral steel pile is directly connected to the bracket with bolts, the adjustability is relatively concrete The embedded bolt foundation is large, reducing the installation difficulty of the photovoltaic bracket.
Problems with spiral steel piles
The current problem of spiral steel piles is the lack of customized standards, such as spiral steel pile pricing methods, steel materials, warranty years, and after-sales service. There are no clear requirements, leading to uneven quality services. In addition, spiral piles have different "algorithms" in terms of pricing methods, steel materials, and warranty years, and there are large differences among spiral pile manufacturers. The lack of such customized standards makes it difficult to guarantee the quality of spiral piles.
In addition, the design life of photovoltaic power stations is generally 25 years, and whether the corrosion protection of spiral steel piles can be guaranteed for 25 years has not been verified. If it cannot be guaranteed, there will be a problem of pile replacement during the operation period, which will increase the operating cost.
Fourth, the connection of spiral steel piles
Spiral steel piles are mainly connected with various types of upper brackets by bolts, flanges, and other types, and they can adjust the height of the column flexibly. The flange of the spiral steel pile is made of a steel sheet of a certain thickness and welded with the central steel shaft as a whole. Bolt holes are reserved on the connecting flange to facilitate connection with the powerhead of the installation machine during installation and to connect with the upper support. Some pile types cannot be flanged, but bolt holes are reserved in the upper part, and bolts are connected to the powerhead through the bolt holes during installation, and can also be used to connect with the upper support after installation.
V. Structural reliability of spiral steel piles
Structural reliability refers to the safety, applicability, and durability of the structure. The following editors will introduce the structural reliability of spiral steel piles from these three aspects.
(1) Safety of spiral steel piles
Safety mainly refers to the bearing capacity of spiral steel piles, which is safe without damage.
According to relevant literature, the change of blade pitch ratio (blade pitch/blade diameter) of spiral steel piles is an important factor affecting the ultimate pullout and compressive bearing capacity of steel piles. According to experimental research, the ultimate bearing capacity of the pile is optimal when the blade spacing ratio S / D = 3 ~ 4. For small spiral steel piles commonly used in photovoltaic foundations, a blade pitch of S / D = 3 is recommended. The pull-out and compressive bearing capacity of a two-blade pile increase with the increase of the blade diameter (blade diameter square), and has an approximately linear relationship. From the perspective of bearing capacity, double-blade piles are better than continuous blade piles and have certain construction advantages. The compressive design value of the single pile bearing capacity of the spiral pile is ≤10KN, and the tensile design value is ≤20KN, which meets the design requirements. The "Design Specification for Photovoltaic Power Stations" also stipulates spiral piles, but only specifies the blade thickness and width: the steel pipe wall thickness of the spiral pile should not be less than 4mm, and the blade extension width (diameter) is greater than the latter. At 20mm, the blade thickness should be greater than 5mm, and when the blade width is less than 20mm, the blade thickness should not be less than 2mm, and the ratio of width to thickness should not be greater than 30. The length of the spiral steel pile is the same as the length of the reinforced concrete cast-in-situ pile, which is generally about 2m.
(2) Applicability of spiral steel piles
Applicability mainly refers to the problems of deformation and cracks of spiral steel piles.
Because the strength of the spiral steel pile itself is relatively high, the photovoltaic bracket system does not have a particularly large load on the spiral steel pile, and the spiral steel pile generally does not have excessive deformation and cracks.
(3) Durability of spiral steel piles
Durability mainly refers to the corrosion protection of spiral steel piles, which is an important issue for spiral steel piles.
At present, the anticorrosion of metal components in photovoltaic power stations generally adopts hot galvanized anticorrosive treatment measures. According to the "Design Specification for Photovoltaic Power Stations", when the hot-dip galvanized anticorrosive treatment is adopted for spiral piles, the galvanized thickness should conform to the current national standard "Technical Requirements and Test Methods for Hot-dip Galvanized Coatings of Metal Covered Steel Parts" According to the regulations, the minimum galvanized thickness should meet the following requirements:
Although there are other standards, the anticorrosion of spiral steel piles must meet the 25-year design service life requirements of photovoltaic power plants, and it will take time to test.
Application range of spiral steel piles
According to the above introduction, the "Design Specification for Photovoltaic Power Stations" also gives the recommended scope of spiral piles: when the groundwater level of the site is low, the depth of the stable holding layer is large, the construction in winter, the terrain is undulating or the site requires high ecological restoration, In this case, the foundation of the support can be a spiral pile.