Construction Scheme of Karst Cave Stratum Rotary Drilling Rig

  When drilling in karst cave area, the mud is easy to leak, and the mud elevation in the hole suddenly drops, which will lead to the failure of the wall protection in the hole and lead to the accident of hole collapse. The filling material in the cave is loose and has collapsed. If the volume of the cave is too large, it is easy to cause the amount of poured concrete to be seriously excessive, and improper treatment will cause the accident of buried drilling, stuck drilling and buried rod for the equipment. Therefore, when each pile encounters the cave, it is necessary to record the geological condition, the depth, height and filling type of the cave in detail, and formulate the corresponding construction plan. For each treatment plan, it is necessary to calculate carefully. When encountering large karst caves, supervision engineers and design units must be asked to check, clarify the treatment plan, and report to the supervisor for approval before implementation.

  According to the height of the cave and the filling in the cave, different treatment methods are adopted for the cave. The main ones are: casting and filling, grouting consolidation, concrete pouring construction, steel casing and so on. The following is a brief introduction of the construction scheme for several karst cave forms for reference.

------------------------------------------------------------------- ○ The cave range is small ○ ---------------------------------------------------------------

------------------------------------------------------------------- ○ The cave is 1-3m high ○ ---------------------------------------------------------------

  And the cave is filled or semi-filled filled cave, when it is found that the mud slurry level drops rapidly, indicating that the cave is opened, after adding mud to the hole, the hole head square clay bag, add a bag of cement and stone pieces, stir in the hole with a drill bucket, is formed through the east of the cement slurry stone wall, after the cement slurry initially blocked the stone gap, stand for 24 hours, After the cement reaches the initial setting state, continue drilling construction through the cave.

------------------------------------------------------------------- ○ The cave is 3m high ○ ---------------------------------------------------------------

  The rotary digging can be combined with the pipe rolling machine for construction, and the lower steel casing can be sunk into the bottom of the cave for wall protection. This construction method can dry the hole without using mud wall protection in the steel casing part. After the completion of perfusion, it is necessary to pull out the casing according to the needs. Using an all-steel casing may increase the cost but is relatively economical compared to filling the hole with concrete. For some karst caves with holes higher than 3m, but which can form holes after throwing filling stones, but cannot be filled with underwater concrete, a 4mm wall thickness protective cylinder can be welded on the steel cage at the location of the karst cave and placed together with the steel cage to pour underwater concrete.

  The above are several commonly used treatment methods for rotary digging after encountering karst cave geology. The above scheme is explained in detail as follows:

• First, the common problems of rotary drilling rig limestone cave formation

1, due to the uneven force during drilling, breaking teeth, bit damage, etc

2, inclined rock face or half of the rock drilling hole skew 3, through the cave roof occurs stuck drilling

4, the development of empty karst caves, cracks geological leakage

5. Cave collapse in the strata above the cave due to slurry leakage in the cave layer or cave collapse in the unstable cave filling layer

6. A large number of super prescriptions are injected into the cave

• Second, construction preparation

  In the preliminary survey, whether the karst cave situation is accurate or not is directly related to the choice of construction scheme, construction cost and construction period, so every area with karst cave development must be guaranteed to be explored. According to the different conditions of karst cave development in geological exploration results, different construction schemes are adopted to make preparations before drilling and ensure that drilling is done in mind.

• Prevent excessive bit damage and tooth breakage

  Drill teeth need to be strengthened before drilling into the cave, and each part of the drill bit needs to be checked frequently for weld cracks. In the cave formation, the drilling principle of "low drilling rate and variable pressure" can avoid accidents such as breaking teeth and bit damage as far as possible.

• Hole skew prevention

When drilling into the karst cave formation, it is necessary to use a barrel drill and a sand dredging bucket to drill together, first by the barrel drill to form a guide and create a free surface, and then by the sand dredging bucket to drill into the dredging slag to avoid the hole inclination. At the same time, it can be considered to increase the height of the drill bucket to strengthen the guidance and prevent the hole deviation. In operation, the method of light pressure or self-weight pressure drilling is used to drill normally until the drill bit all eats into the rock formation to prevent the hole from being tilted.

• Sticking prevention

The guide plate can be repaired on the square rib plate before construction to avoid obvious steps from the top to the bottom of the bit that may contact with the rock mass between the top of the square head and the top of the drill barrel to ensure smooth transition to the maximum extent. If the reinforcement plate has sufficient strength, it can also be welded on one set of diagonal reinforcement plates, the intercepts are oriented towards the rock mass stuck by the drill teeth to cut the part of the rock stuck by the drill teeth, and the drill bit is proposed.

• pulp leakage treatment

  In order to cope with slurry leakage, first of all, it is necessary to estimate the approximate volume of the underground karst cave as far as possible, and then ensure that the slurry capacity of the mud pool is at least twice the volume of the karst cave. The model of the mud pump and water refill pump is larger than that of the conventional one, so as to ensure the rapid grouting and water refill when slurry leakage occurs. At the same time, sufficient filler should be prepared and placed near the hole to be drilled, and the model of the excavator should be at least 200. Guarantee filling speed.

  Filling material:

1. Soft plastic mixture of bentonite, sawdust, clay and cement with water.

2, the stone + packed loose clay, the size of the stone should consider the size of the inlet of the drill bucket used, the stone is best for mudstone blocks, and avoid excessive hardness.

3, block cement, this material is impervious and plastic, viscosity is very good, after filling with drill bucket back drilling compaction, can fully fill the cave space, drilling again is not easy to collapse holes.

  When drilling, it is found that the mud in the hole slowly drops, should be replenished in time, and put forward the drill bit, add filler to the hole, put the drill bit, and the floating reverse of the drill will squeeze the filler 1 into the rock crack to block the mud loss.

  If the mud surface drops rapidly, the leakage is too fast, and it is difficult to achieve the purpose of loose filling, a large amount of grouting and water should be replenishment in time, and the drill should be quickly put in the filling material 2 or 3 in a short time, and put the drill into the floating reverse until the mud slurry level no longer falls.

• Massive infusion of superprescription treatment

  For low cave (less than 1 meter), and at the depth of the cave, there is no distribution of pile holes around the cave, the hole is directly filled with concrete, and a small amount of excess square is normal.

  When the karst cave is slightly high (greater than 1 meter and less than 4 meters) or the depth of the karst cave around the pile hole distribution, and the depth of the karst cave distribution is shallow (such as within 15 meters), the empty karst cave can be filled with filler 2 or 3 and then drilled again.

  For the high karst cave (greater than 4 meters), the above filling material is difficult to fill the empty karst cave completely, and the backfilling efficiency is low, the construction is cumbersome, can be backfilled cement mortar or low grade concrete, to be drilled before the initial setting. This requires that the strata above the cave will not collapse during this period. And if there is rich water in the cave, it is necessary to fully consider that the filling of cement mortar may be diluted by a large amount of groundwater and affect the solidification effect. For the interconnected karst cave group (which can be judged by whether the distribution depth of the adjacent pile holes overlaps with each other), the construction of pile holes can be constructed by group. First, a group of pile holes connected to each other are drilled into the bottom of the cave, and then backfilled and compacted with each other, which can ensure that the connected cave is filled completely and the concrete is saved to the maximum extent. The completion time of this group of pile holes is calculated according to the construction needs of the group to ensure the stability of the hole wall before the pile holes are injected.

• Sludge treatment

  Ensure mud quality. The bottom drill must be used to clean the slag before the steel cage is lowered to ensure that the steel cage is poured immediately after being lowered to minimize the pile hole prevention time. Gas lift reverse circulation process can be used when conventional measures cannot remove slag completely.

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