Experimental study on the stability of foam-conditioned sand under pressure in the EPBM chamber
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)
- et al.
Application ranges of EPB shields in coarse ground based on laboratory research
Tunn. Undergr. Space Technol.
(2015) - et al.
Effect of particles and aggregated structures on the foam stability and aging
C. R. Phys.
(2014) Foams and foam films stabilised by solid particles
Curr. Opin. Colloid Interface Sci.
(2008)- et al.
The role of particles in stabilising foams and emulsions
Adv. Colloid Interface Sci.
(2008) On the equation of the maximum capillary pressure induced by solid particles to stabilize emulsions and foams and on the emulsion stability diagrams
Colloids Surf., A
(2006)- et al.
Bubble size distribution and coarsening of aqueous foams
Chem. Eng. Sci.
(1999) - et al.
Characterizing the influence of stress on foam conditioned sand for EPB tunneling
Tunn. Undergr. Space Technol.
(2018) - et al.
Use of chemical foam for improvements in drilling by earth-pressure balanced shields in granular soils
Tunn. Undergr. Space Technol.
(1998) - et al.
Soil conditioning of sand for EPB applications: a laboratory research
Tunn. Undergr. Space Technol.
(2008) - et al.
An experimental examination of foam stability under pressure for EPB TBM tunneling
Tunn. Undergr. Space Technol.
(2018)
A physicoempirical model to predict the soil moisture characteristic from particle-size distribution and bulk density data 1
Soil Sci. Soc. Am. J.
Additive testing for earth pressure balance shields
12th Eur. conf. on Soil Mech. and Geotech. Engng: Amsterdam
Cited by (30)
Rheological characterization of the conditioned sandy soil under gas-loading pressure for earth pressure balance shield tunnelling
2024, Tunnelling and Underground Space TechnologyApplication of geopolymer in synchronous grouting for reusing of the shield muck in silty clay layer
2024, Construction and Building MaterialsReal-time laser scanning for conditioned coarse-grained soil monitoring on conveyor belt in earth pressure balance shield
2024, Tunnelling and Underground Space TechnologyExperimental study on workability and permeability of sandy soils conditioned with thickened foam
2024, Journal of Rock Mechanics and Geotechnical EngineeringExperimental study on the foam-stabilizing advantages and foam stabilization mechanism of novel microbial polysaccharides
2023, Journal of Molecular LiquidsUndrained vane shear strength of sand-foam mixtures subjected to different shear rates
2023, Journal of Rock Mechanics and Geotechnical Engineering