November 10, 2025 Planning Board Special Meeting - WEHI Transcript

3 of 28 sheets Page 9 to 12 of 112 01/05/2026 03:19:18 PM 9 through the rock.1 The Princeton Hydro writing for the objectors2 were of the opinion that a very low strength index3 should be used and they did perform analysis using their4 stability index.5 It's our opinion that it's prudent to use the6 lower strength index. The complicating factor is that7 while they did use the lowest strength index the safety8 factor came out to be something like 1.7, which in our9 practice would be considered to be adequate. So that,10 that particular zone of slope evaluation where the11 failure surfaces passed through the bedrock, even though12 if you really downgrade the strength of the rock, you13 still come up with an adequate safety factor stability14 based on most of the references. The Princeton Hydro15 references an Army Core of Engineers standard for very,16 very large engineering projects, actually built on17 bedrock slopes, and it's for structures like major dams,18 structures where the failure would result in a19 catastrophe. You know, multiple deaths, you know, in20 the downstream channel below the dam if the structure21 was to fail. That criteria is not, in our, in our22 experience or in our opinion, you know, appropriate for23 this particular analysis case. So we have two24 consultants doing stability analysis. One consultant25 10 using a program called, Slope/W, which is a high powered1 conventional software program for doing this analysis2 and Princeton Hydro used a program called Slide, which3 is also an equally competent tool to use. And they both4 come out with safety factors that exceed 1.5 from5 multiple cases, and so it was our conclusion that, that6 the, that the studies both indicate that the -- that7 none of the analysis performed indicate that there is8 unstable conditions in the slope and yet you have this9 USGS mapping of a slope, potential slope hazard at the10 site. First impression based analysis, it's classified11 as a Hazard Class B3 and for slopes that were up to 1512 degrees from incline from the horizontal.13 The other slope on this site is inclined at slope14 angles between 20 and 30 degrees and so our first15 impression was the mapping was in error, but I think16 it's just a, through the -- they were talking about the17 sandstone slope, that's the lower half of the slope, and18 it's probably due to the effect that if groundwater

ing was in error, but I think16 it's just a, through the -- they were talking about the17 sandstone slope, that's the lower half of the slope, and18 it's probably due to the effect that if groundwater rose19 into it the cemented sands might weaken based on the20 influence of the groundwater and if you get a condition21 called, what's called a Simplified Increment Slope22 Condition, in other words the groundwater flows just23 parallel to the slope, it's established that, that would24 be the controlling safety factor and basically when you25 11 have groundwater -- if you do not have groundwater1 through the slope then the infinite slope is what I2 guess would be conventionally called the angle of3 repose. In other words, if you poured sugar out of a4 jar it would form a certain slope and that would be the5 angle of repose. The theory is that when groundwater6 rises up to the level of the top of the slope and then7 begins to flow downhill towards that, that the stable8 slope reduces to about half of what would ordinarily be9 the angle of repose. In a more complicated, it's called10 the angle of internal friction. It's a much more11 complicated phenomena than the angle of repose, but in12 order for the sheer -- for the soil to fail there's a13 thing called a friction angle, which is roughly related14 to the angle of repose and so, and so the, but the15 analysis performed by both consultants both indicate16 safety factors at 1.5 or better. So it doesn't indicate17 the slope, but the hazard report may be, may be18 considering that on the lower slope the groundwater19 would rise into the slope and could lead to landslides.20 That's our -- we could not find a geological report in21 time for this meeting that would explain why the, the22 hazard classification was made. It's just our opinion23 of what it probably is.24 We felt that the, that the analysis did not, that25 12 we had seen in the reports did not address extreme1 rainfall events where these slope conditions typically2 arise and we recommend that they do that. Langan's3 engineer as of they reported that that analysis was4 included but they didn't include the, the output5 presentation and so, and so that should be provided so6 that we, you know, that we could see it and we would be,7 it would, if the analysis proceeded to the lower slope,8 if you assume that the groundwater is at the surface,9 the slopes, the

e provided so6 that we, you know, that we could see it and we would be,7 it would, if the analysis proceeded to the lower slope,8 if you assume that the groundwater is at the surface,9 the slopes, the safety factors might be reduced below10 the 1.5, but we haven't seen those analysis.11 We, in reviewing the plans we noted that there12 are two best management practice infiltration basins at13 the building location. And so the purpose of the14 infiltration basin is to, is to mound the groundwater15 during a high rainfall event and infiltrate at least, I16 think, I'm not, I don't do this type of design analysis17 work and so, but is they have to, they have to show that18 they're infiltrating water based on an assigned storm, a19 low level storm and they have to prove that they can20 infiltrate that much. The problem here is that for21 slope stability you don't want to be introducing,22 introducing more groundwater at the top of the slope, so23 we recommended that they, that the, that the applicants24 engineer or the applicant instruct his engineer to25