Drilling Grouting Construction

Introduction to Drilling Grouting Technology

With the large-scale development of high-rise buildings in our country and the increasing environmental protection requirements for cities, the number of construction projects using bored cast-in-place piles as engineering piles is increasing. Bored cast-in-place piles offer advantages such as suitability for various geological conditions, large variation in pile diameter and length, and low noise and vibration. However, due to factors like sediment at the pile bottom, mud skin on the pile side, soil relaxation on the pile side, and soil rebound at the pile tip, the quality of the piles is highly variable, with individual pile bearing capacity being relatively unstable and pile foundation costs being high. To improve individual pile bearing capacity, save on construction costs, and reduce the variability in pile quality, the post-grouting technology for cast-in-place piles has gradually been applied and promoted. Its effects on enhancing pile bearing capacity and reducing foundation settlement have been widely recognized. This technology is particularly suitable in regions with large soil voids and good diffusion, such as the soft soil layer or abundant gravel layer in Taizhou, Zhejiang, where it can significantly increase pile strength.

This article takes the Beijing Normal University Taizhou Experimental School project, provided by Qiancheng Building Group in Taizhou, Zhejiang, as an example to showcase the new technology and applications used in the pile foundation construction process, specifically the post-grouting technique for bored pile灌注桩。

I. Technical Introduction and Principles

Drilling grouting construction refers to the technique of injecting cement slurry through pre-buried grouting pipes using a high-pressure grouting pump after the桩 (pile) has been formed and the concrete in the pile has reached the predetermined strength. This method involves the slurry渗透 (penetrating) into the soil around the loose tip of the pile, combining to form a higher-strength concrete. As the amount of grout increases, it continuously penetrates the softened bearing layer at the pile tip, which has been softened by mud slurry, thereby increasing the bearing area at the pile tip and effectively widening the pile base. When the penetration ability of the cement slurry is restricted by the surrounding dense soil layers, continuously increasing pressure compresses, compacts, fills, solidifies, and penetrates the soil layer at the pile tip, effectively reinforcing or compacting the sediment at the pile bottom and the disturbed bearing layer, improving the connection between the pile and soil, enhancing the bearing capacity of the soil around the pile, and thus increasing the single pile bearing capacity and reducing settlement and uneven settlement of the foundation.

II. Construction Techniques and Key Points

Drilled cast-in-situ piles construction → pre-set grouting pipes in steel cage → clear water flushing the grouting pipes within 12 hours after pouring the pile concrete → open the grouting pipes after 7 days to uniformly add slurry and reinforce the soil → stop grouting after the grouting volume (or pressure) reaches the design requirements

(1) Slurry pipe manufacturing

While manufacturing the reinforcing cage, the grouting pipe is also produced. The grouting pipe is made of 25mm black iron pipe, with threaded connections at the joints and both ends sealed with threaded plugs. The grouting pipe is 55cm longer than the reinforcing cage, extending 5cm from the bottom of the pile and 50cm above the concrete surface at the top but not露出地面 to facilitate protection. The lower 20cm of the grouting pipe is made into a grouting nozzle (commonly known as a flower pipe), where 4 rows (with 4 holes each row) of 3mm diameter grouting holes are evenly drilled at a spacing of 3cm to serve as the grouting nozzle. The grouting holes are sealed with tacks, covered with a bicycle inner tube of the same diameter, and sealed at both ends with tape, forming a simple one-way device: when grouting, the pressure in the grouting pipe bursts the tire, pops the tacks, and the cement slurry is pressed into the gravel layer through the grouting holes and the gaps in the tacks. During concrete pouring, the device ensures that the concrete slurry does not block the grouting pipe.

(2) Grouting pipe layout

According to the drawing specifications, symmetrically tie 2 grouting pipes to the outer side of the steel cage. After boring, clean the hole, raise the drill, and lower the steel cage. During the hoisting and placement of the steel cage, pay attention to protecting the grouting pipes; the steel cage must not be twisted to prevent loosening at the threaded connections of the grouting pipes. The nozzle section should be protected with concrete blocks, and it should not rub against the hole wall to avoid tire bursting and blocking the grouting hole. Pour concrete in accordance with the specifications.

(3) Selection of Paste Pumping Time

Based on past engineering practices, the cement slurry has a significant impact area under working pressure within the gravel layer. To prevent the cement slurry from bubbling out from nearby weak points during grouting, it is typically required that the piles be grouted 7 days after the concrete pouring is completed, and there should be no drilling operations within a radius of at least 8 meters around the pile. The concrete pouring for piles within this range should also be completed more than 3 days prior.

(4) Grouting Construction Sequence

It's best to use whole group pile grouting during pile grouting, starting with the peripheral pile positions before the central pile. During grouting, use a two-pile cyclic grouting method, initially grouting the A pipe of the first pile, accounting for about 70% of the total grout volume (111-114 tons of cement). After completing this, proceed to the A pipe of the second pile, followed by the B pipe of the first and then the second pile's B pipe. This ensures a 30-60-minute interval between grouting the two pipes of the same pile, allowing time for the cement slurry to spread within the gravel layer. Keep thorough construction records, which should include the start and end times of the grouting, the amount of grout used, and any abnormal conditions or measures taken to address them.

Section 3: Construction Key Points

(1). Gating system specifications

①. This project's cast-in-place piles are post-grouted, using 42.5 grade fresh ordinary portland cement for grouting, with a water-cement ratio of 0.6, and approximately 1.8 tons of cement per pile (±700 kg).

②. Prior to pouring concrete into the pile shaft, pre-bury two ?50x3.8 steel pipes with one-way raft at the bottom for grouting. The grouting pipes must be accurately positioned to ensure the grouting head is unobstructed.

③. The grouting tube should be tied or welded to the reinforcing cage of the pile casing. The upper grouting tube should be connected with threads, and the nozzle at the bottom of the grouting tube should be inserted into the bottom of the pile hole (≥5cm into the pile end). After the concrete in the pile casing has initially set, use high-pressure water (8.0MPa) to clear the plug, and start grouting after one week, with the pressure controlled between 2~5MPa.

④. The post-grouting valve should possess the following characteristics: a) It should be able to withstand a static water pressure of more than 1Mpa; b) Its external protective layer should be able to resist scratches from sand, stones, and other hard objects without damage; c) It should have a check valve function.

(2). Post-grouting operation requirements

①. Grouting operations should commence within 7 days after pile formation and should not be delayed beyond 30 days after pile formation.

②. The distance between grouting operations and borehole operations should not be less than 8 to 10 meters.

③. Grouting at the pile end should involve sequentially applying equal volumes of grout to each grouting conduit of the same pile.

④. For pile group grouting, it's recommended to start from the outer area before moving to the interior.

⑤ Termination of grouting may occur under any of the following conditions: total grouting volume exceeds 1.8 tons (?700), and grouting pressure reaches 1.2~4MPa; or total grouting volume exceeds 75% and grouting pressure exceeds 4MPa.

⑥. Total grouting volume exceeds 75%, with grouting pressure less than 1.5MPa, the water-cement ratio should be reduced, and continuous grouting should be performed to 1.5MPa with a total grouting volume not less than the design requirements. If the grouting pressure remains below the normal value for an extended period or there is leakage on the ground or cross-grouting between adjacent桩holes, switch to intermittent grouting with an interval of 30-60 minutes. Resuming grouting and reaching the aforementioned termination conditions is required before stopping.

IV. Common Questions and Corresponding Measures

The浆喷头 won't open?

Pressure reaches above 10 MPa and the grout nozzle still cannot be opened, indicating that the nozzle area is damaged. Do not forcibly increase the pressure; make up the grout quantity in another pipe.

Slippage occurring at pile side and other areas?

During grouting, if the pressure remains constant but the grouting speed significantly decreases or stops, it indicates that the pile bottom is saturated, and you can gradually reduce the grouting until stopping the pressure grouting. If there is leakage from the side wall of the pile with a small amount of grout, clean the grouting pipe with clean water or pressured water, and then re-grout after the previously grouted cement slurry has fully cured and sealed the capillary channels the next day.

Insufficient grouting volume for a single pile?

It's best to grout the entire pile group at once during grouting. Start by grouting the peripheral piles to form a closed loop, then proceed to the center. This ensures the grouting quality of the central piles. If the grouting volume of individual piles doesn't meet the design requirements, increase the grouting volume of adjacent piles as a supplement as needed.