王 风，聂如松，李 锐，雷明峰，朱 丽，牛建东

(1.国能朔黄铁路发展有限责任公司，河北 沧州 062350； 2.中南大学土木工程学院，湖南 长沙 410075； 3.中南大学重载铁路工程结构教育部重点实验室，湖南 长沙 410075)

0 Introduction

China is a vast country with complex geological conditions. With the dramatic development of railway engineering, some slope engineering built by filling and excavation engineering inevitably reaches tens of meters in height[1-5]. Due to the complexity and uncertainty of geological and meteorological conditions, massive diseases, such as sliding collapse and poor drainage conditions, occur on the existing slopes under natural variability and anthropic effects. Such diseases can lead to increasingly poor slope stability and railway operation safety. Therefore, reinforcement measures must be taken for some existing slopes[6-8].

One of the effective methods of increasing the stability of slopes is to improve their drainage condition[9]. At present, the most common drainage methods adopted in slope engineering are polyvinyl chloride (PVC) pipes with drilled holes, gutters, underdrains, etc. Although these measures are simple and economical in construction, they are prone to clogging-induced function loss in service[10-11]. For this reason, a new drainage structure named drainage nail, which is a PVC pipe wrapped with a capillary permeable drainage belt, was proposed. Its construction method is similar to that of the soil nail, whereas its function is to drain water inside the slope[12]. The drainage nail is composed of drainage nail segments and connectors.

Fig. 1a shows the drainage nail segment, which is a hollow PVC pipe wrapped with a capillary permeable drainage belt, and its function is to collect, drain, and filter water. One end of the drainage nail is blocked with a plug to avoid clogging caused by surrounding soil particles. The connector is used to connect two drainage nail segments. The structure and drainage path of the drainage nail are illustrated in Fig. 1b. Drainage nails, with the advantages of simple construction, high drainage efficiency, and excellent anti-clogging performance, have been applied to treat existing railway slope diseases[13].

Fig. 1 Drainage nail structure

Considering the practical difficulties in the reinforcement and protection construction on the high and steep slopes of railways, such as the narrow construction site and cumbersome construction equipment movement, the traditional drilling rig and drilling rig platform are improved according to the installation and construction process of drainage nails, and a new drilling rig and drilling rig platform are developed. The specific structures of the developed drilling rig and construction platform are presented, and they have broad application prospects. Then, general provisions and preparations for drainage nail construction are described. A process of drainage nail construction is formulated, and the corresponding construction steps and operation requirements are also put forward. This study can provide a reference for the application of the developed drilling rig, drilling rig platform, and drainage nail.

1 Difficulties in existing high and steep slope reinforcement construction

Scaffolds need to be erected during high and steep slope reinforcement, and this scaffold erection practice is more complex than those in other construction processes. Specifically, the whole scaffold should be adjusted according to the slope shape, angle, and flatness during high and steep slope reinforcement construction. Before the scaffold erection, the bottom construction platform should be cleaned and established. The construction platform should be as flat as possible with a width of more than 4.5 m. It should also be inclined to the slope toe, with a slope angle not exceeding 3°～5° if the ground is hard and inclined outward. Protective fences shall be installed along the construction platform. The construction platform for scaffold erection is required to withstand the vibration force, pullout force, gravity, and wind force during construction, and deformation and settlement are not allowed. The height of the platform should coincide with the equipment height, inclination angle, and position of the hole to be drilled. The scaffold should be erected to a preset height to provide the platform required for drilling construction.

The traditional scaffold construction method for slope reinforcement is demonstrated in Fig. 2. In addition to a complicated construction process, this method also has the main disadvantages of difficult construction equipment movement on the scaffold, complex loading-unloading process, long disassembly time, and resulting long construction period[14]. In this case, a new method must be proposed to reduce production costs and speed up construction to a certain extent.

Fig. 2 Traditional scaffold construction method

2 Platform development for drilling construction on high and steep slopes of existing railways

The existing railways are characterized by high traffic density, short construction time, and limited construction space. The traditional construction method is to set up a platform using scaffold rods, then fix the platform on the slope by an anchor, and finally install the heavy drilling rig onto the platform. After a hole is drilled, the drilling rig is dismantled and then manually moved to the next hole for installation and fixation. The above construction process must consider both the bearing capacity and stability of the construction platform and the position accuracy of the hole drilled. This construction method has the disadvantages of difficult construction, large construction quantity, long construction period, and high production cost. Moreover, a lot of labor is required in the implementation of the construction operation, and their safety is not completely guaranteed.

According to the process of drainage nail construction, the drilling rig and drilling rig platform are specially designed to overcome the difficulties in the traditional construction method. Specifically, a movable drilling rig platform for drilling holes on high and steep slopes of existing railways for drainage nails is designed, and it offers reduced labor intensity, improved construction efficiency, and increased safety during construction. As shown in Fig. 3, the movable platform for drainage nail construction is composed of three parts, including a hanging basket system, a drilling rig installation system, and a moving and positioning system. The hanging basket system consists of a basket base plate, fences, support rods, and position limiting devices. The drilling rig installation system consists of a drilling rig slipway, an inclination angle adjuster for the drilling rig, and connecting joints, and the moving and positioning system is composed of a wire rope, an adjusting hoist, omni-directional wheels, connecting devices, etc.

Fig. 3 Structure of the movable drilling rig platform for drainage nail construction

Besides, the drilling rig is improved to avoid the inconvenience caused by the large size of the traditional drilling rig and the construction of the anchor. The details are as follows: The drilling rig used to drill holes for the drainage nails is composed of a carriage assembly, a scaffold, a console assembly, a power assembly, a cylinder assembly, etc. The size of the slide scaffold and the lengths of the cylinder and the drill rod are improved. The improved slide scaffold is shown in Fig. 4, and the four improved parts of the drilling rig are mainly as follows:

1) The size of the scaffold is reduced from 2 700 mm×400 mm×550 mm to 1 400 mm×400 mm×550 mm to avoid the impact of a rig fuselage too long or too close to the rail.

2) The length of the drill rod is shortened from 1 000 mm to 500 mm to facilitate the addition and subtraction of the drill rod on a narrow platform at both sides of the railway.

3) The length of the cylinder is reduced from 1 000 mm to 650 mm to match the drill rod length of 500 mm.

4) The scaffold is redesigned according to the characteristic of the construction of railway slope reinforcement. The new scaffold is composed of nine lapped steel pipes, including two longitudinal ones on the horizontal plane and five transverse ones and two vertical ones on the vertical plane. The inclination angle of the carriage can be adjusted by changing the positions where the vertical steel pipes connect with the transverse ones of the scaffold, and the angle of the drilled hole can also be adjusted as mentioned above.

3 Construction technology and method for drainage nails

On the basis of the improved drilling rig and drilling rig platform mentioned above, general provisions on drainage nail construction are proposed, and technical preparations, machine preparations, and working conditions required for drainage nail construction are described as well. A process of drainage nail construction on high and steep slopes is formulated, and the corresponding construction steps and operation requirements are put forward in detail.

3.1 General provisions on drainage nail construction

1) If an unstable situation occurs with the retaining structures, such as retaining walls or slopes, it must be reported to the departments concerned in time. Then, the loading exerted by the upper part of the slope can be reduced by cutting the slope, but the lower part of the slope should not be disturbed as much as possible.

2) The fissures on the sliding surface at the top of the slope should be backfilled with raw soil mixed with bentonite.

3) The pneumatic down-the-hole (DTH) hammer without water could be used to drill the hole for the PVC pipe, but the pneumatic DTH hammer with water is strictly prohibited. Skilled workers should be assigned to ensure the drilling quality.

4) The construction of drainage nails and anchor rods should be carried out row by row and from top to bottom. The construction sequence should be determined according to the in-situ conditions on the principle of avoiding mutual influence as much as possible.

5) After the process of hole drilling for drainage nails is completed, the holes should be cleaned in time and the drainage nails should be installed immediately to prevent secondary drilling caused by hole collapse. After the drainage nails are installed, their pipe orifice should be protected in time to prevent clogging. During construction, the drainage nail segments and other materials should be protected to prevent material damage from affecting the drainage performance. Drainage nails should be carefully installed to avoid poor drainage performance caused by improper operation and installation. The water temporarily discharged during construction should also be drained to prevent the performance of the railway from being affected by water scouring.

6) The construction of drainage nails and the drainage pipe network should be completed before the slope protection, such as arch support, on the slope surface is constructed. Special attention should be paid to preventing damages to the drainage nails and the drainage pipe network during the subsequent construction of the support structure. The outlets of the drainage pipe network should be as consistent as possible with the drainage spacings on the slope of the supporting structure.

7) All the outlets of the drainage pipe network shall allow the collected seepage water to be effectively drained into the railway side ditches. If the drainage capacity of the side ditches cannot meet the requirements, the drainage ditches shall be expanded.

8) Clay or sealing materials should be used to block the gap between the drilled hole and the drainage nail for 60 cm near the outlet of the drainage nail. The slope or wall below the outlet of the drainage nail should be reinforced to withstand scouring.

9) Safety assurance measures should be taken during construction.①In the case of geophysical prospecting operations near the existing railways, the provisions in theSafetyRegulationsforConstructionSafetyofRailwayEngineering[15]should be strictly implemented.② Prospecting operations near existing railway lines are geological engineering operations in dangerous areas. Geophysical exploration operators (including surveying and mapping staff) approaching an existing railway line should carry out their engineering work in strict accordance with the railway survey and construction operation regulations and strictly obey the command of the on-site safety officers.

3.2 Construction preparation

3.2.1Technical preparations

1) The design drawings should be carefully consulted, and the requirements of relevant technical construction specifications need to be well acquainted. Then, the construction organization design should be prepared according to the in-situ conditions, and technical disclosure and safety disclosure must be declared to the construction team. Finally, the application for the commencement of sub-divisional works should be prepared and submitted to the departments and leaders concerned for approval.

2) A supervision and protection scheme for the railway route and traffic safety should be specially formulated where the construction is adjacent to the railway route, and the scheme should be incorporated into the construction organization design. Also, special meetings should be held to publicize and assess the team members.

3) Before the construction begins, the construction machines and tools should be reasonably configured according to the arrangement of the construction site and time, and this measure should be taken in a way that the construction would not affect or interrupt the railway operation as much as possible. In-situ construction conditions can be analyzed to determine whether to limit the speed of running trains or to clearly define the speed limit range. The above-mentioned measures shall also be reported to the relevant department for approval. The construction can be carried out only after the approval from the relevant department and the permission and implementation from the officers of the railway construction section are obtained.

4) The safety care during the construction should be undertaken by the special personnel designated by the railway transportation management department.

5) Construction lofting should be conducted, and the position of the drainage nail center should be determined and marked.

3.2.2Construction machine preparations

1) The drilling rig equipment consists of a pneumatic DTH hammer drill, a drilling bench, an air compressor, a ladder, and a winch. The installation of the drilling rig and corresponding preparations are as follows:①After the drilling rig is transported to the construction site, whether the parts and components of the drilling rig are damaged should be checked, and the drilling rig could be assembled and refueled after a confirmation that no damage has occurred is made.②The two front legs of the slide frame are in the operating position, and the support rod is adjusted to the extent that the slide frame reaches the specified angle.③The power supply should be turned off when the motor cable is connected. In principle, the motor cable should be connected before the main power supply is connected. Then, the motor should be tested to check whether its rotation direction is consistent with the controller’s. If not, the motor should be turned off immediately to reverse its rotation direction.④Whether all the bolts are connected firmly and reliably and whether all the pipes are connected firmly should be checked. Air leakage, oil leakage, and other abnormal phenomena are never allowed.⑤An appropriate amount of No.32 engine oil should be refueled in oil mist.⑥All the handles of the pneumatic control valve must be set to the “stop” position.⑦ No-load tests can be carried out after the above preparations are completed.

2) The processing and installation equipment for drainage nail construction includes a cutting machine, an electric screwdriver, and a jack.

3) The power and electrical equipment consists of a transformer, a generator, an air compressor, low-voltage switchgear, lighting equipment required for night construction, etc.

4) The safety equipment contains a series of safety equipment, helmets, face masks, etc., and the equipment should be suitable for railway construction.

3.2.3Working conditions

As for the construction near a railway line, special consideration should be given to whether the construction machines and tools meet the requirements of railway operation safety. If the requirements are not met, the construction machines and tools need to be improved or replaced.

1) Concerning the construction near the railway route, due consideration should be given to whether the installation length of the drainage nail segments can meet the operation safety requirements. If not, the length of the drainage nail segments should be reduced.

2) The steel-stranded wire should be used as the anchor to ensure that the safety line of the railway would not be intruded during the anchor construction for slope reinforcement.

3) After the scaffold is erected, slope or wall surface cleaning is required. The position of the hole to be drilled should be marked and rechecked after it is determined according to the design drawings, and its deviation should not exceed 5 cm. Then, the drilling rig can be moved to the construction face.

4) The construction workers should have passed the professional training and taken part in the training on the technical quality and safety disclosure during drilling and drainage nail construction.

Fig. 5 Process of drainage nail construction

5) The tests for drainage nail construction are as follows: Before the formal construction, drainage nail processing tests shall be carried out to determine the machines, tools, and various technical parameters, and the quality of drainage nail construction should also be tested. The number of tested nails should not be less than 3.

3.3 Process of drainage nail construction

The process of drainage nail construction is shown in Fig. 5.

3.4 Steps of drainage nail construction and operation requirements

During the slope reinforcement construction near an existing railway, special attention should be paid not to damage the railway, the power supply system, and other equipment of the railway. The construction must be carried out under the supervision of the railway department. The detailed steps of drainage nail construction are as follows.

1) Arrange and repair the existing drainage system on the ground.

2) Clean the slope surface, loft the elevation, and mark the position of the hole to be drilled.

3) Prepare the drilling rig platform.

4) Drill holes for drainage nails, and the drilling can be divided into the following steps:① Drill holes for drainage nails.The workers should raise the decelerator to the upper limit position and tighten the drill rod first, turn on the motor, and slowly turn on the propelling cylinder until the no-load test is normal. Next, the drill bit shall be placed on the working face, and the propelling cylinder should be turned off. Then the drill bit could be turned on to drill slowly, and the drill bit can be impacted at full throttle until the drill bit reaches about 100 mm in depth. During drilling construction, the cylinder should be slowly switched on and off according to the speed of footage to keep the drilling bit at a certain propulsive force. Meanwhile, whether the oil atomizer supplies oil should be observed so that the oil supply quantity could be appropriately adjusted.② Extend drill rod.After the first drill rod has been constructed, the drill bit could be turned off. The reducer should be set at a lower limit position, and the drill rod in the drill support should be fixed with a fork. The decelerator should be reversed to disconnect the joint between the drill rod and the decelerator. Then, the decelerator should be raised to the upper limit position, and the second drill rod should be connected with the first one.③ Remove the drill rod.When the drill rod support is in contact with the tool unloader, the drilling construction could be stopped, and a professional round rod shall be inserted into the hole on the outside of the tool unloader. The second flat trough at the end of the drill rod could be aligned with the upper part of the tool unloader by rotating the tool unloader. The joint on the reducer could be separated from the drill rod by inserting the fork and reversing the motor. The first flat trough at the end of the drill rod should be adjusted to be aligned with the rectangular groove of the tool unloader. Another fork would be inserted, and the fork in the tool unloader should be taken out. Then, the drill rod is driven backward by the rod unloader and another fork. When the second flat groove at the end of the next drill rod is aligned with the upper part of the drill rod support, the fork could be inserted, and the motor would reverse. The last drill rod could be taken away when the connecting threads of the upper and lower drill rods are disconnected. Then, the decelerator could be put down slowly, and the rectangular groove of the rod unloader is aligned with the first flat groove at the end of the next drill rod. The above steps should be repeated to remove all the drill rods in turn.

5) The diameter of the drilled hole should not exceed the outer diameter of the drainage nail joint by 5～10 mm. The drilled hole should be round and straight, with a depth not less than the sum of the total lengthLof the drainage nail and 50 cm. The difference between the construction and design spacings of each drainage nail should not be more than 10 cm. The deflection length at the bottom of the drilled hole should not be more than 3% of the designed length of the drilled hole, and the depth of the drilled hole should not be less than the designed value.

6) The drainage nail is installed at the designed depth after the drainage nails are connected by the joints. Meanwhile, a PVC pipe with a length of about 1 m is installed within the outlet of the drain nail. Finally, the sleeve is pulled out.

7) The exposed PVC pipe of each drainage nail is connected with the water-collecting pipe by the two-way and three-way methods, and drainage is thereby conducted within the slope. Finally, the collected water is led out to the side ditch by the water-collecting pipe. An elbow is installed at the end of the water-collecting pipe to prevent the subgrade from being scoured by water.

8) Trenches should be excavated on the slope and retaining wall to install water-collecting pipes.

4 Conclusions

1) In view of the defects in high and steep slope reinforcement engineering, such as complex construction process, great construction difficulty, and long construction period, the drilling rig and the drilling rig platform are improved. The specific structures of the updated drilling rig and drilling rig platform are expounded. They could effectively overcome the difficulties with traditional construction equipment.

2) On the basis of the improved drilling rig and drilling rig platform, general provisions on drainage nail construction are proposed, and the technical preparations, machine preparations, and working conditions required for drainage nail construction are described as well. Before the formal construction, a drainage nail processing test must be carried out to determine the technical parameters, and the quality of drainage nail construction should also be tested.

3) A process of drainage nail construction is formulated, and the corresponding construction steps and operation requirements are put forward for the improved drilling rig and drilling rig platform.

4) The improved drilling rig and drilling rig platform have broad application prospects. This research provides a new method for drainage nail construction on high and steep slopes and serves as a reference for high and steep slope reinforcement construction.