Karst develops in soluble rock areas such as carbonate rocks, and is destroyed and transformed by groundwater and surface water, as well as the resulting hydrological and geomorphological phenomena. In foundation engineering, karst cave treatment is an important work. If it is not handled properly, it will affect the construction period and cause adverse consequences to the whole project quality. Therefore, in the construction of karst area, we must first understand all kinds of situations in which karst caves may occur, formulate corresponding treatment schemes, try our best to avoid accidents and reduce the losses caused by accidents, so as to facilitate the smooth progress of the project.
1 project overview
A project is located in Wuchang Street, Hangzhou. The proposed building mainly includes 3 buildings with 5-8 floors, 2# and 3# buildings with 1 basement, and 1# building without basement. The maximum single column load of the eighth floor is estimated to be 10000kN, and that of the fifth floor is 5500kN.
2 engineering geological conditions
According to the detailed survey data and karst survey data, the lithology of the proposed project is mainly Cambrian interlayer and interbedded dolomite stratum, which belongs to concealed karst and has good concealment. Limestone caves are developed in the site, with different sizes, shapes and burial depths. Mostly grayish yellow-brownish yellow, plastic-soft plastic cohesive soil, containing a small amount of breccia and gravel. Some caves have no fillers, which leads to serious water leakage during drilling construction. Among them, six holes > buried depth: there is a cave at 40m, and the water leakage is serious or the cave is more than 5m, which is difficult to construct; There are broken rod boreholes 1 hole at 48m (there are two 42mm4m drill pipes and 1 φ9 1mm core pipes left in the hole, and the elevation of the pipe top is about-43.03m); There are 1 broken-rod drilling holes of 32-55m in the hole (there are five 42mm4m drill pipes and 1 φ9 1mm 3.3m core pipes in the hole, and the top elevation of the pipes is about -26.90m). Special attention should be paid to pile foundation construction.
3 the determination of the construction scheme
When drilling and grouting method is used to form holes, there is a thick medium-coarse gravel layer in the stratum, gravel and gravel are severely weathered, and the underground water inflow is large, and there is a thick gravel layer above the karst cave, casing follow-up method should be adopted to prevent hole collapse.
Construction technology of bored pile in karst cave 4
4. 1 Punching construction at the top of cavern
During the construction of karst cave (earth cave) pile foundation, the pressing progress should be strictly controlled, with the general progress controlled at 60 cm/h ~ 80 cm/h and the stroke controlled at about 50cm.
According to the pre-drilling data, when the drilling construction is close to the top of the karst cave, lift the drill bit and drill pipe, start the co-rotating drill first, and punch holes with the impact drill. When punching holes with an impact drill, it is required to hammer slowly to make the hole wall smooth and firm, and the lifting height is generally less than 50cm. All buckles and wire ropes must be tested and inspected first, and other construction techniques and precautions are the same as routine.
4.2 Cave drilling
Because the roof and floor of the cave may be uneven and uneven, it can be handled by installing inclined holes in and out of the cave. When drilling holes in caves, cement slurry and clay bags must be filled in advance. If the cavity is small, clay and stone can be filled in layers, and the diameter of stone should be 10 ~ 15 cm, and a stone wall should be formed at the same time to prevent the filler from collapsing and mud loss. When the drill touches the uneven rock surface, it will slide to the soft side. If it is not corrected in time, it will easily lead to inclined holes and sticking accidents. The method of deviation correction is to fill flaky and clay in layers, and cut rocks by means of small stroke drilling, thus achieving the purpose of straight holes and round pipes.
4.3 inspection hole
When drilling rock, it is necessary to check the diameter and inclination of the hole-forming pile to prevent the hole from being skewed and it is difficult to pour the steel skeleton. The method of hole inspection is to use a hole inspector, which has a designed hole diameter such as its outer diameter and a length of more than 5 times the hole diameter. If the hole is stuck, analyze the reason and solve it. If the hole is seriously inclined, riprap and re-drill.
5 karst cave treatment technology in bored pile foundation construction
According to the height of the cave and the filling situation in the cave, different treatment methods are adopted for the cave. The main treatment methods are: throwing and filling material method, grouting reinforcement method, pouring concrete filling method and simple method of protecting large and small steel bars.
(1) The range of karst cave is small, and the height of karst cave is less than1m.. Then throw rubble and clay into the pile hole, impact the pile hole in a short stroke, form a retaining wall within the karst cave, and then continue the construction. A steel plate cylinder of 1 mln is welded on the positioning ribs of the pile reinforcement cage under the karst cave1m, and four positioning ribs are added outside the cylinder.
(2) The height of the cave is between 1.0~3.0m ~ 3.0m, and the cave is filled or semi-filled. First, fill flaky (crushed), sand mixture and water injection mud, and then impact flaky with water hammer to form a gravel external retaining wall. After the flaky cracks are initially blocked by cement mortar, the impact is stopped. After 24 hours, the cement strength reached 2.5MPa, and then it continued to impact through.
(3) The cave is hollow with the thickness of1.0 ~ 3.0m ~ 3.0m.. First, dry concrete is poured into the cave by using advanced drilling holes, and the cave is filled. After the consolidated body reaches a certain strength (2.5MPa), punching construction can be carried out. Dry concrete will be mixed with 425R ordinary portland cement. If the cave space is large and the water conductivity is strong, add a certain amount of water glass to the dry concrete.
(4) The height of the karst cave is 3.0h5.0m, and the spacing of multi-layer karst caves is small, so it needs to be crossed by steel protection slips. First, punch holes and enlarge holes with impact hammer, and then vibrate with vibration hammer to sink the steel protection slips into the bottom of the hole. In order to ensure the strength and rigidity of steel protection slips, a reinforced steel plate hoop is set every 2m.
(5) When the height of the tunnel is more than 5.0m (multi-storey) and the distance between the tunnels is large, it is planned to adopt the card protection method in the tunnel, that is, the card protection method in the tunnel is adopted for construction.
Casing length L=h+2m(h is the height of multi-layer karst cave). The inner diameter of the inner casing should be about 20cm larger than the designed pile diameter, and the outer diameter should be about 5cm smaller than the outer casing. If it is a second-layer karst cave, the inner casing of the second-layer karst cave is 35 cm smaller than the inner casing of the upper layer.
6 conclusion
Practice has proved that when pile foundation construction is carried out under karst geological conditions, geological drilling is first carried out to find out the geological conditions, and then manual digging and impact drilling are selected to carry out hole-forming construction, which can achieve twice the result with half the effort. For the treatment of karst cave, according to the thickness and height of karst cave, concrete and reasonable construction measures are taken to ensure the safe and smooth construction of bored pile foundation.
The detailed information of "Talking about the Treatment Technology of Bored Pile in Karst Area" can be found in Zhong Da Consulting & Construction Link, and all the relevant construction materials you want are available.
For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd