Keywords: cement fly-ash gravel pile; flexible cushion; pile bearing capacity; economic and social benefits 

1. Introduction 

Cement fly-ash gravel piles are developed on the basis of gravel piles. They are pile bodies with certain cementation strength made by mixing stone chips, fly ash, and a small amount of cement with water at a certain mix ratio. This type of pile is a low-strength concrete pile, and the composite foundation composed of it can significantly improve bearing capacity. CFG pile is the abbreviation of cement fly-ash gravel pile. It is a high-bond-strength pile formed by mixing cement, fly ash, crushed stone, stone chips or sand with water, and together with the soil between piles and the cushion, it forms a composite foundation (see Figure 1). The bearing capacity can be adjusted within a large range; the increase in bearing capacity is 20%~30%, and the increase is even greater for soft soil foundations, with rapid deformation stabilization. The CFG cement fly-ash gravel pile machine has stable and reliable performance, is easy to use and maintain, and can self-load, self-unload, and move on walking legs for site transfer. During transportation, the upper main frame of the CFG cement fly-ash gravel pile machine can be retracted and folded, saving a lot of cost and trouble for hoisting and paving. The CFG cement fly-ash gravel pile machine is efficient and economical, truly the best choice for users. 

CFG cement fly-ash gravel pile machine DZJ-90 

Generally, there are three pile-forming construction methods: vibratory driven casing cast-in-place pile (suitable for silt, cohesive soil, and plain fill foundations), long spiral drilling cast-in-place pile (suitable for cohesive soil, silt, plain fill, and medium-dense or denser sand above groundwater level), and long spiral drilling with internal pump pressure mixture cast-in-place pile (suitable for cohesive soil, silt, sand, and sites with strict requirements on noise or mud pollution). 

2. Reinforcement principle of the process 

A long spiral drilling machine is used to form the hole, and a mixture of crushed stone, fly ash, cement, and water in a certain proportion is pumped and poured to form a semi-rigid pile body, which forms a composite foundation with the original stratum and is connected to the foundation through a cushion. Regardless of whether the pile end rests on ordinary soil or hard soil, it can participate in bearing. Since the strength and modulus of the pile body are greater than those of the soil between piles, under load, the stress at the pile top is greater than that at the surface of the soil between piles. The pile can transfer the load it bears to deeper soil layers and correspondingly reduce the load borne by the soil between piles, thereby improving the bearing capacity of the composite foundation. 

3. Construction process flow and operation points 

3.1 Various construction preparations: 

Ensure the three connections and one leveling: connect construction water and electricity, keep roads unobstructed, and level the construction site. Construction materials have been delivered to the site and inspected, the concrete mixer and pile driver have been installed and debugged, and on-site hole-forming tests have been completed. Carefully read the design drawings and specifications, and the geological survey report to understand the engineering characteristics and physical indicators of the soil layers and their impact on the piles. Before construction, a pile-forming process test should be conducted to verify the geological data and whether the mechanical equipment performance, construction technology, and driving sequence are suitable, and to determine various process parameters such as mix ratio, slump, mixing time, and pipe extraction speed. The construction technology should be revised based on problems found in the test piles. 

3.2 Determination of pile positions: Before measuring pile positions, the original ground elevation of the construction site should be leveled and measured, and after leveling and rolling with a grader, the exact position of each pile should be set out. After considering the longitudinal and transverse slopes of the route, the original ground elevation should be controlled within ±5cm. The construction pile top elevation should be controlled at 30cm above the design cushion bottom elevation. Divide the construction area, conduct on-site measurement and setting out according to the drawing design requirements, set control stakes, determine pile positions, and number each pile. Assign machines and personnel for management. 

3.3 Drilling rig positioning: After the drilling rig is in position, adjust the casing to be vertical to the ground, and make the drill pipe vertically aligned with the center of the pile position, with a verticality deviation not greater than 1.5%. On-site control can use the method of hanging a plumb bob on the drill frame to measure the verticality of the hole, or use the verticality adjuster provided with the drilling rig to control the verticality of the drill pipe. Before constructing each pile, on-site engineering technicians should check the pile position centering and verticality. Only after meeting the requirements can drilling start. 

3.4 Drilling to form holes: At the start of drilling, close the drill bit valve, move the drill pipe downward until the drill bit touches the ground, start the motor to drill, first slow then fast, and at the same time check the drilling deviation and correct it in time. If the drill pipe shakes or drilling is difficult during hole formation, the penetration rate should be slowed down to prevent pile hole deviation, displacement, and damage to drilling tools. According to the penetration mark on the drilling rig tower, control the drilling depth to ensure that the pile length deviation is within +100mm. 

3.5 Mixing of mixture: The mixture must be batched according to the mix ratio and mixed centrally. The mixing time of each batch should be controlled according to the parameters determined by the process test, not less than one minute, generally controlled at 90~120 seconds. The specific mixing time should be controlled according to the values determined by the test, and records should be kept. The slump for long spiral drilling and internal pump pressure mixture pile construction should be 160~200mm. The mixed mixture must ensure that the mixture cylinder can smoothly pass through rigid pipes, high-strength flexible pipes, elbows, and reducing pipes to reach the core pipe of the drill pipe. 

3.6 On-site testing: For each batch of mixture, the testing personnel must test the slump, and only after passing can the mixture be fed. During pile formation, samples of mixture test blocks should be taken. Each machine should make one set (3 blocks) of test blocks per shift (150mm cube), standard curing, and determine the 28-day cube compressive strength. 

3.7 Pouring and pipe extraction: After drilling to the design elevation, stop drilling, lift the drill pipe 20~30cm, and then start pumping the mixture for pouring. The feeding amount for each pile should not be less than the design pouring amount. Start lifting the drill pipe after the core pipe of the drill pipe is filled with mixture, and start lifting while feeding. The lifting rate should be controlled at 2~3m/min. The pumping amount of the mixture should match the pipe extraction speed. In saturated sand and silt layers, do not stop pumping to wait for material. It is strictly forbidden to lift the drill pipe before pumping. The mixture in the pipe hole should be kept 0.5m above the drill bit. When pouring the mixture, the control of the pouring amount of the mixture can be done by recording the number of pump pressure times. For the same type of delivery pump, the delivery volume per time is basically a fixed value, and the feeding amount of the mixture can be measured according to the number of pump pressure times. 

3.8 Pile formation and quality inspection: 

When the mixed mixture is injected through the chute into the pump truck storage hopper, it needs to pass through a filter grid of a certain size to prevent large-diameter or flaky stones from entering the storage hopper and causing pipe blockage. To prevent pipe blockage, the mixture delivery pipe should also be cleaned in time, and check in time whether the joints of the delivery pipe are firm, whether the sealing ring is damaged, and whether the drill bit valve and exhaust valve are blocked. 

The pile formation process should be continuous and should avoid saturated sand and silt layers. During construction, try to avoid interruption of pile formation due to waiting for materials to prevent diameter reduction and pile breakage. The pipe extraction speed during pile formation should be controlled at 2~3m/min. After the pouring and pile formation are completed, cover the pile top with soil for curing. 

3.9 Moving the machine: During pouring, use stationary lifting of the drill pipe (do not lift the drill pipe while moving). After pouring reaches the control elevation, proceed to the next pile. When the pile machine moves to the next pile position for construction, the pile position to be constructed should be rechecked according to the axis or the positions of surrounding piles to ensure the pile position is correct. 

3.10 Laying the cushion: The thickness of the cushion should be 150~300mm, determined according to the design. During construction, the loose paving thickness (h): h=△h/λ (where λ is the compaction degree), generally 0.87~0.90. After loose paving, static compaction should be used to reach the design thickness. When the water content of the soil between piles under the foundation bottom is small, dynamic compaction can also be used. For relatively dry sand and gravel materials, after loose paving, appropriate watering can be done before rolling or compaction.