【Excerpts】Problems and treatment of rotary drilling in pile foundation construction of mud-stone strata

1、Project overview

        The construction project of Bijie Maternal and Child Health Hospital (Bijie Children's Hospital and Bijie Obstetrics and Gynecology Hospital) under construction is located in the south of Jinhai Lake, southwest of Shuangshan Group, Shuangshan New District, Bijie City, with a planned total land area of 85337.60m2 and a total construction area of 113541.01m2. The construction of pile foundation adopts mechanical hole and artificial hole, among which the number of mechanical hole filling piles is 409.

2、Engineering geology conditions

① Plain filling: variegous, slightly dense, composed of clay clip gravel. Local distribution, the thinnest is 1.50 m, the thickest is 12.10 m, the average thickness is 5.59 m; the highest elevation is 1479.21 m, the lowest elevation is 1473.54 m, and the average elevation is 1473.98 m.

② Plastic red clay: yellow, brown yellow, more uniform soil, dense structure, plastic state. Local distribution, the thinnest point is 3.10 m, the thickest point is 28.40 m, the average thickness is 15.30 m, the highest elevation is 1472.24 m, the lowest elevation is 1464.10 m, and the average elevation is 1467.99 m.

③ Bedrock: The lithology of the site is Guanling Formation of Triassic system, the bedrock surface is undulating, and the core is sand, block, short column and long column. According to the different degree of weathering, it can be divided into strong weathering layer, medium weathering layer. Strong weathered limestone: gray yellow, core sand, a small amount of block, strong weathering. Sporadic distribution, the thinnest is 1.60 m, the thickest is 1.60 m, average thickness of 1.60 m; the highest elevation is 1457.44 m, the lowest elevation is 1457.44 m, average elevation is 1457.44 m; weathered limestone: gray, light gray, thin ~ medium thick layer, microcrystalline structure, joint development, small holes, long cylindrical and short core, small mass, core 61~89%, rock mass index R.Q.D value is 42~59%. The highest elevation of the level is 1467.28 meters, the lowest elevation of the level is 1438.25 meters, and the average elevation is 1454.23 meters.

④ Karst: a total of 247 boreholes, 19 boreholes encounter karst caves and cracks, encounter rate of 7.7%; karst form mainly dissolution ditch, dissolution trough, cave and fissure.

3、Problems and measures encountered in the construction process

3.1 Hole collapse

        Collapse hole is the most common problem in rotary excavation construction, especially in the rainy season. When hole collapse occurs in the process of drilling, find out the reason and judge how to handle it according to the degree of hole collapse. Treatment method: ① Minor hole collapse can be used by the loader to backfill sand and clay soil mixture into the hole; ② When the hole collapse is serious, low grade commercial concrete can be backfilled, after reaching certain strength (generally after the final setting time), this method can solve most of the hole collapse problems; ③ very serious hole collapse and large karst cave with unknown width and depth, the construction process can be changed, and all steel cylinder can be used to control the concrete volume and ensure the hole quality.

3.2 Card drill

        Card buried drilling tool is also prone to accident in rotary excavation construction, there are many reasons for the stuck drilling:

① drilling loose soft layer, the hole wall collapse caused by the stuck hole; the hole pile wall shrinkage caused by the stuck drilling;

② rotary drilling machine in the cave formation drilling, when the drill head through the cave roof, the cave side, drilling may lead to stuck drilling. The method for handling the drill is generally to pick up the drill directly, and if the drill is stuck and the embedded depth is not deep; if the drill is dead and buried in the depth, it is not easy to dig the wall manually, we can borrow other instruments such as the excavator to assist the rotary excavator to drill, but the hole pile wall has been destroyed and drilled again.

3.3 The sediment at the pile bottom is too thick

        When the geological conditions are not good, such as the soil layer is the soil layer, the backfill area is not compacted, resulting in the soil layer, the rain weather hole wall collapse, the hanging steel cage into too much soil, or the hole pile placed for too long artificially lead to the pile bottom sediment is too thick and so on. In the process of construction, the hole clearing method should be reasonably selected according to different drilling methods, construction equipment, design requirements and formation conditions.

① For impact and rotary drilling hole piles, the initial cleaning method should be used, after the larger particle sediment cleaning, can be used to further clear the hole;

② grouting method is thorough, and the cleaning speed is fast, can be suitable for various soil layers, drilling methods of friction piles or supporting piles, especially for the reverse circulation drilling and the bottom of the hole. However, caution should be used in the pile hole where the hole wall is easy to collapse to prevent the hole collapse.

3.4 Broken pile

        Broken pile, that is, bored cast-in-place pile in the concrete pouring link, concrete mixed with mud, gravel, these substances will separate the pouring concrete and form two sections, resulting in the damage or deterioration of the concrete cross-sectional area, and then affect the quality of the pile.generally speaking, The causes of broken piles are roughly the following points:

① Because the concrete slump is too small, Or due to the large stone particle size and the small catheter diameter, The catheter was blocked during the perfusion process, Can not be dredged before the concrete initial setting, Had to lift the catheter, Finally form a broken pile;

② Concrete segregation occurs due to too long transportation or waiting time, And not perform secondary stirring, Large amount of aggregate is stuck in the catheter during perfusion, Had to propose a catheter for cleaning, Causing broken breaking;

③ The slump hole occurs during the concrete pouring process, Unable to clean up, Or using a suction machine to clean up thoroughly, Irrigation interruption caused by broken pile; In promoting the catheter, Promote the catheter in excess, Pull the pipe bottom out of the concrete surface, Or place the catheter port in a mud layer, Form a broken pile. When the phenomenon of broken pile occurs, the pile connection method can be adopted, that is, the fresh concrete surface with the designed strength is exposed, and the concrete surface is chiseled and cleaned to build the brick mold, straighten the vertical reinforcement of the pile and tie the bearing platform and the ground beam reinforcement, and pour the pile with the cap and the ground beam simultaneously.

3.5 The machine falls into the soft soil layer

        In the process of pile foundation construction, due to geological reasons or the backfill soil is not compacted, and when the rain weather, the rotary drilling rig and concrete tank truck are easy to fall into the soft soil layer. To prevent this situation, it is necessary to make preparations before digging into holes or pouring concrete. First, dig the soft mud layer with a excavator, and then level the site with gravel and beat it with the excavator. If necessary, the roller can be used. It is also necessary to contact the concrete supplier in advance, and the concrete tank truck should not exceed 10 square meters, and it can be poured by day pump if necessary.

4. Conclusion

        The characteristics of loose soil quality and insufficient bearing capacity of the foundation have caused a series of difficulties to the construction of pile foundation. If improperly handled, it will not only affect the quality of the project, but also cause great property losses. Therefore, the site construction personnel should not only find the problem, but also have the ability to solve the problem. In the case of weak strata such as mud and stone, according to the actual situation, comprehensive consideration of various factors, choose safe, economic and effective treatment, to prevent in advance, measures in place, timely solution, the effect is obvious, so as to ensure the quality of pile foundation construction.

——Source: Rotary technology

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