Complexity of the Ground Conditions and Non Compliance with Basic Assumptions in the Trench Stability Analysis: A Case Study in Iran

Transcription

1 Complexity of the Ground Conditions and Non Compliance with Basic Assumptions in the Trench Stability Analysis: A Case Study in Iran A.Eftekhari1 1 ; M.Taromi 2 and M.Saeidi Abbas_eftekhari2003@yahoo.com 2. majidtarami@ymail.com 3. me_saeidi@yahoo.com Paper Reference Number: Name of the Presenter: Abbas Eftekhari Abstract Basic Insufficient Geological and geotechnical studies will be caused the inaccurate parameters for trench stability analysis. Therefore, as the first step, in the trench design, the special considerations for investigations and primary studies are very important. In the Portal of Sabzkuh Water Conveyance Tunnel design, despite the adequate geotechnical investigations, field studies have not been performed with appropriate accuracy, consequently a landslide and collapse is occurred in a part of the portal because of tunnel excavation. The site geology was predicted by taking soil samples from trench soil with underground and surface surveying inside of tunnel as well as trench site, that the Alternating layers of CL, CH, CL-ML, SC-SM and GP-GM with more than 55 degrees slope as well as a buried slide surface under alluvium was observed. But the primary assumptions used in the design by engineers were different from the results that were noted at the recent lines. In this paper, the importance of having a precise and predetermined schedule for selecting site location, Complexities of the ground conditions and its effects on the stability of the trench has been investigated. Key words: Ground complexity, Collapse, Uncertainty, Mistake planning, Sabzkuh tunnel 1 M.sc of Geology, University of Tarbiat Modares, Tehran, Iran. msc.eftekhari@yahoo.com 2 Civil Engineer, South Tehran Branch, Islamic Azad University, Tehran, Iran. majidtarami@ymail.com

2 1. Introduction The water conveyance tunnel, namely Sabzkuh, with a length of km, an excavation diameter of 4.45m, final diameter of 3.6m and ability of 7 to 14 m3/s water transmission is under construction. Mechanized method (using TBM) has been adopted for construction of this tunnel. The portal location of TBM includes masses of fine alluvial soil. Based on data obtained from three drilled boreholes to depth of the tunnel, it was determined that the ground types alluvium about 300 meters from the exit portion of the tunnel. Because the selected TBM is a double shield universal and it is not adaptable for excavating in such ground formations, we decided to excavate the first 300 meters of tunnel by NATM method. Obviously, the flatter slope at the geometric design of trench leads to more stable conditions with regard to sliding or collapsing, but, in the sabzkuh tunnel project, it was not the optimal and economical solution necessarily due to space limitations and not taking (Demising) the land by owner. Therefore, despite these limitations at the geometric design of the trench, the height of tunnel entrance has been considered over 32 meters with benches in 4 meters width and a height of 8 meters (2H: 1V & 1.5H:1V). The stability of this portal was examined by using SLIDE 5.0 and Flac/Slope software and it was found that the slope is stable in both static (FS: 1.5) and pseudo-static (FS: 1.1) conditions. An appropriate safety factor for the stability of the trench was obtained in short term (during construction) in accordance with the assumption of saturated soils in the static mode, as well as in pseudo static mode based on assumption of The horizontal peak acceleration of g for return periods of 75 years and lifetime of 50 years. After excavation of 35m of tunnel, a sliding surface with slope of 75 degrees observed. The existence of this sliding surface with gravel and Aqueous sand midlayers causes reduction in Liquid Limit (LL), increasing displacement potential and finally sliding soil masses on this surface. After sliding and collapsing in the portal of tunnel we found that an error occurred at the one stage of the geological field investigation, geotechnical data or in the trench design. Thus, regardless of the primary data and assumptions, all steps reviewed again and soil sampling and laboratory tests were performed again to discover the truth. The reason of this collapsing was uncertainties in the input parameters in stability analysis, so what is dealt at this paper is the answer of this question: the assumptions used in designing are matched with observations or not? 2. Sources of Uncertainty in Slope Stability Analysis: Principally the methodology adopted in stability analysis methods is of a deterministic nature. If the single value factor of safety is greater than one, the slope is considered safe. If not, it impending failure and modification of design is required. The calculated factor of safety relies on the judgment of engineer when considering input parameters, failure surface mode, assumption and methods used for analysis. Uncertainties in any of the input parameters are not taken into consideration in deterministic approach. As a result, their computation will not provide single unique value for problem. Source of uncertainty in slope stability analysis is uncertainties in soil properties, environmental condition, and theoretical models which are the most important sources for a lack of confidence in deterministic analysis [1]. Morgenstern (1995) divided a geotechnical uncertainty into three distinctive categories: parameter uncertainty, model uncertainty and human uncertainty. Parameter uncertainty is the uncertainty in the inputs of analysis; model

3 uncertainty is due to the limitation of the theories and models used in performance prediction while human uncertainty is due to human errors and mistakes [11]. A-Parameter uncertainty: The sources of variability in soil parameters are illustrated in Figure 1; the parameter uncertainty is divided into data scatter and symmetric error. Data scatter is the dispersion of measurement around the mean. It is emanated from the spatial variation of soil and random testing error. Spatial variability is the true variation of soil properties from one point to another. It is inherent to the soil and cannot be reduced. It must be considered in any analysis of uncertainty. Random testing errors arise from factors related to the measuring process such as operator error or faulty device. Random errors should be removed from measurement prior to analysis. Systematic error results in the mean of the measured property vary from the mean of the desired property. Statistical error is the uncertainty in the estimated mean due to limited sample size. While measurement bias occurs when measured property is consistently overestimated or underestimated at all locations. B-Model uncertainty The gap between the theory adopted in prediction models and reality is called model uncertainly. Analytical models, particular in engineering are usually characterized by simplifying assumptions and approximations. Model uncertainty is probably the major source of uncertainty in geotechnical engineering [11], [2], [3], comparing model predictions with observed performance or predictions of more rigorous and comprehensive model is probably the most direct and reliable approach to quantify model uncertainty. However the data needed for direct comparison with observed performance are seldom available in practice. C-Human uncertainty Human error is caused uncertainty. This results when incorrect mechanism is modeled or failure occurs due to mistake in correctly determining the chosen model or it may due to carelessness and ignorance, misleading information, poor construction, inappropriate contractual relationships and lack of communication between parties involved in the project. Peck (1973) and Sowers (1991) provided examples of the role of human uncertainty into geotechnical failure [5], [4]. The wide variability s and uniqueness of the human contribution from structure to another create difficulties in identifying potential human errors, not to mention assessing their probabilities [12]. Many studies have been developed for probabilistic stability analysis of the slopes that include many uncertainties in soil properties, in recent years. Detailed review of these studies has been done by Mostyn and Li (1993), Wolf (1996), and Baecher and Christian (2003) [6], [10], [7]. But it should be noted that at these methods has not been noticed to all components of design as well as those propose no reliability level, thus for using the appropriate parameters in designing an good engineering judgment is required and it should be recommended:" at the stability analyses of the slopes engineers should pay special attention to field investigations, groundwater and soil shear strength."

4 Figure 1: Uncertainty in soil properties (Christian; Ladd; Beacher, 1996). 4. Difference Between Geological and Geotechnical Surveying According to the site investigations and two exploratory boreholes (Fig. 2), the geology of portal is included the contemporary alluvial and debris. The boreholes log and the profile used in the trench stability analysis are shown at the Figure 3. Figure 2: View of the portal and boreholes location. Figure 3: Predicted geology profile for the portal and boreholes log. After geological mapping of the inside and outside of the tunnel route and reinterpretation of the primary geotechnical reports and comparison between these results with the obtained results from boreholes logs and primary geological surveying at the first stage, fundamental difference was determined, especially at the diversity and the slope angle of soil layers. Then, in accordance with the USCS the new samples, taken from the trench, were classified (Fig. 4&5). According to new results of soil classification, soil categories increased from three (ML, CL-ML, SC-SM) in initial interpretation (Figure 3), into six ones (CL, CH, ML, CL- ML, SC-SM, GP-GM).

5 Figure 4: Soil classification according to USCS. Figure 5: Plasticity graph according to USCS. After that, old sliding surface was detected at the both sides of portal by surface surveying (SS: / ). due to the existence of muddy soils at the surface,the geometrical characteristics of alluvial layers were immeasurable, Thus in the path of waterway that leading to the portal and also the collapse zone in trench, hidden old alluvial layers under contemporary alluviums was observed (50-55/ (Dip/Dip Direction)) (Fig. 6&7). Figure 6: The field mapping of the geometry of soil layers. Figure 7: The geometry of layers at the cavity. In order to complete the geotechnical investigations, geoelectric studies with CRP method was performed in several profiles within trench; from these Profiles the movement of bed rock is easily detectable as a result of sliding. In figures 8 and 9, respectively the CRP profile and the sliding surface on the trench are shown.

6 Figure 8: The CRP profile in Portal. Figure 9: The trace of slide surface on the trench. At the final stage, the physical and mechanical parameters of soil samples have been reassessed by in-situ re-sampling from the soil of cavity and in-situ field tests and reinterpretation of the new geotechnical tests results by statistical approaches. The preliminary geomechanical properties of soil (Highlighted used at the primary trench stability analysis) accordance with the previous geotechnical reports and performed tests and as well as the results of the new classification based on the USCS standard (ASTM D 2487) with physical and mechanical parameters of the trench soil are shown in table 1. At the Figure 10, the previous modified profiles (Fig. 3) as well as at the Figure 11, the occurred collapse at the trench is plotted [8]. Table 1. The physical and mechanical parameters of the soil of the trench. No. Soil unit γ (gr/cm 3 ) φ (deg.) C (kg/cm 2 ) 1 CL CH ML CL-ML SC-SM GP-GM Slide Surface

7 Figure 10: The modified geology profile. Figure 11: Tunnel collapsing and the failure of concrete slab. 5. Monitoring After completion of the trench construction and before the beginning of the tunnel excavation, many cracks were evident at the concrete slabs as well as at the construction and expansion joints. Therefore an adequate monitoring scheme was actually implemented before and during the tunnel construction to monitor the interaction between the tunnel and the trench movements, the monitoring scheme around the study area included the following key elements: A-Monitoring of crack displacements Major cracks pre-existing on the concrete slabs were monitored using a series of glass slides to review the development progress of these cracks (see Fig. 12). Fracturing of the glass slides revealed a bitter truth. Consequently, due to the sensitivity of the trench stability issue and propagation of cracks, Topographical control was conducted. Figure 12: Fracturing of the installed glass slides. B-Topographical control Surveying pins and control points fixed into the potentially unstable ground. The 9 bench marks were installed at the various points of the trench (See Fig. 13) and monitoring of these points were inserted at the work plan of the surveying unit.

8 Figure 13: The location of the monitoring station and overall tendency of displacement along X, Y, Z The preliminary estimation of displacement direction and assessment of the trench stability can be mentioned as two main objectives for continuous trench monitoring. After data acquisition, it was determined the pins at the seven stations have been moved in one directions of the X, Y, Z towards the north to northeast but there was no movement in R3 and R6 stations (see Fig 14). The soil of some parts of the trench consist of such clay type, thus the swelling is predictable as a result of increasing in degree of saturation of the soil due to groundwater and rain-fall that will be caused the soil uplifting.

9 Figure 14: the displacement graphs of the installed pins along X, Y, Z As observed in the Z direction diagram, despite of the trench subsidence, R1 station has been moved upward, the soil at this location contains montmorillonite that has the capability of swelling. 6. The Collapsing Process of the Trench The displacement of installed pins was recorded with mm precision before collapsing, the first signs of sliding were observed after beginning of tunnel excavation at the 35 m from the tunnel so the 4 m of tunnel crown collapsed in 8 hours and 1500 m3 Soil collapsed six times in one month. The monitoring program was conducted in shorter periods as well as more accuracy simultaneously. The results show an inception of ascendant movement of the trench caused the failure of concrete slabs finally (Fig.15). (a) (b) (c)

10 Figure 15: The occurred collapse and failure of the concrete slab: (a): the slide surface and beginning of the collapse (b) : the tunnel face after five days (c) 25 days after collapsing In addition to inaccurate field studies, it was obvious, the trench design parameters possess bad engineering judgment, furthermore the high tunnel advance rate, stresses releasing and existence of Aqueous soils as well as layers with high degree angle caused to soil mass sliding on the slide surface. Therefore, as shown in the chart 16, based on interpretations the overall collapsing factors for this trench can be summarized. Figure 16: Flowchart for trench failure mechanism 7. Trench Stability Reanalysis According to the results obtained from geological and geotechnical studies as noted at the third chapter, Geometric design and the trench stability was reviewed and it was reanalyzed based on new studies. As known, the Slope stability analysis is based on the simplified assumptions; consequently slope designing depends on the field test and investigations significantly. Determining soil properties is the key factor in the stability analysis. On the other hand, soil properties vary spatially even within homogeneous layers as a result of depositional and postdepositional processes that cause variation in properties [8]. Nevertheless, most geotechnical analyses adopt a deterministic approach based on single soil parameters applied to each distinct layer. The conventional tools for dealing with ground heterogeneity in the field of geotechnical engineering have been applied under the use of safety factors and by implementing local experience and engineering judgment [9] and calculations are performed based on the safety factors for unstable (FS 1), boundary (FS =1) and stable (FS> 1) states, in most analysis with little attention to the precision of soil properties and the geometric of the layers. Despite of the uncertainties in the slope stability problems, geotechnical engineers still avoid from adopting of the probabilistic techniques at the analyses, hence most of the design

11 regulations such as Eurocode 7 (2004) consider conservative properties for soils and the relatively large safety factors in the slope stability analyses. With notice to above guidelines and by using of modified geological profile and new geotechnical data, a 3D numerical model of the portal of sabzkuh tunnel with pervious geometry is conducted. Numerical modeling software packages such as FLAC3D (Fast Lagrangian Analysis of Continua, which uses the explicit finite different formulation) can aid in the solution of problems that consist of several stages, large displacements and strains, non-linear material behavior and unstable systems, even cases of yield/failure over large areas, or total collapse. The model geometry for the analyses using FLAC3D (Itasca, 2005) is illustrated in Figures 17. The model is 50m wide and 81m long. The floor of the tunnel is located 15m above the base of the model and the overburden height varies between 3m to 23.5 m. A D-shape tunnel with a width 5.6m and height 5.51m is excavated to collapsing zone (see Fig. 17). The unsupported tunnel length is 1 m. For each step, the excavation is advanced 1 m and the presupport shotcrete (25 cm thickness with two layers of welded wire mesh 8 ) and steel arc support (2IPE140) are installed at the specified distances from the tunnel face. Figure 17: The model geometry based on geology profile for the analyses. The tunnel is constructed in a weak soils that are modeled as a Mohr-Coulomb materials with properties that shown at the table 1. The initial stress state corresponds to gravitational loading with the following relation between vertical and horizontal stresses: 2σzz = σxx = σyy. The pre-support shotcrete is modeled with zones. Note that the steel arch support is not explicitly modeled and its effect is combined with that of the shotcrete. With Assuming 20GPa for modulus of elasticity and 0.2 for Poisson's ratio and using the relations presented by Hoek et al. (2008) the equivalent modulus of elasticity for support system would be 23.2GPa [13]. Before tunnel excavation the factor-of-safety was calculated by the strength reduction method that a value of 1.11 is calculated for Factor of Safety (FOS) under wet condition indicating that the slope is stable, But as observed after beginning the excavation of tunnel, displacements increase and in the vicinity of the sliding surface the maximum relative

12 displacement is 3.358e-001m, the resulting failure surface due to tunnel excavation is depicted by the displacement contour plot shown in Figure 18. While in the areas before the sliding surface displacements are in the permitted limit and increasing is prevented by installation of support system quickly after excavation stage but at the vicinity of sliding surface collapsing has occurred before support installation, at the area near the sliding surface at the tunnel excavation stage the maximum velocity vectors calculated are 7.823e- 007 ms-1, the velocity vector indicates that the soil collapse in wet condition, that this phenomenon has been observed in field also. Figure 19 plots shear strain-rate contours, which allow the failure surface to be identified. In essence, the approach recognizes that field data (such as in-situ stresses, material properties and geological features) will never be known completely. It is futile to expect the model to provide design data, such as expected displacements, when there is massive uncertainty in the input data. However, a numerical model is still useful in providing a picture of the mechanisms that may occur in particular physical systems, but as can be seen if extensive field data are available, then these may be incorporated into a comprehensive model that can yield design information directly. Figure 18: Displacement contours and velocity vectors in the model at the failure state. Figure 19: Shear strain-rate contours in slope model at last non-equilibrium state. 8. Conclusions As the first step in identifying the intrinsic collapsing potential, the Geological considerations are very important. Thus, Perform field identification of soft and unstable soils sediment have high degree of importance in relation to the soil engineering specifications. Thus geological investigations, geotechnical boreholes with results of conducted laboratory tests and data of geography and geomorphology are a major part of the process of identifying unstable soils. In the mentioned project due to inappropriate site selection and insufficient field studies, geologists provided inappropriate information for designers. Regardless of the complexities, designers began to trench stability analysis just with relying on the geotechnical data and based on initial assumptions. By controlling the trench (instrumentation, surveying at the construction and field

13 investigations) during utilization, inconsistencies were observed with the basic design assumptions. Ultimately high advance rate at the excavation without primary support installation leading to collapse of the trench. In conclusion, with study the findings and the available evidence and regardless of the site locating and trench geometry limitations, technical factors that caused the trench collapsing can be summarized as follow: - Failure started by irritation of a point with sliding potential in the tunnel and it has spread to upward. - Water is a major factor in stability of the slopes especially in the soils, as a result of the presence of water within soil masses, reduction of sliding resistant forces that causing a reduction of safety against slope failure or sliding can be considered due to pore water pressure, intensification in erosion process and subsequently a loss of soil shear strength. In fine grain soils, FOS decreases more than 50% from dry to wet condition that leads to instability in the slope. - Failure in the trench due to instability led to the materials especially saturated granular materials slip down, Therefore water by increasing the weight of materials, decreasing the cohesion between the grains and friction reduction will be caused the instability in the trench. - The stability of the trench influenced by deformations due to tunnel excavation and stressreleasing in the soil mass. Finally, these deformations within internal of the tunnel caused instability in the trenches. - Thin layers or sand and gravel lenses can be found in the clayey and silty deposits. A lubricant and slippery surface was created for middle layers due to the instability of these layers, as well as high slope angle has been caused the movement and sliding of soil wedges in during of the tunnel excavations. If the effect of friction on the surface with slide potential reduced the incidence of failure will be possible. So finally with study of site location, type of field investigations, performed analyses and the construction method the main factors of collapsing, as shown in the chart 20; can be summarized. Figure 20: The flowcharts of main factors in trench collapsing.

14 Acknowledgements The authors are most thankful to project manager A.Izadi engineer for his kind assistance throughout the project. References Periodicals: [1] Alonso EE. Risk Analysis of Slopes and Its Application to Slopes in Canadian Sensitive Clays. Geotechnique. Vol.26 (3): , [2] Wu TH, Lee IM, Potter JC, Kjekstad O. Uncertainties in Evaluation of Strength of Marine Sand. Journal of Geotechnical Engineering, ASCE 1987; 113 (7): [3] Whitman VW. Organizing and Evaluating Uncertainty in geotechnical Engineering. In Uncertainty in geologic Environment: From Theory to Practice, Proceedings of Uncertainty 96. Geotechnical Special Publication No 58, ASCE 1996; Vol. 1: [4] Sowers GS. The Human Factor in Failure. Civil Engineering, ASCE. 1991; pp Books: [5] Peck RB. Influence of Non-technical Factors on the Quality of Embankment Dams. In Embankment Dam Engineering, Casagrande. John Wiley & Sons, New York. 1973; pp [6] Mostyn GR. Li KS. Probabilistic Slope Analysis-State of Play. In Probabilistic Methods in geotechnical Engineering, Li and Lo, (eds). 1993; pp [7] Baecher BT, Chritian JT. Reliability and Statistics in Geotechnical Engineering. England, John Wiley & Sons, [8] Lacasse S, Nadim F. Uncertainties in Characterizing Soil Properties, in Uncertainty in the Geologic Environment: From Theory to Practice. Proceedings of Uncertainty 96, ASCE Geotechnical Special publication No. 58, C.D. Shackelford, P.P Nelson, and M.J.S. Roth. 1996; pp [9] Lawton EC, Richard J, Fragaszy RJ, Hetherington MD. Review of wetting-induced collapse in compacted soil. Journal of Geotechnical Engineering, ASCE 1992; 118(9): Technical Reports: [10] Wolff TF. Probabilistic Slope Stability in Theory and Practice. In Uncertainty in The Geologic Environment: From Theory to Practice, Proceeding of Uncertainty Papers from Conference Proceedings (Published): [11] Morgenstern NR. Managing Risk Geotechnical Engineering. Proceeding of the 10th Pan American Conference on Soil Mechanic and Foundation Engineering, 1995, Vol. 4. [12] Cornell CA. First Order Uncertainty Analysis of Soil Deformation and Stability. Proceeding of 1st International Conference on Application of Statistic and Probability to Soil and Structural Engineering, Hong Kong. 1971;13-16, 1971, pp [13] Hoek E, Torres C, Diederichs M, The 2008 Kersten Lecture- Integration of Geotechnical and Structural Design in Tunneling, University of Minnesota 56th Annual Geotechnical Engineering Conference, 2008.

15 Standards: [14] American Society for Testing and Materials, ASTM D