[Home] [Staff] [Special
Projects] [Technical Publications]
[Technical
Awards & Events] [Staff Awards & Accomplishments]
[Corporate Philosophy]
[Client Survey]
[Contact Us] [Explorers
Post] [Map to our Office]
Copyright© Geo/Environmental Associates, Inc. 1995 through 2008
Modified Upstream Construction (MUSC) Method
The use of the upstream construction method offers cost savings and environmental advantages in building coal refuse disposal impoundments. Unfortunately, concerns over the potential loss of strength of hydraulically-placed fine coal refuse due to liquefaction during an earthquake have resulted in some designers refraining from the use of this method. A modified upstream construction (MUSC) method is presented which can mitigate the impacts of a loss of strength of the fine refuse during the design earthquake.
Specifically, after construction of an initial starter dam and a downstream stage, additional stages of the dam are built using the conventional upstream construction method as shown in Figure 1a) (i.e. structural zone Stages 3 through 6). After the dam achieves a given height, the available capacity in the impoundment for disposal of fine refuse will typically increase at a faster rate than the capacity of coarse refuse required to build the structural zone of the dam. Excess coarse refuse not required to build the structural portion of the dam can then be placed downstream of the dam in a buttress zone that is raised intermittently in a single stage as shown in Figure 1b). The required level of the buttress stage at any time is that level that provides adequate resistance to the potential loss of strength of the fine refuse during an earthquake. The downstream slope of the buttress stage can be covered with soil and vegetated as each phase of the buttress is completed. After the buttress zone achieves the level of the structural zone as shown in Figure 2a), the embankment is raised in a single zone as shown by Stages 13 and 14 in Figure 2b). As the final stage in the construction of the facility, coarse refuse is placed upstream of the dam over the settled fine refuse in the final capping of the impoundment.
With regard to the potential impact of a design earthquake on the strength of fine refuse, a post-earthquake static stability analysis can be performed initially presuming that the fine refuse has liquefied during the design earthquake. Figures 3a) and 3b) show the critical potential failure surfaces and factors of safety presuming complete liquefaction (i.e. no strength) for the fine refuse. With such a conservative analysis, an acceptable factor of safety in excess of 1.0 is recommended. For the phases of construction shown in Figures 3a) and 3b), no field verification of the fine refuse strength would be required. In the phase of construction shown by Figure 3c), a nominal undrained steady-state angle of internal friction of 40 with no cohesion is required to provide the required resistance to a failure during an earthquake. As a result, in-situ vane shear testing would be required to verify the available strength of the fine refuse in a completely remolded state as described previously.
.
Figure 1. Down-Valley Profile of Phases I and II of a Coal Refuse Dam Built by the Modified Upstream Construction (MUSC) Method
Figure 2. Down-Valley Profile of Phases III, IV, V, and VI of a Coal Refuse Dam
Built by the Modified Upstream Construction
(MUSC) Method
Use of the Statistical Method for Verifying Placement Criteria
for Coarse Refuse
The Mine Safety and Health Administration (MSHA 1990) requires that coarse refuse placed in the structural portion of a coal refuse impounding structure meet the following criteria:
1. Material should be compacted to at least 95% of the maximum dry density as defined by the standard Proctor test, with the placement water content not exceeding the range of -2% to +3% of optimum.
2. In compacting coarse coal refuse, the lift thickness should not exceed 12 inches.
MSHA (1990) allows less stringent compaction specifications only when justified by extensive testing and analyses or in areas which can be shown to be "non-structural" portions of the dam. In cases where a less stringent compaction criteria is specified, MSHA requires the designer to show that "all potential problems, including settlement, cracking, piping, instability, stratification, and seepage, have been taken into account in the design and that compensating design features have been incorporated."
One method that can be used to justify a reduced compaction criteria is by way of a statistical method of analysis of field moisture content and dry density data. Figure 4 presents statistical compaction data from sites with different minimum allowable compaction criteria. Typically, when a minimum 90% compaction criteria is specified for a given test, the statistical median of the data will be on the order of 95% compaction. When a minimum 95% compaction criteria is specified for a given test, the statistical median of the data will be on the order of 100% compaction. As a result, specifying a minimum 90% compaction criteria with a median 95% compaction criteria can be used to verify compliance with MSHA compaction criteria.
Figure 5 presents statistical placement moisture content data from existing coal refuse disposal sites obtained during different seasons of the year. Generally, the moisture content of coarse refuse as it comes from a given preparation plant will be fairly constant over the various seasons of the year. Variations in placement moisture content will be caused primarily by varying weather conditions. Specifying a narrow moisture range for all field density tests precludes year-round placement which is one of the key design considerations required of most coal refuse disposal impoundments. Specifying a median moisture content in the range of -2% to +3% of optimum moisture content can be used to verify compliance with MSHA criteria and still allow for seasonal variations beyond the specified range.
Figure 4. Statistical Analysis of Field Density Test Data for Coarse Refuse from Sites with Different Minimum Compaction Criteria for a Given Test
Figure 5. Statistical Analysis of Field Moisture Content Test Data for
Coarse Refuse During Different Seasons of the Year
Mine Seal Barriers
The use of the MUSC method of slurry impoundment design and construction requires less coarse refuse than by use of the downstream or centerline construction methods. On several recent project, this excess coarse refuse has been used to construct mine seal barriers to reduce the potential for a release of fine refuse and water in order to respond to the February 11, 1997 MSHA bulletin. Figure 6 illustrates the results of a finite element seepage analysis for a mine barrier constructed using coarse refuse.
Figure 6. Results of Finite Element Seepage Analysis for a Mine Seal Barrier Constructed Using Coarse Refuse