_GENERAL DOCUMENTS - C1981017 (138)

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Memo to Tony Waldron Coal Basin Pile / Ditch Interaction page 2 a consolidated state and remains stable. The timbers rot and contribute to natural revegetation of the pile. The pockets commonly become good sites for volunteer vegetation. The wooden cribbing principle is not amenable to structural analysis . As such, it is no longer an acceptable structural solution under the demanding regulatory atmosphere of SMCRA. Therefore, I sense that the AFO staff considers it to be inherently "suspect" . For all these reasons, cribbing has been relegated to the role of a "landscape architectural technique" . However, pre- law Colorado mining sites, particularly non-coal and AML sites, include numerous, enormous and intricate cribbing examples. With all this in mind, I offer the following comments concerning the Coal Basin Old Refuse Pile / Ditch interaction. (1) As is common, the almost thirty year old cribbing above the ditch transecting the toe of the Coal Basin Old Refuse Pile has reached "maturity" . It is disintegrating in places and intact in others. At three or four locations along the ditch it has significantly weathered and is rotting and composting. The refuse, in response to the action of weather and surface runoff has cascaded down the slope, leaving a few small wedge- shaped mounds of refuse atop the ditch embankment. I saw no refuse material below the strand line of the ditch. In one location a small surficial layer of the refuse (measuring approximately 15 feet laterally, ten feet vertically, and from 0 to 2 feet in thickness) has block glided a few feet down slope. This small surficial failure appears to have overrun some disintegrating cribbing planks and subsequently stabilized itself. This is common in wooden cribbing installations . I could discern no evidence of any deep-seated structural instability effecting the Old Refuse Pile. (2) The limited examples of refuse gliding, unraveling and downslope siltation described above are not the result of the ditch eroding or saturating the refuse. They are the result of normal biodegradation of the wooden cribbing timbers due to weathering. I discerned no evidence that ditch flow had undercut the cribbing in any way. The weathered composting cribbing debris is still evident in place at the toe of the refuse slope. (3) Situations in which wooden cribbing were historically used are often oversteepened. Without cribbing the slope would not have held the grade achieved after installation of the cribbing. After years of consolidation cribbed slopes will commonly stand at steep grades after the cribbing naturally deteriorates and consolidation and revegetation stabilizes the