The foundation design of the tower is closely related to mechanized construction, and the relevant design work should be carried out in detail and in-depth in combination with the construction method, construction load, geological conditions, transportation conditions, etc. The design of the tower foundation should be safe and reliable, and try to be economical and environmentally friendly. Among them, safety and reliability means that the foundation must be stable under various working conditions specified in the regulations, and there must be appropriate safety reserves, even in some abnormal cases, it should have a certain level of reliability, and under the action of external load, the tower foundation can not produce too large, which may cause the deformation of the bearing capacity of the tower to seriously decline. Economic and environmental protection refers to the selection of the basic design scheme with better comprehensive benefits after considering various external factors such as geological conditions and mechanized construction.
1) Excavation and backfill foundation: Excavation and backfill foundation can be divided into two types of foundations: cast-in-place and assembled. Figure 1-1 is a schematic diagram of the cast-in-place foundation form. During its construction, the earthwork excavation is first carried out in the form of large excavation, and then the formwork is supported and concrete poured according to the foundation size.
Figure 1
The prefabricated foundation is generally suitable for areas with water shortage, difficult sand and gravel collection and good transportation conditions. It can be divided into various forms such as straight column single disc type, tower foot embedded type, herringbone type, metal foundation type, etc. As shown in Figure 1-2, it is also necessary to excavate earth and rock in the form of large excavation during construction.
Figure 1
2) Excavation of undisturbed soil foundation: excavation of undisturbed soil foundation is mainly suitable for hard plastic and plastic clay soil conditions with low groundwater level, and excavation foundation form can also be used in strong weathering and medium weathering bedrock. Fig. 1-3 is the shape of the undisturbed soil foundation of excavation and expansion bottom.
Figure 1
3) Rock foundation: The rock foundation is to punch holes in the rock foundation with good integrity, and anchor the anchor rod or steel bar in the rock foundation with cement mortar, so as to make full use of the higher strength characteristics of the rock foundation to bear the external load of the tower foundation, which can be divided into the forms of straight anchor type, cushion bearing type and embedded type, as shown in Figure 1-4.
Figure 1
4) pile foundation: pile foundation is a commonly used foundation form in the transmission line, the amount of earth and stone of pile foundation is small, the consumption of steel and concrete is more, special drilling equipment is required, and Figure 1-5 is the commonly used pile foundation form.
Figure 1
Mechanized construction will be closely combined with design and construction, design should be comprehensively considered from the construction equipment to determine the corresponding design principles, the selection of the foundation form should not only consider the geological conditions and the foundation of its own force requirements, but also combined with the site of traffic conditions, vegetation, civil compensation, equipment performance and topography and other factors, maximize the advantages of mechanical equipment. Generally speaking, the following principles should be considered in the design of different forms of foundations to meet the requirements of mechanized construction:
1) Dig the foundation:
a.When considering mechanized construction, the pile diameter in the soil foundation of the excavated foundation is 06～2.0m, with 200mm as the increment.
b.According to the current construction capacity of rotary drilling rigs, the rock saturated uniaxial compressive strength is greater than 10MPa can not be used to expand the bottom, and the maximum diameter of the soil foundation is 20 times the diameter of the main column.
c.During mechanized construction, the influence of the equipment itself on the stability of the pit wall should be considered, and appropriate construction protection measures should be taken.
2) Bolt foundation. In order to facilitate the provision of the construction operation surface of the bolt drilling rig for drilling construction, the slope of the tower base protection area is gentle, which should not exceed 30 °, and it is required that there are no unfavorable factors such as cliffs and steep ridges within the tower base protection area.
3) Excavated pile foundation:
a.The bearing capacity of the excavated pile is calculated in accordance with the relevant provisions of JGJ 94-2008 "Technical Code for Building Pile Foundation".
b.Considering the construction of rotary drilling rig, the diameter of the foundation pile in the soil foundation is 06～2.0m, with 200mm as the increment. The maximum pile diameter in a rock foundation is 12m。When the single pile does not meet the design requirements, the pile group foundation can be used.
4) Cast-in-place pile foundation:
a.The bearing capacity of cast-in-place piles is calculated in accordance with the relevant provisions of JGJ 94-2008 "Technical Code for Building Pile Foundation".
b.The design diameter of cast-in-place piles is generally 06～2.0m。
5) Prefabricated foundation:
a.The uplift stability of the prefabricated foundation is calculated according to the soil weight method.
b.The prefabricated foundation is suitable for flat hilly areas where the characteristic value of the bearing capacity of the foundation should not be less than 120kpa, and the transportation conditions are convenient.
c.It is not recommended to use prefabricated foundations in strongly corroded areas, and protective measures should be taken and special demonstrations should be carried out when adopting.
The mechanical equipment that can be selected for different basic forms is detailed in Table 3-1
Table 3-1 Mechanical equipment that can be used in different basic forms.
Note: The above mechanical equipment selection is only for learning and understanding, and the specific mechanical equipment is subject to the actual needs of construction.
The mechanized construction of transmission lines includes not only foundation design, but also path selection, site survey, tower design, grounding design and other processes. The whole process mechanized construction technology system of transmission lines is to implement the whole life cycle management concept, innovate design technology and design methods, adapt to the requirements of whole-process mechanized construction, further improve the efficiency of project construction, and improve the safety and quality level of project construction under the premise of ensuring that the project meets the functional requirements of safety, applicability and durability.