Experimental study of the effect of conditioning on abrasive wear and torque requirement of full face tunneling machines
Introduction
Soil conditioning is one of the main factors in success of soft ground tunneling using various pressurized full face machines, especially the earth pressure balance (EPB) TBMs. Proper soil conditioning changes the characteristics of the ground in order to make it suitable for the tunneling process. Soil conditioning is usually applied at different points throughout the tunneling process such as at the face of the tunnel, within the cutting chamber, and inside the screw conveyor. Milligan (2000) summarized the advantages of using soil conditioning as follows:
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Increasing the stability of the tunnel face.
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Improving the flowability of material.
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Reducing the friction and therefore reducing the driving torque.
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Changing the excavated material into a uniform plastic soil which leads to:
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Better control of pressure inside the cutting chamber.
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Better control of groundwater inflow.
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Better control of flow of soil in the screw conveyor.
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Reducing the clogging in the chamber.
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Better handling of excavated soil.
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Improving the safety of the personnel, especially during the maintenance of the cutters/cutterhead.
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Maintaining the above conditions during tunneling operations and maintenance stops, and
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Finally, reduction of wear and tear of the cutters, cutterhead, and other parts of the machine that comes in contact with the muck.
Soil conditioning is often done by injecting foam, polymer, water, and clay (mainly bentonite) into the tunnel face, cutting chamber and screw conveyor. Selection of the type of soil conditioners mainly depends on soil type, geological condition (groundwater and soil permeability), and properties of the tunnel boring machine (injection points, open or closed cutterhead, type of foam generator, etc.). The most important soil conditioners are foam and polymer. However, in some cases due to existing conditions, some other additives like anti-clogging or anti-wear agents are also added to soil conditioners.
In order to study the subject of soil conditioning in soft ground tunneling, several concepts and parameters should be defined as follows:
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Foam: a product generated by the combination of a foaming solution and air
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Foaming solution: basically a mixture made from water and a surfactant, or a so called foaming agent.
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Foam Expansion Ratio (FER): the ratio between the volume of foam at working pressure and the volume of the solution. EFNARC (2005) has specified the range of FER between 5 and 30.
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Foam Injection Ratio (FIR): the ratio between the injected volume of foam at working pressure and the banked volume of ground. EFNARC (2005) has specified the range of FIR between 10% and 80%.
A detailed discussion of the history of soil conditioning and calculation formulas for the foam injection ratios and foam expansion ratios can be found in Williamson et al. (1999). As it was mentioned earlier, foam and conditioned soil should have specific properties during the tunneling operation. In order to understand and control these properties several laboratory tests have been developed that are briefly introduced in the next section.
Meanwhile the use of soil conditioners, particularly foams, for reduction of the wear on the cutterhead of the hard rock tunneling machines have also been considered in several projects, although it is not currently a standard practice. As will be discussed in this paper, in hard rock tunneling, often water is used for dust suppression at very low rate which creates a moist muck, which is more abrasive than dry cuttings (Alavi Gharahbagh et al., 2012). Similarly, use of foam in certain applications can reduce the amount of torque needed to rotate the cutterhead, especially in carbonaceous rocks where a certain amount of moisture can cause compaction of the muck and thus leads to significant increase in torque. The current study evaluates the merits of using foam for reduction of wear and torque on hard rock TBMs in a set of tests conducted on the rock cuttings from two ongoing projects in the US.
In order to characterize the foam used for tunneling purposes, simple laboratory tests have been developed (Quebaud et al., 1998):
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Generation test: to study the relationship between pressure generation and fluid flow in the generator and foam flow rate.
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Consistency test: to quantify the foam quality (bubbles size).
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Half-time test: to measure the necessary time for foam to lose half of the solution originally used for its generation.
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Compressibility test: to understand the foam behavior in a confined environment and under changing pressure.
It must be noted that the above mentioned tests are performed to characterize the foam and foam is tested alone. However, in tunneling operations using EPB machines, foam is mixed with soil, thus there are specific tests that needs to be performed to characterize the soil that is conditioned by foam.
To evaluate conditioned soil there is no universally accepted test. Meanwhile there are some methods used for quantification of conditioned soil that have been adapted from concrete or geotechnical tests. Some of these tests are as follows:
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Foam penetration test: the purpose of this test is evaluation of foam penetration into soil (tunnel face) by pushing pressurized foam into the soil. If foam penetration is high, then foam consumption is increased and the produced pressure may be insufficient. On the other hand, if foam penetration is low, control of groundwater is difficult during the operation.
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Mixing test: in this test, soil and foam are mixed together and the variation of the electric motor power, necessary time to obtain a homogeneous mixture and the quality and behavior of the conditioned soil are evaluated.
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Slump test: in this test (ASTM C143, 2012), soil with a certain amount of water and foam are poured into a concrete mixer and after mixing, poured into a mold. The mold is carefully lifted vertically upwards in such a way that it does not disturb the conditioned soil cone. The amount of subsidence of the top surface of the mixture is measured soon after removing the cone and called the slump value. The overall behavior of conditioned soil is evaluated and classified based on reference shapes (Fig. 1).
The slump test has been widely used to evaluate the behavior of conditioned soil (Peron and Marcheselli, 1994, Quebaud et al., 1998, Jancsecz et al., 1999, Williamson et al., 1999, Langmaack, 2000, Vinai et al., 2008). This test is simple, fast, and low cost. This test provides an overall indication of the rheological behavior of the conditioned soil. The suggested value of slump is in the range of 100–250 mm.
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Permeability test: to evaluate the permeability of the conditioned soil, some methods like constant head test (for coarse-grained soils) or hydraulic compression cell (for fine-grained soils) can be used. In general, conditioned soil is less permeable than ordinary soil.
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Compressibility test: to assess the compressibility of conditioned soil. This test can be done using an apparatus similar to that used for a permeability test and the effects of pressure variation on compressibility of conditioned soil can be measured.
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Adhesion test: to evaluate adhesion between the conditioned soil and a metallic surface. Several measurement methods have been developed (Quebaud et al., 1998, Jancsecz et al., 1999, Milligan, 2000, Thewes and Burger, 2005, Feinendegen et al., 2011, Zumsteg and Puzrin, 2012).
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Cone penetration test: in this test, the effect of a foam-solution type on clay soils is determined. For this purpose, a metallic cone falls down into the conditioned soil sample from a specific height and the penetration depth is measured.
These tests offer some peripheral indications on the soil behavior in the cutting chamber and screw conveyor of the machine but do not have any direct relation to soil abrasion and rheological characteristics of the soil as it pertains to machine operation. Therefore, a systematic study of behavior of conditioned soil relative to wear of various machine components as well as required torque to operate the machine in a given setting deemed to be warranted. The experimental study summarized in this paper was conducted to address this very issue.
Section snippets
Study of the effect of conditioned soil on reducing the wear
As discussed in the previous section, a variety of tests have been developed to look into different properties of the foam and conditioned soil for application in EPB tunneling. However, only a few research studies have been focused on the effect of conditioned soil in reducing the wear of cutters or abrasivity of soil. One of the major advantages of soil conditioners in EPB tunneling is to reduce the wear of the cutters and other components of the machine. The main theory behind the wear
Penn State Soil Abrasion testing system
The Penn State Soil Abrasion study included design and fabrication of a new testing device for soil abrasion that directly measures the wear of metal covers mounted on a three-blade propeller that is submerged in a chamber filled with soil. For each test, the covers are weighed before and after the test; the total weight loss of the covers is the tool wear and can represent the soil abrasion. The device and its various components are shown in Fig. 4.
The device comprises a drill press with 5 hp
Study of the effect of soil conditioners on wear
To examine the effect of soil conditioners on abrasion and torque, a series of 16 tests were performed on three different materials, using selected soil-conditioning agents. The soil types used for this study included Silica sand samples, a crushed rock sample from a tunneling project in the Washington DC area, and a crushed rock sample from an ongoing tunneling operation in Indianapolis. The rock samples were crushed to less than 4.75 mm in size for testing. Testing of the crushed rock samples
Conclusion
Testing of various soil types by the soil abrasion testing device at Penn State University shows that the application of proper soil conditioning can reduce the abrasion, and hence the wear of the tools and inner parts of the tunneling machine as well as significant reduction of the required torque. While the results are much anticipated and confirms the observations in the field, it explains the main reasons for introduction and application of soil conditioners in soft ground tunneling,
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