CN106978807A - Existing bridge pile pulling device and construction method are passed through by a kind of weak soil shield driving area - Google Patents

Abstract

The invention relates to a pile pulling device for crossing an existing bridge in a soft soil shield excavation area, including a crawler crane, a reaction frame, a full casing machine, an inverted triangle hammer and a full casing; the full casing machine drives the full casing to rotate, cut and sink For the original pile, each section of the full casing is connected by a bolt joint, and a cutter head is installed at the bottom of the full casing; an inverted triangle hammer is installed between the full casing and the original pile; the crawler crane connects the inverted triangle through a wire rope Hammer; the reaction force is erected between the crawler crane and the full casing machine. The present invention has the following characteristics and beneficial effects: the old pile is pulled out after the pile body is separated from the soil body by sinking a complete steel sleeve to reduce friction, and has the advantages of high stability and fast construction speed. At the same time, it can guarantee the construction quality and ensure the good prospect of subsequent engineering construction application.

Description

A kind of pile extraction device and construction method for crossing existing bridges in soft soil shield tunneling area

technical field

The invention relates to the field of pile pulling construction for shield tunneling crossing existing bridges in soft soil areas, in particular to the technology and construction method for pulling piles for shield tunneling crossing bridges in soft soil areas.

Background technique

With the continuous acceleration of urbanization, urban rail transit facilities are becoming more and more important. In the process of subway line planning, in principle, the space under the existing arterial roads should be used as much as possible. However, in the process of subway tunnel shield construction, it is inevitable that the tunnel needs to pass through the pile foundation under the existing bridge. In order to make the shield machine move forward smoothly, it is necessary to remove the obstacles in front of the tunnel shield machine, which has also become an important process in the excavation process of urban underground passages.

Generally, pile pulling construction sites are divided into several categories: underground, above ground, and above water. There are different plans and pile pulling equipment for different situations. For example, the pile foundation of bridge pier abutment may be pulled out on water, and it is necessary to use ships as counter force devices to pull out. Pile construction; and pile extraction on land may use special pile extractors, cranes and jacks. When the building above or around the pile foundation to be removed is more important, in order not to affect the safety of the upper building or adjacent buildings, the underpinning technology for removing part of the foundation of the pile foundation should also be used.

Contents of the invention

In order to overcome the difficulty that the tunnel needs to pass through the pile foundation under the existing bridge during the shield construction of the subway tunnel in the soft soil area, so that the shield can be smoothly advanced, the invention provides a shield machine with good stability and strong environmental adaptability. Pile Pulling Device and Construction Method for Shield Tunneling in Soft Soil Area

For realizing above-mentioned technical purpose, the present invention adopts following technical scheme:

A pile extraction device for crossing existing bridges in a soft soil shield excavation area, including a crawler crane, a reaction frame, a full casing machine, an inverted triangle hammer, and a full casing; the full casing machine drives the full casing to rotate and cut into the original pile , each section of the full casing is connected by a bolt joint, and a cutter head is installed at the bottom of the full casing; an inverted triangle hammer is installed between the full casing and the original pile; the crawler crane is connected to the inverted triangle hammer through a wire rope; The power erection is between the crawler crane and the full casing machine.

As a preference: the crawler crane is also connected with a grab bucket through a wire rope.

The construction method of this soft soil shield tunneling area crossing the existing bridge pile pulling device, the main steps are as follows:

1) Construction preparation

Before pulling out piles, the construction platform needs to be prepared to meet the construction requirements of the full casing machine;

2) Measurement and setting out

After removing the bridge deck, measure and record the coordinates of the piles according to the construction drawings of the original piles, carry out stakeout to determine the range and quantity of the pile foundations that need to be removed, and number them;

3) The full-turn equipment is in place and fixed

The full casing machine runs to the pile position, uses the cross positioning plate to align with the center of the original pile, and the drilled pile to be pulled and the clear hole form a concentric circle, and the full casing machine moves into place;

According to the on-site setting-out positioning, the full casing machine is fixed above the center of the bored pile and connected to the power box and the operating room; then the reaction frame is installed, and a crawler crane is parked at the other end of the reaction frame. The crawler presses the reaction frame, and the crawler crane cooperates with the installation of steel sleeves and pile extraction at the same time;

4) The full casing machine drives the full casing to sink

The full casing is hoisted into the full casing machine with the cooperation of the crawler crane, the verticality of the fuselage is adjusted, and the center of the hoop system of the fuselage is kept in the same vertical direction as the center of the clearing hole; the full casing is driven by the full casing machine to rotate and start sinking During the press-in process, each section of casing is put into the clamping device, the clamping hydraulic cylinder is contracted, and the vertical accuracy of the casing is adjusted to the required range by using the drilling rig and the guide correction device; the full casing is driven by the full casing machine to rotate sunk to a predetermined depth;

5) Pull out the bored pile

The entire pile extraction process adopts the method of segmental cutting, segmental hoisting, and multiple repetitions;

Cutting of the original pile: After the full casing is sunk into the predetermined depth, the separation and friction reduction around the pile is realized, and then the triangular hammer is lowered into the full casing, cut into the space between the original pile and the full casing, and the full casing is driven by the full casing machine to rotate, The original pile is driven to rotate eccentrically by the inverted triangle hammer, the original pile is cut off in sections, and the steel cage is also twisted off. By observing the decrease in the control pressure count value of the full casing machine, it is determined that it has been disconnected;

Clear the original pile: When the original pile and the full casing rotate at the same time, remove the inverted triangle hammer, and lower the steel wire rope to pull out the part of the pile lock that has been cut off and separated from the entire pile body at one time; the implementation is completed. Pile body removal; if the pile body is broken or crushed during the removal process, a crane equipped with a grab bucket can be used to grab the remaining pile body;

Press-in cleaning: Before the original pile head is confirmed, clean the sand in the casing; use a grab to remove the soil that fell into the casing during the construction process and expose the pile head again;

Remove all the original piles: Rotate the lower end of the full casing to the original pile design removal position. When the original piles and the full casing are rotated at the same time, use the grab to take out the remaining original piles;

After the full casing rotates to cut the obstacle to a certain depth, use the crawler crane to remove it with a grab bucket; when removing the broken pile, if it is difficult to grab it with the grab bucket, you can use the hammer to break it again and then grab it;

During the pile pulling process, the pulling speed is slow. While using the lifting equipment slowly, the full casing machine is used to drive the full casing to rotate to reduce the friction stress of the pile body; the original piles to be removed are stacked neatly on the site, and cleaned up immediately after a certain amount has been accumulated or Sinotrans;

(6) Pile hole backfilling

After the original piles are pulled out, the pile holes must be backfilled; the strength of the backfill must be slightly higher than that of the original soil, and it is required to be dense and uniform; the cement soil that is uniformly mixed is used for backfill; the cement soil is mixed with plain soil 7-10% cement is repeatedly stirred;

(7) The full casing machine drives the steel casing to be pulled out

The full casing is driven by the full casing machine to rotate and pull out the full casing; the equipment moves to the next pile position.

As preferably: in step 6), the requirement of pile hole cement soil backfilling is as follows:

The earth shall not have blocks larger than 5cm, and shall not contain construction waste and other admixtures, and the cement shall be ordinary Portland cement with a value of 32.5 or more;

The amount of cement is controlled at 7-10%, that is, 130-180kg of cement is added to each cubic soil, and the strength is controlled at 0.5mPa; when mixing, an excavator is used to mix, and the mixing is required to be uniform, and there must be no cement blocks piled up. Finally, it must be used before the initial setting of cement;

During the backfilling process, the removal of the full casing must be carried out simultaneously with the backfilling work. First fill and then pull out. Before backfilling, use a grab to remove the loose soil that fell into the full casing when pulling out the pile, and then fill in the required good soil layer by layer, and timely Measure the backfill depth, pull out the steel casing for 1.5~2.5m after backfilling at a height of 2~3m, and tamp with a heavy hammer several times after each backfill layer to ensure that the backfill is dense and always keep the bottom of the steel casing at Backfill below.

The present invention has the following characteristics and beneficial effects: the old pile is pulled out after the pile body is separated from the soil body by sinking a complete steel sleeve to reduce friction, and has the advantages of high stability and fast construction speed. At the same time, the construction quality can be guaranteed, and the application prospect of subsequent engineering construction is good.

Description of drawings

Figure 1 is a schematic diagram of the construction process of shield excavation over bridge pile extraction in soft soil area.

Figure 2 is a schematic diagram of the lifting of the steel wedge.

Figure 3 is a schematic diagram of a grab bucket for clearing obstacles.

Explanation of reference signs: track sling 1, reaction seat 2, full casing machine 3, inverted triangle hammer 4, full casing 5, reaction plate 6, grab bucket 7, original pile 8.

detailed description

The present invention will be further described below in conjunction with the examples. The description of the following examples is provided only to aid the understanding of the present invention. It should be pointed out that for those skilled in the art, without departing from the principle of the present invention, some improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

As shown in Figures 1 to 3, the soft soil shield tunneling area of this embodiment passes through the existing bridge pile extraction device: including crawler crane 1, reaction frame 2, full casing machine 3, inverted triangle hammer 4 and full casing 5 ; The full casing machine 3 drives the full casing 5 to rotate and cut into the original pile 8, and each section of the full casing 5 is connected by a bolt joint, and a cutter head is installed at the bottom of the full casing 5; An inverted triangle hammer 4 is arranged between the original pile 8; the crawler crane 1 is connected with the inverted triangle hammer 4 through a wire rope; The 360-degree full-rotation casing machine drives the steel casing 360-degree rotation, cuts and sinks, effectively separates the pile body from the soil, and uses special equipment to lower the steel wire rope and securely lock the pile body, and then use large-scale lifting equipment to cooperate with it. Remove old piles. On the one hand, the steel casing transmits the torque and pressing force provided by the top driving device to the cutter head, and on the other hand, it plays a role in supporting the hole wall and preventing the hole wall from collapsing during the drilling process. The length of a single section of the steel casing is 6m, which can be flexibly allocated in actual construction according to different combinations of actual pile lengths, and the diameter of the steel casing can be flexibly selected according to the actual situation. Each section of steel casing is connected by bolt joints, and a cutter head is installed at the bottom of the steel casing. The alloy teeth on the cutter head can be used to penetrate soil layers, concrete and other obstacles. The lifting equipment and slings mainly include cranes, crawler cranes and wire ropes, and corresponding specifications are configured according to actual conditions.

The construction process of pulling piles over bridges by shield tunneling in soft soil areas is shown in Figure 1.

The specific implementation is as follows:

(1) Construction preparation

Before pulling out the piles, the construction platform needs to be prepared to meet the construction requirements of the 360-degree full-rotation casing machine 3. According to the construction characteristics of the project and the site conditions, the construction personnel and mechanical equipment should be organized to enter the site, and the personnel should be trained and explained, and the equipment should be inspected. Commissioning; access to temporary water and electricity at the same time.

(2) Measurement and setting-out

After removing the bridge panel, according to the original pile foundation construction drawing, measure and record the coordinates of the piles, carry out stake out to determine the range and quantity of pile foundations that need to be removed, and number them, so as to draw up a reasonable pile pulling sequence and pile pulling equipment. Implementation of pile pulling.

(3) The full rotation equipment is in place and fixed

The full-rotary casing machine 3 moves to the pile position, uses the cross positioning plate to align with the center of the pile position (the planned drilling and drilling piles form concentric circles with the clearing hole), and the full-rotary casing machine 3 moves into place.

According to the on-site setting-out positioning, the full-rotation drilling rig is fixed above the center of the bored pile, and connected with the power box and the operating room; then install the reaction frame 2 (the function of the reaction frame is to prevent the machine twisting), the other end of the reaction frame 2 stops a crawler crane 1, and the crawler belt of the crane presses the reaction frame 2, and the crane can cooperate with installing steel sleeves and pile pulling work simultaneously.

(4) The full rotary casing machine 3 drives the steel casing to sink

The steel casing 5 is hoisted into the full casing rotary machine 3 under the cooperation of the crawler crane 1, the verticality of the fuselage is adjusted, and the center of the hoop system of the fuselage is kept in the same vertical direction as the center of the cleaning hole. The steel casing 5 is driven to rotate by the full-turn casing machine 3 and begins to sink. During the press-in process, the precision of each section of casing will directly affect the construction quality of the drill hole. Put each section of casing into the clamping device, shrink the clamping hydraulic cylinder, and use the drilling rig and the guiding and correcting device to adjust the vertical accuracy of the casing. Adjust to the required range. During the drilling process, use the level monitoring system that comes with the equipment to check the verticality of the casing at any time, and set up theodolites in the two vertical directions of the casing three times for each hole to check the verticality.

The full-rotary casing machine 3 surrounds and drives the steel casing 5 to rotate and sink to a predetermined depth, generally 3 meters below the bottom elevation of the shield machine, so as to greatly reduce the frictional resistance between the pile body and the soil around the pile.

(5) Pull out the bored pile

Considering factors such as the large diameter of the pile foundation, the deep pile length, and the high pullout resistance of the whole pile in this project, we plan to cut off the piles in sections, lift them in sections, and repeat them many times during the entire pile pulling process.

Cutting of the original pile 8: After the steel casing 5 is sunk into the predetermined depth, the separation and friction reduction around the pile is realized, and then the special inverted triangle hammer 4 (weight 5t, length 5m) is lowered into the casing to cut into the original pile 8 and Between the steel casing 5 pipes, the steel casing 5 is driven to rotate by the full-turn casing machine 3, and the original pile 8 is driven to rotate eccentrically by the inverted triangle hammer 4. The original pile 8 is cut off in sections, and the steel cage is also twisted. Observe that the control pressure count value of the full-turn casing machine 3 decreases, and it is determined that it has been disconnected. According to construction experience, the old piles with steel cages broke into about 7 meters by themselves, and the old piles without steel cages broke into shorter lengths or even shattered.

Clear the original pile 8: When the original pile 8 and the casing 5 rotate at the same time, remove the inverted triangle hammer 4, and lower the wire rope to pull out the part of the pile lock that has been cut off and separated from the entire pile body at one time. ; Implement and complete the removal of the upper pile. If the pile body is broken, crushed, etc. in the process of pulling out, the crane can be equipped with a grab bucket 7 to grab the remaining pile body.

Press-in cleaning: Before the 8 heads of the original pile are confirmed, clean the sand in the casing. Remove the soil falling into the sleeve pipe in the construction process with grab bucket 7 to expose the pile head again.

Remove all the original piles: Rotate the lower end of the steel casing 5 to the original pile design removal position. When the original piles 8 and casing 5 rotate at the same time, use the grab bucket 7 to take out the remaining original piles.

After the steel sleeve pipe 5 rotates and cuts the obstacle to a certain depth, it can be removed with a crane using a grab bucket. When clearing the fragmented pile, if it is difficult to grab it with a grab bucket, you can use the hammer to break it again and then grab it.

In the pile pulling process, the pulling speed should not be too fast, while using the lifting equipment to exert force slowly, while driving the steel casing 5 to rotate with the full-turn casing machine 3 to reduce the friction stress of the pile body. The old piles 8 that have been pulled out are neatly piled up on the spot, and will be cleaned up or shipped out immediately after a certain amount has been accumulated.

(6) Pile hole backfilling

After the original pile 8 was pulled out, the pile hole must be backfilled. The quality of the backfill is very important. It must be ensured that the backfill must reach a certain strength. The strength should be slightly higher than that of the original soil, and it must be dense and uniform. It is proposed to use uniformly prepared and mixed cement soil for backfilling. The mixing of cement soil is made by mixing 7-10% cement into ordinary plain soil and stirring repeatedly. Requirements for pile hole backfilling with cement and soil are as follows:

The earthwork shall not have blocks larger than 5cm, and shall not contain construction waste and other admixtures, and the cement shall be ordinary Portland cement with a value of 32.5 or above.

The amount of cement is controlled at about 7-10%, that is, about 130-180kg of cement is added to each cubic soil, and the strength is controlled at about 0.5mPa. When mixing, excavators are used to mix. It is required to mix evenly, and no cement blocks will pile up. After the cement soil is mixed, it must be used before the initial setting of the cement. The construction of this project requires the cement soil to be used as it is mixed.

Pulling out of the steel casing 5 in the backfilling process must be carried out synchronously with the backfilling work. Pull out after filling first. The hole can not be filled up at one time and then pull out the casing, so as to prevent the backfilled earth from being loosened when the steel casing 5 is pulled out. Before backfilling, use the grab bucket 7 to remove the loose soil that fell into the steel casing when pulling out the pile, and then fill in the required good soil layer by layer, and measure the backfill depth in time. Pull out the steel casing after every 2 to 3m of backfilling 1.5 to 2.5m, but it must be rammed with a heavy hammer several times after each backfill layer is completed to ensure that the backfill is compact and always keep the bottom of the steel casing 5 below the backfill.

(7) The casing machine is driven to pull out the steel casing

The casing machine 3 is driven to rotate and pull out the steel casing 5 . The equipment moves to the next pile position.

In addition, in order to deal with emergencies, the countermeasures for common problems are as follows:

The difficulty and risk of pile extraction in this kind of project are relatively high. The original pile gene construction may have quality problems such as pile diameter expansion, pile body inclination, etc., or local fractures and fractures may be caused by geological, hydrological, age and other reasons after construction. etc. or in the process of pile pulling construction, it is inevitable that the pile body will be pulled out, etc. In view of these possible difficult factors, the following measures can be taken in advance:

(1) It is planned to use a steel sleeve 100cm larger than the diameter of the pile, which can effectively prevent the pile from being cut or cut off by the steel sleeve due to the expansion of the diameter of the pile body or the excessive inclination rate under the conventional construction method and cannot be pulled out;

(2) The old piles and obstacles remaining in the hole caused by chipping or partial crushing and fracture of the pile body are completely removed by grabbing buckets;

(3) When pulling out the pile, the steel wire rope is sent into the lower part of the pile through special equipment. Broken, broken and other situations can also achieve the purpose of pile removal;

(4) If the rope is broken or damaged during the pile pulling process, the upper soil can be removed with a grab bucket, and the piles can be pulled out twice; the piles can still be pulled out;

(5) In case of pile displacement or excessive inclination of the pile body, secondary positioning and correction of the pile center can be implemented so that it can be separated by the steel casing to realize the removal of the entire pile.

To sum up, the solution of the present invention is suitable for pulling out the original piles of tunnel shield excavation over bridges in soft soil areas, so that the shield tunnel can be smoothly advanced. The invention adopts the method of sinking the complete steel sleeve to separate the pile body from the soil body to reduce friction and then pull out the old pile, which has the advantages of high stability and fast construction speed. At the same time, the construction quality can be guaranteed, and the application prospect of subsequent engineering construction is good. Since the tunnel needs to pass through the pile foundation under the existing building during the construction process, it is widely used in engineering examples, and the technical solution of the present invention can be used to remove obstacles in front of the shield in most soft soil areas.

Claims (4)

1. A soft soil shield excavation area passes through an existing bridge pile extraction device, which is characterized in that: it includes a crawler crane (1), a reaction frame (2), a full casing machine (3), an inverted triangle hammer (4) and Full casing (5); the full casing machine (3) drives the full casing (5) to rotate, cut and sink into the original pile (8), and each section of the full casing (5) is connected by a bolt joint, A cutter head is installed at the bottom of the full casing (5); an inverted triangle hammer (4) is provided between the full casing (5) and the original pile (8); the crawler crane (1) is connected with the inverted triangle hammer (4) through a wire rope; The reaction frame (2) is located between the crawler crane (1) and the full casing machine (3). 2. The piling device for crossing existing bridges in soft soil shield excavation area according to claim 1, characterized in that: said crawler crane (1) is also connected with a grab bucket (7) through a wire rope. 3. the construction method of crossing the existing bridge pile pulling device in the soft soil shield tunneling area as claimed in claim 2, is characterized in that, the main steps are as follows: 1) Construction preparation Before pulling out the pile, the construction platform needs to be prepared to meet the construction requirements of the full casing machine (3); 2) Measurement and setting out After removing the bridge panel, measure and record the coordinates of the piles according to the construction drawings of the original piles (8), carry out lofting to determine the range and quantity of the pile foundations that need to be pulled out, and number them; 3) The full-turn equipment is in place and fixed The full casing machine (3) moves to the pile position, uses the cross positioning plate to align with the center of the original pile (8) and positions, and the drilled pile to be pulled out and the clear hole form a concentric circle, and the full casing machine (3) moves bit; According to the on-site setting-out positioning, the full casing machine (3) is fixed above the center of the bored pile and connected with the power box and the operating room; then the reaction frame (2) is installed, and the other end of the reaction frame (2) is parked A crawler crane (1), the crawler belt of the crawler crane (1) presses the reaction frame (2), and the crawler crane (1) cooperates with installing steel sleeves and pulling out piles at the same time; 4) The full casing machine (3) drives the full casing (5) to sink into The full casing (5) is hoisted into the full casing machine (3) with the cooperation of the crawler crane (1), the verticality of the fuselage is adjusted, and the center of the hoop system of the fuselage is kept in the same vertical direction as the center of the cleaning hole; The pipe machine (3) surrounds and drives the full casing (5) to rotate and sink in; during the pressing process, each section of casing is put into the clamping device, shrinks the clamping hydraulic cylinder, and uses the drilling rig and the guiding and correcting device to adjust the vertical accuracy of the casing To the required range; surrounded by the full casing machine (3) to drive the full casing (5) to rotate and sink to the predetermined depth; 5) Pull out the bored pile The entire pile extraction process adopts the method of segmental cutting, segmental hoisting, and multiple repetitions; Cutting of the original pile (8): After the full casing (5) is sunk into the predetermined depth, the separation and friction reduction around the pile is realized, and then the triangular hammer (4) is lowered into the full casing (5) to cut into the original pile (8 ) and the full casing (5), the full casing (5) is driven to rotate by the full casing machine (3), and the original pile (8) is driven to rotate eccentrically by the inverted triangle hammer (4), and the original pile (8) is segmented ) is cut off, and the steel cage is also twisted off, and it is determined that it has been disconnected by observing that the control pressure count value of the full casing machine (3) decreases; Clear the original pile (8): When the original pile (8) and the full casing (5) rotate at the same time, remove the inverted triangle hammer (4), and lower the part of the wire rope that has been cut off and separated from the entire pile body The pile lock is firmly pulled out at one time; the removal of the upper pile body is completed; if the pile body is broken or crushed during the removal process, the crane can be equipped with a grab bucket (7) to grab the remaining pile body; Press-in cleaning: before the original pile (8) head is confirmed, clean the sand in the casing; use the grab (7) to remove the soil that fell into the casing during the construction process and expose the pile head again; Remove all the original piles (8): Rotate the lower end of the full sleeve (5) to the design removal position of the original pile (8), when the original pile (8) and the full sleeve (5) rotate at the same time, use a grab (7) remaining original pile (8) is taken out; After the full casing (5) rotates to cut the obstacle to a certain depth, use the crawler crane (1) to remove it with the grab (7); when removing the broken pile, if the grab (7) is difficult to remove, you can use the hammer again Grab after breaking; During the pile pulling process, the lifting speed is relatively slow. While using the lifting equipment to exert force slowly, the full casing machine (3) is used to drive the full casing (5) to rotate to reduce the friction stress of the pile body; the removed original piles (8) are stacked neatly on the On-site, after a certain amount is accumulated, it will be cleaned up or shipped out immediately; 6) Pile hole backfilling After the original pile (8) is pulled out, the pile hole must be backfilled; the strength of the backfill must be slightly higher than that of the original soil, and it is required to be dense and uniform; the cement soil that is uniformly mixed is used for backfill; the cement soil is mixed with plain soil It is made by mixing 7-10% cement and stirring repeatedly; 7) Full casing machine (3) drives to pull out the steel casing The full casing (5) is driven and rotated by the full casing machine (3); the equipment moves to the next pile position. 4. construction method according to claim 3, is characterized in that: in step 6), the requirement of pile hole cement-soil backfill is as follows: The earth shall not have blocks larger than 5cm, and shall not contain construction waste and other admixtures, and the cement shall be ordinary Portland cement with a value of 32.5 or more; The amount of cement is controlled at 7-10%, that is, about 130-180kg of cement is added to each cubic soil, and the strength is controlled at 0.5mPa; when mixing, an excavator is used to mix, and the mixing is required to be even, and no cement blocks will pile up. After mixing, it must be used before the initial setting of cement; In the process of backfilling, the pulling out of the full casing (5) must be carried out simultaneously with the backfilling work. First fill and then pull out. Before backfilling, use the grab bucket (7) to remove the loose soil that fell into the full casing (5) when pulling out the pile, and then gradually Fill in the required good soil layer by layer, and measure the backfill depth in time. The steel casing must be pulled out for 1.5-2.5m after every 2-3m backfill height. Dense and always keep the bottom of the steel casing (5) below the backfill.