Engineering Analysis of Ground Grouting Controlling Shaft Fracture Deformation

1 Ground grouting reinforcement wellbore mechanism

The vertical additional force hypothesis of shaft rupture suggests that the special aquifer is hydrophobic due to mining and other activities, causing the water level to drop, the effective stress of the soil layer to increase, the consolidation and compression, and the subsidence of the overlying soil. The soil interacts with the wellbore during settling and is applied to the outer surface of the wellbore with an additional force system. When the force is increased to a certain value, the concrete wall will be damaged due to the unbearable. Since the longitudinal additional stress of the borehole wall usually reaches the maximum along the bottom aquifer-bedrock interface, most of the borehole rupture zone is located in the bottom aquifer and the bedrock weathering zone horizon [1-3].

Ground grouting reinforcement is a method of making holes at a certain distance from the ground around the well wall and then grouting the stratum to be consolidated. The method starts from improving the formation properties, grouting and strengthening the root-drainage aquifer that causes the additional force of the well wall, and allowing the slurry with consolidation performance to directly inject into the pores, cracks in the formation, and squeeze or remove the freedom in the formation. Water, which makes the formation compact and lifted up, thus changing the structure and performance of the original formation. When the groundwater level drops, it can slow down the subsidence of the formation and weaken the additional force on the wellbore, thus protecting the wellbore [4-8] .

2 Change law of shaft wall strain during ground grouting

Grouting reinforcement in deep topsoil not only inhibits and reduces the additional strain applied to the borehole by the topsoil, but also reduces the existing vertical strain in the borehole. Figure 1 shows the relationship between the additional strain ε of the wellbore and the aquifer's hydrophobic time t.

The OA segment is the ε-t relationship before the rupture of the borehole wall. The additional strain of the borehole wall is gradually increased due to the effect of drainage and drainage, and finally the ultimate additional strain ε of the borehole wall is reached. In section AB, the well wall is broken, and the additional strain on the well wall is reduced to the residual additional strain εr. The BD section is the continuation of the aquifer drainage and the additional strain on the well wall develops with certain rules. If the aquifer is grouted and reinforced at time t1, due to the intrusion of a large amount of slurry, some of the overlying soil layer will be displaced upward or upward, and the additional strain on the borehole wall will be released slowly. The ε-t relationship is the CE segment in the figure. The additional strain is reduced to εw. If the effect of grouting reinforcement on the hydrophobicity of the aquifer is not considered, the additional strain will develop along the trend of the EF section. In fact, part of the aquifer will be fully compacted, and the slope of the additional strain of the wellbore will change with time. Along the trend of the EG segment. Therefore, the grouting reinforcement of the aquifer has the dual effect of suppressing the additional strain (time t is d) and the sustained release additional strain (time t is h).

3 Ground grouting engineering analysis

3. 1 Project Overview

The depth of the auxiliary wellbore of a mine is 434. 60 m, the inner diameter is 6.50 m, and the topsoil thickness is 168.0 mm. The topsoil freezing section adopts double-layer well wall, the inner wall is 650 mm thick, the outer wall is 500 mm thick, and the outer diameter of the well wall is 8. 8 m. On July 1, 2001, it was found that the borehole wall micro-crack occurred at the depth of 166.5 m, and the cumulative settlement of the abutment surface was 180 mm, and the well wall was in a state of frequent rupture. In order to prevent the situation of the wellbore from further deteriorating, to ensure normal production and improve safety, the ground grouting reinforcement method is selected to prevent cracking of the borehole wall.

The engineer has a total of 8 grouting holes in the ground around the auxiliary shaft, and is evenly arranged around the wellbore. The diameter of the ring is Ф36m, and the outer edge of the well wall is 13.6 m. The slurry diffusion radius is designed to be 6. 6 m, the formation reinforcement width is 13.2 m, the average reinforcement distance is 4.5 m, and the reinforcement section ranges from 148 to 168 m, 128 to 148 m, 108 to 128 m, and 88 to 108 m. . The eight grouting holes are divided into two major sections, which adopt the jumping hole, the segmented ascending, the lower segment of the lower hole, the timed quantitative, and the intermittent grouting method. Grouting material used cement and coal ash powder, pure water plugging slurry is eligible for cement - water glass slurry. 0〜5. 0 MPa。 The final pressure of the grouting orifice is 4. 0~5. 0 MPa.

See Table 1 for the statistics of the amount of slurry injected into the pores.

3. 2 analysis of vertical strain change of shaft wall

In order to control the ground grouting reinforcement process, evaluate the reinforcement effect and long

In the period of prediction and evaluation of the safety of the wellbore, a concrete strain sensor was embedded in four layers of the 100m, 120m, 140m and 160m wellbore depths of the researched mine, and a real-time automatic test system controlled by computer was established. Figure 2 shows the measured vertical additional strain in the 10 months before the grouting, the first stage grouting, the intermittent 8 months, the second stage grouting, and the 10 months after the completion of the grouting project. average of.

In the grouting process on the borehole wall, the vertical additional strain change of the borehole wall is very sensitive. According to the change of the vertical additional strain of the well wall, it can be roughly divided into four stages in the period of 1.5 a before and after the grouting on the ground, and each time period is completely consistent with the actual engineering situation.

(1) The sensor starts working until the first stage of grouting and lasts for about 10 months. During this period, as the aquifer drainage continues, the vertical additional strain of the well wall increases almost linearly, increasing by about 100 με, which is an additional strain growth section after the wall rupture.

(2) The first stage ground grouting was carried out on the 2#, 4#, 6# and 8# holes for more than 3 months. Grouting causes the soil layer to move upwards or move, the additional force of the well wall is restrained, and the vertical additional strain is reduced. This phenomenon is obvious during the whole grouting process until the end of grouting, and the grouting action makes the well The wall vertical additional strain is relieved by about 200 με.

(3) Grouting the 1#, 3#, 5#, and 7# holes for more than 2 months. In a relatively short period of time, the vertical additional strain of the well wall is released by about 200 με, that is, at the end of grouting, the additional force of the well wall is released by about 400 με.

(4) After the grouting, the vertical strain of the borehole wall has increased, but it is not very stable. It will not stabilize until about 200με after more than three months. The actual release of the additional strain on the borehole wall by the second stage grouting is About 100με. The second stage effect is not as obvious as the first stage, mainly due to the decrease in the strength of the grout cement due to the decrease in cement content.

3. 3 grouting curtain test analysis

The grouting high frequency stress wave detection was carried out after 1 a grouting. The test results confirmed that the mortar curtain closed well, and the average thickness was nearly 7.6 m. In the sand and gravel layer with the most serious water loss compression, the mortar spreads more and the stone degree is higher. Observing the core, it was found that the serous stones were scattered in the soil in various phlegm and morphological forms, and some sand layers contained cement. The results of the mechanical indexes of the five core samples after grouting are shown in Table 2. Grouting makes the overall mechanical properties of the soil around the wellbore significantly improved, and some indicators even nearly doubled compared with before grouting.

The formation of the grouting curtain can effectively reduce the sinking of the soil and greatly weaken the additional force of the well wall.

The analysis shows that the ground grouting reduces the existing vertical additional force in the borehole wall, which has the effect of “weakening”, and the additional force weakening effect of the deep wellbore borehole wall is most obvious. After the grouting interval and grouting, the additional strain increase in the borehole wall is obviously weakened, indicating that the ground grouting reinforcement has the effect of suppressing the vertical additional force of the borehole wall.

3. 4 Ground grouting process monitoring

In order to ensure the safety of the well wall during the grouting of the topsoil and to detect the effect of grouting reinforcement, the grouting process must be monitored. The first is to establish an additional strain monitoring system for the borehole wall, collect additional strain values ​​of the borehole wall during grouting reinforcement, and monitor the safety status of the borehole wall in real time and dynamically; secondly, timely grouting parameters according to engineering experience during grouting Adjustments to guide the engineering and control of the grouting reinforcement process. The control parameters of the grouting process include grouting position, grouting pressure, grouting amount and slurry concentration.

When the ground grouting reinforcement, additional load will be generated on the well wall. The additional strain of the well wall can be monitored to embed sensors in different vertical horizontal levels in the borehole wall, which are respectively arranged in the main topsoil reinforcement section. The sensor is typically 630 x 100 m/n and is buried in the borehole wall and destroys the integrity of the borehole wall. Each level is evenly arranged with 4 measuring points along the circumference of the wellbore, and each measuring point is embedded with two sensors in the vertical direction and the circumferential direction. It can be buried at the appropriate level as needed, and then led out to the wellhead through a shielded armored cable to access the junction box and computer. The system uses automatic measurement and recording to obtain corresponding measurements of well walls at different times.

The strain value of the point. The measurement of the data is generally based on the value of the sensor buried for one month, and the additional strain of the well wall is measured.

Ground grouting can change the stress state of the well wall. After the slurry enters the soil layer, the formation can be lifted to different degrees, and the relative tensile strain or compressive strain occurs locally in the well wall, that is, the additional strain of the well wall. During the aquifer grouting reinforcement process, if the grouting range or grouting process and parameters are not properly selected, the slurry pressure may cause the well wall to withstand the uneven horizontal pressure of the original design and cause the well wall to rupture; the overburden of the stratum is excessive, also It may cause the well wall to be pulled vertically, which also poses a threat to the safety of the well wall. Therefore, it is necessary to adjust the grouting parameters and control the grouting process by monitoring the additional strain of the well wall in real time.

4 Conclusion

(1) Ground grouting control wellbore deformation technology is a new wellbore control method. It is a kind of method to restrain and slow down the additional stress on the borehole wall caused by stratum settlement and the resulting wall rupture. An effective method.

(2) The theoretical mechanism of ground grouting to strengthen the borehole wall is to grout the reinforced aquifer to squeeze or replace the free water in the stratum, so that the stratum is compacted and lifted, thus changing the structure and performance of the original stratum. , to reduce the strain of the well wall, thereby protecting the wellbore.

(3) In order to ensure the quality of the grouting project and form a complete grouting curtain, so as to improve the effect of reducing the formation, it is necessary to correctly determine the grouting parameters, grouting time, grouting amount, grouting pressure and other grouting parameters. The key to the success of the wall grouting project.

(4) The implementation process of the whole ground grouting reinforcement project must establish a reliable monitoring system for the wellbore force and surface deformation, so as to adjust the grouting reinforcement parameters according to the additional strain evolution of the borehole wall, and also detect the actual effect of grouting reinforcement. .

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