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]. For the flowability of conditioned soils, slump values in the range of 100–200 mm are accepted and considered reasonable in soft ground tunneling [
2,
3,
18,
19], which means that the overall state of conditioned soils meets the requirement of EPBS machines during tunneling, and the EPBS machines could advance smoothly. The friction coefficient test, imitating the friction process of soil and steel, is used to obtain the frictional coefficient by measuring the friction angle between soil and steel or angle of external friction [
14], which also reflects the effect of soil conditioning on the tested soils. The mixing test, slump test, and friction coefficient test are widely used in evaluating of properties of conditioned soils in laboratory-scale tests because of their simplicity, speed, and low cost.
EPBS machines, which were originally used for fine-grained or mixed -rained soils with at least 30 wt.% fines (
d < 0.06 mm), have been proved to be applicable for coarse-grained and cohesionless soils by the utilization of soil conditioning, such as sand [
19], rock masses [
20], tar sand [
12], weathered granite soil [
3], and so forth. The current research mainly focused on the soil conditioning in single soils, while the feasibility of excavation soils from mix ground composed of cohesive soil and cohesionless soil have rarely been investigated to date, which is more difficult to resolve in EPBS tunneling. The characteristics of the mixed soil, which is abundant in large-diameter EPBS tunneling, are different from those of either clay or sand, as the conditioning agents applicable to single soils cannot be directly used in mixed soils. There are still some knowledge gaps in the application of soil conditioning in mixed soils.
This article presents an experimental investigation of effects of common foam and water on the characteristics of a mixed soil consisting of silty clay, fine-medium sand, and sandy gravel, from an EPBS tunneling project in Beijing. The microstructure of the foam particle was first investigated to explore the decay mechanism, followed by the description of the evaluation methods, testing procedures, and tested soil used in this study. After that, the detailed results and discussion of the laboratory tests, which demonstrates the effect of the soil conditioning, are presented.
3. Soil Samples and Foam Agent Used in Experiments
A mixed soil consisting of silty clay, fine-medium sand, and sandy gravel, with a volume proportion of 2:2:1 encountered in an EPBS tunneling projects in Beijing, was used in this study. The selected properties of the soil were tested based on GB/T 50123 [
23] and are listed in
Table 1. For silty clay, X-ray diffraction tests were performed to determine the mineral constituent, as shown in
Table 2.
Figure 4 shows the grain size distribution of the fine-medium sand and sandy gravel. Based on the liquid-plastic limit combined method [
23], the plastic limit (
WP) and liquid limit (
WL) of the silty clay were 30% and 44%, respectively. For the natural silty clay with a water content of 25%, the plasticity index (I
P) and liquidity index (I
L) were 14% and -0.36, respectively. The gravel particles wrapped by silty clay and sand particles cohere the cutting tools on the cutterhead and it is hard to discharge the mixed soils via screw conveyor. The mechanical behavior of the mixed soil is not suitable for EPBS tunneling. Soil conditioning is needed to modify characteristics of the mixed soil to meet the requirement of EPBS machine. The natural water content of the mixed soil was approximately 25%, and the original water content was set to 25% for a group of soil sample test in this study.
For foam agents, there are two performance indicators which are often used to characterize foaming behaviors and select a foam agent: Foamability and foam stability. Foamability means how many volumes of bubbles with stable structure and moderate size can be produced by a given amount of a foaming agent, which is quantified by the ratio between the resulting volume of the foam and the volume of foaming liquid (foaming agent + water), that is, the foam expansion ratio (FER). Foam stability (or drainage behavior) means how long the foam can remain and how long foamed soils can sustain the desirable properties or behaviors. The half-life period (
T1/2), representing the time for half volume of the liquid matrix flowing out of the foam, can be used to characterize the foam stability. There are several important conditioning parameters for foam agent, which are defined as follows [
2,
15]:
where
Cf is the concentration of the foaming agent (%),
FER is the foam expansion ratio (dimensionless),
FIR is the foaming injection ratio (%),
Vf is the volume of foaming agent (mL),
VF is the volume of foam (mL),
VL is the volume of foaming agent and water (mL), and
VS is the volume of soil (mL).
In order to obtain the foam during laboratory test, a commonly used method called the Waring-Blender method was used in this study. The procedure of foam production in the Waring-Blender test is as follows: (1) 100 mL foaming liquid (foaming agent + water) with different concentration of foaming agent (
Cf,
vol.%) was added into a mixer; (2) the foaming liquid was thoroughly mixed for 1 min at a rotational speed of 3000 rpm (revolutions per minute), as shown in
Figure 5a. Then, the foamability (i.e.,
FER) and foam stability (i.e.,
T1/2) of the generated foam were tested, as shown in
Figure 5b. Finally, the foam agent with
Cf = 4%,
FER = 7 and
T1/2 = 30 min under the room temperature of 25 ℃ was used in this study. The generated foam was used for the observation of the microstructure and subsequent evaluation tests of the conditioned soil. The generated foam was added after thoroughly mixing of tested soils and water (see
Figure 6a). The foam injection ratios (FIR) were set as 0%, 10%, 20%, and 30% for various soil samples. After that, the injected foam and soil were stirred into a homogeneous mixture (see
Figure 6b).
5. Conclusions
This article presents the results of an experimental study, which assessed the influence of foam on a mixed soil consisting of silty clay, fine-medium sand, and sandy gravel utilizing several geotechnical methods to provide a reference for soil conditioning on mixed soils often encountered in EPBS tunneling. Based on the preliminary tests conducted in this study, the main findings specific to the soils and conditioners tested in this study can be summarized as follows:
Foam microstructure changes with the decay of the liquid film. The microstructure test carried out in this study showed that the bubbles decayed as the time went by and the decay mechanism was the drainage of water in the liquid film caused by the gravity and pressure difference, and the foam fusion among various size bubbled due to the pressure difference.
Water plays an important role in modifying the characteristics of the mixed soil. When the water was at the low level (w = 25%), the foam failed to modify the properties of the mixed soil because of its failure behavior of foam particle induced by the tested soil with insufficient water content. However, when w = 40%, the flowability increased and viscosity decreased dramatically.
Foam plays an important role in increasing flowability and decreasing viscosity when the water is sufficient in the mixed soil. The mixed soil in this study could be modified into a proper state to meet the slump requirement of 100–200 mm by mixing the foam with the water content of 40%. Foam could maximum its effect when the mixed soil with sufficient water content (i.e., 40%).
When the water content was 40%, the mixing time, net power value and fluctuation value, slump value, external friction angle and friction coefficient were better than soil samples with water content of 25%. The net power caused by mixed soil with w = 40% and FIR = 30% could decrease by approximately 50%. The foam could improve the properties of mixed soil further when the water is sufficient.