What specifically does drilling construction technology for pile foundation projects include? Below, Zhongda Consulting will introduce relevant content for your reference.
In recent years, with the continuous improvement of national key projects’ requirements for pile hole quality, hole forming speed and construction environmental protection, some small and medium-sized rotary drilling rigs have quietly emerged and attracted more and more attention from constructors. . Rotary drilling rig pile formation is also called rotary bucket pile formation and earth-boring pile formation. The construction in the covering layer has the advantages of good hole formation quality, fast speed, no noise, no pollution or little pollution. For dry hard clay, static mud is not needed Stable liquid wall protection, generally the covering layer adopts mud wall protection. Due to the vast territory of our country and the relatively complex geological conditions, the hole-forming process during rotary drilling rig construction must be formulated in a targeted manner to prevent construction accidents such as buried drilling and collapse and avoid losses. This article combines the construction case of the pile foundation project of a project in Pudong undertaken by our company to talk about the construction experience on how to avoid collapse, buried drill and other accidents during the drilling process of the rotary drilling rig, so as to provide reference for the majority of constructors.
1. Project Overview
Our company undertook the construction of a project in Pudong, with a total number of 2180 piles, pile type cast-in-place piles, pile diameter φ800 mm, and pile depth 36 meters. In swamp areas, there is a small amount of construction waste backfill layer of about 2 meters, clay layer of 2 to 5 meters, sub-clay layer of 5 to 13 meters, sand layer of 13 to 15 meters, sub-clay layer of 15 to 29 meters, and sub-clay layer of 29 to 34 meters. meters is the sand layer, and 34 to 36 meters is the clay layer.
2. Hole forming process
Use a rotary drilling rig to take soil to form holes. The pile forming process is: position the pile → bury the casing → inject mud → drill and take soil → clear the hole once → Place the steel cage → Insert the conduit → Secondary hole cleaning → Concrete pouring → Pull out the casing. The biggest problem during the construction is that when drilling reaches the 5-13-meter clay layer, the diameter of the pile hole shrinks seriously and the hole collapses during the pile formation process. The study found that in addition to the operator's lack of experience in controlling the drilling scale and the lifting speed of the rotary bucket, the biggest impact is that the proportion of static mud, the structure of the drilling tool, and the burial of the casing are unreasonable, which can easily cause the wall to collapse. The mud skin is too thin, the negative pressure under the drilling tool is too high, and the hole penetration leads to collapse accidents.
3. The ratio of static mud
In the hole formed by rotary excavation, the static mud is used as a stabilizing liquid in the hole forming process, and its main function is to protect the wall. A thin layer of mud can be formed on the hole wall, making it impossible for water to penetrate from the inside to the outside or from the outside to the inside. Based on the geological conditions of the project, the mud technology should be strengthened, the mud ratio should be readjusted, the mud proportion should be controlled, the mud powder quality should be improved, the viscosity and lubricity should be increased, treatment agents should be added appropriately, the flocculation capacity should be enhanced, and the thickness and strength of the mud skin should be ensured. When injecting mud for the first time, try to hit it vertically downward in the middle of the pile hole to prevent the mud from flowing down along the side wall of the casing and collapsing the root of the casing, causing the base soil at the root of the casing to become soft. Before formal drilling, pour in 2 to 3 more bags. For bentonite, start the high-speed soil throwing function of the drilling rig and stir thoroughly to increase the content of bentonite and increase the thickness of the protective mud skin at the joint between the bottom of the casing and the base soil to prevent hole leakage and collapse during drilling.
4. Buried protection of the casing
According to the on-site geological conditions, a casing with a height of 4 meters, a thickness of 10 mm and a diameter of φ1.2 meters is specially customized. The inner diameter of the casing is large and can store enough mud. When the drill pipe is lifted out of the pile hole, it can ensure the water pressure in the casing and maintain the stability of the mud on the hole wall. At the same time, the single-sided guillotine gap reaches 200 mm, which can effectively avoid collision and scraping of the protective cylinder during the lifting and lowering of the rotary bucket, and protect the stability of the orifice. During the drilling process, the operator visually locates the hole based on experience, which increases the work intensity, makes it easy to fatigue, and has low accuracy, which can easily cause hole deviations and over-square concrete. Shunjie's drilling rig has a fast rotation automatic positioning function, which can accurately position holes in each working cycle, which reduces the labor intensity of the operator, while ensuring the quality of hole formation, effectively solving the negative impact of the large casing . The specially-made 4-meter ultra-high casing can be buried 500 mm below the clay layer, which can effectively prevent the leakage and collapse of the hole and the impact of vibration and impact of the surrounding environment on the pile hole. The traditional method of burying the casing: first use a φ800 mm rotary bucket to drill to the depth of the casing, install a side knife on the side wall and expand it to the outer diameter of the casing, lift the auxiliary roll, put the casing into the casing, calibrate, and bury and compact it layer by layer. . The traditional method is labor-intensive, low-efficiency, and time-consuming. It usually takes 3 to 4 hours to bury the casing, accounting for almost half of the time of the hole assembly.
A newly developed special driver for the ultra-long casing is fixed on the impact body at the lower end of the power head. The casing is directly installed on the driver through the pin, and the casing is pressed by using the function of the power head to rotate and pressurize. Go to the prescribed burial location and then take soil to form a hole. Effectively improve the bonding degree between the casing and the soil, enhance the ability to resist external vibration and impact, effectively prevent water seepage and grout leakage when grouting or lifting the rotary bucket, reduce the probability of hole collapse, save time, and improve Efficiency, reduced intensity. The distance between the casing and the ground should be controlled at 150 to 300 mm. In addition to protecting the opening to prevent collapse, it is also used to prevent surface water or ground leakage, debris, etc. from sliding into the hole.
5. The structure of the rotary bucket
In the early stages of construction, some equipment rental companies used self-made double-door bottom-opening rotary buckets, cylindrical buckets, and no mud guides on the side walls. The launder and chassis have no side teeth. During use, it was found that the hydraulic system pressure was high, the lifting force of the rotary bucket increased significantly, and the pile diameter shrinkage was serious. According to the analysis of Shunjie Company's technical personnel, the main reason is that the structure of the rotary bucket is unreasonable. When the rotary bucket is lifted, a large negative pressure is generated below, which leads to increased lifting resistance and shrinkage and collapse of the hole wall. Through improvements, the rotary bucket bucket was changed to a conical type, and a diversion groove was welded on the side wall to facilitate guidance and mud flow in the pile hole, and reduce the negative pressure in the pile hole. At the same time, the chassis is welded with side teeth to properly control the distance between the rotary bucket and the tool tip to prevent the rotary bucket from damaging the hole wall when it lifts, lowers, and rotates. On-site results show that during the lifting process of the improved rotary bucket, the pressure of the hydraulic system is significantly reduced, and the shrinkage and collapse of the pile body are alleviated, which has good use effects.
6. Drilling Control of Drilling Rig
During the drilling process, the chassis door of the rotary bucket must be kept closed to prevent sand or clay in the rotary bucket from falling into the retaining mud. , destroy the mud ratio; strictly control the drilling scale in each working cycle to avoid buried drilling accidents; at the same time, the lifting speed of the rotary bucket should be appropriately controlled. Construction practice shows that for a pile diameter of φ800 mm, the lifting speed should be maintained at 0.75~0.85m/s. If the lifting speed is too fast, mud will flow at high speed between the rotary bucket and the hole wall, scouring the hole wall, destroying the mud skin, and damaging the hole. The stability of the wall is unfavorable and can easily cause collapse.
7. Other factors affecting collapse
After the pile hole is completed, the procedures such as hole cleaning, lowering of the steel cage, and concrete pouring should be standardized to avoid collapse of the hole. For example, during the lowering process of the steel cage, it should be lifted by a crane and placed firmly and steadily into the hole to avoid collision with the hole wall, which may cause damage to the mud skin or hole wall, thereby causing the pouring process, collapse of the pile hole, and broken piles and waste. piles and other accidents.
8. Construction Summary
There are many factors that affect the collapse of pile holes during construction. The most important point is how to adapt to local conditions and effectively formulate corresponding construction techniques for different geological conditions. To ensure the smooth progress of drilling holes and avoid construction accidents.
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