Self-drilling Bolts Applying in Underground Car Park Foundation Projects

With the rapid development of the global economy, there is less and less land available for development and utilization in cities. The construction of parking lots and other underground projects alleviated this contradiction. So in the construction of those underground projects, foundation support engineering has been more and more used. In some foundation support projects, the engineering site is very complicated. For example, the miscellaneous fill dominated by construction waste or crushed stones cannot form holes or support in the silt layer. Traditional anchor rods and construction techniques cannot meet the requirements. For engineering needs, self-drilling bolts are born under such circumstances. Now let us see a real case to introduce the application of self-drilling bolts applying in underground parking lot projects.

Self-drilling bolts mainly have the following characteristics:
1. Integrate drilling, grouting and anchoring in one operation;
2. It is most suitable for the formation of broken and easy collapse holes, avoiding the complicated process of casing follow-up;
3. Full and high-pressure grouting can be achieved so that the grout fills the rock and soil cracks;
4. According to construction needs, it can be cut into any length and any connection;
5. It can be easily connected with various types of drilling rigs to realize comprehensive hole-making technology.
Here combined with the concrete case of the foundation support project of the underground parking lot of the China Shipbuilding Industry Science and Technology Building, let us learn about the construction and practical application effect of the self-drilling anchor.
The engineering site of China Shipbuilding Industry Technology Building is located on the north side of the intersection of Haier Road and Tongan Road in Qingdao. The proposed building is 26 to 33 stories high, with two underground floors, a frame-shear structure and a raft foundation. The pit is about 254m long and 80m wide. There is a pond in the middle of the site with an excavation depth of 12m, which was then backfilled.
According to the survey, the stratum in this area mainly includes: The average layer thickness of miscellaneous fill is 18m, mainly construction waste and gravel, with silty soil; wet, medium-density strongly weathered granite, the mechanical properties gradually increase from top to bottom; Because there are many underground fissures in Qingdao area and abundant water sources in the pit, this section is greatly affected by underground seepage during the excavation process.
Because of this geological condition, the design unit adopted R32 self-drilling bolts with a spray surface thickness of 100mm. The grouting pressure is 0.5-2MPa, the grouting water-cement ratio is 0.45-0.50, and the strength is not less than M20. Strictly control during construction. The grouting pressure is generally controlled at about 0.5~1.5Mpa. In the case of special formations, the grouting pressure can be adjusted, and finally the suitable grouting pressure is determined according to the grouting effect. For drilling holes with particularly serious grout leakage, a special method of filling the hole with sand and then grouting or mixing a certain proportion of sandy components in the grout can be used to save the amount of grout.

The foundation is set by 3 monitoring points for tracking and observation throughout the whole process. The maximum settlement of the slope of the foundation pit is 23mm, which is 0.19% of the depth of the foundation pit, which meets the requirements of the specification. Facts have proved that the foundation pit maintenance of this project is generally successful, ensuring the safety of the foundation pit.
In this project, self-drilling anchor rods can well complete the supporting tasks in the formations that are difficult to form holes such as miscellaneous fill and gravel soil and achieve good supporting effects. The efficient construction technology of self-drilling bolts has also been highly recognized by the construction party.

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