SLOPE STABILITY Table 2
F) Use all of the above information for sensitivity analyses: What does it mean if the apparent cohesion is 1, 2, 5, 8, 10 and 15 kPa; how does the global factor of safety change?
G) Then design stable slopes (tempo- rary and final slopes) and compare the stable slope angles with the required slope angles for a profit- able operation of the sand pit.
H) Design the monitoring system. Decide on the basis of the sensitiv- ity analyses of moisture content, the data from the field (moisture content in the field), and the slope angles if monitoring of changes in moisture content is required.
Table 2 - The results of limit equilibrium calculations for the slope presented in figure 2 show (the line in red is the isoline for a global factor of safety equal to 1) that for a sand with a friction angle (φ’) of 35°, an apparent cohesion (c) of about 7.5 kPa is enough to stay at the limit of stability.
A) Actual slope profiles need to be measured. For this today normally drones are used. With a drone a typical sand operation can be “flown” in 1 hour. Processing takes place during a few hours in the office and the next day informa- tion of slope profiles, angles etc. is available for study. Of course traditional measurements with hand held range- finder (with azimuth and inclination) and inclinometers or tachymeter on a tripod can be used alternatively and still do a great job in the field. It is important to catch and analyze the entire slope morphology in sand pits includ- ing the steeper upper part of the slope, the mid-section and the toe of the slope and consider each morphological unit in further analyses.
B) Perform classical field mapping. Identify different lay- ers, hydrological situation. Start creating a geotechnical model of the quarry (i.e. understand and take into account the geomorphology of the area at large (Fookes et al 2015).
C) The information from (A) should then be used to do slope stability back calculations to determine which friction angle and cohesion value might fit to the existing slopes (taking into account the hydrological conditions).
D) Based on (B) and (C): set-up the sampling and in situ testing campaign and the lab program. Samples can only be collected in a good way if the lab program has been well defined. For example, in the case that there are gravel layers, a standard (10cm*10cm) shear box cannot be used. The grains will be too large. In that case an e.g. 50cm*50cm shear box must be used. This device requires however 300 kg of sample or even more for one test location only. It is evident that the collection of small samples (e.g. 5 kg) or larger samples (300 kg) requires different logistics.E) Carry out the lab tests and determine the shear strength. Determine the (apparent) cohesion (e.g. by performing shear box tests at several moisture contents).
E) Carry out the lab tests and determine the shear strength. Determine the (apparent) cohesion (e.g. by performing shear box tests at several moisture contents).
32 TPG •
Oct.Nov.Dec 2020 Summary
This article discussed the background for the design, design- based monitoring (and the standards that allow to do this) in the light of optimum extraction of sand in sand extraction sites. We explained how the apparent cohesion affects the stability of slopes and under which conditions it can be used in the design of slopes in sand quarries (e.g. for temporary slopes). Then a step by step overview was provided that detailed how the design should take place taking the apparent cohesion into account in the design of slopes in sand quarries.
References
Bray, J.D., Sancio, R.B. (2006) Assessment of the Liquefaction Susceptibility of Fine-Grained Soils.
J.Geotech. Geoenviron. Eng., 132(9): 1165-1177
Fookes, P., Pettifer, G., Waltham, T. (2015) Geomodels in Engineering Geology. Whittles Publishing.
Fredlund, D.G., Rahardjo, H., Fredlund, M.D. (2012) Unsaturated Soil Mechanics in Engineering Practice. Wiley.
Schmitz, R.M. (2019) Geotechnical risk management of slopes in quar- ries, mines and dredging operations of Sibelco: geotechnical moni- toring. In: North of England Institute of Mining and Mechanical Engineering Newsletter 2/2020. 7 pages.
About the Authors
Robrecht Schmitz, has a MSc-Eng (Delft University of Technology) and a MSc in advanced studies and a PhD in applied Sciences (both Université de Liège). He has been working for 15 years in different mines in different positions and with different minerals. He is visiting lecturer at Delft University of Technology (since 2009). He is currently Global Leader Mining and Geotechnics with Sibelco. He is registered as CGeol, CEng, CSi, CEnv, CMgr and CPG.
Christian Schroeder is a civil engineer and PhD. He has been a member of the academic staffs of the Université de Liège (ULg), Université Catholique de Louvain (UCL) and, from 2003, Université Libre de Bruxelles (ULB). Presently, he is Invited Professor at the ULB and is the manager of CES Consult, the geotechnical consultancy company he founded in 2008.
www.aipg.org
In a following contribution in TGP we will provide an example of these processes and show the devices Sibelco uses to monitor if changes in appar- ent cohesion take place in active sand extraction operations and the actions that then need to be taken.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64
