## 1 Introduction  
For the sustainable development of the big city, developing more underground space is effective, and the development of intensive tunnels, including metro tunnels, road tunnels, and utility tunnels, is carried out throughout the world. Earth pressure balance shield (EPBS) machines, which were originally developed and used in a close mode for fine grained soils or mixed grained soils with at least 30 wt.% fines (\(d<0.06\) mm) [1,2], have been extensively used in various tunnels because of their high security in driving, high efficiency in tunneling, and high feasibility in stratums. EPBS machines achieve stability of tunnel face by filling the chamber and applying chamber pressure at the back of cutterhead [3], and they have been used in many kinds of excavation soils with complicated characteristics, such as caking property of clayed soils [4], looseness of sandy soils [5], and abrasivity of gravelly soils [6]. Many studies on the application of soil conditioning in various given soils have been carried out in recent years. Today, EPBS tunneling is possible through almost all geological conditions, and more and more EPBS machines work in mixed ground due to their large excavation diameter [7] thanks to the application of the soil conditioning system and soil conditioning agents. Soil conditioning, which is often done by injecting water, foam, bentonite slurry, or polymer in the excavation face, soil chamber, and screw conveyor during EPBS tunneling [8], is used to improve the stability of tunnel face and then minimize the deformation of ground surface, reduce cutting tools wear, and discharge the excavated soils smoothly [9; 10]. The torque of cutterhead and the thrust of EPBS machines increases even if the screw conveyor does not work when the soil chamber and the screw conveyor are filled with excavated soils [11]. The workability of the excavated soils is improved dramatically after the mixing caused by cutters and cutterhead in tunnel face and stirring caused by rods in soil chamber. For soil conditioners, water is a common conditioner which is very easy to access. Foam agent is a widely used soil conditioner, and bentonite slurry are usually used to modify the grain size distribution of the excavated soils with unfavorable particle composition for EPBS tunneling [12]. For challenging geological conditions, in which the use of water is insufficient, foam and slurry, the structuring and water-binding polymers, are warranted to increase the muck cohesion and decrease permeability, respectively [7]. Foam produced by chemical products, named surfactants or surface-active agents, is usually used in two fields of underground works, including cellular concrete and drilling operations, although the industrial applications of foam are various [13]. In most cases, it is necessary to mix the excavated soils with some soil conditioners to modify the original characteristics of the natural soils to meet the requirement of EPBS machines. In order to obtain and evaluate the state and properties of the conditioned soils for various purposes, a series of evaluation methods were proposed, such as a mixing test, slump test, friction coefficient test, adhesive resistance test, penetration test, and compressibility test. Among these tests, the mixing test is used to imitate the real mixing process in the soil chamber [14], and it is necessary to mix the soil conditioner with the tested soils in laboratory tests [15]. The variation of the electric motor power, the mixing time necessary to obtain a homogeneous mixture, and the quality and behavior of the mixture could be observed and evaluated during the evolution of the foam injection in the tested soil [13]. Some mixing tests have been carried out in previous studies [14; 15; 16]. The slump test [17], which is originally used for concrete materials, has been intensively used to evaluate the characteristics of conditioned soils in EPBS tunneling [3; 5; 12; 13; 14; 16; 18; 19; 20; 21; 22].