Photovoltaic array support – screw pile foundation

Photovoltaic array support - screw pile foundation
  1. Structure and Features

1.1 Basic form
Spiral piles are generally cut, deformed, welded, pickled, hot-dipped and other processes to produce qualified piles. Among them, pickling and hot-dip galvanizing are important anti-corrosion treatment processes, which directly affect the service life of screw piles. The processing level of the screw pile directly determines the service life of the metal ground pile, such as the quality of the selected welded pipe, the quality level of the welding, whether there are defects such as pores, slag inclusions, cracks and undercuts, which will affect the future service life and subsequent processing of the screw pile. Pickling is an important anti-corrosion basis, and the quality of hot-dip plating, such as the time of hot-dip plating and the quality of surface treatment, all affect the quality of anti-corrosion treatment of ground piles. Under normal circumstances, the screw pile can be used for 40 to 80 years. The environmental conditions and use methods of the use process also affect the service life of the pile, such as the acidity and alkalinity of the soil, and whether the operation process is appropriate. Improper use will cause damage to the surface of the metal pile, destroy the metal protective layer, accelerate the corrosion of the metal pile, and reduce the service life. Figure 1 shows the spiral base structure

Figure 1 - Spiral infrastructure
Figure 1 – Spiral infrastructure

1.2 Force calculation

The bearing capacity of the screw pile is affected by many factors such as geological conditions, pile type parameters, construction torque, etc. It is still difficult to calculate it accurately. At present, there is no specific calculation method specified in the specification. Generally, the bearing capacity of the screw pile is calculated after modifying the empirical formula according to the change of the pile type parameters (especially the blade spacing). The single-disk bearing capacity method assumes that the bearing capacity failure occurs on each single blade, which is like a steel disk, and the ultimate bearing capacity of a single pile is the sum of the ultimate bearing capacity of each steel disk in the deep foundation failure mode. The soil side friction resistance of the intermediate shaft is no longer considered. When the diameter of the central axis of the upper non-steel disc segment is larger and the length is longer (not less than 5 times the diameter of the last steel disc), the side friction resistance of this segment can be considered. Figure 2 is a schematic diagram of the construction of the screw pile.

Figure 2 - Schematic diagram of screw pile construction
Figure 2 – Schematic diagram of screw pile construction

In the formula: Quk——the vertical compressive/pullout ultimate bearing capacity of a single pile;
Tuk——the standard value of ultimate ultimate bearing capacity of foundation piles when the pile group is non-integrative failure;
Qik——the vertical bearing capacity of a single disc;
Ah – the projected area of ​​the blade;
c——The cohesive force of soil, the parameters of the inner soil layer under/upper 3 times the diameter of the disk are taken for the compression/uplift resistance;
N.c, Nq—the bearing capacity coefficient, the effective stress internal friction angle of the soil in the range of the soil layer under/upper 3 times the diameter of the disk is taken for the compression/pullout resistance;
q——Soil self-weight pressure in the depth range of foundation;
Qs——The upper limit of soil bearing capacity.

1.3 Features

1) Wide applicability
In hard soil layers, due to the limited dead weight of conventional drilling rigs, it is impossible to apply more feed pressure to the drill bit. And the screw pile adopts the power head device, and the feeding capacity of the power head is strong. According to statistics, in the same stratum, the pile forming speed of the screw pile is 5~10 times that of the rotary drilling rig, and the pile forming speed can reach 1m/min, which has obvious advantages compared with the traditional circulating drilling rig, which effectively guarantees the project. progress, saving construction period and reducing construction investment.

2) Easy to move and move
The crawler mechanism of the screw pile can easily move the drilling rig to the desired position, which is not as cumbersome as the traditional circulating drilling rig, thus speeding up the construction progress and being highly adaptable to the site.

3) Pile hole alignment is convenient and accurate
This is impossible for traditional circulating drilling rigs. During the alignment process, the operator can use advanced electronic equipment in the cab to accurately achieve alignment, so that the drilling rig can reach the best drilling state.

4) Not suitable for bedrock areas or areas with more gravel
The bedrock is hard, and the screw pile cannot penetrate the bedrock to provide bearing capacity; the area with more gravel is easy to wear the galvanized layer of the screw pile during the piling process, so that the screw pile loses its anti-corrosion protection. However, for soft soil areas where a small amount of bedrock is distributed, screw pile foundations can still be used. In a small number of bedrock areas, the lead-in construction scheme is adopted, that is, the drilling rig is used to turn the hole on the bedrock according to the design size. After reaching the design depth, fine stone concrete or cement mortar is poured into the hole, and finally the screw pile foundation is inserted into the hole. In this way, the economy and convenience of the screw pile foundation can be used, and the difficulty that the screw pile foundation cannot be directly used for a small amount of bedrock is solved.

5) Outstanding environmental protection features
(a) The traditional Chinese foundation adopts reinforced concrete foundation, which requires a large area of ​​excavation, which can easily cause damage to a large area of ​​natural grasses and plants, and requires a lot of fresh water maintenance. Big waste.
(b) The penetration depth of the screw pile is small, and there is no need for a traditional drilling rig to set up a mud pool in the site, and the civilized construction is controlled.
(c) After use, the steel screw pile can be pulled out for treatment and then recycled, so that the cost of materials and the pollution to the environment can be minimized.

  1. Construction method

The basic construction process of the screw pile is as follows:
(1) First, measure and lay out the line according to the general construction plan provided by the design department and confirmed by Party A, and use the GPS measuring instrument, total station, theodolite and other measuring instruments to lay out the line, and the accuracy of the lay-out needs to be controlled within 10mm;
(2) According to the layout results of the design drawings, determine the position of each pile to be driven, and mark it with easy-to-find objects such as small flags and wooden strips;
(3) The pile driver can quickly reach the pile position through its own walking function and mast luffing mechanism;
(4) Send the screw pile into the guide rod and adjust the verticality of the screw pile;
(5) The power head device of the drilling rig provides torque for the screw pile, and the pressurizing device transmits the pressure to the pile by means of pressurization power until the drilling reaches the design elevation.

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