I. Overview of Pile Foundation
The design diameter of 97#-A pile foundation of the main viaduct of the connecting line of Guangqing Expressway is 200cm, the design pile length is 43.7m, and the concrete grade is C25.
Second, the geological situation
According to the geological survey report, the stratum of this pile is plain fill, cultivated soil, silt, clay, completely weathered silty shale, completely weathered carbonaceous shale, karst cave, slightly weathered carbonaceous limestone, karst cave, slightly weathered carbonaceous limestone, slightly weathered carbonaceous limestone, karst cave, slightly weathered carbonaceous limestone and slightly weathered carbonaceous limestone from top to bottom. There are four layers of karst caves in the pile, which are distributed in the form of beads. The upper three caves are smaller, namely 0.8m, 1.2m and 1.2m, respectively. The height of the bottom cave is 9. 1m, and the total height of the four layers of karst caves is12.3m. The caves are all filled with soft plastic clay.
Third, the determination of the construction scheme
In view of the large karst cave area and beaded distribution of this pile, the construction technology is difficult, and the project is a key project of the city. Our company strictly controls the quality, ensures the progress of the project, and ensures the construction safety. Therefore, our company takes this pile as the top priority and carries out relevant construction monitoring according to the design requirements. Specially organize all technicians to discuss the research scheme and learn the knowledge of karst cave treatment. According to the karst cave of this pile, the karst cave is completely filled with flowing plastic clay, and the karst cave is deep. Finally, it is decided to treat the karst cave by percussion drill and throwing filling materials first, and then consider the scheme of lowering the steel casing.
Fourth, the preparation before construction
1, personnel organization
The project department specially organized relevant technical personnel to study the measures for karst cave treatment, and set up an emergency team for karst cave treatment. Four technicians will be on duty 24 hours a day in two shifts on the construction site, and report any problems found in time. The project department also organized the rig captain of the pile foundation team to learn the knowledge of karst cave treatment, so that they could know well about karst cave treatment. In short, in terms of personnel organization, the division of labor is clear and the responsibilities are clear.
2, material preparation
Flaky 100m3, clay 20m3, cement 10T, 200 woven bags, and steel casing 30m.
3, mechanical equipment
A model 40 loader and a 16T and 10 small hopper car crane.
Five, the construction process control
The elevation of the top of the casing is 1 1.005m, the elevation of the design hole bottom is -35.5m, the drilling depth should be 46.505m, and the hole is formed by impact drilling, and the bit diameter is 2. 10m (increase the bit diameter to make the aperture not less than 2. 1m, which is. The pile was drilled on June 65438+1 October1day, and passed through the last cave on June 16, with a drilling depth of 40.1m.
The upper soft stratum is 9.29 meters thick and the drilling speed is fast. The mud is grayish black and smelly. The above two holes are relatively small, 0.8m and 1.2m respectively, and the spacing is only 10cm, and the two holes are basically connected into a whole. When the drill bit reaches 0.6m above the top roof of the karst cave, throw 0.5m high flaky into the hole to balance the drill bit and prevent well deviation when drilling through the karst cave. The stroke is controlled at 0.8m, and the impact is stable. Careful observation shows that after the drill bit penetrates into the cave, the wire rope does not swing and the hole position does not deviate. After drilling into the cave, there was no slurry leakage, and the bit footage increased slightly. In order to ensure good wall fixation, flaky 1.5m was put into the hole, and the drilling stroke was 0.6m After drilling to 1m, flaky 1.5m was put again, which was mixed with clay, so the drilling footage was small, so the stroke was increased to 1m, and no abnormal phenomenon occurred during drilling. When drilling holes in karst caves, the specific gravity of mud is 1.6.
Between the second and third caves, there is a layer of 5.4m weathered carbonaceous limestone, which is hard and slow, and the drill bit is seriously damaged. Our department timely adjusted and modified the drill bit (mass 4000Kg) and replaced it with the original drill bit of 7000Kg. No slurry leakage was found after the third karst cave was broken. Since the small height of the karst cave in this layer is only 1.2m, the flaky 1.5m is put into the hole immediately after the bit breaks the top, and it is repeatedly crushed with a small stroke of about 0.6m to fix the shaft wall. There is no abnormality during drilling.
The cave in the fourth floor is the largest, with a height of 9. 1m and a hole depth of 3 1m to 40.1m. After the drill bit enters the cave, the length increases. Mud leaks slightly, so the drill bit is put forward, flaky and clay are put into the hole with a thickness of 1.5m, and then it is broken with a stroke of 0.6m After footage 1m, the mud gradually turns from gray-black to yellow-brown. According to the geological survey report, all caves are filled with yellow-brown clay, and the actual strata are consistent with the design. Due to the large karst caves in this layer, most of the flaky thrown out are large-diameter flaky with a size of 25 ~ 40 cm. No large amount of slurry leakage was found in the cave, and the clay was not bagged and thrown, but mixed with flaky. In the treatment of this cave, the method of repeatedly drilling holes with flaky and clay at 1.5m is adopted. The specific gravity of mud in the karst cave is always controlled above 1.6. During the construction process, due to the high quality of the drill bit, the main engine of the drilling rig had a little fault, and there was a period of time in the middle. 9. 1m karst cave treatment lasted for 5 days.
When the four-layer karst cave is treated, the specific gravity of the mud is above 1.6, and the mud is not diluted with water, but continuously circulated with high concentration.
Sixth, the therapeutic effect.
It takes 16 days for the pile to drill through the last karst cave, and the drilling depth is 40.1m. The karst cave treatment lasted for 7 days, the total height of karst cave treatment was 12.3m, 75m3 of flaky and clay were thrown, and 18m3 of clay was thrown. There is no hole collapse, bit suction, bit burying and bit sticking during drilling. The drill bit can move freely up and down, and there is no hole deviation or hole shrinkage.
Seven, concrete pouring
Concrete is poured by vertical conduit method, and commercial concrete is used, and experimenters are sent to concrete manufacturers for quality control, and the performance of concrete is adjusted at any time according to the pouring situation to better meet the construction needs. On-site professional engineers control the grouting progress and buried pipe depth according to the geological conditions to ensure the pile quality. When pouring into the hole, it is necessary to keep a large buried depth to prevent the concrete from suddenly sinking after crushing the retaining wall, resulting in broken piles. During the construction, keep the buried depth of the conduit not less than 5m, and the concrete surface will not drop. However, when pouring into the hole, the concrete surface rises slowly, especially when the hole depth is 35 ~ 33 m, about 8m3 of concrete is needed for each rise 1m, which is a serious waste of concrete. But the boiling is normal and the blanking is smooth. After pouring, the concrete volume is 60m3 more than the design volume.
Eight, experience and experience
1. In order to prevent inclined holes from appearing due to uneven thickness and hardness of the rock on the roof of the karst cave, in addition to the balanced bit filled with rubble, it can also be treated with a high-quality bit.
2. When dealing with karst caves, slowing down or stopping for a period of time can promote the consolidation of flaky and clay to strengthen the wall.
3. It is feasible to treat karst caves with fillings by throwing fillings, but in order to avoid shrinkage cavity, large pieces of flaky can be used.
4. The mud concentration in the hole should be large, and the specific gravity should not be less than 1.6.
5, if there is no large loss of mud in the hole, clay can not be bagged.
6. Because the cave is plastic clay, even if there is no slurry leakage in the hole, in order to increase the slurry concentration and increase the firmness of the solid wall, flaky clay should be added at the ratio of 3: 1.
7. When dealing with slurry leakage or reaching the position where there is no slurry leakage in the karst cave, rubble and clay should be put into the hole to fix the hole wall, and the rubble should be pounded repeatedly in a short trip, so that the rubble can be fully squeezed into the hole wall, and the clay can fully fill the rubble gap, so as to avoid excessive concrete pouring and ensure the pile quality.
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