# A New Soil Conditioner for Highly Permeable Sandy Gravel Stratum in EPBs
## 1 Introduction  
In recent years, with the increasing demand for underground space, underground tunnel excavation technology has transitioned from open excavation to underground excavation and has become a research hotspot [1; 2; 3]. Among all excavation technologies, the earth pressure balance shield (EPBs) is the most commonly used [4]. Under different stratum conditions, specific accidents may occur in the application of EPBs, which makes it difficult to apply this method. For example, different from tunneling in clayey soil, in a sandy gravel stratum accidents, such as groundwater spew and ground collapse, may occur when EPBs is used [5; 6; 7]. Furthermore, since Beijing, Chengdu and other cities that are vigorously developing the metro are rich in sandy pebble stratum, a new soil conditioner for highly permeable sandy gravel to prevent water spewing has become a key technology problem to be solved in underground tunnel construction and in the application of EPBs in China [8]. To ensure the successful excavation of a tunnel in the process of EPBs, an improvement technology to obtain the \"ideal soil\" (low permeability, moderate compressibility, small shear strength and certain fluidity) by adding soil conditioners was proposed [9]. Large particle size, low clay content and low cohesion lead to poor plastic paste and the high permeability of sandy gravel soils. Soil conditioners can decrease the permeability and improve the plastic paste of sandy gravel soils, and many successful cases were obtained [10]. Considerable studies on the improvement of sandy gravel soils have been conducted by researchers. A foam agent was added to enhance the plastic paste of the soil. [PERSON] et al. (2009) [11] studied the effect of different ratios of foam injection on the soil slump of various non-viscous coarse-grained soils. According to the foam loss, slump and water loss, theimprovement results were divided into the non-suitable, the suitable, and the borderline. [PERSON] et al. (2018) [12] concluded that a foam agent can effectively fill the granular soil void, reduce the friction and shear strength between soil particles and improve the plasticity of soil. In the conditioned soil, qualitative evaluation standard considers the slump to be mainly related to the foam injection ratio and water content [13]. However, enhancing the plastic and reducing the permeability are required to prevent water spewing in water-rich sandy gravel stratum [14; 15]. When excavating in a coarse-grained stratum, it is insufficient to obtain a satisfactory conditioning by simply adding a foam agent. Some researchers proposed polymer or bentonite slurry for reducing soil permeability [15]. Adding polymer material to the slurry not only increases the viscosity of the liquid, but also the polymer's long-chain structure can enhance the adhesion between the fine particles of the slurry, so that the conditioned slurry has a better sealing effect on the large-grit sand-gravel soil [16]. During the construction of the earth pressure balance shield tunnel in the high permeability stratum, some methods have been proposed to improve its permeability by adding water-insoluble fine particle solids (such as fine sand, calcium carbonate powder, etc.) or polymer materials. [PERSON] (2007) [17] added fine sand (<1 mm), vermiculite (0.7-4 mm) and a small amount of high-polymer material to the soil of high permeability, which significantly reduced the permeability. In addition, filtration problems always occur in the formation of slurry. [PERSON] (2011) [18] used slurry and high polymer to improve the high-permeability (k > 3 \(\times\) 10\({}^{-3}\) m/s) of gravel stratum in the process of Weinberg tunnel construction. [PERSON] (2006) [19] injected a turbid liquid of calcium carbonate powder as a filler into the excavation chamber, which improved high-permeability coarse sand formation in Turin Metro Line 1.