PUD DESIGN EXACERBATES EXISTING SLOPE STABILITY ISSUES <br />Given that the assessment has been conducted in a manner which is both slip-shod and intentionally <br />limited to the areas of the PUD most likely to be without slope stability issues, the design is <br />seemingly unfettered by geotechnical constraints. Whereas it might be possible to design the PUD <br />to effectively address these real issues, its current design exacerbates existing slope stability <br />problems. Parts of the infrastructure (mostlythe driveways to lots 8/9, 16/17 and 19/20) and the <br />building pads for lots 8 through 19 will be located along the top of the steep east-facing slope. On <br />these twelve lots, the slope of the proposed buildable area ranges from 40 to 73(!) %, with an average <br />of 56%. The result is that fill embankments will need be placed at the top of the steep slope, which, <br />in some cases will be designed with slopes to 1.5H : 1V (67%), which is in itself quite “optimistic”. <br />Normally, fill slopes are designed to not exceed 2H : 1V. The excessively steep fill slopes are <br />employed here to limit the eastward and downslope extent of the toe of the fills which would likely <br />reach the eastproperty line of the PUD or even the Ribbon trail if installed at appropriately <br />conservative slope angles. However, even the 1.5H : 1V slope angle will not suffice at Lots 18 and <br />19 to achieve a catch point within the confines of the buildable envelope for any fill. <br />In essence, constructing fills in the upper portions of the east-facing slope results in loading the top <br />of the slope, which increases driving force. Given that there are already indications both on and <br />immediately adjacent to the PUD property that this slope is undergoing slope failure, it is quite clear <br />that any adverse change to the equilibrium will result in re-activation of existing slope movements <br />or generation of new ones. <br />However, not only does the PUD as currently designed increasethe driving force at the top of <br /> This <br />the slope, but it also decreases the resting force in at least one area at the toe of the slope. <br />will occurin the area immediately east of the SW corner of Lot 13 and the SE corner of Lot 14, <br />where the two 38-ft. level spreaders are proposed to be located. Per the proposed plan these storm <br />water discharge systems are to dispose of the storm water which is generated from most of Cupola <br />Drive and the 19 or 21 lots adjacent to it (Lots 7 – 17, 33, 34, and 27 – 32, plus possibly Lots 18 and <br />19). The Level Spreaders are proposed to be designed for a maximum discharge of 0.16 cfs (cubic <br />feet per second) each for a total discharge of 0.32 cfs over an approximately 70-ft. wide stretch of <br />slope. While this doesn’t seem like a large number, one must consider the units used. One cubic <br />foot is 7.48 gallons, and there are 60 seconds in a minute. As a result, this seemingly small number <br />is actually 144 gallons per minute. In comparison, a garden hose runningat full capacity is around <br />5 gpm. Therefore, the design discharge of these two level spreaders is the equivalent of nearly 29 <br />garden hoses running full bore. And all of this water is discharged onto a segment of 50% slope that <br />is located 104’ from the Ribbon Trailand above several residences along Floral Hill Drive (see <br />figures on the following pages). <br />It is impossible to see how this design: <br />Does not result in severe soil erosion and running muddy wateronto and across the Ribbon <br />A) <br />Trail; and <br />Does not result in a localized increase in pore pressures near the base of the steep slope. <br />B) <br />8 <br />Capital Hill PUD Geotechnical Review, GeoScience, Inc. 3/7/18 <br /> <br />