202 ALDER ST 2025-08-25 - Laserfiche WebLink

Based on LIDAR data, the slopes along Pigeon Creek that meet the geometric requirements for treatftientuas critical areas are located between approximately 110 and 200 feet east of the east boundary of the View Ridge Elementary School site. A copy of the LIDAR data we reviewed is attached with this letter as Figure 1. Conclusions and Recommendations The off-site steep slopes to the east of the project need to be considered in the context of slope stability considerations, as well as storm water infiltration potential of the site. From a geotechnical perspective, the slopes east of the site are expected to consist of glacially consolidated advance outwash sediments, based on our on-site explorations, as well as on published mapping referenced in our earlier preliminary report for the project. Dense glacially consolidated soils typically possess relatively high strength characteristics, and tend to perform well with respect to slope stability. The project limits are 125 feet or more from the existing off-site steep slopes of the Pigeon Creek drainage. Significant proposed site improvements are substantially farther away from the steep slopes, on the order of 300 feet or more. The total height of the steep slopes adjacent to the project area is on the order of 75 feet. Considering the height of the steep slopes, the distance from the project to the steep slopes, and the assumed dense, glacially consolidated nature of the subsurface materials in the slope area, the off-site slopes pose a low risk of damage to the proposed site improvements, in our opinion. Additionally, the project as currently proposed does not call for thick structural fill, or other substantial loads in close proximity to the off-site steep slopes that would be likely to adversely affect the existing stability of those slopes, in our opinion. No detailed quantitative slope stability analysis was completed as part of our current study, and none appears warranted based on geotechnical parameters, in our opinion. The stability of the off-site steep slope is also a consideration with respect to storm water disposal on-site by infiltration. Based on our recent discussions with Coughlin Porter Lundeen, the project civil engineer, we understand that the storm water management plan is expected to include smaller, distributed infiltration structures, such as rain gardens and areas of permeable paving. These areas are expected to be located primarily on the west part of the site, distant from the off-site steep slope. All proposed infiltration features would be approximately 200 feet or farther from existing off-site steep slopes. The infiltration structures may include gravel-filled "pit drains" that enhance ground water infiltration in stratified sediments, and also result in delivery of infiltrated storm water deeper below the ground surface. All of these features tend to limit increases in ground water levels distant from the site, which is desirable. It is conceivable that infiltration of storm water on-site could result in higher ground water levels at the face of the off-site slope,potentially resulting in reduction in slope stability factors of safety and/or increased erosion at the off-site steep slope area. The magnitude of such risks is dependent on the nature of the proposed infiltration system, the location of the infiltration system, and the characteristics of the storm water receptor. At this site, considering the observed and mapped subsurface conditions, the observed ground water levels, and the nature of the planned infiltration system components, it is unlikely, in our opinion, that substantial adverse impacts will result with respect to stability of off-site steep slopes. This opinion is based on our previous experience with 2