Keywords: jet grouting pile    low-pile cap    application

　　1. Introduction

　　The conventional method for deep-water low-pile cap construction generally adopts single- or double-wall steel cofferdams sunk to the design elevation followed by concrete sealing at the bottom. However, for the main pier of the Bengbu Chaoyang Road Huaihe River Highway Bridge, the riverbed surface geology consists of hard clay, and the pile cap base rests in sand and silty sand permeable layers. Sinking the cofferdam to the design elevation would be complex and difficult. Drawing on the principle that high-pressure jet grouting piles used for onshore embankments can both seal water and provide support, this method was boldly applied in deep water, organically combining the cofferdam with high-pressure jet grouting piles. This not only solved the difficulty of sinking the cofferdam through the relatively thick hard clay layer but also overcame the challenges of water sealing and support when excavating the pile cap into the sand and silty sand layers. This paper focuses on the application of vertical curtains of high-pressure jet grouting piles for water sealing and support in deep-water low-pile cap construction.

　　2. Project Overview

　　The Bengbu Chaoyang Road Huaihe River Highway Bridge is a major highway bridge crossing the Huaihe River, with a total length of 1730 m, dual carriageway with four lanes. The main span adopts a 140 m prestressed reinforced concrete continuous rigid frame. The main piers are located in the Huaihe River, with foundations consisting of 14 φ2.0 m bored rock-socketed column piles and low-pile caps. The construction water depth is 4–6 m. The structural configuration is shown in Figure 1.

　　(1) Engineering Geology

　　The geology at this pier from the riverbed surface downward is as follows:

　　(a) Hard clay layer: bearing capacity 260 kPa, thickness 2–4 m, ultimate friction resistance 60 kPa.

　　(b) Silty sand and sand layers: sand layer along 2/3 of the cofferdam perimeter, bearing capacity 80 kPa, thickness 2.2 m; the remainder is silty sand in soft plastic state, average thickness 1.5 m, SPT blow count 13, bearing capacity 180 kPa.

　　(c) Hard plastic silty clay layer: average thickness 5.75 m, SPT blow count 13, bearing capacity 220 kPa.

　　(d) Below are silty sand and rock layers.

　　The specific geological layout is shown in Figure 1.

　　(2) Groundwater

　　The main pier is located in the Huaihe River. The cohesive soils encountered are all impermeable layers; only the sand and silty sand layers are permeable, and these permeable layers are in a complementary relationship with the Huaihe River water.

　　2. Construction Scheme

　　(1) Cofferdam Selection

　　The cofferdam adopts a double-wall steel cofferdam for the bottom section and single-wall steel cofferdam for the rest, with an outer diameter of φ26.6 m, inner diameter of 25 m, height of 16 m, and self-weight of 237 t. Considering the light self-weight of the cofferdam, it is relatively difficult to overcome the huge external friction by using the internal soil excavation and ballasting method to sink the cofferdam. Based on the geology, the cofferdam construction adopts sinking by 1.5–2.0 m, followed by vertical curtain support and water sealing using high-pressure jet grouting piles for pile cap construction.

　　(2) Design of High-Pressure Jet Grouting Piles

　　① Pile Layout

　　According to the design, two rows of piles are arranged along the outer side of the cofferdam in the silty sand layer area, and three rows in the sand layer area. The piles are 0.1 m from the cofferdam edge, with a spacing of 501 mm, length of 9.0 m, diameter of 0.7 m, and the pile tip penetrates more than 1 m into the hard plastic silty clay. The specific layout is shown in Figure 2, and the calculation models are shown in Figures 3 and 4.

　　Calculations show that the forces meet the requirements.

　　② Construction Mechanism of High-Pressure Jet Grouting Piles

　　The drilling rig adopts XPZ-50 single-tube high-pressure jet grouting pile equipment and BWT100/30 high-pressure grouting pump. After the drilling rig reaches the design elevation, the high-pressure jet grouting machine lowers the grouting pipe equipped with horizontal nozzles to the bottom of the hole. High-pressure equipment is used to jet the grout at a pressure of 20 MPa through the nozzles, impacting and cutting the soil and mixing it with the soil. As the grouting pipe rotates and lifts, a cylindrical pile body is formed. Through a series of physical and chemical reactions, the grout and soil solidify into a pile, thereby achieving support and water sealing.

　　③ Design Requirements

　　The permeability coefficient of the sand and silty sand layers at the high-pressure jet grouting pile curtain is K ≤ 1×10⁻⁷ cm/s, and the compressive strength of the pile body is taken as 3 MPa.

　　4. Construction Process

　　(1) Process Flow

　　Rig positioning → pilot hole (reaming) to design elevation → sealing vertical nozzles → grout mixing → jet grouting from bottom to top to design top elevation → flushing → relocation.

　　(2) Construction Sequence

　　First, drive 2–3 test piles to understand the construction process and jet grouting effect, and finally determine the construction parameters. Then proceed with the construction of jet grouting piles. During construction, the inner row of piles should be constructed first, followed by the outer row. To ensure good interlocking between piles, the skip-one-pile method should be adopted, and the interval should be greater than 36 hours.

　　(3) Selection of Construction Parameters

　　Pump pressure: 20 MPa

　　Nozzle diameter and quantity: 2.3 mm×1–2.4 mm×1

　　Flow rate: 37–40 L/min

　　Drill rod lifting speed: cohesive soil 30 cm/min

　          　sandy soil 15 cm/min

　　Drill rod rotation speed: cohesive soil 30 r/min

　          　sandy soil 15 r/min

　　Water-cement ratio: 1.1–1.2

　　Cement: ordinary Portland 32.5, cement dosage 125 kg/m per meter

　　Admixtures: sodium chloride 1.5%, triethanolamine 0.03% or water glass 1.5%, water glass modulus 2.4–3.4, concentration 30–40 Baume degrees.

　　Note: Triethanolamine and water glass must not be used together, otherwise a chemical reaction will occur.