06 C8-2025-0085.0A - Arbor View Subdivision District 10 - Staff Geotech Presentation — original pdf

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Geotechnical Report 1 Insufficient information and analysis has been provided with the updated Geotechnical report to confirm whether the underlying rock is structurally competent. • Per the updated report “The structural details must indicate the penetration requirements on the downslope side of the footing excavation. Deeper penetration might be required due to structural requirements associated with the lateral load analyses...” However, geotechnical data has not been collected that would allow a lateral load analysis. • Additionally, per the report a “cursory analysis of potential planar type failure did not yield any concerns.” However, it is not clear if all relevant data was factored into this analysis. 2 Geotechnical Report Staff Concerns 1) Test borings were not performed a) Truck-mounted boring equipment allows sampling several feet into bedrock material, whereas hand-dug pits are limited to excavation of loose or fractured material. b) Test pits were hand dug through fractured rock, with most pits terminating prior to reaching hard limestone c) No test pits extended to the recommended pier depth of 4 ft d) Due to lack of borings, no sampling and evaluation of bedrock material was performed, which limits the applicability and accuracy of any analyses performed 3 Geotechnical Report Staff Concerns (cont.) 2) There is evidence of soil creep – the gradual movement of soil and other surface materials – on the slope. a) As soil gradually moves over time vegetation can be carried with it, causing trees to change their resting angle on the slope. The trees then continue their vertical growth, creating bends in what would otherwise be a straight trunk. b) This is an indicator of slope instability and potential planar failure. It’s not clear if this was factored into the geotechnical analysis. 4 Geotechnical Report Summary Insufficient information and analysis has been provided with the updated geotechnical report to confirm whether the underlying rock is structurally competent. Recommendation Staff does not recommend approval of the variance due to the environmental risks of construction on steep slopes. If the commission approves the variance, staff recommends applying the previously identified staff conditions, including the requirement for a full geotechnical investigation.