Using the Slump Test to Assess the Behavior of Conditioned Soil for EPB Tunneling

Using the Slump Test to Assess the Behavior of Conditioned Soil for EPB Tunneling DANIELE PEILA CLAUDIO OGGERI LUCA BORIO Department of Land, Environment and Geoengineering, Tunnelling and Underground Space Center, Politecnico di Torino, Corso Duca degli, Abrutzi, 24, 10129, Torino, Italy Key Terms: Tunnels, Soil Mechanics, Laboratory Testing, Tunnel Boring Machine, Soil Conditioning ABSTRACT In order to extend the application field of Earth Pressure Balance (EPB) tunnel machines to various soil conditions, the soil to be excavated has to be treated with additives in order to modify its mechanical properties, changing it into a plastic paste. Sometimes the grain size distribution is also changed with the use of fine-sized materials. The performance of the conditioned soil should be evaluated with tests that are able to describe its mass behavior, but very little research has been carried out in this field. Often the choice of the conditioning agent set and its control during the excavation is made on a trial-and-error basis during the excavation process. The slump cone test performed on conditioned material is a fast and lowcost way of checking this behavior both in the laboratory and on the job site. The results of a test program on different conditioned non-cohesive soils using the slump cone test are presented and discussed. The influence of the water content and the amount of conditioning foam has been studied, and the feasibility of this type of test for the control of EPB conditioned soil has been assessed. INTRODUCTION Full-face Earth Pressure Balance shield (EPBS) machines have been widely applied in urban environments, and at present they can be considered the most commonly used mechanized equipment for soil tunneling. Excavation using an EPBS is made by rotating a cutterhead fitted with picks or disk cutters or a combination of both (Figure 1). The soil excavated at the face enters into a bulk chamber (also named a plenum ) directly behind the cutterhead, where the soil is mixed with conditioning agents to give it a plastic, pulpy consistency. The pressure of the soil in the bulk chamber, which is maintained by a combination of the moving thrust and the volume rate removal of the material, provides a stabilizing action at the tunnel face to counteract the underground water and soil pressure (Maidl et al., 1995; Anagnostou and Kovari, 1996; and Guglielmetti et al., 2007). The conditioned soil is then removed, in a controlled manner, from the bulk chamber with a screw conveyor. The conditioned soil inside the screw conveyor forms an impervious plug, which ensures that there is no loss of pressure in the bulk chamber and that no water enters (Yoshikawa, 1996). At the end of the screw drive, the spoil is discharged onto a conveyor belt that transports it, usually using trains, outside the tunnel. Conditioning is done by modifying the soil into a plastic, pulpy, impermeable paste that is able to correctly control tunnel face ground movements; to apply a stabilizing pressure to the face; to control the water flow; to reduce wear of the tools, the cutterhead, and the screw conveyor; and to permit easy handling of the muck during transport. Determining the optimum amount of conditioning agents that will control the properties of the pulpy paste and the conditioned soil behavior during excavation is therefore a key problem in EPB technology. Very few laboratory tests on conditioning agents have been carried out, and conditioning design is often based on the workers experience or is determined according to a trial-and-error procedure performed directly on the job sites in the first stretch of the tunnel. However, with reference to water content and amount of conditioning agent, it is much more suitable that the properties of the conditioned soil should instead be assessed using laboratory tests to give some reference data to the job site operations in order to shorten and simplify the usual trial-and-error procedure done on site at the beginning of the excavation. These tests must be able to provide an easy comparison of the various Environmental & Engineering Geoscience, Vol. XV, No. 3, August 2009, pp. 167 174 167

Peila, Oggeri, and Borio Figure 1. Schematic drawing of the main elements that constitute an EPB shield machine. The photographs show the cutting wheel of an EPB machine (a) (Herrenknecht News, March 2001); the shield (b) (machine that excavated the Line 1 of Turin Metro courtesy Gruppo Torinese Trasporti); and an example of a segment lining (c). The white arrows indicate the path of the excavated soil. additives available on the market, the definition of the correct amount of conditioning agents, and an easy control of the conditioning quality during excavation. The characterization of conditioned soil is usually obtained using tests derived from geotechnical or concrete measurement technologies; these tests include the mixing test, the cone penetration test, the permeability test, the compressibility test, the shear test, and the slump test. Some large-scale tests using a laboratory screw conveyor device have recently been proposed and they have proved feasible since they allow many parameters directly linked to the EPB excavation process to be measured (Mair et al., 2003; Merritt and Mair, 2006; Peila et al., 2007; and Vinai et al., 2007). At present, this type of test appears to be the best tool for conditioning design but it requires a large volume of soil to be handled and it is not suitable for carrying out a systematic comparison of various conditioning sets of various types of products. It is therefore necessary for researchers and job site engineers to have a simpler test procedure that can be 168 used to conduct both round-robin tests and more complex tests. The round-robin test allows a preliminary choice of the conditioning set on which to conduct more complex tests and allows one to keep the conditioning quality under control on the job site during tunneling. The various possibilities for tests include the slump cone test, which is usually performed on fresh concrete. However, the slump cone test has been used by many in the tunneling industry and this test provides a simple, quick procedure for quality control (Peron and Marcheselli, 1994; Quebaud, 1996; Jancsecz et al., 1999; Williamson et al., 1999; Leinala et al., 2000; Pen a, 2003; Hanamura et al., 2007; and Vinai et al., 2007). SLUMP CONE TEST FOR THE CHARACTERIZATION OF CONDITIONED SOIL Slump cone tests have been used by several researchers to provide a measure of the plasticity Environmental & Engineering Geoscience, Vol. XV, No. 3, August 2009, pp. 167 174

Soil Conditioning for EPB Tunneling and mass behavior of a conditioned soil. The test involves mixing the soil with the foreseen amount of foam and water in a concrete mixer and then pounding it inside two slump cones. After 1 minute, without stroking or mixing the soil with the tamping rod, the cone is lifted up. The fall value and the global behavior of the mix are observed. Peron and Marcheselli (1994) described the application of an EPBS to excavate a twin tube tunnel in Milan (Italy) in an alluvium containing 75 percent sand, 20 percent gravel and cobbles, and less than 5 percent clay. They reported that, for a correct excavation procedure, it was necessary to maintain the slump between 50 mm and 100 mm. When tunneling above the water table, a FIR (ratio between the volume of foam and the volume of conditioned soil) ranging from 50 percent to 80 percent and 5 percent water were added to the bulk chamber, while below the water table it was necessary to inject foam with a FIR of 50 percent. Quebaud (1996) and Quebaud et al. (1998) carried out tests on a homogeneous fine sand with a grain size distribution curve ranging from 0.2 mm to 0.4 mm and on a sand with a grain size distribution curve ranging from 0.01 mm to 4 mm. Quebaud et al. stated that the optimal slump value was 120 mm and that, in order to obtain this result, when the water content varied between 6 percent and 23 percent, it was necessary to use a FIR ranging between 5 percent and 35 percent for both soil types. Peña (2003) compared the effects of different foaming agents to condition a reference sand with a grain size distribution curve ranging from 0.002 mm to 2 mm. He observed that with a water content of 22 percent, a concentration of foaming agent ranging from 1.5 percent to 2.5 percent, and a FIR of 65 percent, the slump was 100 150 mm (which he considered the correct range for EBPS application), while with a FIR of 80 percent, the slump rose to 150 200 mm. Similar results were obtained by Leinala et al. (2000), who carried out tests on the different types of soil encountered during excavation of the Toronto Sheppard Subway Project. For the studied silty sand with an initial water content from 8 percent to 11 percent, it was necessary to use a FIR of 50 percent to obtain a slump of 100 mm. Vinai et al. (2007) carried out tests on a homogeneous sand to obtain a workable mix, and they found that there is a close correlation between the water content and the FIR. All the presented research shows that the slump cone test can offer a good indication of the workability of the conditioned soils and that there is a close correlation between the water content and the necessary FIR. For this reason, in this research, the Figure 2. Photo (a) and schematic drawing (b) with the principal dimension (mm) of the slump test cone used in the study. slump cone test was systematically applied to different cohesionless soils, varying the amount of conditioning foam and the water content, to investigate the applicability and feasibility of slump tests to characterize conditioned soil and to study the influence of the soil granulometry, moisture, and foam content on the behavior of the conditioned soil. The foam was produced using a foam generator that is able to control precisely the quantities of air, water, and foaming agent in a generation cylinder full of glass fragments; a commercial foaming agent was used with a surfactant concentration of 2.5 percent. The foam was produced with a FER (ratio of the obtained volume of foam and the volume of the generator fluid: water + foaming agent) of 16, which is an average value that is usually used in tunnelling, and it had a half-time life of 390 seconds with an average bubble size of 0.50 mm. Since a great variability of FIR can be observed in real tunnel excavation (ranging from 10 percent to 80 percent), the tests were carried out using a FIR range of 10 60 percent. The slump cone test was performed following the Standard Test Method for Slump, as suggested by ASTM 143C (ASTM, 2003), and the test was carried out as follows: the soil was mixed with the desired amount of foam and water in a concrete mixer and it was then poured inside two slump cones (see Figure 2). After 1 minute the cone was lifted, without stroking or mixing the soil with a tamping rod. The fall value and the mass behavior of the mix were then observed. The shape and eventual rupture of the soil cone and the drainage of water and foam were observed and taken into account in order to define the behavior of the material, and the following features were identified: N failure to form a plastic paste, as defined by irregular collapse of the cone (not suitable), due to insufficient water or foam content or both (dry mix) or too much foam but not enough water. A loss of foam was observed, the grain size distribu- Environmental & Engineering Geoscience, Vol. XV, No. 3, August 2009, pp. 167 174 169

Peila, Oggeri, and Borio RESULTS The tests were carried out on soils that were artificially prepared in the laboratory by mixing different percentages of silt, sand, and gravel to obtain the grain size distribution curves shown in Figure 3. The most relevant results for the various soils are here presented and discussed. Results of Tests on Soil 1 Figure 3. Grain size distribution of the conditioned soils: soil 1: a medium-size sand, with D10 5 0.12 mm and D60 5 0.5 mm; soil 2: a mix with the same sand as soil 1 and gravel with a grain size of 4 to 8 mm, with D10 5 0.2 mm and D60 5 5 mm; soil 3: a mix with the same sand as soil 1 and gravel with a grain size of 8 15 mm, with D10 5 0.2 mm and D60 5 9 mm; soil 4: a mix with 47 percent sand, 45 percent gravel (4 to 8 mm), and 8 percent silt, with D10 5 0.2 mm and D60 5 3.5 mm. tion curve was not suitable for the creation of the paste (i.e., not enough sand, silt, or clay in the mix); N a stiff behavior with a reduced slump value but with the creation of a plastic paste (borderline behavior), mainly due to an insufficient foam content; N a too-fluid mix with a relevant loss of water and/or foam (not suitable behavior) due to the presence of too much water and/or foam; N a plastic behavior with a reduced water loss from the soil (borderline behavior); and N a correct behavior of the mix (suitable behavior) that consisted of a slump cone fall of 140 200 mm with a regular shape of the mass and little or no water loss. The results of the tests on soil 1, classified, using the Unified Soil Classification System (ASTM, 2009), as SP, are summarized in Figure 4, and they indicate that the conditioned soil only showed suitable behavior and a slump within the 150 200-mm range, for specific combinations of water and FIR contents, and this FIR range (20 50 percent) is quite close to the values suggested in EFNARC (2005) for sand (20 40 percent). Referring to the water content, it can be observed that with a water content below 3 percent, the conditioned soil is too dry, even though large foam injection ratios were used, while with a water content of more than 18 percent, the conditioned soil was usually too wet, even though low foam injection ratios were used. The suitable mixes are therefore restricted to a limited area that defines, respectively, the upper threshold over which the material is too wet and fluid and a lower threshold under which the material is too dry and does not show the required plastic and pulpy features. The central position of this area indicates the optimum conditioning parameters, in which FIR 5 40 percent and water content 5 10 percent. Figure 4. Tests carried out on soil 1 (clean sand SP) and definition of the suitable area. 170 Environmental & Engineering Geoscience, Vol. XV, No. 3, August 2009, pp. 167 174

Soil Conditioning for EPB Tunneling Figure 5. Tests carried out on soil 2 (gravelly sand SW) and definition of the suitable area. Results of Tests on Soil 2 The results of the tests on soil 2, classified, using the Unified Soil Classification System, as SW, are summarized in Figure 5, and they indicate that, with a water content below 8 percent, the conditioned soil was too dry, even though high values of FIR were used, while, with a water content higher than 12 percent, the conditioned soil was wet, and for high FIR values (.50 percent), the mix was not able to absorb all the added foam, which both flowed away and made the mix too fluid. The suitable mixes for this type of soil are located in the water content versus FIR plane in a smaller area than for soil 1 as a result of the presence of the gravel grains and the reduction in the amount of sand that can be conditioned by the foam bubbles, since the gravel grains do not interact with the foam to create the paste and since they have the negative effect of breaking the paste structure made by the conditioned sand. Results of the Tests on Soil 3 The results of the tests on soil 3, classified, using the Unified Soil Classification System, as GP, are summarized in Figure 6, and they indicate that none Figure 6. Tests carried out on soil 3 (gravel sand mixture GP). It is not possible to define a suitable area. Environmental & Engineering Geoscience, Vol. XV, No. 3, August 2009, pp. 167 174 171

Peila, Oggeri, and Borio Figure 7. Tests carried out on soil 4 (gravel sand silt mixture GM) and definition of the suitable area. of the tested mixes showed suitable behavior and that some of them can be described only as borderline. This is because, with a water content of lower than 8 percent, the conditioned soil is too dry and it is not able to absorb the foam, which flowed out from the mix during the test, while with a water content of about 15 percent, the conditioned soil is too wet with any FIR and there is a great loss of water from the mix. It is not possible to determine an optimum value combination for this type of soil. The overall behavior of this mix indicates that the large gravel grains completely break the paste, preventing the foam from correctly conditioning the sand, and, therefore, the overall behavior appears too rigid or there is a loss of water. Results of the Tests on Soil 4 The test results on soil 4, classified using the Unified Soil Classification System, as GM, are summarized in Figure 7 and they indicate that, with a water content of less than 3 percent, the conditioned soil was too dry, not pulpy enough, and with no FIR value, while, with a water content higher than 12 percent, the conditioned soil is too wet. The suitable mixes are located in a larger area than the mix without silt (soil 2) and with lower FIR values. The overall behavior of this mix is influenced by the presence of the siliceous silt that fills the voids between the sand and gravel grains, thus permitting one to reduce the amount of foam, at the same water content, compared to soils 1 and 2, in order to obtain a suitable mix. If the slump values are plotted versus FIR for the same water content (8 10 percent), the data for the various soils are located on parallel lines. Figure 8 reports slump values versus FIR for the tested soils, the data reported in technical literature, and the value (V1 point) suggested by Obayashi Company and used by Peron and Marcheselli (1994) and by Maidl at al. (1995) (FIR 5 a/2[(60 2 4.0X0.8) + (80 2 3.3Y0.8) + (90 2 2.7Z0.8)], where FIR 5 ratio between the volume of foam and the volume of conditioned soil; X 5 fraction passing a 0.075-mm sieve; Y 5 fraction passing a 0.420-mm sieve; Z 5 fraction passing a 2.0- mm sieve; and a 5 empirical coefficient that depends on the uniformity coefficient of the soil [U]: U, 4a 5 1.6; 4, U, 15a 5 1.2 ; U. 15a 5 1.0) and shows that the proposed evaluations are in good agreement with those reported in technical literature. CONCLUSIONS When tunneling with an EPBS machine in cohesionless ground, soil behavior needs to be optimized in order to use the machine properly and to control tunnel face stability. This change is obtained through injections of special additives, such as foam and polymers, inside the bulk chamber and on the cutterhead. The protocol for determining the correct amount of conditioning agents and the control of the mix quality and behavior during the excavation process are key factors in the design and management of this mechanized tunneling method. Among the various possibilities, the slump cone test appears to be a simple and inexpensive procedure that can be used both for job site control and for preliminary design. Recent 172 Environmental & Engineering Geoscience, Vol. XV, No. 3, August 2009, pp. 167 174

Soil Conditioning for EPB Tunneling Figure 8. Plot of FIR and slump for soils 1, 2, and 4 that met acceptable behavior (indicated by darkened diamond, square, and triangle). Acceptable performance based on previous study (Quebaud, 1996; Williamson et al., 1999; EFNARC, 2005) is also shown. Lines represent a regression for each soil, and the hatching shown on each line represents the range of acceptable FIR values. research has shown that a laboratory screw conveyor device also can be used for the complete description of the conditioned soil behavior and properties with reference to EPBS tunneling (Merritt and Mair, 2006; Peila et al., 2007; and Vinai et al., 2007). The test program carried out on various types of cohesionless soil (from silty sand to sandy gravel) has led to some relevant information on the behavior of conditioned soil and to the design of an assessment procedure. The tests made it possible to highlight the great influence of large-sized soil grains on the use of foam in conditioning cohesionless soils, since it is the sand size fraction of the grain size distribution that interacts with the foam bubbles to create the pulpy paste, which encompasses the larger grains. If there are too many gravel grains and they are too large,theybreaktheconditionedmixanddonot allow a plastic paste to form. Therefore, in some conditions, it is necessary to add a filler to the bulk chamber to plasticize the paste. This was done, for example, during excavation of the Turin Metro (Carrieri et al., 2004; Grandori, 2004). Furthermore, it was shown that it is possible to create a pulpy paste for a defined mix only if the water content and the FIR fall within specific ranges, which are a function of the grain size distribution curve of the soil. The presence of gravel reduces the suitable area, and control of the conditioning is therefore influenced to a great extent by the water content; this is a problem when tunneling below the water table in soils with different permeability and moisture levels. On the other hand, the presence of silt in the soil enlarges the suitable area, even when gravel is present, since the fine granulometric fraction fills the voids between the larger grains, interacts with the water, and thus makes it possible to use less foam (reduced FIR values) to obtain a pulpy consistency. Finally, on the basis of the tests, it can be concluded that the slump test provides a good indicator of the overall behavior of the conditioned soil and, as a result of its simplicity and economy, can be profitably used both in the preliminary design stage and at the job site to keep the conditioning under control during excavation. ACKNOWLEDGMENTS The authors would like to thank MAPEI S.p.A. Company for the technical and financial support for this research. Special thanks are offered to Prof. S. Pelizza for his suggestions and comments. The research has been financed by the Italian Ministry of University and Research (National Research Projects PRIN 2006) within the project: Optimization of the structural, technological and functional performances of tunneling methods and materials in tunnel linings National Coordinator G. Plizzari, Environmental & Engineering Geoscience, Vol. XV, No. 3, August 2009, pp. 167 174 173

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