Abstract: The underground part of transmission line tower is collectively called foundation. Combined with the actual situation of transmission line foundation construction, this paper probes into the technical measures of transmission line foundation construction.

Keywords: transmission line; Foundation; Architecture; measure

The underground part of the transmission line tower is collectively called the foundation. It bears all kinds of loads of transmission line towers and transfers them to the surrounding foundations, thus achieving the purpose of stabilizing transmission line towers. At present, the common foundation forms in transmission lines are: stepped foundation, flat foundation, oblique inserted foundation, dug foundation, rock foundation and pile foundation, among which stepped foundation, flat foundation and oblique inserted foundation are customarily called "large excavation" foundation because of their foundation pit formation characteristics. In the construction process, different foundation forms have different characteristics and technical requirements. In order to effectively control the quality of foundation construction, it is necessary to formulate corresponding construction technical measures.

Before the construction of transmission line foundation, retest and pit separation must be done well. Transmission line survey construction refers to the survey construction carried out by the construction unit before the line construction, to check whether the tower list and plane cross-section provided by the design unit are consistent with the site, whether the design piles are lost or moved, and whether the position, span and cross-section elevation of the middle piles and corner piles of the tower meet the design and specification requirements. If it is found that the deviation exceeds the allowable range of the specification during the second interview, the reason must be found out and corrected. After the path measurement is determined, the pit mouth is set out according to the foundation and tower type, the opening of the foundation root (front and side), the diagonal of the foundation (including the far point, the near point and the center point of the foundation pit) and the size of the pit mouth, which is called sub-pit measurement. Generally, these two steps are collectively referred to as retest and pit separation.

1 technical measures for construction of transmission line foundation

1. 1 Digging foundation

At present, there are three common excavation foundations: full excavation foundation, semi-excavation foundation and inclined excavation foundation, which are suitable for clay, hard plastic, gravel and rock with different weathering degrees, and the groundwater level is lower than the bottom elevation of concrete foundation. This foundation can give full play to the characteristics of undisturbed soil and has good uplift and overturning stability. At the same time, it also shows high economic and environmental benefits, saves materials and reduces environmental damage, but the construction is difficult and limited by soil conditions. In the process of transmission line construction, digging foundation brings two unfavorable factors to our constructors: (1) the appearance inspection cannot be carried out after concrete pouring; (2) If it is defective, it cannot be repaired.

In view of the above unfavorable factors, in order to ensure the construction quality of excavation foundation, the following construction technical measures should be taken: (1) When batching, 0.5~4cm continuous grading should be adopted, or 85% of 2~4cm gravel mixture 15% of 0.5~ 1.0cm gravel should be adopted; (2) In order to ensure that the earth wall whose foundation is on the ground is destroyed, measures should be taken to line it with plastic sheets with a height of about 0.5cm, which will be removed after pouring into the column; (3) In order to ensure that the concrete at the enlarged head of the foundation is easy to be compacted, the concrete slump can be selected at a higher level. At the same time, in order to meet the requirements of concrete workability, under the premise of keeping the water-cement ratio unchanged, the sand ratio can be appropriately adjusted or the amount of cement slurry can be increased. When the enlarged head is filled with concrete and vibrated, it is necessary to observe and judge whether there is residual gas around it, and if necessary, mortar can be added to fill the gap, so that the concrete slump at the column can be smaller; (4) Strengthening the vibration of concrete is the key link to ensure the concrete quality of the dug foundation, and the dug foundation should be vibrated with an internal vibrator to improve its strength and compactness; (5) Concrete shall be mechanically stirred. If it is necessary to mix concrete manually due to terrain restrictions, the mixing method of "three drying and four wetting" should be strictly implemented to ensure the uniform mixing of concrete ingredients.

1.2 "large excavation" foundation

"Large excavation" foundation includes stepped foundation, slab foundation and oblique inserted foundation. The so-called "large excavation" means that the earthwork excavated in the foundation pit is much larger than the volume of the foundation itself. The characteristic of this foundation is that it needs formwork casting, and the formed foundation is buried in the foundation pit and compacted after backfilling.

Stepped foundation: the foundation slab is rigid and compressive, and there is no steel bar. Because of the large amount of concrete and the deep buried depth of stepped foundation, it is difficult to reach the design depth in areas prone to landslides and quicksand.

Plate foundation: the foundation slab is elastic and compressive, and has been reinforced. Plate foundation slab has great rigidity and large amount of concrete, which is suitable for soft geological conditions and effectively prevents the foundation from sinking or tilting.

Oblique insertion foundation: The main feature of this foundation is that the slope of the main column of the foundation is the same as that 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. This kind of foundation is widely used in plain and river network areas, and its biggest advantage is that it saves foundation materials and is convenient for construction. Its disadvantage is high construction precision, and it is difficult to treat the foundation if it sinks or deviates after forming.

This kind of foundation basically has steps, and the quality of transition joints between steps is the most difficult to control during concrete pouring construction. If it is not well controlled, it will run away, causing beehives, dog eyes and even exposed tendons. Therefore, the technical measures to be taken in this foundation construction are as follows: (1) When concrete is poured into the transition joint of the steps, concrete will accumulate in the gap between the outer bottom of the upper formwork and the concrete of the lower steps. Then pouring concrete into the upper step mold, tamping after pouring to a certain height, stopping vibrating when cement slurry starts to appear around the template where concrete is piled up, and continuing pouring concrete; (2) In the process of concrete pouring and vibrating of stepped foundation, slab foundation and oblique inserted foundation, we must pay close attention to whether the formwork and support are deformed, sinking, moving and leaking, and whether the reinforcement keeps a certain distance from the formwork. If there is any problem, stop pouring and deal with it immediately; (3) Oblique insertion of foundation is easy to cause more slurry leakage in the inner corner and less slurry leakage in the outer corner, resulting in honeycomb formation in the gap. Therefore, when pouring the concrete of the column, it is not necessary to directly push the material into the inner corner, but to feed it from the outer corner with a square shovel to achieve uniform concrete slurry at the inner and outer corners of the column. At the same time, it is difficult to put the vibrator near the inclined plane of the main column of the inclined foundation for pouring and tamping, which makes the concrete tamping at the inclined plane insufficient. Therefore, in addition to mechanical tamping, column concrete should also be tamped with brazing to tamp the concrete around the main column, so as to avoid honeycomb and dog eye phenomenon; (4) The terrain of this foundation is good, and the construction unit will generally use excavators to excavate and backfill the foundation pit. After the foundation pit is excavated and formed, the loose soil layer in the foundation pit needs to be cleaned out of the pit manually to prevent the quality problems caused by the foundation after the foundation is formed; When the foundation is backfilled, in order to prevent the foundation from shifting or tilting, it should be backfilled evenly around the foundation. In particular, the inner side of the inclined column should be backfilled first, and then the outer corner side and side surface should be backfilled. When backfilling, the dumping height of earthwork should be as low as possible to avoid earthwork impacting the foundation. During backfilling, surveyors should also check the size and height difference of the root opening, and adjust it in time if there are any changes; (5) The key is to control the position of the main reinforcement in the oblique insertion foundation, so it is necessary to calculate the opening and diagonal dimension of the lower root of the main reinforcement and the height of the main reinforcement exposed from the top surface of the column before the foundation construction; At the same time, in order to fix the bottom of the protagonist steel for the oblique insertion foundation, a 390mm×390mm×80mm thick concrete pad with an angle steel groove in the middle is needed. Put the cushion block into the reserved pit on the cushion layer, the surveyors will use theodolite to measure the relevant data of the cushion block, level and align it, and then fill it with mortar and gravel around the cushion block to stabilize it and avoid the deviation of the bottom of the main steel when pouring concrete; (6) During the pouring construction of oblique inserted foundation, surveyors often check the opening of the top root of the foundation, the opening of the top edge of the inserted angle steel to the opposite half root, the height difference and inclination angle, etc. If there is any error, adjust it in time.

1.3 Rock foundation

Rock foundation can be divided into buried foundation and excavated foundation. This kind of foundation is suitable for strongly weathered rock foundation with shallow overburden or no overburden, with stable uplift capacity and strong uplift capacity. The technical measures of rock foundation construction are the same as those of excavation foundation construction. However, it is worth noting that rock foundations are generally used in strongly weathered rock areas, and foundation pits can be constructed by manual excavation or loose blasting, which will lead to large forming size of foundation pits, over-pouring of concrete and waste of materials and labor. Therefore, when excavating the foundation pit of rock foundation, the construction personnel are required to dig down once every 1.0m, and hang the foundation pit in the middle to prevent excavation deviation. In loose blasting construction, the dosage must be strictly controlled, and it is strictly forbidden to destroy the integrity of the rocks around the foundation pit due to blasting construction. 1.4 Pile foundation

Pile foundation in transmission lines is generally divided into bored pile foundation and manual digging pile foundation. This foundation mainly depends on the friction between pile and soil and the bearing capacity of pile tip to bear the uplift force and uplift force, which is convenient, safe and reliable in construction. The disadvantage is high construction cost. The quality problem of pile foundation is broken pile, and the common phenomena of cast-in-place pile foundation include borehole deviation, paste drilling, shrinkage cavity, hole wall collapse, casing leakage and so on. The technical measures of manual digging pile construction are the same as those of digging foundation construction, so I won't repeat them here.

The technical measures we should take in the construction of bored pile foundation are as follows: (1) In case of borehole deviation, we should first find out the position and degree of deviation. Generally, we can hang a drill bit in the deviated place to sweep the hole up and down repeatedly to make the hole vertical. If the deviation is serious, gravel or clay mixed stone should be backfilled in the hole above the deviation, and re-drilled after sinking and compaction; (2) When drilling in soft clay layer, if the footage is too fast, it is easy to block the slurry outlet, so the footage speed should be controlled in general drilling; (3) Plastic soil will shrink and bond when it expands in water. At this time, repeated hole sweeping should be adopted. If the drill hole is smaller than the pile diameter due to serious wear of the drill bit, the drill bit should be repaired by welding before sweeping the hole. (4) There are many reasons for hole wall collapse. According to the reasons, the hole-forming speed of cast-in-place pile should be selected according to the geological conditions, and at the same time, pay attention to the change of groundwater level, and take connection measures such as heightening the casing, increasing the water head or using siphon. In addition, when binding and hoisting the reinforcing cage, aim at the center of the hole to avoid collision with the hole wall. If the orifice collapses, first find out the position, fill the mixture of sand and clay to 1~2m above the collapse position, and then. If the hole collapse is serious, it should be completely backfilled before drilling; (5) The soil around the casing is not dense, which is easy to cause water leakage. Therefore, clay with appropriate water content should be selected around the casing for filling and reinforcement, and compacted in layers. (6) The reasons for pile breakage are: the concrete slump is large, the aggregate particle size is large, the conduit cannot be lifted in time, the conduit is tilted, which causes the conduit to be blocked and the pile concrete to be suspended, and the lifting conduit collides with the reinforcing cage, so that the soil on the hole wall is mixed into the concrete as a whole, forming a concrete pile interlayer.

The following technical measures should be taken for the reasons of broken piles: (1) The slump of concrete should meet the design requirements after inspection, and the coarse aggregate must be controlled according to the specification requirements; (2) Pull out the catheter when pouring concrete, and track the depth of the catheter buried in concrete at any time to ensure that the catheter is always buried in concrete; (3) When the conduit is blocked and the concrete has not solidified, a section of rail or other heavy objects can be hoisted to impact the conduit, and the blocked concrete will be washed away, and then the concrete will continue to be poured; (4) If the concrete is interrupted below the groundwater level, a drill bit slightly smaller than the original pile diameter can be used to drill a hole at the original pile position to an appropriate depth below the broken pile, and then the hole can be cleaned; Add a reinforcing cage to the broken pile, bury it in a new borehole, and then continue pouring concrete. (5) Broken pile is the main quality problem of underwater concrete pouring, and any treatment method should be studied and determined with the supervision engineer and the site design representative before implementation.

2. Technical treatment measures for soft foundation problems of transmission lines

The gravity of various loads on the transmission line tower acts on the foundation and is transmitted to the surrounding foundation through the foundation. The geological conditions of foundation directly affect the foundation form, cost, quality, construction period and safe operation of transmission line engineering. Among all kinds of foundations, soft soil foundation has the most obvious influence on transmission lines. A little carelessness will often lead to accidents such as foundation sinking, tower tilting and even tower falling, so we must attach great importance to soft foundation problems in all aspects of engineering construction. The key to the construction of tower foundation on soft soil foundation is to do well the drainage measures in the process of foundation pit excavation and concrete pouring, so as to avoid the undisturbed soil at the basement from being disturbed as much as possible.

For the soft foundation, the measures of filling stones are adopted, that is, when the last layer of soil is dug to the design depth, the stones prepared in advance are put into the soil, and the stones are rammed into the soil to be dense, and the soft soil on the extrusion surface is cleaned up, and then gravel is laid; Laying concrete cushion.

When excavating a foundation pit whose bottom is lower than the groundwater level, groundwater will continuously penetrate into the pit. If the water flowing into the pit is not discharged in time, the pit wall will collapse and the bearing capacity of the foundation will be reduced after the soil is softened by blisters. Therefore, drainage during foundation construction is the basic requirement of soft foundation construction. There are many methods for foundation pit drainage, and the drainage method can be determined according to the drainage capacity of foundation pit and its own drainage equipment during construction. For mud and water sand pits, in order to prevent the pit wall from collapsing and reduce the amount of water flowing into the pit bottom, measures such as water retaining plate or caisson can be taken before excavation.

In the process of foundation pit excavation, construction personnel should pay attention to whether the actual geology on site is consistent with the geological data provided by the design. In case of inconsistency, it shall report to the design and supervision departments in time, and invite the geological representative to the site for identification and treatment, and the foundation construction shall not be carried out blindly.

Although soft soil foundation is the difficulty of transmission line construction, as long as the survey, design, construction and supervision personnel are highly responsible, closely cooperate and manage scientifically, the line investment in soft soil foundation will be controlled, the quality will be guaranteed and the line will run safely and reliably.

refer to

Li Qinglin. Overhead transmission line construction manual [M]. Beijing: China Electric Power Press.

[2] Huang Jianhui. Treatment of soft foundation problems in the construction of high voltage transmission lines [Z].

Chen. Treatment of sand pit in power line construction [J]. Petroleum engineering construction, 1995.

[4] Li Lixin. Preliminary study on construction method of main steel plug-in foundation of high voltage transmission line [J] Inner Mongolia Petrochemical, 2005.

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