3 Methods to Solve Hole Collapsing During Rock Anchorage Construction

Time:2022-06-23From:sinorock View:

In geotechnical anchoring construction, we often encounter geological conditions that are easy to collapse holes and difficult to form holes. Drilling in such geological conditions will frequently cause hole collapse. So how to solve the problem of hole collapse in rock anchoring construction? What preventive measures can be taken to reduce the occurrence of hole collapse problems?

Drill with Casing Pipe

Drilling with casing is a special drilling method for geological core drilling. That is, the casing is pressed into the casing while drilling, or the casing is pressed in ahead of time, and then the drilling tool follows. There are two types of follow-pipe drilling, eccentric follow-pipe, and concentric follow-pipe, which are different in process principle.

Eccentric Follow-pipe Drilling

During normal drilling, the eccentric drilling tool is driven to drill through the vibration and impact of the impactor. During drilling, due to the centrifugal force and frictional force, the eccentric wheel deflects outward to achieve the purpose of expanding the hole diameter and then passes through the rod stabilizer. The impact drives the casing to follow up, and the rock powder produced by the drilling is blown out of the hole through the keyway on the stabilizer. After the drilling is completed, the eccentric is retracted and the casing is pulled out by reversing, and the casing is left in the hole to protect the wall to form a hole. Eccentric follow-pipe drilling should be used when the hole depth is within 40m and there are no large boulders in the formation.

Concentric Follow-pipe Drilling

The center drill bit and the concentric sleeve outside the center drill are used to impact and break the rock to make a hole. At the same time, the casing is brought into the hole by the reaming action of the concentric sleeve. The concentric sleeve is provided with a keyway. Exit in concentric sleeves. The concentric follow-up pipe can better solve the problems of sticking and boulder drilling caused by complex geological conditions during eccentric follow-up pipe drilling, which is more conducive to construction operations and improves drilling efficiency. Concentric follow-pipe drilling should be used when the hole depth exceeds 40m and there are many large boulders in the formation.

Advantages of Follow-through Drilling

1. There are many types of reinforcement materials: hot-rolled ribbed steel bars, fine-rolled rebar, steel strands;

2. Mature construction technology: there are many application projects and many engineering cases that can be referenced;

3. Reinforcement is cheap and easy to buy.

Disadvantages of Follow-through Drilling

1. The casing is pressed into the casing while drilling, or the casing is pressed into the drilling tool in advance to form a hole, the construction process is cumbersome, and it takes up a lot of labor;

2. The completion time of a single root is long and the construction speed is slow;

3. It is necessary to configure grouting pipe or exhaust pipe and casing extraction equipment separately, which requires many equipment and workers, and the cost is high;

4. The site required for construction is large, which is not conducive to the development of other construction work;

5. The dust generated in the process of construction is large, and the pollution of the environment is serious;

6. When pulling out the tube, the tube is easily broken, resulting in economic losses.

Drill holes after grouting reinforcement

In civil construction, especially in foundation and foundation construction, anti-floating and uplifting measures are required. Commonly designed pile foundations include uplift anchors, pipe piles, cast-in-place piles, etc. In the same kind of engineering piles, compared with cast-in-place piles, pipe piles, and other anchors, the cost is only a fraction of the cost, and the required construction machinery and equipment are small, occupying less land, and less noise, etc., and have good engineering performance. Low cost, widely used in engineering. But in some special geology, it is more difficult to form a hole, and it is easy to collapse. In a coastal island project, an uplift bolt design is adopted, but due to the complex geology, the upper part contains a thicker sandy soil layer, the middle is mixed with a silty soil layer, and the lower part is various layers of weathered rock layers, and the soil moisture content is large. The bottom of the steel bar is designed to enter the moderately weathered rock stratum. Due to the small cohesion between sand and silt, poor self-support, and the drilling rig needs to use impact rock breaking during the construction of the rock stratum, a certain vibration is generated, which leads to the completion of the upper part. The hole is necked and deformed, and even the collapsed hole is seriously unable to form.

1. Cut off the external water source flowing into the anchor construction site;

2. Dig drainage ditches around the anchor construction site, and set up dewatering wells in the ditch, with a distance of no more than 10 meters;

3. Arrange dewatering wells in the middle of the bolt construction site, and the well spacing should not exceed 10 meters;

4. Measure and place the anchor pile position;

5. Taking the anchor pile position as the center, carry out the reinforcement operation of split grouting, stirring pile or rotary jetting pile, and the prepared cement slurry contains a quick-setting agent and early-strength agent;

6. Take the standard test block with added solid for a compression test, and carry out hole-forming construction in time after the strength is not less than 5mpa; if the strength of the added solid increases rapidly due to environmental climate and other factors, the use of admixtures such as accelerators can be reduced, and speed up the construction speed of the bolt to prevent the difficulty of forming holes due to the high strength of the reinforcement;

7. Use an auger pipe to borrow soil, and take out the upper soil layer;

8. When the drill pipe is drilled to the rock layer and cannot be drilled any more, lift the drill pipe and replace the impact drill bit. The top of the drill bit is provided with an air outlet. The top of the drill pipe is connected to the compressed air equipment to inflate the hole, and the drill pipe goes deep into the bottom of the hole to carry out Impact rock breaking construction, and at the same time, a water pipe is installed at the top of the pile hole to irrigate the pile hole; thus, the broken rock fragments and debris in the pile hole are taken out of the bottom of the hole by the air as power and water to achieve the purpose of slag removal and hole cleaning.

During the construction, dewatering wells combined with drainage ditches were used to reduce the water content of the soil layer at the construction site, so that the sand layer and the silt soil layer were consolidated, and the strength of the soil layer was improved, thereby improving the stability of the soil layer. At the same time, pouring water into the pile hole can reduce the noise and vibration generated by the drill bit impacting the rock formation, as well as cooling the drill bit and reducing wear.

Precautions

For areas with complex geology, the casing should be inserted when drilling about 1m. In the subsequent process, the casing should be pressed down regularly until it cannot sink easily;

Adjust the concentration of the cement slurry, and properly increase the water-cement ratio when the fine sand layer is drilled to form a hole so that it has a good wall protection effect;

When drilling into the fine sand layer, the drilling speed should be slowed down, and the hole should be swept frequently to form a mud skin on the hole wall to protect the wall and meet the quality requirements of the mud wall.

Self-drilling Hollow Rock Bolt Construction

Components

The self-drilling rock bolt system is composed of a hollow anchor bar, coupler, nut, drill bit, plate, and centralizer. It can combine drilling, grouting, and anchoring in one process. A self-drilling rock bolt system is safe, efficient, and convenient. It is suitable for broken rock, loose soil, and geological conditions where is difficult to drill holes. It helps to ensure the anchoring effect in complex ground conditions and achieve the best construction result.

self drilling rock bolt in rock anchoring construction - Sinorock

Construction Process

1. Construction preparation

The working surface needs to be kept flat and solid to prevent the drilling rig from retreating due to the reaction force generated by the construction of the self-drilling hollow bolt. The width of the working surface should not be less than 6 meters, so as to avoid affecting the self-drilling hollow bolt connecting rod and personnel operations. Since the quicksand layer has the characteristics of containing groundwater, it is necessary to set up a drainage ditch at the foot of the slope to drain the water in time.

2. Positioning

The self-drilling hollow bolt needs to be measured and positioned according to the design before construction. In the positioning of the quicksand layer, attention should be paid to the distance between the bolt and the bottom of the pit, because during the construction process, mud sand, and the slurry will flow out of the anchor hole. The self-drilling hollow bolt needs to be 200-300mm from the bottom of the pit.

3. Drilling and Grouting Simultaneous

The self-drilling hollow bolt can realize the function of grouting while drilling. Because the quicksand layer is unstable, it is necessary to always pay attention to whether the inclination of the bolt meets the design requirements during drilling. If not, the drilling rig needs to be adjusted. Grouting while the self-drilling hollow bolt is being drilled can effectively reinforce the soil, increase the bond strength and improve the bearing capacity.

4. Secondary Grouting

Due to a large amount of slurry eaten in the quicksand layer, secondary grouting needs to be carried out after the grouting is completed. After the anchor body reaches a certain strength, it is very important to carry out secondary high-pressure grouting, which will directly affect the bearing capacity of the self-drilling hollow bolt.

5. Fixed

After the drilling and grouting is completed, if there is a leakage of slurry at the orifice, it needs to be replenished in time. After completion, install the backing plate and nut on the self-drilling hollow bolt, fix the rod body, and prohibit the movement of the bolt during maintenance to avoid displacement.

6. Pull Test

After the construction is completed, the pull-out test of the bolts needs to be carried out. Before tensioning, the tensioning equipment needs to be calibrated to prevent the mutual influence between adjacent bolts. The interval tensioning construction method is adopted, and the total number of bolts is randomly selected. 5% for the acceptance pull-out test.

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