Piles can be classified by construction method into precast piles (prestressed pipe piles) and cast-in-place piles (bored cast-in-place piles). Both are widely used in foundations with soft soil and relatively thick bearing strata, and feature high bearing capacity, good stability, small settlement, and low material consumption, effectively meeting the requirements of buildings for foundation strength, deformation, and stability. The two pile types each have their own characteristics; different construction methods use different machinery and equipment and construction techniques, and the interaction mechanisms between pile and soil also differ, resulting in different applicable scopes. This article compares these two pile types, analyzes their respective advantages and disadvantages and applicability, so as to determine whether to choose prestressed pipe piles or bored cast-in-place piles.

△ Prestressed pipe piles are hollow cylindrical slender precast concrete members made by the pretensioning prestressing process, with high-efficiency water-reducing admixture added, and formed by high-speed centrifugal steam curing, mainly consisting of a cylindrical pile body, end plates, and steel hoops.

△ Bored cast-in-place piles are piles formed on the construction site by using drilling equipment to drill into the soil, breaking up the soil while removing spoil to form a hole, then placing a reinforcing steel cage in the pile hole and pouring concrete.

This article provides a comparative analysis of prestressed pipe piles and bored cast-in-place piles from the perspectives of pile action mechanism, construction conditions, construction technology, and construction cost

From the perspective of action mechanism

Prestressed pipe piles are driven or pressed into the foundation to the required depth. During pile sinking, the surrounding soil is squeezed by the pile body, causing pore water pressure to rise in the short term, which makes the soil heave and squeeze laterally, deforming or even damaging existing buildings (structures) and roads within the stress influence zone. At the same time, it also squeezes already completed engineering piles, causing them to shift and float upward.

Bored cast-in-place piles use the dry method or mud slurry wall protection method to form holes. During hole formation and pile construction, there is no squeezing effect on the surrounding soil between piles, and no increase in excess pore water pressure in the soil is caused, so pile construction will not endanger the safety of adjacent buildings (structures) and roads. Therefore, compared with prestressed pipe piles, bored cast-in-place piles have the characteristics of no vibration, no soil squeezing effect, and little impact on surrounding buildings, but the pile concrete strength 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 of soft soil, sandy soil, plastic soil, silt, fine sand, and loose gravelly cobble soils free of large boulders or erratic stones. They are not easy to penetrate thick hard interlayers such as sand, and can only enter firm bearing strata such as sand, gravel, hard clay, and strongly weathered rock to a limited depth. When pile sinking is difficult due to sand layers or boulders, pre-boring measures can be adopted. When driven or statically pressed prestressed pipe piles use strongly weathered rock as the pile foundation bearing stratum, most of the pile body passes through soft soil, cohesive soil, and completely weathered rock, so overall pile sinking resistance is not large. If erratic stones are distributed regularly in local residual soil and completely weathered layers, certain difficulties may be caused during pile formation. Because construction requires large machinery such as vibratory pile hammers and lifting equipment, the required construction site is relatively large.

Bored cast-in-place piles are suitable for various sandy soils and cohesive soils, and also for gravel and cobble soil layers and rock layers. However, construction is relatively difficult in mud and foundations where quicksand or confined water may occur. Therefore, compared with prestressed pipe piles, bored cast-in-place piles have the characteristics of simple construction equipment, convenient operation, and no restriction from narrow sites, but the construction period is longer and construction quality is less stable than that of prestressed pipe piles.

From the perspective of construction technology

The construction process of prestressed pipe piles is: survey positioning → pile machine positioning and centering → pile pressing → pile splicing → pile feeding or pile cutting → static pressing to design elevation.

(1) Survey positioning: before construction, set out the axis and each pile position, and apply paint to make the marks obvious.

(2) Pile machine positioning and centering: start the longitudinal and transverse traveling cylinders of the pile pressing machine 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 two directions.

(3) Pile pressing: clamp the pile tightly with the clamping cylinder, then extend the pile pressing cylinder to apply pressure to the pile.

(4) Pile splicing: the single-section length of the pile should be determined according to equipment conditions and construction technology. When the soil layer penetrated by the pile contains thin sand layers, the length of the single-section pile should be determined so as to avoid stopping the pile end in the sand layer for pile splicing. When the next section of pile is pressed to 0.8–1.0 m above the ground, the previous section can be spliced.

(5) Pile feeding or pile cutting: if the pile top is close to the ground and the pressing force has not yet reached the specified value, pile feeding can be carried out. If the pile top is a certain distance above the ground and the pressing force has reached the specified value, the pile should be cut so that the pile pressing machine can move.

(6) Completion of pile pressing: when the pressure gauge reading reaches the predetermined value, pile pressing can be stopped.

The construction process of bored cast-in-place piles is: pile position setting out → steel casing installation → mud system layout → drilling machine positioning → drilling → hole cleaning → lifting and installing reinforcing steel cage → pouring underwater concrete → completion of pile foundation construction, dismantling the drilling machine.

(1) Pile position setting out: use the measuring method to set out and measure pile positions, protect the measurement baseline during construction and recheck it regularly to ensure that measurement control accuracy meets design requirements.

(2) Steel casing installation: according to the pile position, make the casing center coincide with the pile center, with deviation less than 50 mm. Fill the space between the casing and the pit wall with cohesive soil to ensure the accuracy and stability of the casing position, recalibrate the casing center deviation, and check the verticality of the casing with a spirit level so that the casing is level and firm.

(3) Mud circulation system layout: equip a settling tank and circulation tank to purify the mud and then return it to the hole, ensuring that the various indicators of the mud in the hole meet requirements before pouring after final hole formation.

(4) Drilling machine positioning: the base of the drilling machine should be firmly supported, and the base frame should be adjusted horizontally with a spirit level until the drilling frame is vertical to the ground and the crown block pulley, rotary table, drill bit, and pile position centers are on the same plumb line.

(5) Drilling: during hole formation, the drilling tools (drill bit, active drill pipe, and auxiliary drill pipe) must ensure verticality, concentricity, and reliable connection integrity to prevent accidents such as hole deviation and drilling tool detachment. When drilling reaches the design elevation for final hole formation, the drilling machine should be kept idling without penetration for 10 minutes until the mud indicators meet the requirements for primary hole cleaning.

(6) Hole cleaning: remove settled drill cuttings and mud at the bottom of the hole to ensure the quality of the poured reinforced concrete and ensure the bearing capacity of the pile.

(7) Lifting and installing the reinforcing steel cage: during lifting and transport, because the longitudinal bending resistance of the reinforcing steel cage is relatively poor, supports should be added inside the cage when necessary to improve its rigidity. When the cage is lifted to the hole opening, align the cage center with the pile position center, straighten it, and lower it slowly and uniformly into the hole. Swinging and collision with the hole wall are strictly prohibited, and protective blocks should be installed while lowering the reinforcing steel cage. When the last lifting bar is lowered to the hole opening, fix it on the positioning frame at the hole opening and temporarily suspend the steel cage at the hole opening.

(8) Pouring underwater concrete: before pouring, place a water-stop cover plate under the hopper. When pouring begins, open the hopper and immediately supply material continuously so that the first batch of concrete presses down the water-stop cover plate and falls rapidly. As concrete is continuously poured, the concrete surface in the hole rises continuously, and the embedded depth of the tremie pipe also increases continuously, so the rise of the concrete surface needs to be measured regularly. During pouring, the tremie pipe should be continuously lifted and lowered to prevent voids caused by excessive speed and prevent concrete from failing to flow outside the steel cage.

From the perspective of construction cost

Under normal circumstances, within the same construction site conditions, the engineering cost of prestressed pipe piles is lower than that of bored cast-in-place piles. If in areas with poor geological conditions, where erratic stones are distributed regularly in local residual soil and completely weathered layers, prestressed pipe piles will require pre-boring measures, which will greatly increase project investment and make the cost 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 limited by soil quality or site conditions, but the construction period is long and the cost is relatively high.