Abstract: The characteristic of secant pile construction using Luoyang spade is that hole formation is carried out in sections according to variable cutoff joints, i.e., all A and B piles within a variable cutoff joint are drilled and then concrete is poured uniformly, eliminating the need for full casing drilling rigs and super-retarding concrete for plain piles used in conventional secant piles. This improves the secant quality, greatly reduces engineering costs, and yields considerable economic benefits. The article elaborates on the advantages and key construction technology points of Luoyang spade secant piles in practical engineering, with emphasis on the extremely considerable economic benefits formed by the cost comparison between Luoyang spade secant piles and conventional secant piles.

 

　　Keywords: Luoyang spade   secant pile   super-retarding   economic benefits

 

        1. Introduction

        With the development of China's economy, many cities are developing into international metropolises. Urban widening and reconstruction have become the mainstream of current infrastructure construction. To alleviate the growing population pressure in cities and improve urban traffic congestion, subways, underpass bridges, underground parking lots, underground shopping malls, various underground passages, and civil air defense projects have become major urban infrastructure projects. Secant pile engineering serves both as retaining wall and water stop, and is widely used in such projects. Therefore, discussing the construction technology of Luoyang spade secant piles is of great significance and has good prospects.

 

        2. Characteristics of Luoyang Spade Secant Piles

        Secant reinforced concrete piles achieve soil retention and water stop through pile interlocking. Through uneven distribution of reinforcement within the pile, maximum bending strength is obtained under the same reinforcement ratio. This construction method adopts sectional hole formation according to variable cutoff joints, i.e., all A and B piles within a variable cutoff joint are drilled and then concrete is poured uniformly, improving the secant quality.

Construction sequence of secant piles: A1～A7     B1～B8 (construction in 10m sections)

 

 

        3. Project Overview  

        The Harbin Hashuang South Road Widening and Reconstruction Project, with a construction period from June 11, 2011, to May 30, 2012, is located on Hashuang South Road, Nangang District, Harbin. The road mileage is K0+660—K1+900, starting from Wanggang Street in the west, passing under three existing railways including the Wangsun up and down connecting lines and the Hatun Line, and ending at Dongchilun Road K1+900 in the east, with a total length of 1240m. The center of the new underpass bridge coincides with the centerline of the existing road, with a curve radius of R=1000m. The secant pile mileage is K0+970—K1+900, with a total length of 930 meters. The secant piles have a diameter of 800mm and are divided into A piles (without reinforcement) and B piles (with reinforcement), interlocking by 15cm. Each section is 10m, with 7 A piles and 8 B piles, totaling 15 piles. For the entire bridge, there are 1306 A piles and 1469 B piles, totaling 2775 piles and 27997m. In the Harbin Hashuang South Road Widening and Reconstruction Project, a relatively complete construction method for Luoyang spade secant piles was adopted. It overcame conventional secant pile construction difficulties such as low-height construction under railway catenary, simultaneous multi-point operations to rush the schedule, and low-noise all-weather construction, achieving excellent economic and social benefits.

 

         4. Process Principle

         Secant reinforced concrete piles achieve soil retention and water stop through pile interlocking. Through uneven distribution of reinforcement within the pile, maximum bending strength is obtained under the same reinforcement ratio. This construction method adopts sectional hole formation according to variable cutoff joints, i.e., all A and B piles within a variable cutoff joint are drilled and then concrete is poured uniformly, improving the secant quality.

         For Luoyang spade hole formation, a tripod is supported above the hole position. A steel wire rope at the center of the tripod connects to the spade head through a pulley, with the center of the spade head coinciding with the center of the hole position. The other end of the steel wire rope (outside the tripod) is connected to a winch. The spade head free-falls to excavate soil. After excavation, the winch lifts the spade head, a wheelbarrow is pushed directly under the spade head, and the spade head is shaken to discharge the soil into the wheelbarrow for removal. The Luoyang spade consists of a winch, powered spade head, electric motor, and tripod. The winch crane is 2 tons, equipped with Φ16mm high-strength steel wire rope and a 114 kW electric motor. The spade head weighs 300Kg～1000 Kg, the tripod height is 5 meters, the entire machinery covers about 6m2, and requires two workers to operate.

 

         5. Scope of Application

        This mechanical hole formation is applicable for diameters between Φ300mm and 2000mm, suitable for hole formation in silty clay, not suitable for soils with high sand content or water-bearing strata. It can replace rotary drilling rigs in narrow sites and sites with height restrictions.

 

        6. Construction Method

        6.1 Site Leveling

        Use a level to measure the natural ground elevation, calculate the difference from the pile top elevation to determine the leveling depth. The site should be basically level and free of debris.

        6.2 Survey and Positioning

        Use a total station to set out the center point of each pile, measure the elevation with a level, calculate the actual pile length, and form a technical disclosure for issuance.

        6.3 Rig Positioning and Centering

        Use a crowbar to move the Luoyang spade so that it is located 1.5～2.0m outside the pile. Move the two legs of the Luoyang spade so that the crown block is directly above the pile position, and the angle between the legs and the ground should not exceed 75°. If greater than 75°, the Luoyang spade should be moved back. Lift the Luoyang spade and check whether the central axis of the Luoyang spade is vertical to the wooden stake at the center of the pile hole. If not vertical, move the two legs of the Luoyang spade to make it vertical, and the leg angle should meet the requirements. Finally, reinforce the adjusted legs to ensure sufficient stability during drilling.

        6.4 Soil Excavation and Hole Formation

        The opening of the Luoyang spade is centered on the wooden stake at the pile position, and any deviation should be corrected promptly. The diameter of the hole formed by the Luoyang spade is 0.8m, with a deviation not exceeding ±20mm. When opening, check the maximum opening size of the first impact of the Luoyang spade on the ground; this size is the hole formation size. If it meets the requirements, continue construction; if not, adjust the connecting plate of the Luoyang spade to make the hole size meet the requirements. The verticality of the guide hole of the Luoyang spade is 0.3%. During hole formation, check the verticality every 5m and correct any deviation promptly. Pile position deviation ≤20mm. To ensure sufficient interlocking thickness at the bottom of the bored secant piles, in addition to strict control of the hole positioning error, the verticality should also be strictly controlled. According to China's "Code for Construction and Acceptance of Metro Engineering," the verticality standard for piles should be 3‰. The verticality of the pile should be well controlled during hole formation.

        6.5 Hole Inspection

        Pull a string line to determine the pile position and check whether the displacement of the pile hole meets the requirements. If it exceeds the specification, repair the hole. For pile holes that meet the requirements, continue to inspect their verticality. Weld a cross frame with rebar, place the intersection point of the cross frame at the actual center of the pile hole, and lower a plumb bob along the center of the pile hole to the bottom of the pile to check whether the verticality of the pile hole meets the requirements. If it exceeds the specification, repair the hole. For pile holes that meet the requirements, continue to inspect their pile diameter. Make a lifting ring with a diameter of 780mm using rebar. Lower it into the pile hole with a rope. If the lifting ring smoothly reaches the bottom of the pile, the pile diameter meets the requirements; otherwise, it does not meet the requirements and the hole needs to be repaired. After the pile hole is accepted, the bottom of the pile should be compacted with a heavy hammer. Before concrete pouring, the pile hole should be covered and protected in time to prevent falling accidents and to protect against rain.

        6.6 Fabrication and Installation of Reinforcement Cage and Concrete Pouring

        Each construction section is divided into 10m according to the design drawings. Successively complete the hole formation of 15 piles, and install all 8 reinforcement cages for B piles in each section. Concrete is placed using tremie pipes. A tremie pipe is lowered into each B pile. To reduce the impact of concrete on the hole wall, the height between the bottom of the tremie pipe and the hole bottom should not exceed 10cm. During concrete pouring, the concrete must be kept at the same level.

 

        7. Conclusion:

        In summary, compared with conventional secant piles, the Luoyang spade secant pile adopts sectional hole formation according to variable cutoff joints, i.e., all A and B piles within a variable cutoff joint are drilled and then concrete is poured uniformly, improving the secant quality of the secant piles; it eliminates the entry costs of large equipment such as full casing drilling rigs and the super-retarding concrete for plain piles used in conventional secant piles, and does not require preparatory work such as working platforms and guide walls. The drilling rig has low manufacturing cost and requires fewer operators, reducing engineering costs by about 60%. The Luoyang spade has a small working area, overcoming conventional secant pile construction difficulties such as low-height construction under railway catenary and existing bridges, construction adjacent to existing lines, and low-noise all-weather construction. The Luoyang spade equipment is extremely simple to operate and has fast construction speed. One person controls it and one person assists with a small cart to transport soil. It enables simultaneous multi-point operations to rush the schedule, greatly shortening the construction period and saving costs. With the continuous improvement and perfection of construction technology, more and more bridge designs adopt this technology. Therefore, in construction, quality control should be strengthened, precautions should be clarified, experience should be seriously summarized, lessons should be learned in time, and construction quality should be gradually improved and controlled to bring China's bridge construction to a new level. The Harbin Hashuang South Road Widening and Reconstruction Project applied this technology in construction, overcame many difficulties, and fully met the construction specifications and acceptance standards.

 

References:

[1] "Quality Acceptance Standard for Construction of Railway Bridge and Culvert Engineering" TB10415-2003 J286-2004

[2] "Quality Acceptance Standard for Construction of Railway Concrete and Masonry Engineering" TB10424-2003 J283-2004

 

Author Introduction

    Ren Wei, male, born in 1980, assistant engineer, bachelor's degree. (Project Management Department of the Third Section of the Harbin Hashuang South Road Widening and Reconstruction Project, China Railway 23rd Bureau Group Co., Ltd. Second Company)