A vertical soil deep mixing wall, known as the TRD (Trench cutting Remixing Deep wall) method was developed in Japan in the early 1990s as a technology for constructing soil cement diaphragm continuous underground walls. In this method, a cutter post built into the ground is moved laterally to excavate the ground with a cutter bit attached to a cutter chain, while mixing and stirring the solidified liquid and in-situ soil in a vertical direction to create a wall-like solidified body in the ground. One of the characteristics of this method is the continuity of the wall and high water sealing performance due to its unique excavation mechanism, and it is applied not only to earth retaining walls but also to impervious walls such as impervious revetments and underground counterfort revetments, as well as to containment of contaminated soil.
▍INTRODUCTION
The positioning of soil cement underground continuous walls is classified as shown in Fig. 1. In this paper, we report on the outline of the TRD method and a case study of the application of the wide type of TRD method to the earth retaining wall of an open-cut road tunnel.
2.1 Overview and Application
The TRD method is a relatively quiet, clean, and compact method. In this method, as shown in Fig. 2, a cutter post built into the ground is moved laterally to excavate the ground with a cutter bit attached to a cutter chain while mixing and stirring the solidification fluid and in-situ soil in the vertical direction to create a wall-like solidified body in the ground. In the case of earth retaining walls, as shown in Fig. 3, the ground is excavated ahead of time while injecting excavation fluid such as bentonite, and after returning to the starting point by return excavation, walls are built while injecting solidification fluid, and after inserting core materials, the ground is excavated ahead of time again. This sequence of steps is referred to as “3-pass” construction.
▲ Fig. 4. Application of TRD Method
In recent years, a new type of TRD method has been developed that is capable of building walls as wide as 900-1200 mm, as shown in Fig. 5, in contrast to the 450-850 mm wall thickness of conventional types, and each model can be employed according to the scale,purpose, and site conditions of the wall construction.
▲ Fig. 5. Scope of application by TRD method model
2.2 Characteristics of the Construction Method
The features of this construction method are as follows
(a) Stability of the machine
The low center of gravity design enables safe construction with a constant machine height of approximately 10 m（8.5-12m）, regardless of the wall depth, by adding a cutter post in the ground.
(b) Wall continuity and core spacing can be set as desired.
Since the walls of uniform thickness are stirred and mixed while moving continuously in the horizontal direction, there are no blind spots, and high-quality walls with high impermeability can be realized. In addition, because the walls are of uniform thickness, they can be erected at any core material interval, and steel sheet piles, steel members for underground continuous walls, concrete sheet piles, PC walls, etc. other than H-shaped steel members can be inserted and erected.
(c) Hard ground and homogeneous wall quality in the depth direction
The excavation mechanism can excavate hard ground with a normalized N-value of 100 or higher by rotating the cutter chain in the vertical direction while the cutter post inserted in the ground is pressed against the ground in the horizontal direction. By mixing the excavated soil and solidified solution in the vertical direction, it is possible to construct a homogeneous wall with a high permeability of 1×10-8 m/sec or higher, with little variation in wall strength or permeability in the direction of depth.
(d) High-precision construction
Since high vertical accuracy is required when constructing underground structures, the standard value for this method is within 1/250. The behavior of the cutter post in the ground is measured in two directions on the X-axis and Y-axis by a multi-stage inclinometer installed in the cutter post on a dedicated monitor on the operator’s seat, allowing the wall construction work to be performed under constant monitoring. In addition,a newly developed software that outputs the measured displacement data during excavation from a specified screen is in operation.
2.3 Construction Management
When constructing a soil cement continuous wall using the TRD method, the purpose of construction,design intent, and characteristics of the construction method should be fully understood, and construction management should be conducted according to the construction management chart for continuous walls shown in Table 1.
▲ Table 1. Management Chart of TRD Continuous Wall
Data source：Japanese Geotechnical Society Special Publication
Editor：AlanXiang
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