Technical problems and treatment measures in the c

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Technical problems and treatment measures in the construction of transmission line engineering

1 about the foundation engineering

the foundation of high-voltage transmission line, that is, the buried part of the pole and tower, is to ensure that the pole and tower will not sink during operation or collapse or deform under the action of external forces. The quality of foundation construction has a great impact on the safe operation of high-voltage transmission lines. Therefore, it is very important to ensure the quality of foundation construction. In the work of on-site construction, the necessary technical means should be used to control the quality required by the construction drawing design

1.1 rock embedded foundation this foundation type is suitable for strongly weathered rock foundation with shallow overburden or no overburden. Its characteristics are that the bottom plate is not reinforced and the foundation pit is completely excavated. It is stable in uplift and has strong uplift bearing capacity. If necessary, the slope of the main column can be set the same as that of the main material of the tower leg to reduce the eccentric bending moment, and the anchor bolt can also be omitted. Because this foundation type makes full use of the shear strength of the rock itself, the amount of concrete and reinforcement is small, and the amount of earthwork and stonework of the foundation pit is reduced. No formwork is required for pouring concrete, and the construction cost is low

1.2 rock bolt foundation

this foundation type is suitable for the integrity of moderately weathered and above. According to the specification and length of the sample, there can be grade adjustment of hard rock with good tensile space. The foundation type is to directly drill holes in the rock, insert anchor bolts, and then grout, so that the product quality is not forgotten to improve. The anchor bolts are closely bonded with the rock, making full use of the strength of the rock, thus greatly reducing the amount of foundation concrete and steel. However, rock bolt foundation needs to identify the integrity of rock foundation by foundation

1.3 excavation foundation

this foundation type is divided into full excavation and half excavation, which is suitable for hard plastic cohesive soil foundation without groundwater. When the foundation pit construction can be formed, the undisturbed soil will not be disturbed when excavating the foundation pit, so as to avoid filling after large excavation. When the foundation bears uplift load, the internal friction angle and cohesion of undisturbed soil can be brought into full play. This type of foundation also shows high economic and environmental benefits. According to the statistics of previous projects, due to the different geological conditions of each line and other reasons, the steel and concrete savings of using full cut foundation are 3 ~ 7% and 8 ~ 20% respectively compared with using stepped foundation. There are two types of cut Foundation: straight column type and inclined plug type. The slope setting of the main column of the inclined cut foundation is the same as that of the main material of the tower leg, which reduces the eccentric bending moment caused by the horizontal force of the foundation and eliminates the anchor bolts

1.4 stepped foundation

this foundation is a traditional foundation type, which is suitable for all kinds of Geology and tower types. It is characterized by large excavation, formwork pouring, backfill after forming, uplift resistance by using the weight of soil and concrete, rigid compression resistance of foundation slab, and no reinforcement. Because the stepped foundation has a large amount of concrete and is deeply buried, it is difficult to reach the design depth in areas prone to collapse and quicksand, so it should be used as little as possible in such areas

1.5 large plate foundation

the main design features of large plate foundation are: the bottom plate is large, the buried depth is shallow, and the bottom plate is thin. The two-way reinforcement of the bottom plate bears the bending moment and shear force caused by the tower pulling up, pressing down and horizontal force. The calculation of the main column is the same as that of the stepped foundation. Compared with the stepped foundation, the buried depth is shallow, easy to excavate and form, and the amount of concrete can be appropriately reduced, but the amount of reinforcement can be increased more. Compared with cast-in-place pile, it is widely used in soft foundation. It is convenient for construction, especially for tower sites where the foundation pit is not easy to form, such as soft, fluid plastic cohesive soil, silt and silty fine sand. During the design, the height thickness ratio of the base plate should be controlled to a certain extent (cantilever length: base plate thickness 3:1). When it is insufficient, steps can be added under the main column to reduce the cantilever length and base plate thickness of the plate. In order to reduce the amount of concrete, the center of the main column and the center of the base plate are set with eccentricity to offset the horizontal bending moment, so as to achieve the effect of reducing the base plate and reinforcement. During the design of the large slab foundation, the settlement and uneven settlement shall be controlled, and the foundation settlement and deformation of the corner tower and the tangent tower with large load shall be checked. During the construction, the foundation soil shall be disturbed as little as possible, all the excavated topsoil shall be removed and the cushion shall be made, and the block stone grouting shall be used when necessary

1.6 inclined insert plate foundation

the main feature of this foundation is that the slope of the main column of the foundation is consistent with the slope of the main material of the tower leg, and the angle steel of the main material of the tower leg is directly inserted into the foundation concrete, so that the influence of the horizontal force of the foundation on the foundation slab is minimized. Under normal conditions, the results of uplift stability, compression stability and safety of foundation soil may be different. The influence of horizontal force can be ignored in the calculation of foundation strength. Compared with the large slab foundation, the eccentric bending moment is greatly reduced, and the size of the foundation slab under the stability control of pressing can be reduced accordingly, thereby reducing the amount of concrete and the amount of reinforcement in the slab. Because the tower base plate and anchor bolt are omitted, the comprehensive index of its steel is reduced by about 25%

1.7 cast in place pile foundation

for tension towers or tangent towers with fluid plastic geological conditions, deep foundation bearing layer and large foundation force, the use of bored cast-in-place pile foundation is a widely used method in design. It mainly bears the uplift force and downforce of the foundation based on the friction between the pile circumference and the soil and the bearing capacity of the pile end. The construction is convenient, safe and reliable. The disadvantage is that the construction cost is high

1.8 combined foundation

combined foundation is mainly applicable to weak soil towers with small foundation root opening, difficult excavation of foundation pit and overlapping soil on slab foundation. Its design feature is that the buried depth is shallow, the four foundations are cast as a whole, the longitudinal and transverse stiffened concrete beams on the foundation slab bear the bending moments caused by the uplift force, downward pressure and horizontal force on the foundation, and the bottom slab is reinforced with longitudinal and transverse stiffeners, which has good integrity. The disadvantage is that the amount of basic materials is large, the construction is cumbersome, and the design is not easy to form a series

1.9 composite caisson foundation

composite caisson foundation is a new type of foundation for soft soil foundation with high groundwater level, especially soft soil foundation prone to "quicksand". The composite caisson foundation is composed of upper and lower parts: the upper part is a square step foundation, and the lower part is a ring-shaped reinforced concrete caisson. The top of the caisson is exposed and embedded into the step foundation to form a whole. The buried depth of the foundation is about 4m, and the diameter of the sinking simple is about 2.5m. From the perspective of the depth width ratio of the foundation (generally 1.5, with good damping performance), it is still a shallow foundation. Concrete and ordinary reinforced concrete cast-in-place foundations are commonly used on high-voltage transmission lines and are suitable for sections with sufficient sand, stone and water sources near the lines. Among them, due to the large uplift force, the corner tower should choose concrete foundation, which is large in volume, weight and resistance to uplift force, and is relatively stable. Sometimes, in order to save the amount of concrete, reinforced concrete foundation can be used. The type of rock is class I, that is, unweathered or slightly weathered rock. The slightly weathered rock has only slightly weathered cracks, and its color is dimmer than that of the unweathered, without obvious physical and chemical changes. It is buried in the overburden as a whole, and it is difficult to split with a hammer. When drilling with a steel drill, there is rebound, and its sound is loud without noise. The debris in the hole is powdery. Such rocks include granite, solid porphyry, andesite, basalt, Cemented Sandstone, solid limestone and marble. Class II, slightly weathered rock. Class III, moderately weathered rock. Class IV, light weathered rock. No matter what method is adopted for the excavation of rock foundation, the integrity of rock structure shall not be damaged. The stone powder, floating soil and loose piston in the hole wall should be cleaned up. The installation size and position of anchor bar shall be checked repeatedly, and it shall be poured after being correct and temporarily fixed. The mortar shall be marked, poured and braced in layers, and cured according to the requirements of cast-in-place concrete

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