CN112001014B - Method for treating karst cave area foundation by dynamic compaction - Google Patents

Abstract

The invention discloses a method for dynamic compaction treatment of the foundation of a karst cave area, comprising the following steps: step 1, determining the characteristics of the karst cave in the construction section; step 2, selecting preset dynamic compaction parameters to perform large-area dynamic compaction in the karst cave area: 3. Obtain the upper limit of the roof span of the unbroken karst cave in the construction section according to the thickness of the covering layer and the dynamic compaction parameters; Step 4. According to the upper limit of the roof span of the karst cave and the geotechnical parameters, apply the embankment and vehicle load to the foundation of the construction section to check the stability ; If the stability check does not pass, set up two-way reinforced continuous concrete slabs on the pavement base for reinforcement and reinforcement, so as to achieve the purpose of treatment. The invention can quickly determine the upper limit of the roof span of the unbroken karst cave according to the thickness of the covering layer and the ramming ability, so that a large number of drilling holes are not needed to investigate the karst cave span, and the investigation cost and investigation time are saved; It can speed up the construction period and save the engineering cost and survey cost.

Description

A method of dynamic compaction for treating the foundation of karst cave area

technical field

The invention belongs to the field of transportation, and in particular relates to a method for treating the foundation of a karst cave area by dynamic compaction.

Background technique

The roadbed in the karst cave area will face the risk of collapse of the underlying unexplored karst caves during the operation process, so the hidden karst caves need to be detected, evaluated and treated during construction. The difficulty of treating hidden karst caves is not how to deal with them, but how to accurately determine the geometric size of the karst caves. Commonly used cave detection methods include drilling and geophysical exploration. Geophysical exploration can theoretically detect the spatial characteristics of karst caves, but due to the limitation of technical development level and the complexity of geotechnical media, the actual effect is difficult to be satisfied. Drilling can identify parameters such as the depth, height and roof thickness of the karst cave, but even a large number of drilling cannot accurately identify the span of the karst cave. The span is the most important factor affecting the stability of the cave roof. The lack of accurate roof span data of karst caves is an important reason for the difficulty in evaluating its stability.

Among many foundation treatment methods, the huge impact force of the dynamic compaction method can break the karst cave, and then as long as the karst cave is backfilled, the karst foundation with questionable stability can be properly treated; on the contrary, if the huge impact force does not break the karst cave, then It shows that the roof of the cave has a certain bearing capacity, and it may not be necessary to deal with it. Therefore, the dynamic compaction method is used to treat the foundation of the karst cave area, which takes into account the foundation treatment and the evaluation of the stability of the roadbed, which is economical and efficient.

Under the action of dynamic compaction, the effect of the karst cave roof can be divided into the following two situations: (1) the karst cave roof is broken down; (2) the karst cave roof is not broken down. For the karst cave where the roof is broken down, it can be filled with rock and crushed soil to eliminate the risk of collapse. Compared with the traditional grouting method, the cost of dynamic filling of rock and crushed soil is much lower. For unbroken karst caves, they are generally considered to have a certain bearing capacity and are usually not treated. However, the stability of some karst caves cannot actually meet the road requirements, which leads to the risk of collapse.

SUMMARY OF THE INVENTION

In order to solve the above problems, the present invention provides a method for treating the foundation of a karst cave area by dynamic compaction.

For solving the above-mentioned technical problems, the technical scheme of the present invention is:

A method for dynamic compaction treatment of the foundation of a karst cave area, comprising the following steps:

Step 1. Determine the characteristics of the karst cave in the construction section through the preliminary investigation of the borehole. The characteristics of the karst cave include the height of the cave, the covering layer and the thickness of the roof of the cave;

Step 2. Select the preset dynamic compaction parameters to carry out large-scale dynamic compaction in the karst cave area: if the karst cave is broken down, then through the perforation, backfill with rubble, boulder, and gravel soil and compact it; if the karst cave is not penetrated , it will not be processed for the time being;

Step 3: Obtain the upper limit of the roof span of the unbroken karst cave in the construction section according to the thickness of the covering layer and the dynamic compaction parameters; The maximum span of ; the dynamic compaction parameter is the impact energy;

Step 4. According to the upper limit of the roof span of the karst cave and the geotechnical parameters, apply embankment and vehicle load to the foundation of the construction section for stability check calculation. If the stability check is passed, no additional processing is required; if the stability check fails, then The pavement base is provided with two-way reinforced continuous concrete slabs for reinforcement and reinforcement, so as to achieve the purpose of treatment.

For a further improvement, in the third step, firstly obtain a comparison table of the thickness of the covering layer and the upper limit of the span of the unbroken karst cave roof corresponding to the dynamic compaction parameters; then estimate the unbroken karst cave in the construction section according to the thickness of the covering layer and the dynamic compaction parameters The upper limit of the roof span.

A further improvement, in the third step, the data in the comparison table of the thickness of the covering layer obtained by the test and the upper limit of the upper span of the unbroken karst cave roof corresponding to the dynamic compaction parameters, take the thickness of the covering layer as the abscissa, and take the unbroken karst cave as the abscissa. The upper limit of the roof span is the ordinate, and data fitting is performed to obtain the corresponding curve of the thickness of the covering layer under the set dynamic compaction parameters and the upper limit of the roof span of the unbroken cave; Get the upper limit of the span of the unbroken cave roof.

Further improvement, the comparison table is as follows:

For a further improvement, in the fourth step, the foundation stability is checked and calculated by the finite element method according to the upper limit of the roof span of the karst cave and the geotechnical parameters, and then the foundation stability safety factor is calculated by the strength reduction method.

In a further improvement, the geotechnical parameters include the thickness, strength parameter index and modulus of various rocks at the top of the karst cave.

Advantages of the present invention:

1. The upper limit of the roof span of the unbroken karst cave can be quickly determined according to the thickness of the covering layer and the ramming effect, so that there is no need to drill a lot of holes to investigate the karst cave span, which saves the investigation cost and investigation time.

2. For the broken karst cave, the filling material is block stone, gravel soil and other materials, which are very cheap, and the filling is compacted, and the reinforcement effect is good; force and avoid touching the cave. Therefore, the method has the advantages of low price and fast construction speed, which can speed up the construction period and save the construction cost and survey cost.

3. Effectively prevent the possible collapse risk of unpenetrated karst caves.

Description of drawings

Figure 1a shows the development process of microcracks in the roof of the cave with a span-thickness ratio of 8;

Figure 1b shows the development process of micro-cracks in the roof of the cave with a span-thickness ratio of 8;

Figure 1c shows the development process of microcracks in the roof of the cave with a span-thickness ratio of 8; Figure 3;

Figure 2 is a diagram of the failure morphology of the roof of the karst cave with a span-thickness ratio of 8;

Figure 3 is the final distribution diagram of micro-cracks in the roof of the cave with a span-thickness ratio of 7;

Figure 4 is a diagram of the failure morphology of the roof of the karst cave with a span-thickness ratio of 7;

Figure 5 shows the final distribution of micro-cracks in the cave roof with a span-thickness ratio of 6;

Figure 6 is a graph showing the relationship between the span of the karst cave and the breaking energy E under the conditions of the covering layer thickness H=4, 5, and 6m;

Figure 7 is a model diagram of the finite element verification model for the stability of the foundation.

Detailed ways

The technical solutions of the present invention will be specifically described below through specific embodiments.

1. In order to find out the breakdown mechanism of different karst cave spans, the thickness of the karst cave covering layer is 4m, the thickness of the karst cave roof is 1m, the span-thickness ratio λ is 4~8 respectively, and 6000kN m (the weight of the rammer is 200kN, and the diameter of the rammer is 200kN). 2.6m) The ramming energy can carry out numerical simulation of dynamic compaction to analyze the failure mode of the cave roof under different conditions.

The breakdown of the rock on the roof of the karst cave is the result of the accumulation of internal damage during the dynamic compaction process. In order to reveal the damage and even the breakdown process of the cave roof under the action of dynamic compaction, the present invention uses the FISH language to write subprograms, and uses a disk to indicate the occurrence of inter-particle fractures. The micro-cracks of the rock are monitored, and the cohesion between the rock particles is monitored, and the tracking and display of the rock micro-cracks are realized, revealing the failure mode of the cave roof during the dynamic compaction process.

1.1 The situation of dynamic compaction breaking the roof of the karst cave

Figures 1a-1c show the development process of microcracks in the roof of the cave foundation with a span-thickness ratio λ of 8 during the dynamic compaction process.

The breakdown process of the roof shown in Figures 1a-1c is as follows: In dynamic compaction, damage microcracks (as shown in Figure 1a) are first generated in the upper part (at the layer of rock and soil), the reason is that the impact energy generated by dynamic compaction is eroded by waves. When the shock wave reaches the layer of soil and rock, soil particles and rock particles dislocate each other, resulting in micro-cracks. Then damage micro-cracks appeared on the bottom surface of the roof span and the roof support of the karst cave (as shown in Figure 1b). The damaged micro-cracks in the roof mid-span continued to extend upward to form macro-cracks, and the roof mid-span was damaged. distribution, bear a large bending moment, a large number of damaged micro-cracks appear on the top surface of the support (as shown in Figure 1c), the micro-cracks penetrate, and the support is damaged. As shown in Figure 2, a typical bending-pull failure is formed.

Figure 3 shows the micro-crack morphology of the roof during the dynamic compaction process of the karst cave with a span-thickness ratio λ of 7. The breakdown process of the roof is as follows: During the dynamic compaction process, damage micro-cracks first appear at the rock-soil layer, and then the roof is in the middle of the span. Damaged micro-cracks appeared on the bottom surface and the supports of the cave roof, and the cracks continued to develop. Finally, damage cracks appeared in the middle of the span but did not penetrate, while the micro-cracks of the support penetrated (as shown in Figure 3), forming macro-cracks, and finally the support was destroyed. The time from the appearance of microcracks on the top plate to the final failure is 66ms, and the final failure pattern is shown in Figure 4, forming a typical punching shear failure.

To sum up, the failure modes of the cave roof under the action of dynamic compaction are mainly divided into two types. When the span-thickness ratio of the roof of the karst cave is relatively large, the bending-tension failure of the roof will occur under the action of dynamic compaction, which is a progressive failure mode and the failure time is long. , the destruction time is short.

1.2 The situation where dynamic compaction fails to break the roof of the karst cave

Figure 5 shows the final morphology of the microcracks in the roof of the cave with a span-thickness ratio of 6 during the dynamic compaction process. During the dynamic compaction process, the development process of micro-cracks is similar to the development process in Section 2.1. First, damage micro-cracks appear at the rock and soil layers, and then damage micro-cracks appear in the roof mid-span and support. The cracks continue to develop, but do not penetrate through. With the dissipation of impact energy, the microcracks of the roof no longer increase (as shown in Figure 5), and finally the roof of the cave is not broken down.

When the span-thickness ratio of the cave roof is 5, no breakdown will occur under the action of dynamic compaction. During the dynamic compaction process, only a small amount of damaged micro-cracks appeared at the rock and soil layers, and the mid-span bottom surface of the roof and the supports were not affected. The damage of the cave roof was mainly caused by the upward extension of the micro-cracks on the mid-span bottom surface and the supports. Therefore, it can be considered that under the action of dynamic compaction, the flexural and shear bearing capacity of the karst cave roof will not be affected.

2. Dynamic compaction treatment of karst cave foundation

2.1 The relationship between the breaking energy and the roof span of the cave

The breaking energy E of the cave roof is defined as the minimum ramming energy when the discrete element particle velocity of the cave roof cannot converge to zero with time. By analyzing the relationship between the breaking energy and the span of the roof, a method of dynamic compaction to treat the cave foundation is proposed.

The rammer weight is 200kN, the diameter of the rammer is 2.6m, and the thickness of the cave roof is 1m, and the numerical simulation of dynamic compaction is carried out with different tamping energy. The relationship between W and breaking energy E. It can be seen from Fig. 6 that the breaking energy E decreases with the increase of the span W of the cave.

2.2 The method of dynamic compaction to treat the foundation of the karst cave area

Under the action of dynamic compaction, the effect of the karst cave roof can be divided into the following two situations: (1) the karst cave roof is broken down; (2) the karst cave roof is not broken down. For the karst cave where the roof is broken down, it can be filled with rock and crushed soil to eliminate the risk of collapse. Compared with the traditional grouting method, the cost of dynamic filling of rock and crushed soil is much lower. For karst caves that have not been broken down, further stability evaluation is required before considering how to deal with them.

In the evaluation of cave stability, the most difficult data to obtain is the span of the cave. It can be seen from Figure 6 that, under the action of a certain ramming energy, the larger the roof span of the cave with a certain covering layer thickness and the thickness of the cave roof, the easier it is to break. Therefore, under certain ramming energy, the span of the unbroken cave has an upper limit. By finding the upper limit of the span, the stability of the unbroken cave can be estimated; and because this span is the upper limit, the evaluation result is conservative and meets the engineering requirements.

Based on the above analysis, the methods of dynamic compaction to treat the foundation of the karst cave area are summarized as follows: 1. Through the preliminary investigation of a small number of boreholes, the general characteristics of the karst cave in a certain road section are determined: the height of the cave, the covering layer and the thickness of the roof of the karst cave, etc. It is not required to identify all karst caves Distribution and span; ② Select certain dynamic compaction parameters to carry out large-scale dynamic compaction in the karst cave area: if a karst cave is broken down, it will be backfilled with rubble, boulder, and gravel soil through perforation; if the karst cave is not penetrated , it will not be processed for the time being. ③ According to the stratum and dynamic compaction parameters, estimate the upper limit of the roof span of the unbroken karst cave in this road section. ④According to the upper limit of the span of the karst cave and the geotechnical parameters, apply the embankment and vehicle loads to this section of the foundation for stability check calculation. If the stability check is passed, no additional treatment is required; The reinforced continuous concrete slab is reinforced to achieve the purpose of treatment.

2.3 Advantages of dynamic compaction in treating the foundation of karst caves

In the traditional evaluation and treatment of karst caves, a large number of drilling holes are required to find out the span of all karst caves, which not only requires a lot of time, but also requires a large amount of survey costs. The cave treatment usually adopts the grouting method, which has problems such as high treatment cost and uncontrollable grouting amount during construction. The use of dynamic compaction for the foundation treatment of the karst cave area, the economic benefits are reflected in: for the broken karst cave, the filling material is rock, crushed stone soil and other materials, which are very cheap, and the filling is compacted, and the reinforcement effect is good; for the unbroken karst cave, choose the The pavement base is reinforced, and the bearing capacity of the roof itself is used to avoid touching the karst cave. Therefore, the method has the advantages of low price and fast construction speed, which can speed up the construction period and save the construction cost and survey cost.

3 Estimation method of upper limit of unbroken karst cave span

Unbroken karst caves are a serious threat to future highway operation, and the difficulty in evaluating the stability is how to determine the span of the cave roof. Since the larger the span of the karst cave, the easier it is to break. Therefore, under the condition of a certain ramming energy, the span of the unbroken karst cave should be less than a certain value. upper limit".

The commonly used ramming energies in practical engineering are 4000kN m and 6000kN m. For the convenience of application, this paper gives the roof thickness of 1m and the upper limit of the span of unbroken karst caves under different covering layer thicknesses, as shown in Table 1. For other geological conditions, the numerical method recommended by the invention can be used for calculation and determination.

Table 1 Upper limit of unbroken karst cave span under common ramming energy

According to the data in the table above, a curve of the thickness of the covering layer under the corresponding ramming energy and the maximum span of the unbroken karst cave can be produced, so as to obtain the maximum span of the unbroken karst cave corresponding to the thickness of the covering layer under a certain ramming energy according to the curve.

4 Engineering applications

The present invention takes the foundation treatment of the section K273+600-K274+000 of the Hunan Ningdao section of the Xiarong Expressway as an example to illustrate the application method of the results.

Section K273+600~K274+300 is the road section for filling the embankment. The height of the embankment is 4m. The preliminary design and investigation found that there are many karst caves in this section, including 10 drill holes in the road section, and 6 drill holes for karst caves. The geological conditions are from the top The bottom are: ①silty clay, with a thickness of 1-9m; ②medium weathered limestone, with a thickness of 1-3m and a compressive strength of 35.5MPa. The karst cave in this road section is in a semi-filled state, the filling material is soft plastic silty clay, the thickness of the karst cave roof is 1-3m, the height is 0.4-3.8m, the thickness of the covering layer is 2.8-5m, and the span of the karst cave is unknown. It is decided to use (200×20)kN m tamping energy for one time tamping on this road section, the diameter of the tamper is 2.6m, the distance between the tamping points is 5m, and each place has 2 blows. Three karst caves were broken down by dynamic ramming, and the collapsed position of the karst caves was filled and compacted with on-site rubble. Among them, the thickness of the roof of the unbroken karst cave is 1m, and the thickness of the covering layer is 5m. It can be seen from Table 1 that the upper limit of the karst cave span is 9m. Then use the finite element method to check the stability of the foundation (see: Dai Zihang. Numerical analysis of the stability of highway embankments and karst cave roofs in karst areas [J]. Rock and Soil Mechanics, 2014, 35(S1): 382-390; Zhang Lin, Yang Zhigang. Orthogonal finite element method analysis of factors affecting the stability of karst cave roof [J]. China Karst, 2005(02): 156-159), the calculation model is shown in Figure 7, and the stability of the foundation is calculated according to the strength reduction method. The coefficient is 1.52, which indicates that the unbroken karst cave in this section is in a stable state under the load of the embankment, no damage will occur, and no additional treatment is required.

The above-mentioned expressways were all opened to traffic in 2012, and the cave collapse problem has not occurred so far, which shows that the treatment method of the karst foundation in this paper is feasible.

The above is only a specific guiding embodiment of the present invention, but the design concept of the present invention is not limited to this, and any non-substantial modification of the present invention by using this concept shall be an act infringing the protection scope of the present invention.

Claims (4)

1. A method for treating a karst cave region foundation by dynamic compaction is characterized by comprising the following steps:

determining the characteristics of a karst cave of a constructed road section through preliminary investigation of drilling, wherein the characteristics of the karst cave comprise the height of the cave, a covering layer and the thickness of a karst cave top plate;

selecting preset dynamic compaction parameters to carry out large-area dynamic compaction on the karst cave area: if the karst cave is punctured, tamping after backfilling with rubbles, stones and gravel soil through punching; if the karst cave is not punctured, the treatment is not carried out temporarily;

step three, obtaining a comparison table of the thickness of the covering layer and the upper limit of the unbreaked karst cave roof span corresponding to the dynamic compaction parameter through a test; then estimating the upper limit of the span of the top plate of the undamaged karst cave of the constructed road section according to the thickness of the covering layer and the dynamic compaction parameters; then, performing data fitting on data in a comparison table of the thickness of the covering layer obtained by the test and the upper limit of the span of the top plate of the undamaged karst cave corresponding to the dynamic compaction parameter by taking the thickness of the covering layer as a horizontal coordinate and the upper limit of the span of the top plate of the undamaged karst cave as a vertical coordinate to obtain a corresponding curve of the thickness of the covering layer under the set dynamic compaction parameter and the upper limit of the span of the top plate of the undamaged karst cave; then searching a corresponding curve according to the thickness of the covering layer and the dynamic compaction parameters to obtain the upper limit of the span of the top plate of the undamaged karst cave;

obtaining the upper limit of the span of the top plate of the undamaged karst cave of the constructed road section according to the thickness of the covering layer and the dynamic compaction parameters; the upper limit of the top plate span of the undamaged karst cave is the maximum span of the undamaged karst cave under the thickness and the dynamic compaction parameters of the approved covering layer; the dynamic compaction parameter is the compaction energy;

the method comprises the following specific steps:

step four, according to the upper limit of the top plate span of the karst cave and rock-soil parameters, applying embankment and vehicle load to the foundation of the constructed road section for stability checking calculation, and if the stability checking calculation is passed, no additional processing is needed; if the stability check calculation is not passed, the bidirectional reinforcing bar continuous concrete slab is arranged on the pavement base layer for reinforcing and reinforcing, so that the treatment purpose is achieved.

2. The method for treating a karst cave area foundation by dynamic compaction according to claim 1, wherein the comparison table is as follows:

3. the method for treating the karst cave area foundation by dynamic compaction of claim 1, wherein in the fourth step, the foundation stability is checked by adopting a finite element method according to the upper limit of the karst cave roof span and geotechnical parameters, and then the foundation stability safety coefficient is obtained by calculating according to a strength reduction method.

4. The method for treating a foundation of a cavern region by dynamic compaction of claim 1, wherein the geotechnical parameters include thickness, strength parameter index and modulus of various rocks at the top of the cavern.

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