During mechanical tunnel driving it is known that the excavated soils can stick to the metallic part of the tunnel boring machine (TBM). Despite the fact that several test methodologies have been developed during the last 15 years to provide an assessment or quantification of the clogging risk, no standardized method is available yet. This study evaluated three natural clay samples with different grain size distribution and mineralogical content considering two widespread test set ups for the evaluation of the clogging tendency: the Hobart mixing and the plate pull-out test. After a comprehensive geotechnical characterization of the three clay samples, adherence and pull-out force have been measured at different water content/consistency index. Results show that adherence and pull-out force in terms of water content \(w\) and Consistency index \({I}_{C}\) follow normal distribution curves which, as the percentage of clay, the content of clay minerals and the plasticity increases, have higher amplitudes and higher peak values. in general, the existence of a correlation between geotechnical characteristic parameters and the behavior in terms of clogging potential risk for natural clays makes it possible to predict these risks in the design phase of the tunnel excavation and to plan appropriate mitigation actions.
Research Article
Geotechnical characterization of natural clays for the prediction of clogging risk for TBM
https://doi.org/10.21203/rs.3.rs-1645803/v1
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Clays
TBM
shear strength
clogging
mineralogy
- No standardized method is available yet for clogging measurement
- Three natural clay samples were tested considering two widespread test set ups for the evaluation of the clogging tendency
- The overall results of the mixing tests on the three soils seems more similar both for the recorded peak adherence values and for the position of the peak in terms of Ic.
Earth Pressure Balance (EPB) Tunnel Boring Machines (TBM) are often used to excavate tunnels in soft grounds, ranging from sands and gravels to clays. As the name suggests, adopting the EPB technology the excavated material is used to balance the pressure at the tunnel face in the excavation chamber (Spagnoli 2011). Mechanical tunneling in cohesive soil is often affected by clogging phenomena of the excavated material at the cutting wheel (Fig. 1), in the excavation chamber or in the conveyor belt of TBM, which leads to significant additional expenses and delays in the construction process (Feinendegen and Ziegler 2016), which in turn can create problems between the contracting authorities and executing companies. According to Thewes and Burger (2005), the profitability of a tunneling project can be strongly influenced by the clogging potential of the materials involved. In the most serious cases, clogging can also cause damages to internal components and the blocking of the TBM. The type and the extent of the problem depend on the geological and geotechnical conditions (mineralogy, grain size distribution, plasticity, water inflow, etc.), the machine technical specifics (cutting wheel design, pump capacity, etc.), as well as the construction operation (drive mode, downtime, soil conditioning process, etc.).
Hence, assessing the clogging potential of the excavated material is of importance. Although several laboratory-based approaches were developed to estimate the clogging potential in TBM tunnelling (e.g. Geodata SPA 1995; Sass and Burbaum 2008; Feinendegen et al. 2011; Thewes and Hollmann 2016; Basmenj et al. 2017; Spagnoli et al. 2019, Sebastiani et al. 2019a; Pirone et. al. 2020), and numerous studies investigated the adhesion phenomena (e.g. Schlick 1989; Atkinson et al. 2003; Zumsteg and Puzrin 2012), the prognosis of the predicted impacts on the construction process still remains troublesome.
Among the different test set-ups, the Hobart mixing (Zumsteg and Puzrin 2012) and the (plate) pull-out test (Thewes and Burger 2005; Khabbazi et al. 2019) are currently the most widespread, although the cone pull-out test developed at the RWTH Aachen University (see Feinendegen and Spagnoli 2021) is widely used in the German tunnelling environment.
A precise and reliable prediction of the clogging behavior is, in fact, not yet possible. Furthermore, the methods and laboratory tests currently proposed in the technical literature do not provide a consistent set of information on how to predict the occurrence of clogging and how this phenomenon is connected to some characteristic parameters of the soil.
This paper presents the results of a research activity developed in the geotechnical laboratory of “Sapienza”, University of Rome. The main goal of the study is to provide a deeper understanding of the information regarding the potential clogging risk that can be deduced from an adequate geotechnical characterization and from the execution of specifically developed tests, the Hobart mixing test and pull-out test, on fine grained soils. Detailed geotechnical characterization of the to-be excavated soil alone is either not sufficient or not properly related to the clogging risk during tunnel driving; in this regard, the systematic comparison of the results provided by the two previously mentioned tests is extremely useful.
After a description of the main characteristics of the three natural fine-grained soils selected for this study, the results of an extensive experimental campaign will be presented. Results of fall cone, oedometer tests, direct and ring shear tests and mixing and pull-out tests, performed at different water contents, will be illustrated and discussed.
The results highlight the differences in clogging potential for different soil samples and, for each soil, for different values of water content w and consistency index \({I}_{C}\). Moreover, the obtained data will be helpful to define some correlations between clogging phenomena and some mechanical properties of soils. Finally, some relevant differences in the outcomes obtained from the mixing test and pull-out test will be highlighted proving that part of the inconsistencies between the data produced in literature are due to the lack of single, reliable and shared methodology and laboratory test for measuring clogging risk.
Geotechnical characterization
The experimental study was divided into two different parts: 1) geotechnical characterization of the fine-grained soil samples, which included grain size distribution analysis, Atterberg limits, mineralogical contents, fall-cone tests, oedometer test, direct and ring shear tests; 2) evaluations on clogging potential risk, which included the execution of two of the main tests, the Hobart mixing and the pull-out tests, suggested in literature for the measurement of potential clogging of clays.
The grain size distribution measure was performed following the ASTM D7928 (2021), the mineralogical content according with UNI EN 13925-2 (2006), the fall cone test according with ASTM D4318 (2017), the oedometer tests according with ASTM D2435 / D2435M-11 (2020), the direct shear tests with ASTM D3080 / D3080M (2011) and the ring shear tests with ASTM D6467 (2021) / BS 1377.
Below a brief description of the mixing and pull-out tests which, even if still not included in international standards, were proposed in literature and are currently performed (Sebastiani et al. 2019c) for clogging potential evaluations.
The mixing test
The Hobart mortar mixer is often used to empirically quantify the potential clogging of soft clayey soil mixtures by measuring the soil sticking to the mixing tool after a fixed time of mixing.
The adherence is measured through the stickiness ratio \(\lambda\), defined as:
$$\lambda =\frac{{G}_{MT}}{{G}_{TOT}}$$
1
where \({G}_{MT}\) is the weight of soil sticking to the mixing tool and \({G}_{TOT}\) is the total weight of soil involved in the mixing process, usually about 1000 g.
The stickiness ratio quantifies the tendency of the soil to remain stuck on a mixing tool after a mixing process in a mortar mixer.
The pull-out test
The pull-out test represents a family of widespread systems for measuring the adhesion between a metallic element and the soil. Different authors, as Thewes and Burger (2005), Sass and Burbaum (2008) and Khabbazi et al. (2019), with different approaches, used this test to measure the clogging tendency of a soil and, by comparison, the beneficial effect obtained by injecting chemicals into the soil.
The test is performed by placing a metal tool in contact with the soil sample and then measuring the force necessary to separate the tool from the soil, extracting it vertically.
The modified version of the pull-out test performed at the Sapienza University geotechnical laboratory (Fig. 2) was set up applying the same principle using a convex surface plate instead of the conical or flat tool. The main reason of this configuration is to improve the homogeneity of the contact between the steel tool and the soil: the plate, in fact, is pushed directly on the other half of the spherical joint leaving a thin layer of soil in between and thus eliminating possible unevenness or air bubbles, which can be created pre-drilling the cavity for the cone insertion. The execution modality of this modified configuration consists in the progressive approach of the tool to the soil sample, until they adhere completely, and the subsequent extraction of the tool at a constant speed of 5 mm/min (which is comparable to extraction rate of the cone pull-out test being 5.83 mm/min, see Spagnoli et al. 2019), measuring the force at regular intervals. The maximum measured extraction force is the pull-out force, PF.
As mentioned above, the first part of this study was the characterization of the three fine-grained soils chosen for this experimental analysis: London (which is well-known to be challenging for TBM excavation, e.g. Spagnoli et al. 2012), Malnome and Viterbo clays.
The grain size distribution of London clay, Malnome and Viterbo, shown in Fig. 3, was obtained from the granulometric analysis. As it can be seen, London clay is made up of 48% silt, 27% sand, 18% clay and the remaining 7% gravel. The uniformity coefficient Cu is equal to 33 and classifies this soil as well graded. From Malnome grain size distribution it can be seen that this fine-grained soil is made up of 49% clay, 47% silt and the remaining 4% sand. Viterbo, instead, is made up of 69% silt, 25% clay and the remaining 6% sand.
In the following Table 1 are listed the Atterberg limits (liquid limit \({w}_{l}\) and plastic limit \({w}_{p}\)), the plasticity index (\({I}_{p}\)), the Activity (A) and the specific gravity of soil (\({G}_{s}\)) of the three soil samples mentioned. The results show similar \({I}_{p}\) and Activity values for London and Malnome samples while a very lower \({I}_{p}\) with the same Activity for Viterbo samples.
The results, plotted in the Casagrande plasticity chart shown in Fig. 4, highlight the differences of the three samples: Viterbo samples could be classified as low plasticity clay, on the opposite Malnome samples is classified as high plasticity clay and London sample has an intermediate position. It is worth mentioning that the three clay samples fall between the A-line and the U-line. The latter represents the upper limit for natural soils (Carter and Bentley 2016).
Table 2shows the results of the analysis of the mineralogical composition of the three soils obtained through the X-ray diffractometer (XRD) following the Rietveld method. Particularly, and coherent with its grain size distribution, the London sample has an higher quartz content, mainly due to the sand particles, high amount of illite/smectite and kaolinite and low amount of calcite; Malnome, as Viterbo, has low amount of quartz (14%-16%) and high amount of calcite (22%-23%) and while Malnome has 15% of illite/smectite Viterbo has 4%.
|
London |
Malnome |
Viterbo |
|
|---|---|---|---|
|
Quartz |
26% |
14% |
16% |
|
Illite/smectite |
15% |
15% |
4% |
|
Kaolinite |
12% |
7% |
6% |
|
Mica/illite |
10% |
13% |
16% |
|
Calcite |
8% |
23% |
22% |
|
K-feldspar |
8% |
4% |
4% |
|
Chlorite |
8% |
10% |
12% |
|
Plagioclase |
6% |
8% |
10% |
|
Others |
7% |
6% |
10% |
The samples used to perform direct shear tests and oedometer tests were prepared according with the reconstituted soil preparation methods of slurry consolidation, reaching 98 kPa of vertical stress. In Table 3 the results of the direct shear tests and ring shear tests are presented, while in Fig. 5 the compressibility curves, as a result of the oedometer tests performed, are shown. The three soils seem to be quite similar in terms of critical state shear behaviour considering the difference in consistency index \({I}_{c}\) (higher for Viterbo and lower for London) and the difference in grain size distribution (30% for London and 0% for Malnome and Viterbo). Values of residual friction angle, \({\varphi }_{res.}\), are somehow comparable with those reported by Kenney (1967) although values of the Viterbo clays are slightly lower if the mineralogy is considered. On the opposite, very relevant differences were recorded between London/Malnome soils and Viterbo soil in terms of compressibility (oedometer test) due to the differences in terms of plasticity.
Figure 6 shows the correlation between liquidity index \({I}_{L}\), which is the inverse of the consistency index (and it used for scaling the natural water content of a soil sample to its limits) and undrained shear strength values \({c}_{u}\), measured by the fall cone test. Data are plotted following the range of results presented by Mitchell (1976). Apart of four points falling outside the range (likely due to the very low \({c}_{u}\) values not well detectable by the apparatus), all the data follow the range suggested by Mitchell (1976). The results show that high \({c}_{u}\) values are obtained for low \({I}_{L}\) values (hence high \({I}_{C}\)values).
Considering the Hobart mixing test, the results of experiments performed on London, Malnome and Viterbo clays samples at different water content \(w\) (a) and consistency index \({I}_{c}\) (b) are reported in Fig. 7, along the clogging potential fields proposed by Thewes (1999).
The theoretical normal distributions of the three soils, derived from the punctual results measured at different water contents, are necessary to understand how the clogging risk is affected by water contents. For the London clay it can be observed that the maximum adherence is measured for values of water content approximately between 42% and 52% and the mean of the distribution is equal to 47.5%. For Malnome clay the high clogging risk area is achieved by a larger range of water content: from 35–58% and the mean of the distribution is equal to 47.0%. These results show a natural higher clogging tendency if compared to London and Viterbo clays. Furthermore, between all the three soils, even if the peak values reach similar values, Malnome achieves the highest values in terms of adherence. The results obtained for Viterbo clay, instead, show that the maximum adherence is measured for values of water content between 30% and 37%, with the mean of the distribution equal to 33%, much lower than the other two soils examined.
However, if the water content is scaled to the limits and the adherence is plotted against \({I}_{C}\), it is possible to observe that the peak is between 0.3 and 0.7, which is comparable with data provided, among the others, by Spagnoli (2011) and Kang et al. (2020). London clay shows the highest adherence for \({I}_{C}\) values of about 0.3, which is comparable with a set of London clay samples tested by Spagnoli et al. (2019) whereby the highest peak in terms of g/m2 was also about 0.3.
The results obtained through the plate pull-out test performed on London, Malnome and Viterbo clays samples at different water content \(w\) are shown in Fig. 8.
As expected, these results provide information about the soils adherence substantially in line with those of the mixing test in terms of the range of water contents in which the pull-out forces are higher: wider for Malnome clay and smaller for London clay and Viterbo. Tests scaled against \({I}_{C}\) also provide similar results compared with the Hobart mixing method, however, the pull-out test amplifies the differences between the three soils: for London clay the maximum pull-out force is approximately equal to 50 N, for Malnome 250 N and for Viterbo 170 N. The presence in London clay of about 30% of sand leads to much lower pull-out force values than the other two soils, in which the fine graine parts play a key role.
An experimental research activity was performed including standard geotechnical characterization tests,and clogging potential risk tests on three natural fine-grained soil samples coming from real sites.
Geotechnical characterization of soils can provide useful information about several parameters, which seems to be well correlated to clogging phenomenon. More in details, in accordance with what has already been highlighted in the literature higher amount of clay in the soil lead to higher clogging risk and even a small amount of sand is able to drastically reduce this phenomenon.
Regarding the two clogging tests performed, Hobart mixing test and pull-out test: the data presented in this study allow to highlight a normal distribution trend between the potential clogging and the water content \(w\) and consistency index \({I}_{C}\). From a qualitative point of view, the tests provide consistent results with the clay with higher plasticity related to high peak adherence and pull-out force and on the opposite the low plasticity clay related with lower value recorded for the two parameters. In general, in terms of consistency index, peak values are aligned in a range between 0.3 and 0.7, which seem to be in line with other values provided by other authors. This is probably the reason for the diffusion of both tests as a potential clogging risk assessment tool.
Going into greater detail in quantitative assessments, the pull-out test proved to be more reliable in amplifying the differences between the three soils investigated. It provided lower clogging potential peak values for the London clay sample, likely due to its sandy content an higher quartz content and a lower amount of clayey minerals and coherently lower compressibility; on the other side, it provided higher clogging potential peak and higher corresponding \({I}_{c}\) values for Malnome clay, the soil sample with higher clayey mineralogical content, higher plasticity and no sandy fraction while the results on Viterbo clay are recorded in an intermediate position. The overall results of the mixing tests on the three soils, on the opposite, seems more similar both for the recorded peak adherence values and for the position of the peak in terms of \({I}_{c}\).
As a final comment, the authors suggest more interexchange regarding different methods for the evaluation of the clogging potential risk, in order to reach a more standardized (or recognized) methodology to assess clogging behavior during TBM excavation.
Conflict of interests
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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