Comprehensive Interpretation of the Construction Method of Gravel Pile

0 Gravel pile is a kind of loose pile. It is made of gravel (pebbles) without bonding strength as the main material. After drilling holes in the soft foundation by vibration, impact or water flushing, the gravel (pebbles) are squeezed into the soil holes to form a dense pile body composed of large-diameter gravel. It is a composite foundation reinforcement pile composed of gravel (pebbles) piles and soil between piles.

According to its pile-making process, it can be divided into two categories: vibrating (wet) gravel piles and dry gravel piles. Gravel piles made by vibration and water flushing are called vibrating gravel piles or wet gravel piles. Gravel piles made by various waterless flushing processes (such as dry vibration, vibration extrusion, hammering, etc.) are collectively called dry gravel piles. Piles made with gravel sand, coarse sand, medium sand, round gravel, angular gravel, pebbles, crushed stones, etc. as filling materials are called gravel piles.

 

The vibratory water flushing method was designed and manufactured by the German Keller Company in 1937. It was a machine with the prototype of a modern vibrator and was successfully used to compact sand and gravel foundations. In the early 1960s, the vibratory water flushing method began to be used to reinforce clay foundations. Since the material used was crushed stone, it was called a crushed stone pile. The application of the vibratory water flushing method in my country began in 1977. Because vibratory crushed stone piles pollute the environment with mud and water, their application in cities and areas with existing buildings is limited, and they also have the effect of softening the soil. Therefore, since the 1980s, various construction techniques have emerged accordingly, such as the hammer method, the vibratory squeezing method, the dry vibration method, the immersed tube method, the vibratory air flushing method, the bagged gravel method, the dynamic compaction gravel pile replacement method, etc.

Application of gravel piles in soft soil foundation

The so-called soft soil foundation refers to a foundation whose compression layer is mainly composed of silt, silty soil, backfill, miscellaneous fill, or other highly compressible soil layers. Soft soil foundations are widely distributed in my country, especially in the southeast coastal areas. Therefore, the correct treatment of soft soil foundations is an important part of my country’s current highway construction.

The so-called gravel pile method refers to a method of reinforcing the foundation when building structures such as embankments on soft foundations. The purpose of foundation reinforcement can be divided into the gravel drainage well method and the gravel pile method. The gravel pile method has been widely used in areas lacking well-graded sand. The reinforced foundation soil can meet the requirements of bearing capacity, deformation, and anti-liquefaction. The gravel pile is a good drainage channel, which can reduce the excess pore water pressure in the event of an earthquake in the area in the future. Therefore, since the introduction of the gravel pile foundation treatment method in my country, it has been rapidly promoted in the foundation reinforcement of a large number of industrial, civil buildings, and water conservancy and transportation projects, and has achieved good results.

Vibroflotation for soft soil reinforcement

The so-called vibroflotation is the abbreviation of the vibrating water-flushing method. It uses a crane to lift the vibrator, start the submersible motor, and drive the eccentric block to make the vibrator generate high-frequency vibration, and start the water pump at the same time to make high-pressure water spray through the high-pressure water flow. Under the combined action of vibration and flushing, the vibrator is sunk to the predetermined depth in the soil. After cleaning the hole, gravel can be gradually filled into the hole from the ground. Each section of filler is vibrated and compacted under the action of vibration. After reaching the required density, the vibrator is lifted, and this is repeated, filling and vibrating until the ground, thus forming a large-diameter and very dense pile body in the foundation. Thereby achieving the effect of strengthening the soft foundation.

1. Reinforcement mechanism for clay soil

For clay soil foundation (especially saturated soft soil), due to the high clay content, strong inter-particle bonding force, and low permeability, the water in the soil is not easy to drain under the action of vibration force, so the role of gravel pile is not to compact the foundation, but to replace it. During construction, the vibrator uses its weight, horizontal vibration force, and high-pressure water to turn the clay soil into mud water and discharge it out of the hole to form a hole slightly larger than the diameter of the vibrator. Gravel is poured into the hole, and under the lateral force of the vibrator, the gravel is squeezed into the surrounding holes to form a pile body with a certain density and large diameter. It forms a composite foundation with the original clay and works together. Since the stiffness of the gravel pile is much greater than that of the clay soil around the pile, the stress in the foundation is redistributed according to the material deformation modulus. Therefore, most of the load will be borne by the gravel pile. The ratio of the pile stress to the cohesive soil stress between the piles is called the pile-soil stress ratio, which is generally 3 to 5. Although the bearing capacity of the cohesive soil is not improved, the bearing capacity of the entire foundation is improved.

Crushed stone piles are sometimes also used to improve the anti-sliding ability of soil slopes. At this time, the role of the pile body is to improve the shear strength of the soil body, just like the general anti-sliding piles, forcing the sliding surface to move away from the slope surface and move deeper.

2. Reinforcement mechanism for sandy soil

Gravel piles mainly play a compaction role in sandy soil, and their main purpose is to increase the bearing capacity of foundation soil, reduce deformation, and increase resistance to liquefaction. During the construction of gravel piles, the saturated loose sand particles are rearranged and compacted under strong high-frequency forced vibration, and after a large amount of coarse aggregate is filled in the vibration hole, it is squeezed into the surrounding soil by the strong horizontal vibration force. This kind of forced compaction increases the relative density of sand, reduces the porosity, increases the dry soil weight and internal friction angle, improves the physical and mechanical properties of the soil, and greatly increases the bearing capacity of the foundation (generally by 2 to 5 times). In addition, due to the significant increase in foundation width and the corresponding increase in relative density, the liquefaction resistance is also improved.

Construction and quality control of gravel piles

1. Construction of gravel piles

(1) Introduction to construction machinery and tools

The main machinery and tools are vibrators, cranes, or special flat cars for construction and water pumps.

Currently, the main vibrators are ZCQ-13, ZCQ-30, and ZCQ-55, among which ZCQ-30 is the most commonly used machine and tool.

(2) Construction preparation work

① Three connections and one leveling

The three connections and one leveling of the construction site refer to water connection, power connection, material connection, and site leveling, which are important guarantees for the smooth progress of construction.

Water connection means that on the one hand, the water required for construction should be supplied, and on the other hand, the muddy water generated during construction should be dug and drained away. The pressurized water is pumped out by the water pump through the hose and enters the water pipes of each vibrator. The water pressure needs to be 400~600KPa. At the same time, it should be noted that water should be reused as much as possible during construction.

The so-called power connection means that there are two power sources, three-phase and single-phase, during construction. Both are indispensable, and the voltage cannot be too high, but it cannot be too low, about ±20V.

Material access means that several stockpiling yards should be set up near the reinforcement area, and the material transportation route should be as short and smooth as possible without interfering with the working route. Material access also includes a sufficient number of materials, and try not to stop work and wait for materials.

② Positioning of piles

After leveling the site, measure the ground elevation. The elevation of the reinforcement area should be 1m above the designed piletop elevation. If this elevation is lower than the groundwater level, it is necessary to equip precipitation facilities or appropriately increase the ground elevation. Finally, mark the pile position with small wooden stakes on site according to the pile position design drawing, and the pile position deviation shall not exceed 3cm. First, align the vibrator with the pile position, and require the vibrator to fall vertically and not tilt, otherwise it will deviate and damage the direction section. After aligning the pile position, start the water source again, and check whether the power supply is normal. (The rated current of the ECQ-30 vibrator is 60A, and the no-load current is 25A.)

③ Vibration drilling

After everything is ready, start the vibrator and start the crane at the same time to lower the vibrator. After the vibrator opens the hole, the hole should be expanded near the hole mouth. The process of the vibrator descending to form a hole should have the necessary steps of stopping the vibrator to solidify the wall. At the same time, the descent speed must be controlled during construction, generally controlled at about 1-2m/min. The water pressure for drilling should be kept above 60N/cm2. The water volume can be reduced as required during work, but the water should not be stopped to prevent mud and sand from flowing back into the water pipe. However, the water pressure is appropriately reduced when approaching the bottom elevation of the hole. For every 1m of penetration, the vibrator should be lifted and suspended for vibration for 5-8s, and then continue to penetrate when the mud overflows. The mud overflows from the hole and flows into the mud pool. It is transported away in time by a special transport vehicle. When the vibrator is vibrated to 30cm above the designed pile bottom elevation, the vibrator is lifted to the hole mouth at a lifting speed of 2-3m/min and then sinks to the bottom of the hole. Repeat 1-2 times. It not only plays the role of cleaning the hole but also achieves the purpose of hole expansion.

(3) Production of gravel piles

① Filling method

After the foundation hole is formed, fillers are added to the hole as required. In the past, there were three ways of adding materials. The first one is to lift the vibrator out of the hole mouth, pour about 1m of fillers into the hole, and then lower the vibrator to vibrate the fillers. Then repeat this operation. The second one is that the vibrator is not lifted out of the hole mouth, but is only lifted about 1m, then the material is poured into the hole mouth, and then the vibrator is lowered to vibrate the fillers. The third one is to lift the vibrator slowly upward while continuously adding materials at the hole mouth. For clay foundations, most of them adopt the first way of adding materials, because the quality of the pile body is not easy to guarantee in the latter two ways.

② Construction sequence of piles

The construction sequence of piles is generally “from inside to outside” or “pushing from one side to the other side”, because this is conducive to squeezing out some soft soil. If the piles are made “from outside to inside”, it is difficult to make piles in the center area. For soft clay foundations with very low shear strength, to reduce the disturbance of the original soil during pile making, it is advisable to use the method of interval skipping.

③ Pile-making operation steps

The so-called operation steps also refer to the construction process of gravel piles, which includes hole making, hole cleaning, material addition, and vibration compaction. The steps are as follows:

Step 1: Aim the vibrator at the pile position, turn on the water and electricity, and check whether the water pressure, voltage, and no-load current value of the vibrator are normal.

Step 2: Start the winch of the construction vehicle or crane to make the vibrator sink in the soil layer at a speed of 1-2m/min. Be careful not to let the current value exceed the rated value of the motor when sinking. Because the change in current value qualitatively reflects the change in soil strength. Once exceeded, it means that the strength of the soil is too large. At this time, corresponding measures must be taken to loosen the soil before continuing.

Step 3: When the vibrator reaches 30-50cm above the designed reinforcement depth, start to lift the vibrator up to the hole mouth.

Step 4: Repeat steps 2 and 3 one or two times. If there is mud blocking the hole, it should be dug out. Finally, stop the vibrator 30-50cm above the designed reinforcement depth to clean the hole.

Step 5: After cleaning the hole for one or two minutes, pour 0.15-0.5 of crushed stone into the hole, and then vibrate it with a vibrator to compact it. At the same time, some stones squeeze into the hole wall for reinforcement. Repeat the above steps, and a pile body is formed from bottom to top.

In addition, in the filling process, “continuous filling, more filling, and slow vibration” should be achieved. After repeated filling and vibration, the crushed stone is squeezed and compacted into a pile. When the filling is vibrated, the compaction current should reach 50-60A, the water pressure should also reach 60-80N/cm2, and the vibration time should be greater than 30. When the vibrator is working, when the current of the vibrator motor exceeds the specified value (50-60), it can automatically alarm the bell sounds, and at this time remind the operator to lift the vibrator upwards. The compaction quality is controlled by the compaction current. During construction, you should pay attention to keeping records, that is, filling in the pile and making a statistics chart. The content includes: pile number, pile-making depth, filling amount, time, and completion date.

2. Construction quality control

Two controls and one management: construction quality control, construction progress control, and strengthening on-site construction management. The quality of vibro-compacted stone piles should be based on the principle of full compaction of the pile body, which is closely related to the progress of hole formation, filler quantity, and vibration retention time. Only with a certain filler quantity can a certain compaction current be achieved. A certain vibration retention time is also required to vibrate the filler into a pile.

The quality of materials must be strictly controlled. Therefore, when selecting gravel, the gravel particle size should be selected according to the design requirements. Generally, 2 to 4 cm gravel should be selected, and the grading should be moderate. The maximum size should not exceed 5 cm, because large particle size is not compacted and is not good for machinery. The mud content of gravel should not exceed 10%. In addition, the filler quantity of each pile should also be carefully verified and controlled. During construction, it should be noted that when drilling, the current value will suddenly rise due to the obstruction of the vibrator, and the on-site alarm will keep ringing. In this case, do not mistake it for correctness. You should carefully analyze the cause and take corresponding measures after finding out the cause. At the same time, vibrate repeatedly several times until the current value on the ammeter stabilizes, and then continue construction.

Matters needing attention:

(1) The vibrator must not be powered off and stopped when it is deep in the soil layer. Therefore, understand the power supply situation before construction, and prepare a generator if necessary.

(2) It is difficult to drill a hole, and the vibrator is not easy to sink. The main reason is that the water pressure is not enough, and the water pressure should be increased. When the upper layer is harder, the water pressure should be increased, and the descending speed should be slower. The water pressure should be maintained at 6-8kg/cm2. A valve should be installed on the on-site pipeline to adjust the water pressure and water volume.

(3) The compaction current cannot be reached for a long time. The usual measures are to reduce the water pressure. Appropriately increase the amount of filler each time. Repeat the vibration several times to make it meet the compaction current value.
(4) If there are other existing buildings around the construction site, attention should be paid to whether the existing buildings have cracks or subsidence during construction.

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