What are the construction quality control measures for MJS piles?

MJS method pile (Metro Jet System), also known as all-round high-pressure jetting method, was originally developed to solve the problems of slurry discharge and environmental impact in the process of horizontal rotary jet construction. It is currently mostly used for foundation treatment, treatment of leakage and quality problems of foundation pit retaining water-stopping curtain, and treatment of water seepage on the exterior wall of basement structure. Due to the use of unique porous pipes and front-end forced slurry suction devices, forced slurry discharge in the hole and ground pressure monitoring are realized, and the ground pressure is controlled by adjusting the forced slurry discharge volume, so that deep mud discharge and ground pressure are reasonably controlled, and the ground pressure is stabilized, which reduces the possibility of surface deformation during construction and greatly reduces the impact on the environment. The reduction in ground pressure also further guarantees the diameter of the pile.

Pre-control

Since the MJS pile construction technology is relatively complex and more difficult than other grouting methods, it is necessary to strictly follow the design requirements during the construction process, do a good job of corresponding technical and safety briefing, and comply with the corresponding operating procedures to ensure the construction quality. 

After the drilling rig is in place, the pile position should be controlled well. Generally, the deviation from the design position should not exceed 50mm, and the vertical deviation should not exceed 1/200. 

Before the formal construction, the pressure and flow of high-pressure water, high-pressure grouting pump and air compressor, as well as the lifting speed, grouting volume, and final hole conditions of the grouting pipe during the injection process are determined through trial piles. During the formal construction, the centralized management console can be used for automatic tracking and control. Make detailed records of various construction records on site, including: drilling inclination, drilling depth, drilling obstacles, collapse, working parameters during slurry injection, slurry return, etc., and leave key image data. At the same time, the construction records should be sorted out in time, and problems should be reported and handled in time. 

In order to ensure that there is no pile breakage when the drill rod is disassembled or the work is interrupted for a long time due to some reasons, the overlap length of the upper and lower piles is generally not less than 100mm when normal injection is resumed. 

Maintain the construction machinery before construction to minimize the quality problems caused by equipment failure during construction. Conduct pre-construction training for machine operators to familiarize them with the performance and operation points of the equipment. During construction, a dedicated person is responsible for the operation of the equipment. 

Inspection before construction 

Before construction, raw materials, machinery and equipment, and spraying process should be inspected, mainly in the following aspects: 

1 Quality certificates and witness test reports of various raw materials (including cement, etc.), mixing water should meet the corresponding regulations; 

2 Whether the slurry mix ratio is suitable for the actual soil conditions of the project; 

3 Whether the machinery and equipment are normal. Before construction, the MJS all-round high-pressure rotary jet equipment, hole drilling rig, high-pressure mud pump, slurry mixing background, water pump, etc. should be tested and run, and the drill rod (especially multiple drill rods), drill bit and guide device should be unobstructed; 

4 Check whether the spraying process is suitable for geological conditions. Before construction, the process test spraying should also be carried out. The test spraying should be carried out at the original pile position. The number of test spraying pile holes should not be less than 2 holes. If necessary, adjust the spraying process parameters. 

5 Before construction, the underground obstacles should be uniformly checked to ensure that the drilling and spraying meet the design requirements. 

6 Check the accuracy and sensitivity of the pile position, pressure gauge and flow meter before construction. 

In-process control

During the construction process, the following should be paid attention to: 

1 Check the verticality of the drill rod, the drilling speed, the drilling depth, the drilling speed and the rotation speed at any time to see if they are consistent with the requirements of the pile test report; 

2 Check the cement slurry mix ratio and the measurement of various materials and admixtures, and truthfully record the injection pressure, injection speed and injection volume during injection grouting; 

3 Whether the construction records are complete. The construction records should record the pressure and flow data once every 1m of lifting or at the junction of soil layer changes, and leave image data if necessary. 

Post-control

After the construction is completed, the reinforced soil should be inspected, including: the integrity and uniformity of the consolidated soil; the effective diameter of the consolidated soil; the strength, average diameter, and pile center position of the consolidated soil; the impermeability of the consolidated soil, etc.

1 Quality inspection time and content 

Since cement soil solidification requires a certain amount of time, generally more than 28 days, the specific requirements should be based on the design documents. Therefore, the inspection of the quality of MJS spraying construction should generally be carried out after the MJS high-pressure jet grouting is completed and the age reaches the specified time in the design. 

2 Quality inspection quantity and location 

The number of inspection points is 1% to 2% of the number of construction spraying holes. For projects with less than 20 holes, at least one point should be inspected, and those that fail should be sprayed again. Inspection points should be arranged in the following locations: locations with large loads, pile center lines, and locations where abnormal conditions occur during construction. 

3 Inspection methods 

The inspection of jet grouting piles is mainly mechanical property inspection. Generally, the compressive strength index of cement soil is measured. The sample is obtained by drilling and coring method, and it is made into a standard test piece. After meeting the requirements, indoor physical and mechanical property testing is carried out to check the uniformity of the cement soil and its mechanical properties.