Laserfiche WebLink
Gained only 1.7Z of [his fraction in [he unweathered material. As shown in Table <br />2.7-4 the shale tended to weather [o a greater degree than the sandstone plus <br />siltstone. After weathering, over twice as much of the shale appeared in the <br />smallest size fraction (-0.053 mm) compared to the sandstone plus siltstone, <br />indicating that the shale tends to break down faster and into finer particles <br />than sandstone plus siltstone. Thus, the initial overburden should be quite <br />resistant to erosion, but as weathering proceeds, the erosion potential will <br />increase. Therefore, significant predictions about erosion hazards of the over- <br />burden depend on the degree of weathering. <br />The potential problems with some of the weathered overburden will be eliminated <br />or reduced once topsoil is placed on the spoil surface. <br />Water Holding Capacity, Hydraulic Conductivity and Settling Volume <br />The data for water holding capacity, hydraulic conductivity and settling volume <br />after weathering appear in Table 2.7-5. Large differences occurred in the water <br />holding capacity at 1/3 bar suction and at IS bars suction. The difference <br />between these two values represents a measure of the amount of water available to <br />plants. The water contents in each column re Elect differences in the overburden <br />due mainly to texture. By linear regression calculations, the clay contents were <br />be estimated to range from 2Z in a layer (71'-96') in CR213 to 64 Y, in a Layer <br />(64'-109') in CR220. <br />The settling volume was very high for three overburden layers with clay contents <br />above 40$ i.e. 64'-109' and 161'-172' in CR220 and 114'-145' in CR223. It was <br />high for two other samples which contained an estimated 359. clay (150'-184' in <br />CR222) and 407. clay (96'-111' in CR223). These Eive samples with clay contents <br />of 359.' ar more were the only samples with ESP values greater than 15. These same <br />five samples had the Lowest values (<O,OOL cm/hr) foc hydraulic conductivity.. <br />The very low values for hydraulic conductivity in some of the overburden samples <br />weathered from shale indicate that problems could arise from erosion and poor <br />water infiltration if these materials were no[ going to be retopsoiled. The <br />resulting condition shoiiLd be very similar to many pre-mining areas where very <br />clayey subsoils occur naturally. <br />2-365 <br />