PC-Encased Steel Pipe Piles are a new type of retaining pile technology. Their predecessor is the traditional Larsen steel sheet pile. Simple steel sheet piles, whether U-shaped, Z-shaped or straight-belly, have low cross-sectional stiffness and require multi-layer support and anchor structures when used in deep foundation pits, which increases the difficulty of construction and makes it difficult to give full play to their advantages of fast and efficient construction. PC-Encased steel pipe piles combine Larsen piles with steel pipes, steel sections and other materials to form combined piles of various cross sections for retaining and water stopping. During the construction process, the combination of construction piles suitable for the construction can be selected according to the actual situation of the foundation pit project.
Before formal construction, trial driving is required. Generally speaking, steel pipe piles can be barely squeezed into the soil by a high-power hammer due to their high rigidity, while Larsen steel sheets are prone to distortion.
Due to the uneven hardness of the soil layer, the distortion occurs in both the horizontal and vertical directions, causing the steel pipe piles to be locked when they are locked into the soil along the Larsen steel sheet piles. This causes the steel pipe piles to be locked due to the distortion of the lock mouth, making it extremely difficult for them to sink. The lock mouths may even dissolve due to excessive heat generated by friction, and finally the lock mouths between the steel pipe piles and the Larsen steel sheet piles may tear, allowing the steel pipe piles to continue to sink into the soil. To avoid this phenomenon, steel pipe piles with appropriate rigidity should be used.
The first-section steel pipe pile and Larsen steel sheet pile sinking equipment can be injected with a hydraulic long-arm vibrator. The equipment has a large operating radius and flexible loading. It can freely adjust the direction and verticality of the steel pipe piles, and has high construction efficiency.
When using a hydraulic vibrating pile hammer, first clamp the steel pipe pile (or steel sheet pile) with the hydraulic vibrating pile hammer, lift the pile to make it suspended, and then continue to lift it until the pile body is vertical. Then move the steel pipe pile (or steel sheet pile) to the pile sinking position, make its locking mouth interlock with the locking mouth of the steel sheet pile (or steel pipe pile) that has been sunk, and slowly lower it until it enters the soil layer without sinking and is self-stable.
During the driving process, clamp the pile wall with a hammer clamp, and the sinking speed must be controlled until it stops at the designed depth. At the same time, the hammer body and the pile body should be kept vertical.
When the steel pipe pile cannot be driven to the designed depth, the pile can be assisted by water injection.
If there is a deviation, it should be corrected in time. The verticality of the first steel pipe pile after it is sunk affects the verticality of other piles of the entire retaining structure. Therefore, it is necessary to drive the pile slowly. When the pile is driven to half of the designed depth, stop driving the pile and check whether the verticality of the pile body is within 0.5%L. If the requirement is met, continue to use the vibrating hammer to drive the pile. Otherwise, pull out and drive it again.
Under the joint action of the lock, other piles generally do not produce large deviations. It is only necessary to check every 5-10 piles to ensure that the verticality of the pile body is within 0.5%L.
The high-pressure water jet boring method pile technology uses high-pressure water flow to impact the stratum to soften the clay and sandy soil, so as to achieve the purpose of boring:
The high-pressure water jet needs to use a method pile as the "boring pile", and two seamless steel pipes with a diameter of 25MM are welded and fixed on the pile body.
Two water jet nozzles are connected to the seamless steel pipe head at the insertion end, and the upper part of the seamless steel pipe is connected to the high-pressure pump with a high-pressure hose.
During construction, the high-pressure water jet is fixed on a "pilot hole pile" and inserted together with the construction method pile.During insertion, the high-pressure water pump is turned on, and the high-pressure water flow is used to impact the stratum to soften the clay and sandy soil. After being driven to the designed position, it is pulled out, and a common construction method pile is replaced and re-inserted. This cycle can be repeated.
Install the water jet device → connect the high-pressure device → test run the machine → insert and pull out the pilot hole of the pilot hole → insert and drive the steel sheet pile (or steel pipe pile) → cycle the pilot hole and insert the steel sheet pile (steel pipe pile).
1. Construction method of PC-Encased Steel Pipe Piles
(1) Determine the position
Set control measurement points at a certain distance along the foundation pit, use a total station (or GPS) to measure and lay out the inner contour of the support structure, and determine the position of the pipe piles and sheet piles according to the designed combination method.
(2) Trial pile construction
Before formal construction, trial driving is required. Generally speaking, steel pipe piles can be barely squeezed into the soil by a high-power hammer due to their high rigidity, while Larsen steel sheets are prone to distortion.Due to the uneven hardness of the soil layer, the distortion occurs in both the horizontal and vertical directions, causing the steel pipe piles to be locked when they are locked into the soil along the Larsen steel sheet piles. This causes the steel pipe piles to be locked due to the distortion of the lock mouth, making it extremely difficult for them to sink. The lock mouths may even dissolve due to excessive heat generated by friction, and finally the lock mouths between the steel pipe piles and the Larsen steel sheet piles may tear, allowing the steel pipe piles to continue to sink into the soil. To avoid this phenomenon, steel pipe piles with appropriate rigidity should be used.
(3) First-section injection
The first-section steel pipe pile and Larsen steel sheet pile sinking equipment can be injected with a hydraulic long-arm vibrator. The equipment has a large operating radius and flexible loading. It can freely adjust the direction and verticality of the steel pipe piles, and has high construction efficiency.
(4) Driving
When using a hydraulic vibrating pile hammer, first clamp the steel pipe pile (or steel sheet pile) with the hydraulic vibrating pile hammer, lift the pile to make it suspended, and then continue to lift it until the pile body is vertical. Then move the steel pipe pile (or steel sheet pile) to the pile sinking position, make its locking mouth interlock with the locking mouth of the steel sheet pile (or steel pipe pile) that has been sunk, and slowly lower it until it enters the soil layer without sinking and is self-stable.
(5) Control the sinking speed
During the driving process, clamp the pile wall with a hammer clamp, and the sinking speed must be controlled until it stops at the designed depth. At the same time, the hammer body and the pile body should be kept vertical.
(6) Assisted sinking
When the steel pipe pile cannot be driven to the designed depth, the pile can be assisted by water injection.
(7) Verticality correction
If there is a deviation, it should be corrected in time. The verticality of the first steel pipe pile after it is sunk affects the verticality of other piles of the entire retaining structure. Therefore, it is necessary to drive the pile slowly. When the pile is driven to half of the designed depth, stop driving the pile and check whether the verticality of the pile body is within 0.5%L. If the requirement is met, continue to use the vibrating hammer to drive the pile. Otherwise, pull out and drive it again.
(8) Verticality check
Under the joint action of the lock, other piles generally do not produce large deviations. It is only necessary to check every 5-10 piles to ensure that the verticality of the pile body is within 0.5%L.
2. Construction technology of PC method combined pile boring
(1) Construction method
When the soil layer is clay layer and fine sand layer, if conventional mechanical boring method pile boring is used, it is often difficult to advance the method pile.The high-pressure water jet boring method pile technology uses high-pressure water flow to impact the stratum to soften the clay and sandy soil, so as to achieve the purpose of boring:
The high-pressure water jet needs to use a method pile as the "boring pile", and two seamless steel pipes with a diameter of 25MM are welded and fixed on the pile body.
Two water jet nozzles are connected to the seamless steel pipe head at the insertion end, and the upper part of the seamless steel pipe is connected to the high-pressure pump with a high-pressure hose.
During construction, the high-pressure water jet is fixed on a "pilot hole pile" and inserted together with the construction method pile.During insertion, the high-pressure water pump is turned on, and the high-pressure water flow is used to impact the stratum to soften the clay and sandy soil. After being driven to the designed position, it is pulled out, and a common construction method pile is replaced and re-inserted. This cycle can be repeated.
