APPENDIX D. Slope Stability Analysis Results for Soil and Overburden Storage Mounds

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1 Geotechnical Assessment Report APPENDIX D Slope Stability Analysis Results for Soil and Overburden Storage Mounds DABGeot/09059GA/Final

2 Geotechnical Assessment Report STABILITY OF SOIL AND OVERBURDEN STORAGE MOUNDS D. Shear Strength Parameters Most glacial tills are fissured or contain other directional voids that permit pore pressure dissipation. Preference has therefore been given to the use of effective stress tests in determining the shear strength of the glacial cover at Highthorn site. The results of the consolidated undrained triaxial compression tests are summarized in Tables and 3. Both the cohesion (c') and friction angle (ø') values have been derived from linear envelopes drawn to sets of Mohr circles of failure. Whilst this is in accordance with BS377 Part 8 (990), Section 7.6.3, Note 4, some of the intercept values (c') are considered to be too high. This conclusion is based upon the examination and back analysis of a number of failures at surface mines in the UK and the application of a curved failure envelope. Further analysis has therefore been undertaken by plotting the successive stress states of each of the test specimens in p, q space where: p = σ + σ 3 and q = σ - σ 3 Data from Ferneybeds site and the National Coal Board s offshore investigations at Highthorn have been included in the analysis. The soil types have been classified in accordance with the units defined by Robertson (99) so that direct comparison can be made with the shear strengths similarly derived using the combined data from a large number of sites in Northumberland. The data are plotted in Figures D to D3. D DABGeot/09059GA/Final

3 Geotechnical Assessment Report Unit Unit Figure D p -q Plots for Units and DAB Geotechnics Ltd. Scheme Geotechnical Assessment Report Date: 6 th May 05 D DABGeot/09059GA/Final

4 Geotechnical Assessment Report Unit 3 Unit 4 Figure D p -q Plots for Units 3 and 4 DAB Geotechnics Ltd. Scheme Geotechnical Assessment Report Date: 6 th May 05 D3 DABGeot/09059GA/Final

5 Geotechnical Assessment Report Remoulded Units and 3 Figure D3 p -q Plots for Remoulded Units and 3 DAB Geotechnics Ltd. Scheme Geotechnical Assessment Report Date: 6 th May 05 Linear regression has provided values for the intercept (A') and the slope (tan a ) for the best fit linear curve. The parameters c' and ø' have then determined using the following equations: ø' = sin - (tan α'), where α' is the slope of the linear curve; and c' - A' where A' is the intercept (set at zero). cosø' The results are summarized in Table D, most of which are distinctly lower than those derived by Robertson (99), but nevertheless have been used to model the ground conditions at Highthorn. D4 DABGeot/09059GA/Final

6 Geotechnical Assessment Report General Description Unit Highthorn Site Northumberland * c' (knm - ) ø' ( ) c' (knm - ) ø' ( ) Soft to firm, orange or reddish brown, mottled grey, silty or sandy CLAY Firm to very stiff, reddish brown, brown or grey brown, silty sandy to gravelly CLAY with occasional laminations of silt or sand Very stiff, grey silty CLAY Soft to stiff, brown thinly to thickly laminated CLAY Remoulded mixture of Units and * Robertson (99). Table D Summary of Calculated c' and ø' Values D. Topsoil Storage Mounds The stability of the topsoil mounds has been assessed using Bishop s Simplified method for circular failure, the most likely potential failure mechanism, and the computer program, Talren. Non-circular failure has also been considered. Shear strength parameters for remoulded topsoil were determined by analysing triaxial test data from Shotton site in Northumberland and are as follows: c = 0 kn/m and = 35º (bulk density.84 Mg/m 3 ) The topsoil mounds will be founded on natural ground (glacial material). Three scenarios were considered in accessing their stability: (i) (ii) (iii) a foundation comprising Unit material (weathered glacial till); a foundation consisting of Unit (i.e. in the absence of any weathered glacial till or Unit material); and a mound founded on a soil succession comprising solely Unit 4 (laminated clay). The cable percussion boreholes clearly show that the third scenario will not occur at Highthorn site, but laminated clay is present and the analysis results serve to demonstrate the likely effects on stability. Unit 3 only occurs are depth. The shear strengths for all the foundation materials were taken from Tables D with bulk densities derived from the data provided in Appendix B. Characterization of the laminated clay would have benefited from a number of shear box tests, but this omission from the test programme is not thought to be critical. A mound height of 6m was used in all the calculations. D5 DABGeot/09059GA/Final

7 Geotechnical Assessment Report The mounds will be constructed with outer slopes of not greater than v in h (6.6º) and this profile was used in the analysis. Induced pore pressures will be very low at shallow depth due to fissuring of the glacial material, but a range of values (expressed in terms of the pore pressure ratio, r u ) were used for comparative purposes and to the assess the likely impact of an adjacent subsoil or overburden mound. The pore pressures will dissipate with time and at depth this process can be quite slow owing to the low permeability of the glacial till. It will be enhanced where the adjacent ground is excavated. An assessment has been made of the stability of the mounds where they lie adjacent to the highwalls, but it is the security of the slopes themselves which will be critical. Further details are given in Appendix E. For the purposes of this exercise, a slope gradient of v in h (45 ) was used regardless of the thickness of the glacial deposits. This is the steepest profile recommended in Section 5.4. and represents the most conservative condition. The factors of safety (FoS) were determined using a grid of circle centres and the results are shown in Figures D4 to D3. A summary is provided in Tables D to D4 where the following classification has been applied: <.00 Unstable.0 to. Barely stable. to.3 Short term stability (i.e. a few days or weeks).4 Medium term stability (i.e. a few weeks or months).5 Long term stability (i.e. months and years) The analysis results indicate that stability can be maintained if the outer slopes of the topsoil mounds are formed at not greater than v in h (6.6º) as planned. Where they are formed at a lower angle the factors of safety will be higher. The same will apply where the mounds are constructed at a lower height. If any pore pressures are induced they will be relatively low and will slowly dissipate. This and the consolidation of the deposited material will also improve stability. Minor slumping or sliding of the outer surfaces of the mounds may occur, especially if the topsoil has a high moisture content when it is stripped, but as experience has shown this will not endanger plant and personnel. The recommended standoff from the site boundary (3m) will ensure that the security of adjacent properties is not compromised. D6 DABGeot/09059GA/Final

8 Geotechnical Assessment Report Location of Slip Circle Topsoil Mound Founded on Unit Soil Slip circles with overall minimum factor of safety. Slip circles forced through point m behind crest of outer slope. Slip circles forced through point m behind crest of outer slope. Slip circles forced through foundation. Fully drained conditions. Slip circles forced through foundation (r u = 0.). Slip circles forced through foundation (r u = 0.4). Slip circles forced through foundation (r u = 0.5). Slip circles forced through foundation with adjacent excavation highwall. Fully drained conditions. Slip circles forced through foundation with adjacent excavation highwall. Partly drained conditions. Non-circular failure surface extending from the crest of the mound and through the upper part of the foundation to the highwall. Fully drained conditions. Ditto with r u = 0.. Ditto with r u = 0.4. Factor of Safety Notes Slip circles confined to the outer surface of the topsoil mound and are representative of minor unravelling only. Stable in medium term. Slip circles with the minimum factor of safety extend through the full height of the outer slope. Stable in long term. Slip circle with the minimum factor of safety extends through the full height of the outer slope. Stable in long term. Slip circles with the minimum factor of safety extend through the full height of the mound. They penetrate up to 0.5m into the foundation and extend about 3m from the toe. Stable in long term. Slip circles with the minimum factor of safety extend through the full height of the mound. They penetrate up to 0.5m into the foundation and extend about 3m from the toe. Stable in long term. Slip circles with the minimum factor of safety daylight in the central part of the outer slope. They penetrate up to 0.5m into the foundation and extend about.5m from the toe. Stable in medium term. Slip circles with the minimum factor of safety daylight in the central part of the outer slope. They penetrate up to m into the foundation and extend about.5m from the toe. Stable in short term. Slip circles with the minimum factor of safety extend through the full height of the mound. They penetrate up to 0.5m into the foundation and extend about 3m from the toe. Stable in long term. Not possible to generate plausible slip surfaces that extend through the mound and slope. Stable in long term. Stable in long term. Stable in long term. Table D Summary of Slope Stability Analysis Results for Topsoil Storage Mounds D7 DABGeot/09059GA/Final

9 Geotechnical Assessment Report Location of Slip Circle Topsoil Mound Founded on Unit Soil Slip circles forced through foundation. Fully drained conditions. Slip circles forced through foundation (r u = 0.). Slip circles forced through foundation (r u = 0.4). Slip circles forced through foundation (r u = 0.5). Slip circles forced through foundation with adjacent excavation highwall. Fully drained conditions. Slip circles forced through foundation with adjacent excavation highwall (r u = 0.). Non-circular failure surface extending from the crest of the mound and through the upper part of the foundation to the highwall. Fully drained conditions. Ditto with r u = 0.. Ditto with r u = 0.4. Factor of Safety Notes Slip circles with the minimum factor of safety extend through the full height of the mound. They penetrate up to 0.5m into the foundation and extend about 3m from the toe. Stable in long term. Slip circles with the minimum factor of safety extend through the full height of the mound or from the upper part of the outer slope. They penetrate up to 0.5m into the foundation and extend about 3m from the toe. Stable in long term. Slip circle with the minimum factor of safety extends from the upper part of the outer slope. It penetrates up to.0m into the foundation and extends about.5m from the toe. Stable in short term. Slip circle with the minimum factor of safety extends from the upper part of the outer slope. It penetrates up to.5m into the foundation and extends about 3.5m from the toe. Barely stable. Slip circles with the minimum factor of safety extend through the full height of the mound. They penetrate up to 0.5m into the foundation and extend about 3m from the toe. Stable in long term. Slip circles with the minimum factor of safety extend through the full height of the mound and from the upper part of the outer slope. They penetrate up to 0.5m into the foundation and extend about 3m from the toe. Stable in long term. Other slip circles extend at depth through the foundation and daylight in the excavation slope. Not possible to generate plausible slip surfaces at higher pore pressure ratios. Stable in long term. Stable in long term. Stable in long term. Table D3 Summary of Slope Stability Analysis Results for Topsoil Storage Mounds D8 DABGeot/09059GA/Final

10 Geotechnical Assessment Report Location of Slip Circle Topsoil Mound Founded on Unit 4 Soil Slip circles forced through foundation. Fully drained conditions. Slip circles forced through foundation (r u = 0.). Slip circles forced through foundation (r u = 0.4). Slip circles forced through foundation (r u = 0.5). Slip circles forced through foundation with adjacent excavation highwall. Fully drained conditions. Slip circles forced through foundation with adjacent excavation highwall. Partly drained conditions. Non-circular failure surface extending from the crest of the mound and through the upper part of the foundation to the highwall. Fully drained conditions. Ditto with r u = 0.. Ditto with r u = 0.4. Factor of Safety Notes Slip circles with the minimum factor of safety extend through the full height of the mound. They penetrate up to 0.5m into the foundation and extend about 3m from the toe. Stable in long term. Slip circle with the minimum factor of safety extends from the central part of the outer slope. It penetrates up to 0.5m into the foundation and extends about.5m from the toe. Stable in medium term. Slip circles with the minimum factor of safety extend through the full height of the mound or the crest of the outer slope. They penetrate up to.5m into the foundation and extend about 3.5m from the toe. Stable in short term. Slip circle with the minimum factor of safety extends from the upper part of the outer slope. It penetrates up to.5m into the foundation and extends about 3.5m from the toe. Barely stable. Slip circles with the minimum factor of safety extend through the full height of the mound. They penetrate up to 0.5m into the foundation and extend about.5m from the toe. Stable in long term. Not possible to generate plausible slip surfaces that extend through the mound and slope. Stable in long term. Stable in long term. Stable in long term. Table D4 Summary of Slope Stability Analysis Results for Topsoil Storage Mounds D9 DABGeot/09059GA/Final

11 Geotechnical Assessment Report D3. Subsoil and Drift Material Storage Mounds The stability of the subsoil mounds was also assessed using Bishop s Simplified method for circular failure. This is again the most likely potential failure mechanism. Complete remoulding of the subsoil material is unlikely to occur during mound construction because of the method of emplacement: namely, excavation by backactor, end tipping from rigid and/or articulated dumptrucks and spreading by dozers. Investigations at the former Acklington and Colliersdean surface mines in Northumberland suggest that the shear strength of the mound material can closely match that of the in situ weathered till, more so after a period of consolidation. The parameters used in these calculations have been based on p -q analysis of Unit test data as follows: c = 0 kn/m and = 30º. A bulk density of.00 Mg/m 3 was used to represent the deposited material, but this will increase as consolidation ensues. The same foundations were considered as for the topsoil mounds and where Unit has been used, this is based on the reasonable assumption that not all of this material will have been removed during the stripping of the subsoil. A mound height of m was adopted with outer slopes of v in h (6.6 ). Fully drained conditions were assumed, but the effect of induced pore pressures was considered. These will again be very low at shallow depth due to the presence of fissures. The minimum factors of safety were determined using a grid of circle centres and the results are presented in Figure D3 to D57. A summary is provided in Tables D5 to D7. The same classification has been applied as in Section D. The results show that in general the stability of the deposited subsoil can be maintained if the outer slopes of the mounds are formed at not greater than v in h (6.6º). The mound foundations will comprise a combination of the soil types identified in the cable percussion boreholes, although Unit appears to be predominant. This may or may not be capped by a layer of Unit material. Layers or beds of laminated clay (Unit 4) will only impact the factors of safety where they lie at very shallow depth (less than 4m), which is generally not the case. Minor slumping of the outer slopes of the mounds may occur during or shortly after placement, but stability will improve as the subsoil consolidates as experience has shown. Such occurrences will not compromise the safety of plant, personnel and members of the public. Adjacent properties will remain unaffected because of the standoffs that will be provided. Nevertheless, it is recommended that the outer slopes of the mounds are graded to the required profiles as soon as the deposited material reaches 3m in height. This operation should be continued until construction is completed. The analysis results are based on a m high structure and not all mounds will be constructed to this height nor will the side slopes be formed as steeply as v in h (6.6 ). Both these factors will serve to increase the factors of safety. The drift material storage mound will be constructed to a maximum height of m, but its eastern slope will be graded down to about v in 5h (.3 ). The stored material will comprise a mixture of Units and and as such the shear strength should be close to that used in the calculations. However, the recommendations made with regard to the construction of the subsoil mounds equally apply. D DABGeot/09059GA/Final

12 Geotechnical Assessment Report Location of Slip Circle Subsoil Mound Founded on Unit Soil Slip circles with overall minimum factor of safety. Slip circles forced through point m behind crest of outer slope. Slip circles forced through point m behind crest of outer slope. Slip circles forced through point 4m behind crest of outer slope. Slip circles forced through foundation. Fully drained conditions. Slip circles forced through foundation (r u = 0.). Slip circles forced through foundation (r u = 0.4). Slip circles forced through foundation (r u = 0.5). Slip circles forced through foundation with adjacent excavation highwall. Fully drained conditions. Slip circles forced through foundation with adjacent excavation highwall. Partly drained conditions. Non-circular failure surface extending from the crest of the mound and through the upper part of the foundation to the highwall. Fully drained conditions. Ditto with r u = 0.. Ditto with r u = 0.4. Factor of Safety Notes Slip circles confined to the outer surface of the subsoil mound and are representative of minor unravelling only. Stable in short term. Slip circles with the minimum factor of safety extend through almost the full length of the outer slope. Stable in short term. Slip circles with the minimum factor of safety extend through the full length of the outer slope. Stable in short term. As above. Stable in short term Slip circles with the minimum factor of safety extend from around the crest of the outer slope. They penetrate up to 0.5m into the foundation and extend about 8.5m from the toe. Stable in medium term. Slip circles with the minimum factor of safety extend from around the crest of the outer slope. They penetrate up to 0.5m into the foundation and extend about 5.5m from the toe. Stable in short term. Slip circles with the minimum factor of safety extend from around the crest and upper part of the outer slope. They penetrate up to m into the foundation and extend about 4.5m from the toe. Stable in short term. Slip circle with the minimum factor of safety extends from the upper part of the outer slope. It penetrates up to.5m into the foundation and extends about 4.5m from the toe. Barely stable. Slip circles with the minimum factor of safety extend through the toe of the mound and the upper part of the adjacent safety bench. Stable in short term. (Barely plausible). Not possible to generate plausible slip surfaces that extend through the mound and slope. Stable in medium term. Stable in short term. Stable in short term. Table D5 Summary of Slope Stability Analysis Results for Subsoil Storage Mounds D DABGeot/09059GA/Final

13 Geotechnical Assessment Report Location of Slip Circle Subsoil Mound Founded on Unit Soil Slip circles forced through foundation. Fully drained conditions. Slip circles forced through foundation (r u = 0.). Slip circles forced through foundation (r u = 0.4). Slip circles forced through foundation (r u = 0.5). Slip circles forced through foundation with adjacent excavation highwall. Fully and partly drained conditions. Non-circular failure surface extending from the crest of the mound and through the upper part of the foundation to the highwall. Fully drained conditions. Ditto with r u = 0.. Ditto with r u = 0.4. Factor of Safety Notes Slip circles with the minimum factor of safety extend from around the crest of the outer slope. They penetrate up to 0.5m into the foundation and extend about 7m from the toe. Stable in short term. Slip circles with the minimum factor of safety extend from the upper part of the outer slope. They penetrate up to 0.5m into the foundation and extend about 4m from the toe. Stable in short term. Slip circles with the minimum factor of safety extend from the upper part of the outer slope. They penetrate up to.5m into the foundation and extend about 4m from the toe. Barely stable. Slip circles with the minimum factor of safety extend from the upper part of the outer slope. They penetrate up to.5m into the foundation and extend about 5.5m from the toe. Barely stable. Not possible to generate plausible slip surfaces that extend through the mound and slope. Stable in short term. Stable in short term. Stable in short term. Table D6 Summary of Slope Stability Analysis Results for Subsoil Storage Mounds D DABGeot/09059GA/Final

14 Geotechnical Assessment Report Location of Slip Circle Subsoil Mound Founded on Unit 4 Soil Slip circles forced through foundation. Fully drained conditions. Slip circles forced through foundation (r u = 0.). Slip circles forced through foundation (r u = 0.4). Slip circles forced through foundation (r u = 0.5). Slip circles forced through foundation with adjacent excavation highwall. Fully and partly drained conditions. Non-circular failure surface extending from the crest of the mound and through the upper part of the foundation to the highwall. Fully drained conditions. Ditto with r u = 0.. Ditto with r u = 0.4. Factor of Safety Notes Slip circles with the minimum factor of safety extend from around the crest and the upper part of the outer slope. They penetrate up to 0.5m into the foundation and extend about 5.5m from the toe. Stable in short term. Slip circles with the minimum factor of safety extend from around the crest and the upper part of the outer slope. They penetrate up to m into the foundation and extend about 4.5m from the toe. Stable in short term. Slip circles with the minimum factor of safety extend from the upper part of the outer slope. They penetrate up to.5m into the foundation and extend about 5.5m from the toe. Barely stable. Slip circle with the minimum factor of safety extends from the upper part of the outer slope. It penetrates up to.5m into the foundation and extends about 3.5m from the toe. Unstable. Not possible to generate plausible slip surfaces that extend through the mound and slope. Stable in short term. Stable in short term. Stable in short term. Table D7 Summary of Slope Stability Analysis Results for Subsoil Storage Mounds D3 DABGeot/09059GA/Final

15 Geotechnical Assessment Report D4. Overburden Mounds Two overburden mounds will be constructed on the site: OBM located in the north, which will be partly founded on opencast backfill and will formed at a maximum height of 5m; and OBM in the western part of the site resting on natural ground at a height of not greater than 7m. The stability of these two mounds has again been assessed using Bishop s Simplified method for circular failure. The same foundation models were used as for the assessment of the topsoil and subsoil mounds, but with the addition of old opencast backfill, for which suitable shear strength parameters have been taken from the test records for the permitted Ferneybeds site: c = 6 kn/m and = 30º (bulk density. Mg/m 3 ). These parameters are representative of a clay rich fill and may well be conservative. The shear strength will be higher if more rock fragments are present. Two types of overburden material were considered: a moderately compacted weak rockfill and a clayfill comprising a remoulded mixture of Units and 3. The shear strength parameters for the rockfill were estimated using the data provided by Charles and Watts (980) and an average vertical stress value for each of the overburden mounds. A bulk density of.0 Mg/m 3 was adopted based on experience. The shear strength of the rockfill that will be produced at the site will be higher because it will contain significant proportions of moderately strong to strong sandstone. The parameters for the remoulded clay were taken from Table D and Appendix B. Fully drained and partly drained conditions were considered for the foundation. Section profiles were determined for the outer slopes of the mounds and the steepest overall slope was assessed for Mound OBM. This is almost identical to the eastern slope of OBM, but the western side of this mound was assessed because of the adjacent A68. Banks have designed the mounds with an 8 to m high, v in h (6.6 ) outer slope along the toe, but with a gently graded upper part to minimize visual intrusion. The topsoil and subsoil storage mounds will also provide a screen. The analysis results have been determined using a grid of circle centres and are again expressed as minimum factors of safety (Figures D58 to D). They are summarized in Tables D8 and D3 and are classified in the same manner as in Sections D and D3. D4 DABGeot/09059GA/Final

16 Geotechnical Assessment Report Location of Slip Circle Mainly Clayfill Founded on Old Opencast Backfill Slip circles with overall minimum factor of safety. Slip circles forced through point m behind crest of v in h slope. Slip circles forced through foundation. Fully drained conditions. Slip circles forced through foundation (r u = 0.). Slip circles forced through foundation (r u = 0.4). Mainly Clayfill Founded on Unit Factor of Safety Notes Slip circles confined to the outer surface of the mound and are representative of minor unravelling only. Barely stable conditions. Slip circles with the minimum factor of safety extend through the v in h section of the outer slope. Barely stable conditions. Slip circle with the minimum factor of safety extends through the full height of the v in h section of slope. It penetrates up to 0.5m into the foundation and extends about 4.5m from the toe. Stable in medium term. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to 0.5m into the foundation and extend about 4.5m from the toe. Stable in short term. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to 0.5m into the foundation and extend about.5m from the toe. Stable in short term. Slip circles forced through foundation. Fully drained conditions. Slip circles forced through foundation (r u = 0.). Slip circles forced through foundation (r u = 0.4). Slip circles forced through foundation (r u = 0.5) Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to 0.5m into the foundation and extend about 8m from the toe. Stable in short term. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to 0.5m into the foundation and extend about 5.5m from the toe. Stable in short term. Slip circles with the minimum factor of safety extend from the upper part of the v in h section of slope. They penetrate up to.5m into the foundation and extend about 4.5m from the toe. Barely stable. Slip circles with the minimum factor of safety extend from the upper part of the v in h section of slope. They penetrate up to m into the foundation and extend about 6m from the toe. Barely stable. Table D8 Summary of Slope Stability Analysis Results for Overburden Mound OBM D5 DABGeot/09059GA/Final

17 Geotechnical Assessment Report Location of Slip Circle Mainly Clayfill Founded on Unit 4 Slip circles forced through foundation. Fully drained conditions. Slip circles forced through foundation (r u = 0.). Slip circles forced through foundation (r u = 0.4). Slip circles forced through foundation (r u = 0.5). Rockfill Founded on Opencast Backfill Slip circles with overall minimum factor of safety. Slip circles forced through foundation. Fully drained conditions. Slip circles forced through foundation (r u = 0.). Slip circles forced through foundation (r u = 0.4). Factor of Safety Notes Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to 0.5m into the foundation and extend about 5.5m from the toe. Stable in short term. Slip circles with the minimum factor of safety extend from the crest and upper part of the v in h section of slope. They penetrate up to 0.5m into the foundation and extend about 4.5m from the toe. Barely stable. Slip circles with the minimum factor of safety extend from the upper part of the v in h section of slope. They penetrate up to m into the foundation and extend about 5.5m from the toe. Barely stable. Slip circles with the minimum factor of safety extend from the upper part of the v in h section of slope. They penetrate up to.5m into the foundation and extend about 4.5m from the toe. Unstable. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to.5m into the foundation and extend about 4.5m from the toe. Stable in long term. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to.5m into the foundation and extend about 4.5m from the toe. Stable in long term. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to.5m into the foundation and extend about 5m from the toe. Stable in long term. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to 3.5m into the foundation and extend about 5.5m from the toe. Stable in long term. Table D9 Summary of Slope Stability Analysis Results for Overburden Mound OBM D6 DABGeot/09059GA/Final

18 Geotechnical Assessment Report Location of Slip Circle Rockfill Founded on Unit Slip circles forced through foundation. Fully drained conditions. Slip circles forced through foundation (r u = 0.). Slip circles forced through foundation (r u = 0.4). Slip circles forced through foundation (r u = 0.5). Rockfill Founded on Unit 4 Slip circles forced through foundation. Fully drained conditions. Slip circles forced through foundation (r u = 0.). Slip circles forced through foundation (r u = 0.4). Slip circles forced through foundation (r u = 0.5). Factor of Safety Notes Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to.5m into the foundation and extend about 5m from the toe. Stable in long term. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to 3m into the foundation and extend about 5.5m from the toe. Stable in long term. Slip circle with the minimum factor of safety extends through the full height of the v in h section of slope. It penetrates up to 4m into the foundation and extends about 7.5m from the toe. Stable in long term. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to 5.5m into the foundation and extend about 7m from the toe. Stable in short term. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to.5m into the foundation and extend about 5.5m from the toe. Stable in long term. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to 4m into the foundation and extend about 7.5m from the toe. Stable in long term. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to 5.5m into the foundation and extend about 7m from the toe. Stable in short term. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to 7.5m into the foundation and extend about 6m from the toe. Barely stable. Table D Summary of Slope Stability Analysis Results for Overburden Mound OBM D7 DABGeot/09059GA/Final

19 Geotechnical Assessment Report Location of Slip Circle Mainly Clayfill Founded on Unit Slip circles with overall minimum factor of safety. Slip circles forced through point m behind crest of v in h slope. Slip circles forced through point 4m behind crest of v in h slope. Slip circles forced through foundation. Fully drained conditions. Slip circles forced through foundation (r u = 0.). Slip circles forced through foundation (r u = 0.4). Slip circles forced through foundation (r u = 0.5). Factor of Safety Notes Slip circles confined to the outer surface of the mound and are representative of minor unravelling only. Barely stable conditions. Slip circles with the minimum factor of safety extend through the full height of the v in h section of the outer slope. Barely stable conditions. Slip circles with the minimum factor of safety extend through the full height of the v in h section of the outer slope. Stable in short term. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to 0.5m into the foundation and extend about 6.5m from the toe. Stable in short term. Slip circles with the minimum factor of safety extend from the crest of the v in h section of slope. They penetrate up to 0.5m into the foundation and extend about 4m from the toe. Stable in short term. Slip circles with the minimum factor of safety extend from the crest and upper part of the v in h section of slope. They penetrate up to.5m into the foundation and extend about 4m from the toe. Barely stable conditions. Slip circles with the minimum factor of safety extend from the crest and the upper part of the v in h section of slope. They penetrate up to.5m into the foundation and extend about 5.5m from the toe. Barely stable conditions. Table D Summary of Slope Stability Analysis Results for Overburden Mound OBM D8 DABGeot/09059GA/Final

20 Geotechnical Assessment Report Location of Slip Circle Mainly Clayfill Founded on Unit 4 Slip circles forced through foundation. Fully drained conditions. Slip circles forced through foundation (r u = 0.). Slip circles forced through foundation (r u = 0.4). Slip circles forced through foundation (r u = 0.5). Rockfill Founded on Unit Slip circles with overall minimum factor of safety. Slip circles forced through foundation. Fully drained conditions. Slip circles forced through foundation (r u = 0.). Slip circles forced through foundation (r u = 0.4). Slip circles forced through foundation (r u = 0.5). Factor of Safety Notes Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to 0.5m into the foundation and extend about 4.5m from the toe. Stable in short term. Slip circles with the minimum factor of safety extend from around the crest and upper part of the v in h section of slope. They penetrate up to 4m into the foundation and extend about 7.5m from the toe. Barely stable conditions. Slip circles with the minimum factor of safety extend from the upper part of the v in h section of slope. They penetrate up to.5m into the foundation and extend about 4.5m from the toe. Barely stable conditions. Slip circles with the minimum factor of safety extend from the upper part of the v in h section of slope. They penetrate up to m into the foundation and extend about 3.5m from the toe. Unstable. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to.5m into the foundation and extend about 3m from the toe. Stable in long term. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to.5m into the foundation and extend about 3m from the toe. Stable in long term. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to.5m into the foundation and extend about 3.5m from the toe. Stable in long term. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to 4m into the foundation and extend about 6m from the toe. Stable in long term. Slip circle with the minimum factor of safety extends through the full height of the v in h section of slope. It penetrates up to 4m into the foundation and extends about 3.5m from the toe. Stable in short term. Table D Summary of Slope Stability Analysis Results for Overburden Mound OBM D9 DABGeot/09059GA/Final

21 Geotechnical Assessment Report Location of Slip Circle Rockfill Founded on Unit 4 Slip circles forced through foundation. Fully drained conditions. Slip circles forced through foundation (r u = 0.). Slip circles forced through foundation (r u = 0.4). Slip circles forced through foundation (r u = 0.5). Factor of Safety Notes Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to.5m into the foundation and extend about 3m from the toe. Stable in long term. Slip circles with the minimum factor of safety extend through the full height of the v in h section of slope. They penetrate up to 3.5m into the foundation and extend about 6m from the toe. Stable in long term. Slip circles with the minimum factor of safety extend through the full height of the v in h slope. They penetrate up to 3.5m into the foundation and extend about 6m from the toe. Stable in medium term. Slip circle with the minimum factor of safety extends through the full height of the v in h section of slope. It penetrates up to 3.5m into the foundation and extends about 4m from the toe. Stable in short term. Table D3 Summary of Slope Stability Analysis Results for Overburden Mound OBM The analysis results are considered to reflect very conservative conditions and the factors of safety are likely to be higher. In all cases, however, they show that the slip circles with the minimum factors of safety are focused on the steeper, v in h (6.6 ) section of the mounds outer slopes. Clayfill is the least stable overburden material and there is a risk that the outer slopes will slump where they are formed at v in h (6.6 ). The risk of bearing capacity failure will diminish as the pore pressures dissipate and the material consolidates, but any adverse effects will not be felt beyond the site boundary if the recommended standoffs are applied. The pore pressures used in the analysis will be lower at shallower depth and where drainage is facilitated by granular opencast backfill. The presence of topsoil and subsoil mounds will also serve to load the toe and increase the factors of safety. The quality of the overburden will also be variable. Remoulding of the clay material will only be partial and in many instances it will retain its in situ strength. High factors of safety and satisfactory levels of stability were achieved using the shear strength parameters for rockfill, which may also prove to be conservative. A mixture of clay and rockfill should therefore achieve the necessary degree of security for the two mounds. However, it is recommended that as much rockfill as possible is incorporated into the outer perimeters of the two mounds where the steeper profiles are to be formed. This bund should be at least 5m wide and should be formed as the mound level is raised to between 8 and m in height. The standoff between the mounds and the excavations will be sufficient to prevent significant interaction. D0 DABGeot/09059GA/Final

22 γ c 0 0 φ Topsoil Gmin.39 Unit Date : 05/07/00 Time : 00:6:00 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Gen.tal Proj : Scale : / 00 D4

23 γ c 0 0 φ Gmin.47 Topsoil Unit Date : 05/07/00 Time : 00:8:5 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Gen.tal Proj : Scale : / 00 D5

24 γ c 0 0 φ Gmin.56 Topsoil Unit Date : 05/07/00 Time : 00:0:05 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Gen.tal Proj : Scale : / 00 D6

25 γ c 0 0 φ Conditions concerning soils and are verified simultaneously. Topsoil Gmin.67 Unit Date : 05/07/00 Time : 00::37 Scale : / 00 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Gen.tal Proj : D7

26 γ c 0 0 φ ru 0. Conditions concerning soils and are verified simultaneously. Topsoil Gmin.54 Unit Date : 05/07/00 Time : 00:4:48 Scale : / 00 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Gen.tal Proj : D8

27 γ c 0 0 φ ru 0.4 Conditions concerning soils and are verified simultaneously. Topsoil Gmin.37 Unit Date : 05/07/00 Time : 00:8:09 Scale : / 00 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Gen.tal Proj : D9

28 γ c 0 0 φ ru 0.5 Conditions concerning soils and are verified simultaneously. Topsoil Gmin.5 Unit Date : 05/07/00 Time : 00:7:0 Scale : / 00 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Gen.tal Proj : D

29 γ c 0 0 φ Conditions concerning soils and are verified simultaneously Gmin.67 Topsoil Unit Date : 05/07/00 Time : 00:3:03 Scale : / 00 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Exc.tal Proj : D

30 γ c 0 0 φ Conditions concerning soils and are verified simultaneously. Ga-z.88 a Topsoil z Unit Date : 05/07/00 Time : 00:4:35 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Exc.tal Proj : Scale : / 00 D

31 γ c 0 0 φ ru 0. Conditions concerning soils and are verified simultaneously. Ga-z.7 a Topsoil z Unit Date : 05/07/00 Time : 00:4:6 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Exc.tal Proj : Scale : / 00 D3

32 γ c 0 0 φ ru 0.4 Conditions concerning soils and are verified simultaneously. Ga-z.5 a Topsoil z Unit Date : 05/07/00 Time : 00:4:5 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Exc.tal Proj : Scale : / 00 D4

33 γ c 0 0 φ Conditions concerning soils and are verified simultaneously Gmin.57 Topsoil Unit Date : 05/07/00 Time : 0:5:4 Scale : / 00 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Gen.tal Proj : D5

34 γ c 0 0 φ ru 0. Conditions concerning soils and are verified simultaneously Gmin.45 Topsoil Unit Date : 05/07/00 Time : 0:53:30 Scale : / 00 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Gen.tal Proj : D6

35 γ c 0 0 φ ru 0.4 Conditions concerning soils and are verified simultaneously Gmin.6 Topsoil Unit Date : 05/07/00 Time : 0:0:0 Scale : / 00 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Gen.tal Proj : D7

36 γ c 0 0 φ ru 0.5 Conditions concerning soils and are verified simultaneously. Topsoil Gmin.3 Unit Date : 05/07/00 Time : 0:05: Scale : / 00 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Gen.tal Proj : D8

37 γ c 0 0 φ Conditions concerning soils and are verified simultaneously Gmin.57 Topsoil Unit Date : 05/07/00 Time : 0::57 Scale : / 00 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Exc.tal Proj : D9

38 γ c 0 0 φ ru 0. Conditions concerning soils and are verified simultaneously Gmin.45 Topsoil Unit Date : 05/07/00 Time : 0::59 Scale : / 00 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Exc.tal Proj : D0

39 γ c 0 0 φ Conditions concerning soils and are verified simultaneously. Ga-z.75 a Topsoil z Unit Date : 05/07/00 Time : 00:44:4 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Exc.tal Proj : Scale : / 00 D

40 γ c 0 0 φ ru 0. Conditions concerning soils and are verified simultaneously. Ga-z.59 a Topsoil z Unit Date : 05/07/00 Time : 00:45:6 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Exc.tal Proj : Scale : / 00 D

41 γ c 0 0 φ ru 0.4 Conditions concerning soils and are verified simultaneously. Ga-z.44 a Topsoil z Unit Date : 05/07/00 Time : 00:45:59 6m High Topsoil Mound Unit Foundation v in h Outer Slopes File : Topsoil Exc.tal Proj : Scale : / 00 D3

42 γ c 0 0 φ Conditions concerning soils and are verified simultaneously. Topsoil Gmin.48 Unit 4 Date : 05/07/00 Time : 0:55:47 Scale : / 00 6m High Topsoil Mound Unit 4 Foundation v in h Outer Slopes File : Topsoil Gen.tal Proj : D4

43 γ c 0 0 φ ru 0. Conditions concerning soils and are verified simultaneously. Topsoil Gmin.34 Unit 4 Date : 05/07/00 Time : 0:57:56 Scale : / 00 6m High Topsoil Mound Unit 4 Foundation v in h Outer Slopes File : Topsoil Gen.tal Proj : D5