1. Prospects and Advantages of Steel Sheet Pile Application
1.1 Application Prospects of Steel Sheet Piles
The construction of steel sheet piles refers to the process of using steel sheet piles to provide support for excavation pits during construction. This method is frequently applied in the foundation support projects of thermal power plants, high-rise civilian and industrial buildings, high-speed railway projects, subway projects, highways, and large structures, as well as in cofferdam engineering for buildings and structures in the sea, lakes, and rivers. Steel sheet piles are a type of section steel with interlocking, available in various shapes such as straight, grooved, and Z-shaped, and come in different sizes and interlocking forms. Common types include Larssen, Rackwanna, etc. Larssen steel sheet piles are currently the most widely used.
1.2 Advantages of Steel Sheet Piles
(1)Light weight structure with strong load-bearing capacity. The support structure composed of steel sheet piles has high strength and rigidity, large deformation adaptability, and can provide necessary safety with strong timeliness for disaster prevention and emergency rescue, suitable for the prevention and treatment of various geological disasters.(2)Simple construction, compared with support schemes such as rotary jet piles and diaphragm walls, it can not only shorten the construction period but also adapt to different geological conditions and soil types, reducing the amount of excavation.
(3)Good water tightness, the interlocking of steel sheet piles is tightly combined, which can naturally prevent seepage. It can also be used as a water stop curtain in the support structure.
(4)Durable, with a service life that varies according to the environment but can last up to 40 years at a minimum.
(5)Environmentally friendly, the construction of steel sheet piles greatly reduces the amount of excavation and concrete used, effectively protecting land resources. The construction does not produce mud, causing less environmental pollution.
(6)Efficient construction, short duration, not limited by weather conditions, especially suitable for rapid implementation of disaster relief and natural disaster prevention such as earthquakes, floods, quicksand, and collapses.
(7)It can reduce the spatial requirements of the working face, mainly suitable for areas with limited space and important buildings around.
(8)Reusable. After the project is completed, the steel sheet piles can be extracted and recycled, and can be reused 20 to 30 times in temporary projects, saving funds.
2.Common Problems in Larsen Steel Sheet Pile Construction
2.1 High resistance, poor penetration or difficulty in driving the sheet piles
Reasons: When constructing in sand or gravel layers, if the piles are not fully driven in one go, it becomes difficult to penetrate after stopping and restarting due to increased grip between the piles and the sand and gravel; severe corrosion or deformation of the interlocking connections of the steel sheet piles, reducing their strength and rigidity; large rocks or unknown obstructions underground at the pile driving location, resulting in insufficient driving depth of the steel sheet piles.
2.2 Pile body inclination, failure to meet the required verticality specifications
Reasons: During the construction of steel sheet piles, the resistance at the interlocking connections is greater than the resistance around the piles, causing the head of the pile to lean towards the direction of less resistance, leading to pile body inclination. Inclining occurs as the steel sheet piles are squeezed into the process, subject to different degrees of lateral squeezing effects from rocks and other objects in the backfilled land section.
2.3 Pile body torsion issues
Reasons: The interlocking of the steel sheet piles is hinged, and under the action of insertion and hammering, the pile body deviates from the central axis and twists, turning the centerline into a zigzag shape.
2.4 Problem of pile dragging down with sinking
Reasons: Due to the inclination and bending of the steel sheet piles, the resistance of the interlocking connections increases, causing the pile body to exert a pulling force on the adjacent piles, dragging the adjacent steel sheet piles down as the pile sinks.
2.5 Difficulty in pile extraction issues
Reasons: Severe corrosion and deformation of the connecting interlocks; steel sheet piles are driven into dense sandy soil layers; the support is not timely during excavation, leading to significant deformation of the steel sheet piles.
3.Methods for Handling Construction Issues of Larsen Steel Sheet Piles
3.1 Preventive Measures for Pile Driving Obstruction Issues
Before construction, conduct a detailed analysis of the geological conditions based on the geological report to understand the geology within the penetration depth of the steel sheet piles. Before pile driving, inspect each steel sheet pile individually, remove those with corroded and severely deformed interlocking connections, and apply grease inside the locks. During the lifting and transportation of steel sheet piles, two lifting points should be used for lifting. When lifting, the number of piles lifted each time should not be too many, and the interlocking connections should be protected to prevent damage. There are usually two lifting methods: bundled lifting and single lifting. Bundled lifting often uses slings for binding, while single lifting commonly uses special lifting equipment. The storage location for steel sheet piles should be chosen on a flat and solid site to prevent significant settlement deformation due to pressure. If large obstacles such as concrete blocks prevent construction, a 3-meter long and 3-meter wide area should use the U-shape construction method. If obstacles prevent driving, use a long-armed excavator to dig, driving and digging the steel sheet piles at the same time until they reach the designed depth.
3.2 Preventive Measures for Pile Body Inclination Phenomenon
(1) During construction, use instruments to inspect and control the verticality of the steel sheet piles, correcting as construction progresses.(2) If steel sheet piles are found to be inclined, use a steel wire rope to pull the pile body at the part close to the ground, correcting gradually while driving, ensuring the wire rope always stays in close contact with the ground.
(3) After the pile body is inclined, pull up the steel sheet pile by 1.0m, then hammer it down, repeating this up and down several times to shift the large rocks below the pile and correct its position, reducing the inclination of the steel sheet pile.
(4) Use a screen-type driving method to ensure the accuracy of steel sheet pile driving. First, use a crane to lift the steel sheet pile to the insertion point for insertion, making sure the locks are aligned, and gently hammer after inserting each piece. During the driving process, use two theodolites in two directions to control and ensure the verticality of the pile body. To prevent displacement of the central plane of the lock, set a blocking plate at the lock of the steel sheet pile in the driving direction to prevent displacement. At the same time, pre-calculate the position of each plate on the wale to facilitate inspection and correction at any time.
Drive the steel sheet piles in several stages, the first time to one-third of the position, the second time to two-thirds of the position, and the third time to the elevation marked on the drawings. When driving, ensure the accuracy of the position and direction of the first and second steel sheet piles, measuring every 1.5m of driving.
3.3 Preventive Measures for Pile Body Twisting Issues
(1) Lock the front lock of the steel sheet pile with a blocking plate in the direction of driving.(2) Use a guide frame to ensure verticality. In steel sheet pile construction, to ensure the correct position of the pile axis and the verticality of the pile, control the driving accuracy, prevent bending deformation of the sheet piles, and improve the driving ability of the piles, a guide frame with certain strength and rigidity is usually required, also known as "construction wale." The guide frame is a single-layer, double-row form, usually composed of guide beams and wale piles, with wale pile spacing generally at 2-3 meters. The spacing between the double wales should not be too large, generally slightly greater than the thickness of the sheet pile wall by 10-20mm.
When installing the guide frame, pay attention to the following points:
① When installing the guide beam, a level and theodolite must be used to control and adjust the installation position of the guide beam.
② The height of the guide beam should be appropriate to facilitate the control of the construction height of the steel sheet piles and improve construction efficiency.
③ The guide beam must be securely fixed and not deform or sink with the driving of the steel sheet piles.
① The position of the guide beam must be vertical and not collide with the steel sheet piles.
3.4 Preventive Measures for Pile Dragging Down Issues
(1) Correct the steel sheet pile in a timely manner when it is inclined.(2) Weld together the steel sheet piles that are sinking together with one or several others using profile steel.
(3) Apply lubricating grease at the connection lock position to reduce resistance.
(4) After the steel sheet pile is dragged down, weld the same type of steel sheet pile to the head to make up for the length.
3.5 Preventive Measures for Difficult Pile Extraction Issues
(1) Vibration driving and extraction: When extracting, first use a vibrating hammer to loosen the soil at the lock of the sheet pile, and the rust between the bite of the piles will also fall off with the vibration, which can reduce the adhesion of the soil, and then extract while vibrating. For more difficult piles, use a diesel hammer to first vibrate down 150-200mm, and then alternate between vibration driving and extraction.(2) Pile extraction order and starting point: For a closed steel sheet pile wall, the extraction starting point should be more than 4 piles away from the corner pile. According to the difficulty of sinking, determine the position where it is easy to enter the soil when driving piles as the starting point for extraction, and the extraction order is the opposite of the driving order. Remove the steel sheet piles one by one from the starting point, and if there is difficulty, the piles can also be removed by skipping.
(3) When the soil on both sides of the steel sheet pile is compacted and bears a large earth pressure, making it difficult to extract the steel sheet pile, drive another pile in parallel nearby to facilitate the smooth extraction of the original pile. It is also possible to open a slot on the side and inject bentonite mud to reduce the resistance during extraction.
(4) For steel sheet piles with large extraction resistance, intermittent vibration methods can be used, vibrating for 20 minutes each time, with continuous vibration not exceeding 1 hour.
