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HomeMy WebLinkAboutCrowson Res Report 1999 B.G. HICKS CONSULTING ENGINEERING GEOLOGIST 190 VISTA STREET OREGON E-729 ASHLAND, OR 541-482-8451 FAX/PHONE 541-482-8638 TO: CHIEF KEITH WOODLEY ASHLAND FIRE AND RESCUE 455 SISKYOU BLVD. ASHLAND, OR 97520 CC: MARTY MAIN 1305 BUTTE FALLS HWY. EAGLE PT., OR 97524 CROWSON RESERVOIR AREA REPORT JANUARY 19, 1999 CITY OF ASHLAND PROPERTY CROWSON RESERVOIR AREA STABILITY MAPPING AND VEGETATION MANAGEMENT RECOMMEDATIONS OUTLINE INTRODUCTION II MAP DISCUSSION - STABILITY III SEPARATE ISSUES TRAIL DRAINAGE DROP INLET IV HIGH RISK AREA V OPTIONS: FAILURE HAZARD MITIGATION J~ 2~' 111~ ~~ 2 CITY OF ASHLAND PROPERTY CROWSON RESERVOIR AREA STABILITY MAPPING AND VEGETATION MANAGEMENT RECOMMEDATlONS I INTRODUCTION: This report covers the area of City of Ashland property that is located east of Crowson Reservoir and generally southwest of Ashland Loop Road. My field work was completed in December 1998 and January 1999 and traverses covered the approximately four acres of City land in this area. (See attached map.) The request for my detailed stability mapping input was primarily triggered by the presence of the slope failure above Ashland Loop Road near the junction with Waterline Road. This failure occurred during the December 31, 1996 - January 1, 1997 period of landslides and debris flows. (Earlier mapping that I completed for the City of Ashland Planning Department included this failure zone as well as additional failures further south on the Loop Road and east below Waterline Road.) This report covers the relationship of vegetation manipulation/removal to the interpreted and estimated effects on slope stability. The issue is the concern that this portion of City land could become a fire corridor unless the relatively dense vegetation in this area is modified. Contrasted with the fire risk is the concern that this vegetation removal could result in additional failures that have the potential for impact to the homes downslope. This report presents a point-of-view which calls for minimization of the risk of failure-based on my observations and interpretations. High, medium, and low stability risk zones or areas are described below. The fire risk must be judged in relationship to the hazards and unknowns of assessing slope stability. II MAP DISCUSSION.. STABILITY: 1. Mapped Slope Failure Along Loop Road -- The map shows the area along Loop Road where failures have caused retreat of the cut slope. These failures are due to a process that involves pulses of high velocity (relatively) water flow through the uppermost more permeable zone (P1 zone); commonly at its base. The material below this permeable (and 'weaker' P1 zone) is characterized as stronger and definitely less permeablel (i.e., the P2 zone). The 'failure' occurs as the water emerges from the 'free' face (Le., the cut slope face), causing liquefaction of the granular granitic P1 zone. At liquefaction, the saturated material flows downslope - as a thick, viscous liquid. (During specific circumstances these 'failures' occur instantaneously, almost explosively.) At this Loop Road location the material flows down onto the Loop Road ditch line and is either carried down the road to the first drop inlet or, if the volume or velocity of failure is sufficient. carried directly across the road to cascade downslope to other roads, or cause slope erosion or impact downslope structures. The failure zone (the P1 material) is relatively thin,(about 2 to 3 feet deep) even in the area of current maximum failure (i.e., f1agline[B] to [C] along the cutslope). The loss of the lower portion ofthe original cutslope (in the P2 zone) is due to the gouging effect of the saturated, Iiquified granular moving downslope to the Loop road pavement (Le., not directly failure of this zone). North of this 3 zone (i.e., [B) to [A] and farther north - the depth ofthe weak zone (P1) is less, about one foot. This is the reason the assigned failure risk north of [B) is less. The source for the ground water causing the failures is obviously from infiltration - from upslope and directly into this area. (Additional sources may be present or ground water may be diverted into this area by fault or fracture zones.) It should be noted that the extent and severity of the failure zone shown and the surficial characteristics of older, long term, progressive failures is obvious. I interpret these signs of earlier incipient failure as indicative of long term low stability, and partial failure - before the excavation of the cutslope really destabilized this area. 2. Hydrologic Boundary (Top of Failure Influence Area) - The nominal landslide 'influence area' boundary for the slope facing Loop Road extends from the hydrologic boundary (see map) to the Loop Road. It is within this area that management activities can influence stability (or erosion). 3. RedJ White Striped Flagging Line -I installed a red/white 'candy striped' flag line from the upslope trail intersection with the property line, downslope to Loop Road and extending along the current top of the main failure-slope or cutslope. The key points along this flag line are [A], at the beginning; [B), at north end of high risk zone; [C), at the property stake on the old cutslope; and [E], the upslope end at the junction with the trail. 4. High Risk Area .. The shaded area shown on the map between [B) and [EJ, which extends upslope to the 'hydrologic boundary' is considered the high risk zone along this slope. If it is desired to minimize all failures in this area, I recommend all existing vegetation not be cut and appropriate native vegetation be planted. 5. Medium Risk Area .. The area between [A] and [B), extending directly upslope to the hydrologic boundary is the zone classified as medium risk of failure, principally because of the shallow depth of the upper weak zone (P1). Most ofthis area is occupied by brush. I believe impact due to removal of this vegetation would result in some additional raveling of the uppermost cutslope and unless modified by planting, short-term site impact. 6. Low Risk Area - The entire area north of [A] toward and beyond the (1) (= water bar no. 1) is classified as low risk.( From the cutslope to the hydrologic boundary.) Brush clearing for this area is considered to present the minimum risk of cut slope failure. The obvious thin P1 zone is apparent in the exposed cutslope. III SEPARATE ISSUES: A. TRAIL DRAINAGE: The map shows three recommended water bars and one existing water bar. The construction of these three additional water bars would reduce the water flow and erosion down the old trail. I believe that reduction of erosion and the planting/encouragement of native species along this trail should result in its eventual rehabilitation. B. DROP INLET: A drop inlet is located along Loop Road a short distance northeast of Crowson Reservoir. Water and debris flowing along the ditch enters this inlet. In addition, as shown on the map, flow from the trails and City property sloping toward the Reservoir probably enters the _...............".._.._..~.- 4 back side of this inlet. I believe a trash rack should be installed at this inlet to help insure that it does not plug during high flow and debris load events. IV HIGH RISK AREA The high-risk area, as defined above, is relatively clear and straightfOlward. The difficulty in assessing whether or not any additional failures from this slope area could cause damage to downslope homes is that the range of unknowns is large. In most cases, any 'slow', small failures would be deposited on Loop Road and remain there - or be washed on down the road by surface water flow - toward the drop inlet. However, the slow progressive, continual failure of this zone (there was small-scale active failure occurring on January 18, 1999) is interpreted as indicating that the P1 failure zone will migrate upslope. As this point of failure becomes farther from and at a greater vertical distance above Loop Road, I interpret that the relative hazard of a 'slug' of saturated material being launched with sufficient energy to reach a downslope home is possible. Alternatively, a slug of granitic material reaching the road can be washed over the side of the road by water flowing down Loop Road. Both of these types of events occurred during the 1996-1997 high precipitation/flooding. The risk of these occurrences is low but the potential is real. (An option of 'failure control' is possible as discussed below.) V OPTIONS: FAILURE HAZARD MITIGATION STOPPING FAILURES FROM DOWNSLOPE TRANSPORT A. RETAINING WALUOUTSIDE ROAD-EDGE BARRIER Although not the first or probably the best option, it is possible to protect downslope structures from landslides/debris surges by construction of a barrier at the outer road edge. I do not at present have a cost estimate for this construction. I can prepare this estimate is requested. B. FILTER-BUTTRESS A more realistic solution to the issue of stabilization of this approximately 120 foot long failing cutslope is the construction of a filter-buttress. (I have constructed a smaller size buttress ofthis design for the identical failure situation.) This buttress would stabilize the upper three to four feet of cutslope. In practice, an excavation would be made at the uppermost +/- four feet of the existing cutslope to create a new slope at +/- 80 degrees to which is attached filter-fabric covered by a wire- mesh/rock blanket. This outer rock blanket (and entire filter assembly) is "stapled" to the soil/P1 slope by driven rebar. The outer end of the rebar is bent into a 'hook' to bind the wire-mesh/rock blanket to the filter-fabric thence to the previously failing ground. This type of remedial construction is designed to allow the ground water to flow through the soillP1 zone without dislodging or moving the soil, thus stoppi e "failure". The estimated cost of this approximately 480 square feet of filter-buttress 00 to $4500. A refined cost estimate can be made upon request. ORE30~.. !J O{;i'~i'"'t L...... . ;n~, ''-It 1 ii.r_oJ~+-4 . .. .. ~ (J \.,,~ * ~,/ ~IiING G~o .~Q.~~ ' ~~S I I /;/ ~- :::: ;- " ~ ,- " ------- 24DO ( ',,- '''-'''- "" ~ \'.. ' DROP INLET CROWSONRESERVOlR,\ "--"'---..~ "'r--:~ \ ) .1t~~! - ,~ -- "'. ~ \ \ .. ~, fr - - ,-, \. \.. ~ I'- ~" ACCESS TO 37~ '" ./ \ '\ "" AND 395 LOOP ROAD ~ .- \ \ \\-.- -.-' ....-----,~ I \ 450.---- /------~I \ \ _~V // ~1' <0 \1 __--~ ...Jt;f ./ -1-~ \ -,11 ~- 6"S"'y~) i:;.~ ! ) ,/ CITY PROPERTY _ -f h '. .I 'I I 1- 500 / t) ,{ - ~ t-') / \ \ ;t6){1 A-II~ \~ I /<, < /' ~ : I \\ I } ; . i \ /n) ) ~,; I I II '. .! ( - // / J I . i ' / _ _~ / i / ) ,~StILAND LOOP \1 SCALE: 1 INCH = 100 FEET .L.v-:;-"- ,," (I ROAD '" ~6"0//C:: \ I /:F; \ u BGHlc::=25;:ARYI999 / I / J / / . ! .,1 / // / I ! / / I II . I / I ~ iRAe! STREET. / I ! /~ \ \ / / , \ \ '--'""'" \// / ~ \ , ,/, /~ (/1 IV'? .-// 2300 '------- 135.0 "",-,,,,"'1; -..... LEGEND , rK\ REDIWHITE STRIPED ~ FLAGGING UNE Af -""f SLOPE FAILURE ALONG f 4cp LOOP ROAD ~ _ C~PROPERTY UNE APPROX. HYDROLOOIC /"-- BOUNDARY(TOPOF ~/ FAILURE INFLUENCE ( AREA) ~~ RECOMMENDED ADOI-. TIONAL WATER BARS o T I.. .,.. CITY OF ASlll.AND PROPERTY CROWSON RESERVOIR AREA STABILITY MAPPING