Keywords

1 Introduction

The level of mechanization of overhead transmission lines is an important symbol of modern power construction [1]. Overhead transmission line has the characteristics of long distance, crossing different region, complex and diverse terrain and harsh field construction environment, etc. All the time, affected by traditional habits and equipment investment, the mechanization degree of transmission line construction is low in the past. With the innovation of the State Grid to promote the mechanized construction of transmission lines, China has also achieved more results in the field of mechanized construction. Foundation mechanized construction application rate reached more than 90% on UHV from Longdong to Shandong ±800 kV transmission line project and Hami – Chongqing ±800 kV transmission line project. At the same time, the domestic counterparts have more innovation in mechanized construction, Wang Yuan etc. [2] according to design innovation to overcome the unfavorable conditions in the river network area, improve the level of construction mechanization, and achieve a new model of mechanized construction of each process in the harsh environment.

For temporary road construction methods, Ma Li etc. [3] based on typical whole-process mechanized construction cases, proposed that the key influencing factors of temporary road construction include: topography and geomorphic conditions, road width and subgrade bearing capacity. Wang Daojing etc. [4] summarized the requirements of mechanized construction equipment on road conditions, giving the suggestion to transmission line construction. Chen Xing etc. [5] studied the mechanized construction temporary road construction scheme of paddy field terrain, and used gravel paving to build temporary roads for good geological conditions, and some towers needed to be widened. Ma Mingzhi etc. [6] proposed a comprehensive evaluation method of construction road network to realize efficient use of existing roads and rationalization of temporary road construction, while optimizing the route.

In terms of quantitative evaluation of mechanized construction, Ge Zhaojun etc. [1] proposed an evaluation method of mechanization rate applicable to overhead transmission lines, calculates the average score rate of each sub-process according to the use of mechanized equipment of each sub-process of tower construction. Zeng Shoujian etc. [7] conducted technical and economic analysis, and initially found out the cost and benefit of the whole process of mechanized construction of transmission line engineering. In terms of equipment research, Wang Xichen etc. [8] analyzed and studied the economy and necessity of using centralized mixing concrete in UHV transmission lines, and applied it in the foundation construction of 1000kV Jindongnan - Nanyang - Jingmen UHV AC transmission line project. Qin Qingzhi etc. [9] developed the special equipment of rotary digging drill for cutting and digging basic machinery of transmission tower in combination with the basic design characteristics of Xiangjiaba-Shanghai ±800 kV UHV DC transmission line project.

Although a lot of research have been carried out on mechanization, for the transmission line on mountain area is still difficult to carry out mechanization construction. The main reason is that a large area of temporary road construction and foundation excavation are required for the equipment to enter the tower site, which causes damage to the environment. The lack of planning on road construction and foundation resulted in the slope slip phenomenon along with the rainy season (Fig. 1).

To sum up, although domestic counterparts have carried out some researches on mechanization, there are few researches on typical design of temporary road construction and foundation construction. Good design and planning of road construction and typical planning of construction foundation can minimize the disturbance of mechanical equipment to the foundation and reduce the impact on the environment, reducing the cost of environmental remediation and improving the reliability of soil and water conservation measures.

Fig. 1.
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Road slope slip on mechanized construction

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2 Introduction of Mechanized Construction Foundation

The core of influencing on temporary road and foundation construction lies in the selection of foundation type and foundation construction equipment. This section focuses on the research of foundation equipment commonly used in domestic transmission lines. Other equipment such as temporary road construction, material site transportation, concrete construction, grounding construction, tower erection, stringing and construction auxiliary can be selected on the basis of the selection of foundation excavation equipment.

2.1 Pile Foundation

The common excavation equipment for the pile foundation on mountain is the rotary drilling rig, and different bits can be selected according to different geological conditions. Short auger, suitable for clay, silt, fill, sand and weathered rock above the water table; Core auger drill, suitable for gravel soil, medium hardness rock and weathered rock; Core rotary bucket, suitable for weathered rock and cracked rock. The functional parameters of commonly equipment used in transmission line construction are shown in Tables 1 and 2.

Table 1. The functional parameter of integral rotary drilling rig
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Table 2. The functional parameter of split rotary drilling rig
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2.2 Rock Anchor Foundation

The mechanized construction of rock anchor foundation mainly focuses on the drilling of rock anchor. Air compressor + light anchor drill is the key equipment of drilling, which affects the quality of the foundation - the orientation, depth and accuracy of the drilling diameter. Through investigation, the related parameters on drilling equipment of rock anchor foundation commonly used in transmission line are shown in the Table 3.

Table 3. The functional parameter of rock anchor equipment
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2.3 Mountain Micro Pile Foundation

Mountain micro pile foundation can reduce the amount of foundation, improve the efficiency of construction and reduce the risk of operation. Through deeply investigation of scientific research institutions and equipment manufacturers, the latest mountain micro pile foundation construction equipment and related parameters are shown in the Table 4.

Table 4. Comment micro pile equipment on transmission line
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3 Typical Application of Foundation Design in Mountain Areas

Considering that the selection of foundation and construction equipment in mountain areas is a major difficulty in mechanized construction, the typical application scenarios of foundation design in mountain areas are sorted out as follows according to tower topography, geology and equipment approach conditions. Typical design parameters is shown on Table 5.

Table 5. Typical design parameters
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4 Typical Design Scheme of Temporary Road Construction

4.1 Plain Area

The plain area has a flat terrain and good general traffic conditions. It is usually possible to use the existing road or earthed road, and some roads need to be widened and reinforced.

Road scheme: If heavy machinery such as tanker truck, crane or rotary drilling rig passes by, wooden boards should be laid on the cement road surface, and old tires can also be padded when the crawler equipment walks. When padding the wooden board, the board shall cover the width of the wheel or track, as shown in Fig. 2(a); When padding old tires, place two old tires per meter, as shown in Fig. 2(b).

Fig. 2.
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Diagram of cement road reinforcement

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4.2 Coastal Areas

Some towers in coastal areas are located in beach areas or ponds, and general construction machinery cannot pass through, so approach roads and construction platforms need to be built. The common foundation is cast-in-place pile foundation, and the main machinery is rotary drilling rig, transport truck, crane, commercial mixing tank truck, etc.

Approach road scheme 1 Filling the construction road and construction platform to the shore elevation, with road width is 4m and slope is not more than 1:1.5. The size of the construction platform is related to the distance of the tower root, and normally can be considered by 20–30 m. Before filling, measures such as pumping, cofferdam and silting need to be carried out according to the situation. The filling the construction road is shown on Fig. 3.

Fig. 3.
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Schematic diagram of construction road and construction platform constructed by filling

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Approach road scheme 2: Build temporary steel trestle and steel platform. The height of trestle and platform top is the same as the top elevation of foundation cap. The length of steel trestle is determined according to the distance between each pile position and the shore. Steel trestle and steel platform are composed of steel pipe piles, bridge panels, steel beams, guardrail and so on. The steel trestle platform in the sea and the beach shall be dismantled after the construction is completed. Temporary steel trestle is shown on Fig. 4.

Fig. 4.
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Temporary steel trestle

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4.3 River and Swamp Area

The tower is located in the river and swamp area, including fields where water is often accumulated, areas where mud is silted, and areas where rivers cross and affect land traffic. With Abundant surface water and soft soil, they are not conducive to mechanical passage, and general vehicle passage is prone to subsidence. The foundation types are mainly cast-in-place pile foundation and plate foundation, and the main entering machinery is rotary drilling rig, transport truck, crane, commercial mixing tank truck, etc.

Approach road scheme: When there is silt on the surface, the 0.3 m silt should be replaced and filled by 0.3 m thick pond slag and compacted. The thickness of Lay steel plate is not less than 10 mm. The steel plate is shown on Fig. 5.

Fig. 5.
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The steel plate diagram

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4.4 Mountainous Areas

The terrain of mountain area is complex, with a certain distance from the existing road to tower site. The overburden soil is mostly silty clay and sandy clay, and the bearing capacity is high. The temporary road construction in the mountain areas should make use of the original cultivator road, gravel road, etc., and use bulldozers, excavators, road rollers and other machinery to broaden and strengthen to meet the equipment entry requirements. Part of the weathered rock in the mountain area is exposed and broken, and the surface soil is loose and shallow. Under the action of concentrated rainwater, some surfaces soil on temporary are muddy and prone to soil erosion. The foundation type suitable for mechanized construction in mountain areas mainly include pile foundation, micro pile foundation, and rock anchor foundation; Main machinery are rotary drilling rig, micro pile drilling rig, transport vehicle, excavator etc.

Typical scheme 1: The original machine plowing road is 1-2m long, with slopes on one or both sides of the road, with slopes of less than 10°, 10–25°, 25–35°, and greater than 35°.

Approach road scheme1: Expand the original organic farming road, excavate soil and rock, and trim the slope. The new slope formed after excavation has a slope of 45–80°, a road width of 3.5 m, and a turning back site is 6m. After leveling and compacting the ground, construction equipment can pass through.

Typical scheme 2：There are no existing roads, and existing slopes are less than 10°, 10–25°, 25–35°, and greater than 35°.

Approach road scheme 2：Excavate the earth and rock and trim the slope, and the new slope formed after excavation has a slope of 45–80°. The road width is 3.5 m, and a turning back site is 6m, the ground is smooth and compact, and some muddy road need to be reinforced with steel plates. Detail is shown on Fig. 6 and Table 6.

Fig. 6.
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Road scheme on Mountain area

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Table 6. Technical parameter of approach road scheme in mountain area (per meter)
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5 220 kV Typical Design Scheme on Tower Site

5.1 Typical Scheme 1 - Rock Anchor Foundation with Slope ≤10°

The equipment of Rock anchor foundation can be divided into self-propelled and disassembly. The climbing capacity of self-propelled equipment is 20°, and requirement of the road width is 2 m. If the disassembly type is used, it is necessary to use crawler transporters for transportation, and the road construction width is 2 m.

The self-propelled equipment of rock anchor foundation can be transport on the base surface, and the length of temporary road is 4.8 times of tower root value when transferred between tower legs (assuming the tower root value is 10 m). Since the climbing capacity of equipment is 20°, the earth excavation between the tower leg transition is not considered. Taking into account the requirements of the flatness of the construction surface, the excavation area of the working surface is 16 m2. The diagram of transition between tower legs is shown on Fig. 7 and Table 7.

Fig. 7.
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The diagram of transition between tower legs on 10° slope

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Table 7. The amount of excavation work of rock anchor foundation with slope ≤10°
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5.2 Typical Scheme 2 - Pile Foundation with Slope ≤10°

The equipment of mechanical pile foundations are mainly divided into two types: split rotary drilling equipment and integral rotary drilling equipment. Specific parameters are detailed in Tables 1 and 2. Due to different equipment, the requirements for temporary roads are also different. Therefore, the following will elaborate on the tower site scheme for different equipment.

Split Rotary Drilling Equipment

The width of the temporary road construction is 2 m, and it is transported to tower site using tracked transport vehicles. Taking into account factors such as construction efficiency, the split type rotary drilling rig adopts a self-propelled method for the tower legs transfer. When transferring between tower legs, the length of road should be 4.8 times of tower root. As the climbing capacity of the rotary drilling rig is 25°, the excavation of surface soil during tower leg transfer is not considered. With the requirement for flatness of the construction work surface, the area of the work surface is 30 m2. Detail is shown on Fig. 8 and Table 8.

Fig. 8.
figure 8

The diagram of transition between tower legs on 10° slope (split rotary drilling equipment)

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Table 8. The amount of excavation work of pile foundation with slope ≤ 10° (split rotary drilling equipment)
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It can be seen on Table 8 that under the condition of a 10° slope, the excavation volume of the split type on tower site is not large, accounting for 5–30% of the total volume. The proportion of earthwork excavated on tower site is higher than that of rock anchor foundation under the same conditions.

Integral Rotary Drilling Equipment

Taking KR150D rotary drilling rig as an example, the equipment needs to build a temporary road with a width of 3.5m and walk up to tower site on its own.

Considering factors such as construction efficiency, the KR150D rotary drilling rig adopts a self-propelled method for tower legs transfer. When transferring between tower legs, the base is leveled within the range of the tower legs (taking a 10 m tower as an example).

Fig. 9.
figure 9

The diagram of transition between tower legs on 10° slope (integral rotary drilling equipment)

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It can be seen on Fig. 9 that due to the small distance of tower legs, the transfer road of equipment basically occupies the land area inside the tower legs. At this point, the corresponding earthwork volume will be slightly greater than that of split equipment.

Table 9. The amount of excavation work of pile foundation with slope ≤10° (integral rotary drilling equipment)
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From Table 9, it can be seen that under the condition of a 10° slope, the excavation volume of tower site surface is relatively large, accounting for about 5–50% of the total volume. Due to the high width requirements of the integrated rotary drilling rig for temporary roads, the earthwork volume of temporary roads is 1.75 times that of rock anchor temporary roads.

5.3 Typical Scheme 3 - Micro Pile Foundation with Slope ≤10°

Micro pile foundation drilling equipment are mainly divided into two types: split type and integral type, with specific parameters detailed in Table 4. Due to different equipment, the requirements for temporary roads are also different. Therefore, the following will elaborate on the schemes for different equipment.

Split Micro Pile Drilling Rig

The equipment adopts JK-MD-400/15-A split type micro pile drilling rig. As the equipment can be assembled and disassembled, the temporary road construction width is 2 m, and it is transported to tower site by tracked transport vehicles.

Considering factors such as construction efficiency, the split type micro pile drilling machine adopts a self-propelled method for the transfer of the foundation. Considering the requirement for flatness of the construction work surface, the area of the work surface is 25 m2.

The excavation quantity of the foundation surface for the split micro pile foundation is the same as that of the split type drilled pile foundation under a 10° slope condition. The proportion of earthwork excavated on its foundation is higher than that of rock anchor foundation under the same conditions.

Integral Micro Pile Drilling Rig

Taking the WZL400/10 A micro pile drilling rig as an example, the equipment needs to build a temporary road with a width of 3.5 m.

Considering factors such as construction efficiency, the WZL400/10 A micro pile drilling rig adopts a self-propelled method for the transfer of the foundation. The excavation quantity of the foundation surface for the integral micro pile foundation is the same as that of the integral type drilled pile foundation under a 10° slope condition.

5.4 Typical Scheme 4–10-25° Rock Anchor Foundation

As the climbing capacity of the anchor rod drilling rig is 20°, it is necessary to consider increasing the length of the temporary road between tower legs to reduce the climbing slope. The length of temporary road should be 5.0 times tower legs. Due to the small size of the 220kV foundation, if a transfer is carried out within the tower leg range, it will involve excavation of a large amount of soil. Therefore, the transfer road for rock anchor rod foundation should be selected from outside the tower leg. Considering the requirement for the flatness of the construction work surface, the area of the work surface is 16m2. Considering factors such as sloping, the excavation volume of the tower site surface is 40 cubic meters. Detail is shown on Fig. 10.

Fig. 10.
figure 10

The diagram of transition between tower legs on 25° slope

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Table 10. The amount of excavation work of rock anchor foundation with slope 25°
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From Table 10, it can be seen that under the condition of a 25° slope, the amount of earthwork for temporary road construction is about 2.39 times that of a 10° slope under the same conditions, and the amount of earthwork increases significantly. Compared to the 10° slope under the same conditions, the amount of earthwork on the tower site increases by 2.33 times. Under this typical scheme, the main amount of earthwork occurs in the construction of temporary roads.

5.5 Typical Scheme 5–10-25° Pile and Micro Pile Foundation

Split Equipment

When transferring between tower legs, the length of temporary road should be 2.5 times of tower legs distance. Due to the small 220 kV tower legs distance, the method of constructing a platform with two tower legs is considered, resulting in a larger amount of foundation soil. Due to the climbing capacity of the rotary drilling rig being 25°, earthwork excavation during tower leg transfer is not considered. Detail is shown on Fig. 11.

Fig. 11.
figure 11

The diagram of transition between tower legs on 25° slope (split equipment)

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Table 11. The amount of excavation work of split equipment with slope 25°
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From Table 11, it can be seen that under the condition of a 25° slope, the excavation volume of the split equipment increases by 2 times compared to the rock anchor foundation under the same conditions, and the excavation volume of the temporary road increases by about 2.14 times compared to a 10° slope.

Integrated Equipment

Considering factors such as construction efficiency, integrated equipment adopts a self-propelled approach for the transfer of the foundation. Considering the small tower root opening, a work platform is set up for the two tower legs, and the length of the road construction during the transfer between the tower legs is 2.5 times tower legs distance. Due to the climbing ability of the rotary drilling rig at 25°, soil excavation during the transfer of the tower legs is not considered. Detail is shown on Fig. 12.

Fig. 12.
figure 12

The diagram of transition between tower legs on 25° slope (integral equipment)

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Table 12. The amount of excavation work of integral equipment with slope 25°
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From Table 12, it can be seen that under the condition of a 25° slope, the excavation volume of the foundation surface increases by 94% compared to the condition of a 10° slope, and the earthwork volume for road construction increases by 2.38 times compared to the condition of a 10° slope. Due to the high width requirements of the integrated rotary drilling rig for temporary roads, the earthwork volume of the temporary road is 1.75 times that of the split type temporary road.

6 Conclusions

This article is based on the sorting of commonly used mechanical construction foundation types in mountainous areas, through equipment research, typical schemes for access road and foundation design are studied, and the main conclusions are as follows:

  1. (1)

    The most famous mechanized foundation types in mountain area are rock anchor foundation, pile foundation and micro pile foundation. The equipment of different foundation type can divide into split and integral type.

  2. (2)

    Typical design scheme of temporary road construction include plain area, coastal areas, river and swamp area and mountain area. The scheme of plain area are wooden boards and old tires. The scheme of coastal areas and river area are filling and temporary steel trestle. The scheme of mountain area are expand the original road and excavating new road.

  3. (3)

    The excavation quantity of rock anchor foundation with slope ≤10° is the less than that of pile foundation and micro pile foundation, because of the small equipment. The excavation quantity of split equipment is less than integral equipment because of the requirement of the road width.

  4. (4)

    The excavation quantity of rock anchor foundation with slope 25° is 2.33 times than that on slope 10°. The amount of earthwork on the tower site increases by 2.33 times with slope 25° of split equipment on pile and micro pile foundation. Compared to the 10° slope under the same conditions, the amount of earthwork of integral equipment on the tower site increases by 2.33 times.