Experimental study on the stability of foam-conditioned sand under pressure in the EPBM chamber

https://doi.org/10.1016/j.tust.2020.103590Get rights and content

Highlights

  • Foam is more stable when it is mixed with soil than foam itself.

  • Soil particles help stabilize foam bubbles.

  • Engineering properties of foam-conditioned soil are relatively stable over time.

  • Accumulated air in the mixing chamber is not due to the de-stabilization of foam bubbles.

Abstract

Proper soil conditioning is critical for effective earth pressure balanced tunnel boring machine (EPBM) operation. Foam as a soil conditioner has been extensively used in EPBM tunneling to modify the excavated soil properties. A critical characteristic of foam-conditioned soil is stability, i.e., the ability to maintain the engineering properties throughout the residency time (30–90 min) in the mixing chamber. A comprehensive suite of experiments on foam-conditioned soil was conducted at both microscale and macroscale to investigate the fundamentals of foam-soil interaction and engineering properties of foam-conditioned soil. A foam-soil capture device and an optical microscope were used to capture bubble-grain images at a microscale under pressure. A pressurized testing chamber (PTC) was used to examine the stability of the mechanical properties of foam-conditioned soil. The compressibility, vane shear strength, pore pressure, and effective stress of foam-conditioned soil with elapsed time were obtained from the PTC test. The bubble-grain image analysis results reveal that foam is more stable when mixed with soil than by itself, indicating that soil particles help stabilize foam bubbles. Soil particles create a barrier to bubble coalescence and coarsening. The PTC test results show that the engineering properties of foam-conditioned soil remained constant over the 60-minute test duration. The results suggest that the accumulated “air bubble” at the top of the mixing chamber during EPBM tunneling is not due to the de-stabilization of foam bubbles when mixed with soil.

Introduction

In earth pressure balanced tunnel boring machine (EPBM) operation, soil conditioning is critical for effective performance. Foam as a soil conditioning agent has been widely used in EPBM tunneling to modify the mechanical and hydraulic properties of excavated soils. High compressibility and elasticity, low shearing resistance, abrasivity, and permeability, and improved flowability are the desired properties of foam-conditioned soil (Budach and Thewes, 2015, Milligan, 2000, Mori et al., 2018, Peila, 2014, Thewes et al., 2012, Vinai et al., 2008). Considerable research has been conducted to study the properties of foam-conditioned soil including compressibility, shear strength, abrasivity and rheological properties (Bezuijen et al., 1999, Budach and Thewes, 2015, Mooney et al., 2016, Mori et al., 2018, Peila, 2014, Peila et al., 2007, Psomas and Houlsby, 2002, Quebaud et al., 1998, Thewes et al., 2012, Vinai et al., 2008). Research has also been conducted on the stability of foam itself, both under atmospheric pressure (Rand and Kraynik, 1983, Schramm and Wassmuth, 1994) and at pressures experienced in the excavation chamber (Wu et al., 2018). For foam stability under pressure, Wu et al. (2018) conducted a series of experiments on foam bubble sizes and foam liquid drainage (or half-life). The authors found that foam liquid drainage is significantly slowed for foam with smaller and more uniform bubbles. Uniformity and smaller bubble size lead to less air diffusion and coalescence, as well as a longer, more tortuous drainage path. However, little is mentioned in the literature about the stability of the foam while mixed with soil in the EPBM chamber. After all, the foam is not designed to exist by itself; it is designed to exist within the soil/muck mass.

The stability of foam-conditioned soil, defined as the persistence of desired engineering properties, is a critical characteristic. In EPB tunneling, foam must maintain its desired engineering properties from the time of injection at the cutterhead, through the mixing process in the excavation chamber, and into the screw conveyor for transport to the belt conveyor. This period constitutes 30–90 min depending on TBM size, production rates and cycle times (excavation plus ring build). In addition, the accumulated air in the crown of the excavation chamber is a common and significant concern, and it is unclear if this comes from the collapse of foam bubbles (instability) when mixed with soil or from bubble migration upwards through soil void space during mixing where heavier soil particles move downwards and push or migrate foam upward. And for each case, what are the mechanics and characteristics that govern instability and migration.

To improve understanding of the stability of foam-conditioned soil in terms of bubble instability and migration, a series of soil conditioning experiments was performed under pressure with investigation at both the bubble scale and macro sample scale. A foam-soil capture device was developed to investigate foam bubble stability in foam-soil mixtures under pressure. Foam bubble size distributions of foam-soil mixtures were obtained with elapsed time, and the results were compared with the foam-only scenario. Further, a pressurized testing chamber (PTC) was used to investigate the stability of the mechanical properties of foam-conditioned soil including compressibility, vane shear strength, and effective stress with elapsed time.

Section snippets

Background

Previous research has found that pressure has a significant influence on both foam and foam-conditioned soil properties (Mooney et al., 2016, Mooney et al., 2017, Mori et al., 2018, Williamson et al., 1999, Wu et al., 2018). A foam-conditioned soil that shows ideal properties under atmospheric pressure (p = 0 bar gauge) can behave poorly at higher pressure. Mooney et al. (2017) conclude that the foam-conditioned soil exhibits foam-controlled behavior when the chamber pressure is below a

Testing devices

A foam-soil capture device was developed and an optical microscope with high resolution (1 µm/pixel) was used to investigate the grain-bubble interaction and bubble size distribution of foam-conditioned soil under pressure. The device, shown in Fig. 1, is an extension of the foam capture device developed to examine foam-only behavior (Wu et al., 2018). The foam-soil capture device is made of clear acrylic to visualize the foam-conditioned soil sample. The sample container captures a 5 cm

Test results

Fig. 8a shows an image of foam-conditioned sand at atmospheric pressure (p = 0 bar gauge) with conditioning parameters of FIR0 = 50% and FER0 = 10. The void ratio of this conditioned sand sample is 0.80. As shown, the foam bubbles appear in the pores between soil grains and serve to expand the grain structure of the sand. Some large bubbles exhibit non-spherical shapes because of surrounding soil grains. According to the scale in Fig. 8a, most of the bubble sizes are around 0.10–0.20 mm, though

Stability of engineering properties

The study described above investigates the stability of foam-conditioned soil at the microscale. This section discusses the stability of the engineering properties of foam-conditioned soil under pressure. The properties including compressibility, vane shear strength, and pore (liquid) pressure of conditioned soil were measured through a series of PTC tests. Fig. 13 shows the PTC test results for foam-conditioned soil samples at different pressure conditions.

Fig. 13a shows the test results of

Conclusions

The stability of foam-conditioned soil was investigated through a series of tests at both the microscale and macroscale. At the microscale, time elapsed bubble stability of foam-conditioned soil under pressure was evaluated using a foam-soil capture device and an optical microscope. At macroscale, PTC tests were conducted to assess the stability of the engineering properties of foam-conditioned soil under pressure.

The bubble-grain interaction image analysis shows that foam is more stable when

CRediT authorship contribution statement

Yuanli Wu: Conceptualization, Methodology, Investigation, Data curation, Writing - original draft, Writing - review & editing. Ali Nazem: Investigation, Validation, Resources. Fanyan Meng: Investigation, Validation. Michael A. Mooney: Supervision, Project administration, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Financial support for this research was provided in part by BASF. We are grateful to BASF for the support and help to make this research possible.

References (27)

  • Arya, L.M., Leij, F.J., Shouse, P.J., Van Genuchten, M., 1999, Relationship between the Hydraulic Conductivity Function...
  • L.M. Arya et al.

    A physicoempirical model to predict the soil moisture characteristic from particle-size distribution and bulk density data 1

    Soil Sci. Soc. Am. J.

    (1981)
  • A. Bezuijen et al.

    Additive testing for earth pressure balance shields

    12th Eur. conf. on Soil Mech. and Geotech. Engng: Amsterdam

    (1999)
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