Post-grouting construction technology of bored pile?

Introduction of post-grouting technology for bored pile

With the large-scale development of high-rise buildings in China and the continuous improvement of urban environmental protection requirements, the number of construction projects using bored piles as engineering piles is increasing. Bored cast-in-place pile has the advantages of adapting to different geological conditions, great changes in pile diameter and length, and low noise and vibration. However, due to the influence of many factors, such as sediment at the bottom of the pile, mud skin at the side of the pile, soil relaxation at the side of the pile and soil rebound at the end of the pile, the quality of the finished pile is high, the bearing capacity of the single pile is unstable and the pile foundation cost is high. In order to improve the bearing capacity of single pile, save the project cost and reduce the discreteness of pile quality, the post-grouting technology of cast-in-place pile has been gradually applied and popularized, and its role in improving the bearing capacity of bored pile and reducing the settlement of pile foundation has been widely recognized.

I. Technical Introduction and Principle

Post-grouting of bored pile refers to the construction technology of injecting cement slurry by high-pressure grouting pump through embedded grouting pipe after bored pile is completed and the concrete of pile body reaches the predetermined strength. This technology forms high-strength concrete by infiltrating the slurry into the soil around the loose virtual tip of the pile end. With the increase of grouting amount, the slurry continuously penetrates into the bearing layer of the pile end softened by mud soaking, which increases the bearing area of the pile end, which is equivalent to expanding the bottom of the bored pile. When the permeability of cement slurry is limited by the surrounding dense soil layer, the pressure is rising continuously. Compacting, tamping, filling, consolidating and infiltrating the soil layer at the pile end will effectively reinforce or tamp the sediment at the pile bottom and the disturbed bearing layer at the pile end, improve the relationship between the pile and the soil, and improve the bearing capacity of the soil around the pile, thus improving the bearing capacity of the single pile and the settlement and uneven settlement of the foundation.

Second, the construction technology and key points

Bored pile construction → reinforcing cage embeds grouting pipe → clear water dredges grouting pipe within 12h after pouring pile body concrete → after 7 days, open grouting pipe, make grout join evenly, reinforce soil → stop grouting after grouting amount (or grouting pressure) meets the design requirements.

(1) Grouting pipe manufacturing

When making reinforcing cages, make grouting pipe. Grouting pipe adopts a black iron pipe with a diameter of 25mm, and the joints are connected with threads, and both ends are sealed with plugs. The length of grouting pipe is 55 cm longer than the reinforcing cage. The reinforcing cage grows 5cm at the bottom of the pile, and the upper part is 50cm higher than the concrete surface at the top of the pile, but it shall not be exposed to the ground for protection. Grouting pipe made a grouting nozzle (commonly known as flower tube) at the bottom of 20cm. In this part, the drill bit is used as the grouting nozzle, and four rows (four in each row) of grouting holes are drilled evenly, with a spacing of 3cm and a diameter of 3 mm.

(2) Layout of grouting pipe

According to the drawing requirements, two grouting pipe are symmetrically tied to the outside of the reinforcing cage. After the hole is formed, clean the hole, lift the drill and lower the reinforcing cage. Attention should be paid to protecting the grouting pipe during the hoisting and placing of the reinforcing cage. The reinforcing cage shall not be twisted, so as not to cause the grouting pipe to loosen at the threaded joint. The nozzle part should be protected by concrete pad, and the hole wall should not be rubbed to avoid tire rupture and blocking the grouting hole. Pouring concrete according to the specification requirements.

(3) Selection of grouting time

According to the previous engineering practice, in the gravel layer, the influence area of cement slurry under the action of working pressure is large. In order to prevent cement slurry from emerging from weak points nearby during grouting, grouting piles should normally be completed more than 3 days after concrete pouring is completed 7 days, and there should be no drilling rig within at least 8m around the piles.

(4) Grouting construction sequence

When grouting, it is best to use the whole pile group of pile caps for one-time grouting, and the surrounding piles should be constructed before the middle piles; During grouting, two piles are used for circular grouting, that is, pipe A of pile 1 is pressed first, and the grouting amount accounts for about 70% of the total (11~1/4t cement), and then the other/kloc-0 is pressed after grouting. Construction records should be made during grouting, which should include construction time, grouting start and end time, grouting quantity, abnormal situation and treatment measures.

Third, the construction points

(1). Various standards of grouting technology.

(1). The cast-in-place pile in this project is post-grouting at the pile end, and the cement used for grouting is 42.5 fresh ordinary portland cement, and the water-cement ratio of the slurry is controlled at 0.6.

(3) The grouting pipe shall be fixed or welded with the stiffening rib of the reinforcing cage, the upper grouting pipe shall be connected by screw thread, and the sprayed concrete head at the bottom of grouting pipe shall be inserted into the bottom of the pile hole (entering the pile end ≥5cm). After the initial setting of pile concrete, use high-pressure water (8.0MPa) to open the plug, and start grouting one week later, with the pressure controlled at 2~5MPa.

④ The post-grouting valve should have the following properties: a) It should be able to withstand the hydrostatic pressure above 1Mpa; B) Its outer protective layer should be able to resist scratching by hard objects such as sand and gravel without being damaged; C) It shall have inspection function.

(2) Post-grouting operation requirements

(1). Grouting operation shall start 7 days after pile formation, but not later than 30 days after pile formation;

② The distance between grouting operation and hole-forming operation point should not be less than 8 ~10m;

(3) Each grouting conduit of the same pile shall be grouted at the pile end in equal amount in turn;

(4) group pile grouting should be outside first and then inside;

Four, common problems and corresponding measures

The grouting nozzle won't open?

When the pressure reaches above 10MPa, the grouting nozzle can't be opened, indicating that the nozzle part has been damaged. Don't force the pressure, but make up the grouting amount in another pipe.

Grouting occurs at the pile side and other parts?

During grouting, when the grouting pressure is constant but the grouting speed obviously drops or stops, it means that the pile bottom is saturated, and grouting can be gradually reduced until grouting is stopped; If there is little grouting in the side wall of the pile, the grouting pipe can be washed clean with clean water or pressure water, and grouting can be carried out after the cement slurry originally pressed into the pile finally solidifies and blocks the grouting capillary channel the next day.

Insufficient grouting quantity of single pile?

When grouting, it is best to use the whole pile group of pile cap for one-time grouting. When grouting, the grouting quality of the middle pile can be guaranteed by constructing the surrounding piles to form a closed circle first and then constructing the middle pile. If the grouting quantity of a single pile cannot meet the design requirements, the grouting quantity of adjacent piles can be increased as a supplement as appropriate.

The above are collected and sorted by Zhong Da Consulting Company.

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