Piles can be divided into precast concrete piles (prestressed pipe piles) and cast-in-place piles (bored cast-in-place piles) according to different construction methods. Both are widely used in foundations with soft soil and thick burial. They have high bearing capacity, good stability, small settlement, and low consumption of materials. They can effectively meet the requirements of buildings for foundation strength, deformation, and stability. The two pile types have their own characteristics, different construction methods, different mechanical equipment and construction techniques, different interaction mechanisms between piles and soil, and different scopes of application. This article will compare these two pile types and analyze their respective advantages, disadvantages, and applicability to determine whether to choose prestressed pipe piles or bored piles.
△ The precast concrete pile is a hollow cylindrical slender concrete precast member made by pre-tensioning prestressing process, adding a high-efficiency water-reducing agent, and high-speed centrifugal steam curing molding method. It is mainly composed of a cylindrical pile body and end plate and is composed of steel hoops, etc.
△ Bored cast-in-place piles are piles made by using drilling tools to drill into the soil at the project site, crushing the soil while digging out soil residue to form a hole, and then placing a steel frame in the pile hole, and then pouring concrete.
From the perspective of the mechanism of action
Prestressed pipe piles are driven or pressed into the foundation to reach the required depth. During the pile driving process, the surrounding soil is squeezed by the pile, causing the pore water pressure to rise in a short period of time, causing the soil to bulge and move toward the ground. Lateral extrusion will cause deformation or even destruction of existing buildings (structures) and roads within the range affected by the stress. At the same time, it will also squeeze the completed.
engineering piles, causing them to shift and float.
Bored cast-in-place piles are formed by the dry working method or mud wall protection method. During the process of hole formation and pile formation, there is no squeezing effect on the surrounding soil between the piles, and it will not cause an increase of excess pore water pressure in the soil, so the pile's construction will not endanger the safety of surrounding adjacent buildings (structures) and roads. Therefore, compared with prestressed pipe piles, bored cast-in-place piles have the characteristics of no vibration, no impact of soil squeezing, and little impact on surrounding buildings. However, the concrete strength of the pile body is lower, the single pile bearing capacity is lower, and the settlement is larger.
From the perspective of construction conditions
Prestressed pipe piles are suitable for foundation soils such as soft soil, sandy soil, plastic soil, silt, fine sand, and loose gravel soil that does not contain large pebbles or boulders. It is difficult to penetrate thick sand and other hard soil. The interlayer can only enter a small depth of solid supporting layers such as sand, gravel, hard clay, and strongly weathered rock formations. When it is difficult to sink piles in sand layers, boulders, etc., guide hole measures can be used. When driving or static pressure prestressed pipe piles are used, and the strongly weathered rock stratum is used as the bearing layer of the pile foundation, most of the pile bodies pass through soft soil, clay soil, and fully weathered rock strata. Overall, the pile sinking resistance is not large. For example, the regularly distributed boulders in the local eluvial layer and the entire weathered layer may cause certain difficulties during the pile formation process. Since construction requires large machinery such as vibrating pile hammers and lifting equipment, a large construction site is required.
Bored cast-in-place piles are suitable for various sandy soils and clay soils, as well as crushed and pebble soils and rock formations. However, construction is more difficult for foundations with silt and where quicksand or pressurized water may occur. Therefore, compared with prestressed pipe piles, bored cast-in-place piles have the characteristics of simple construction equipment, convenient operation, and are not restricted by small sites. However, the construction period is longer and the construction quality is not as stable as prestressed pipe piles.
From the perspective of construction technology:
The construction process of prestressed pipe piles is measurement and positioning → pile driver positioning and centering → pile pressing → pile connection → pile delivery or pile cutting → static pressure pile to the design elevation.
(1) Measurement and positioning: Place the axis and each pile position before construction, and apply paint to make the markings obvious.
(2) Pile driver positioning and centering: Start the longitudinal and transverse traveling cylinders through the pile driver to align the pile tip with the pile position. Start the pile-pressing cylinder to press the pile into the soil. After the pile sinks to a stable state, adjust the verticality of the pile in both directions.
(3) Pile pressing: The pile is clamped by the clamping cylinder, and then the pile pressing cylinder is extended to apply pressure to the pile.
(4) Pile connection: The length of a single section of the pile should be determined based on equipment conditions and construction technology. When the soil layer that the pile penetrates contains a thin layer of sand, the length of the single-section pile should be determined to avoid stopping the pile end in the sand layer for pile connection. When the next pile is pressed to 0.8~1.0m above the ground, the next pile can be connected.
(5) Pile feeding or pile cutting: If the top of the pile is close to the ground and the pile pressing force has not yet reached the specified value, the pile can be fed. If the top of the pile is a certain distance above the ground and the pile driving force has reached the specified value, the pile must be cut to facilitate the displacement of the pile driver.
(6) End of pile pressing: When the pressure gauge reading reaches the predetermined value, pile pressing can be stopped.
The construction technology of bored piles is bored pile construction process: stake setting out → steel casing buried → mud system arrangement → pile driver in place → drilling → hole cleaning → installing and lifting steel skeleton → pouring underwater concrete → piles After the foundation construction is completed, the drilling rig is disassembled.
(1) Pile position setting out: Use the measurement method to measure and place the pile positions. During the construction period, the measurement baseline shall be protected and reviewed regularly to ensure that the measurement control accuracy meets the design requirements.
(2) Steel casing embedding: According to the pile position, the center of the casing coincides with the center of the pile, and the deviation is less than 50mm. Fill the space between the casing and the pit wall with cohesive soil to ensure the accuracy and stability of the casing position. Correct the center deviation of the casing again, and use a level to check the verticality of the casing to ensure that the casing is level and firm.
(3) Arrangement of the mud circulation system: Equipped with a sedimentation tank and a circulation tank to purify the mud and then flow it back into the hole to ensure the requirements for various indicators of the mud in the front hole after the final hole is poured.
(4) Drilling rig in place: The base of the drilling rig in place should be firmly padded, and use a spirit level to adjust the base frame horizontally until the drilling frame is vertical to the ground, and the four-point centers of the crane wheel, return drill plate, drill bit, and pile position are aligned. on the same plumb line.
(5) Drilling: During hole construction, the drilling tools (drill bit, active drill pipe, and auxiliary drill pipe) must ensure verticality, concentricity, and reliable integration during connection to prevent accidents such as hole inclination and drilling tool falling off. When drilling the final hole at the design elevation, the drill should be kept idling for 10 minutes until the mud index reaches the first hole cleaning requirement.
(6) Hole cleaning: Remove the drilling residue and mud deposited at the bottom of the hole to ensure the quality of poured reinforced concrete and the bearing capacity of the pile.
(7) Installing and hoisting the steel frame: During the lifting process, the steel cage has poor longitudinal bending resistance. If necessary, add supports inside the cage to improve the rigidity of the cage. When the cage is hoisted to the hole, align the center of the cage with the center of the pile position. After righting it, slowly lower it into the hole at a constant speed. It is strictly forbidden to swing and hit the whole wall. Install the protective block while lowering the steel cage. When the final lifting bar is lowered to the opening, it is fixed on the positioning frame of the opening, and the steel cage is temporarily suspended at the opening.
(8) Filling of underwater concrete: Before pouring, the hopper should be covered with a waterproof cover. When starting pouring, open the hopper and immediately supply materials continuously so that the first batch of concrete presses the waterproof cover and falls quickly. As the concrete is poured continuously, the concrete surface in the hole continues to rise, and the buried depth of the conduit also continues to increase. It is necessary to regularly measure the rise of the concrete surface. During pouring, the conduit must be constantly lifted up and down to prevent holes from being formed due to excessive speed and the inability of concrete to flow outside the steel cage.
From a construction cost perspective
In general, within the same construction site conditions, the project cost of prestressed pipe piles is lower than that of bored piles. If the geological conditions are not very good in an area where there are regularly distributed boulders in the local eluvial layer and the weathered layer, the prestressed pipe piles will need to use pilot hole measures, which will increase the investment of the project and the cost will be relatively high.
Prestressed pipe piles have the advantages of economical cost, short construction period, and guaranteed quality, but they have certain requirements for soil quality and site conditions.
Bored cast-in-place piles are not restricted by soil quality and site conditions, but the construction period is long and the cost is high.
