1994-01-01_PERMIT FILE - C1981017 ‎(5)‎

• Permit No: C1981017
• IBM Index Class Name: Permit File
• Doc Date: 1/1/1994
• Doc Name: Road Reclamation at Entire Coal Basin Complex
• Media Type: D
• Archive: No

In soils of little or no cohesion, three important physical
phenomena must be considered when interpreting the results of the
direct shear tests:
1) If, during reclamation, the material cannot be compacted to 90%
of the maximum dry density, which is a real possibility at a 45
degree angle, then the strength parameters in the field would be
less than those reported from the tests. This will be accounted for
through "what if" cases in the slope stability analyses.
2) When this type of material is tested in the direct shear
apparatus under a compacted condition, it initially requires a
slightly higher stress to shear the sample when compared to a
sample that was initially loose. The shear stresses for these two
conditions are referred to as the peak strength and the ultimate or
residual strength, respectively. The extra shear strength from the
initially dense sample is attributed to the greater degree of
interlocking of the particles. Both are reported in the test
analyses. The choice for a practical b a c k f i l l i n g problem depends on
the amount of deformation expected in the backfilled material. If
significant deformation can occur, the lower ultimate strength
values should be used. In the case of such a steep backfilling
angle for these roads, deformation can be expected, therefore
ultimate or residual strength values should be used.
3) Since some moisture is present in the samples during shearing,
the resulting failure envelope, when plotted, can show an "apparent
cohesion ". With damp, low cohesion soils, this extra strength is
due to compressive forces exerted on the soil particles as a result
of surface tensions where water menisci have formed between soil
particles. The extra shearing strength exists as long as this
amount of water is present. If the soil dries out or becomes,
saturated, this extra strength is lost. To account for this in the
stability analyses, some cases will be run at slightly lower
cohesions.
Although the direct shear tests were conducted in technically the
" undrained" condition, the samples had approximately 8 to 10%
moisture, which is near optimum. Undrained tests conducted in the
saturated condition do not use an internal angle of friction or
pore pressure ratio in the analysis, which is typical for clays.
Since the material was not saturated and is more like a low
cohesion soil, internal angles of friction and pore pressure ratios
should be used in the total stress analysis.
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