Construction Technology of Bidirectional Cement Mixing Pile

1. Design Requirements

(1) The diameter of the bidirectional cement-soil mixing pile is 0.7m, and the spacing is generally 1.9–2.3m. The mixing piles are arranged in an equilateral triangle pattern in plan. The arrangement extends to outside the subgrade slope toe or the cone slope toe.

(2) The mixing pile treatment section adopts equal-load preloading, with a preloading period of 120 days.

(3) When the pile diameter is 0.7m, the cement content is 100kg/m; for the enlarged-head pile with a diameter of 1.0m, the cement content is 200 kg/m. The 28-day unconfined compressive strength of the cement-soil from laboratory tests is: R28≥1.0MPa.

2. Construction Technology (two-spray, four-mix, one-stop)

(1) After the mixing machinery is transported to the site, it shall be installed and debugged first. Once the rotation speed, air pressure, and metering facilities are normal, positioning may begin.

(2) Align the mixing head with the designed pile position, start the motor, and after the mixing head reaches normal rotation speed, sink while rotating and cutting the soil until the reinforcement depth is reached.

(3) Spray grout upward from the pile bottom while mixing and lifting until 30cm above the ground; then re-spray grout while mixing down to the pile bottom, stop drilling, spray grout continuously for 1 minute, and finally mix and lift to the ground surface.

(4) Turn off the power, move the equipment, and repeat the above steps.

(5) Backfill the 30cm below ground that was not sprayed with cement using cement-soil.

3. Trial Piles

Before mixing pile construction, trial piles shall be carried out according to actual site conditions to obtain various machinery parameters and ensure the quality of large-scale construction. The trial pile work requires the following:

(1) Master various technical parameters satisfying the designed grout injection volume (converted from cement content and cement slurry water-cement ratio), such as drilling speed, lifting speed, mixing speed, and grout injection pressure.

(2) Understand the resistance during drilling and lifting, and select reasonable technical measures.

(3) Verify whether the cement-soil mix proportion determined by laboratory tests is suitable for the site.

(4) Check whether the unconfined compressive strength of the pile body meets the design requirements, i.e., the 28-day strength and standard penetration test blow count shall not be lower than the design values.

(5) Verify whether the uniformity of the reinforcement agent distribution and the effective reinforcement length meet the design requirements.

(6) The number of process trial piles is generally first determined by the supervisory engineer by dividing the soft foundation of this contract section into different representative road sections, with 20–30 trial piles per representative section, or determined according to actual needs.

(7) Trial piles must be carried out before formal construction, and large-scale construction may begin only after confirmation by the construction unit, supervisor, and designer.

4. Construction Method

(1) Original ground treatment

If the original ground is an ordinary road section, mixing piles can be constructed after clearing the surface; within 30cm of the pile top, 6% lime is added, then plowed, turned, and rolled to shape. If the original ground is a river-pond section, first remove the silt to the undisturbed soil, then backfill with plain soil and roll to the original ground level, and then construct the mixing piles.

(2) Construction setting-out

Based on the survey control points and construction drawings, use a theodolite to set out the pile positions and make marks to ensure accurate positioning of the pile rig.

(3) Pile rig positioning

Using the walking device at the bottom of the pile rig, under the command of a dedicated person, calibrate the pile rig with a spirit level and positioning plumb. When the ground is uneven, manually level or fill and compact the site so that the pile rig is level and the mixer is vertical, with a tilt rate of less than 0.5%. To ensure accurate pile positions, positioning clamps must be used; the pile position centering error shall not exceed 10cm, and the guide frame and mixing shaft shall be perpendicular to the ground.

(4) Grout preparation

The slurry for cement mixing piles uses ordinary Portland cement of strength grade 32.5. The water-cement ratio of the cement slurry shall be strictly controlled according to the mix proportion test results. The prepared cement slurry shall not be left standing too long; if it exceeds 2h, it shall be used at a reduced grade. The slurry shall be continuously stirred in the grout mixer until just before delivery. During delivery, the prepared cement slurry is fed into the hopper.

(5) Grout injection and pile formation

Start the mixer so that the mixing head mixes and cuts the soil along the guide frame and drills down. When the reinforcement depth is reached, start the grout pump; after confirming that the slurry is sprayed from the nozzle, mix and lift while spraying grout until 30cm above the ground; then re-spray grout while mixing down to the pile bottom, stop drilling, spray grout continuously for 1 minute, and finally mix and lift to the ground surface.

5. Construction Quality Control

Quality control during construction shall comply with the following provisions:

(1) The curing agent slurry shall be mixed strictly according to the predetermined mix proportion. The prepared slurry shall not segregate and shall not stand too long (no more than 2h). The slurry shall be filtered through a screen when poured to avoid lumps in the slurry damaging the pump body.

(2) Before pumping the slurry, the pipeline shall be kept moist to facilitate grout delivery. For on-site slurry mixing, a dedicated person shall record the amounts of curing agent and admixture, and record the start and end times of grout pumping.

(3) Construction shall be carried out according to the technical parameters determined by the pile formation test. Operators shall record the sinking time and lifting time per meter, and record changes in relevant parameters such as grout delivery time and grout stop time.

(4) Grout supply must be continuous, and mixing must be uniform. Once grout supply stops for any reason, to prevent broken piles and grout deficiency, the grout-spraying mixer shall be sunk to 1.0m below the grout stop level, and grout spraying and lifting shall resume after grout supply is restored. If the machine is stopped for more than 3h for any reason, to prevent the slurry from hardening and blocking the pipe, the grout delivery pipeline shall first be dismantled, cleaned, and kept for standby.

(5) When the mixer is lifted to 2m below the ground surface, a slow speed should be used. When the grout nozzle is about to be raised out of the ground, lifting shall be stopped and mixing continued for several seconds to ensure the pile head is uniform and dense.

(6) During mixing pile construction, if insufficient grout injection is found, an additional pile shall be added alongside; the grout injection volume of the re-drilled pile shall still be no less than the design amount.

(7) The subgrade may be filled only after the mixing pile construction is completed for 28 days and has passed inspection.

6. Quality Inspection

(1) Construction allowable deviations

The construction allowable deviation inspection includes pile diameter, pile spacing, pile length, verticality, grout injection volume per pile, and strength, as detailed in the table below.

Construction Allowable Deviations for Bidirectional Cement-Soil Mixing Piles

(2) 28-day core drilling

For 28-day core drilling, the inspection quantity is 0.5% of the total number of piles. Standard penetration tests shall be carried out simultaneously with coring, with one SPT every 2 m for each inspected pile; the SPT blow count shall not be less than the design value. Where the SPT blow count is less than the design value, cores shall be taken for strength testing. For each inspected pile, core tests must be carried out at three points: the upper, middle, and lower parts of the pile body.