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SEP-18-1997 12~aa HRRDING LAw60N B61 363 aa76 P.05 Page Four Mr. Norm Every Mountain Coal Company Harding Lawson Associates 1. Since these workings would collect water from a large surface area of fractured rock and coal and then permit the water to flow unrestricted through a large open void to the area immediately behind the slide mass through the floor, it should significantly shorten the time required to deliver significant pore pressure to the back of the slide. 'Ibis characteristic should manifest itself as a more mpid response during wet years (or otherwise staled as increase in the acceleration rates during the fall and winter months reducing lag time tiom the onset of significant snow melt activity). 2. The storage of a large volume of water within the old workings should extrnd the time over which water and high pore pressure is delivered to the back of the slide. Manifestation of this etFect should show up as an extended duration of the period over which acceleration occurs (or otherwise stated as an increase in the acceleration rate through the spriag and into the early summer). A review of the monitoring data does disclose that movements on many of the monitoring points have continued to steadily accelerate from year to year since monitoring began in 1993 and especially since 1995. We have included several plots of the monitoring data to visually present this information. This data also discloses that the acceleration is greater duriag the wiater and spring periods and to an extent during the fall seasoas with minimal acceleration over the summer periods. This behavior would tend to support the scenazio of the collection of a large volume of water and development of pore pressure in the old workings. 1n our opinion direct infiltration of rainwater and snow melt from the ground surface above the slide mass itself either does not contribute or contributes minimally to pore pressure causing movement along the slide plane. Wetting liont progression from the surface at a unit gradient is simply too slow to cover the full 75 to 100 feet of depth in a single season and cannot produce ffie rapid response and short lag time observed between the atuet of snow melt and the acceleration of the slide movement. Therefore it is believed that the water and pore pressure responsible For accelernting slide movements is sourced in areas off the slide mass, higher on the mountain where exposed cracks and joints near outcrop areas permit rapid infiltration into fractured rock and the fractured rock permits relatively rapid delivery of the water to the base of the slide m:us undrr pressure. This same mechanism could apply to old workings which collect water from the same jointed rock masses, from coal seams. and from tension cracks and rubble zones above subsidence areas to deliver the water and excess pore pressures rapidly to the area fx:hind the slide. Both of these mechanisms are felt to be potentially viable mechanisms for reactivating of the existing slide mass.