GM22-16_Final_Conceptual_Design_Geotechnical_Memorandum_15July2016.pdf

GEOLOGIC SETTING AND LANDSLIDE DESCRIPTION

The Wells Creek watershed, and to a larger extent the area in Douglas County underlain by the Elkton Formation, is marginally unstable when groundwater is concentrated in an area, or the bedding of the Elkton Formation sandstone and shales, in combination with high groundwater conditions, are dipping toward the open face of a hillside. This is shown in several locations in the recent (2015) LiDAR acquisition by the Bureau of Land Management (BLM) in early 2016 through the Oregon Department of Geology and Mineral Industries (DOGAMI) (Figures 2 and 3).

The landslide is approximately 375 feet wide by 450 feet long and significant spring activity is prevalent in the middle and eastern portion of the landslide as high as 995 feet above mean sea level.

Two recent landslide events have occurred within a much larger prehistoric landslide feature at this location. The two recent landslide events occurred in January 2012 during a heavy precipitation event and the second event followed reconstruction of the roadway in December 2014 during a heavy precipitation event. The December 2014 landslide event retrogressed to the north and west approximately 50 to 100 feet, expanding the area of the landslide.

The constructed road repair in 2014 included raising the grade approximately 6 feet in the middle of the landslide mass, removing landslide debris from the approximate location of the original roadway, and cutting a new 1V:1.5H cut slope in the landslide debris. A shallow cutoff trench drain in the ditch line was included to intercept cut slope and roadway generated runoff water. During construction of the roadway and cut, several springs where encountered that ran heavy and dissipated over time for construction to continue (Figure 4). In one case near the east end, the landslide material was so wet and unstable that a riprap rock armoring was used to provide surface stabilization of the cut slope materials (Figure 5). No test borings or instrumentation was installed as part of this initial ERFO project work.

GEOTECHNICAL INVESTIGATION

After discussion with the BLM and our ERFO coordinator, it was determined that the landslide at Wells Creek is complex and requires a moderate level of geotechnical investigation beyond a site reconnaissance to reduce the likelihood of short-term failure of the reestablished roadway after the December 2014 reactivation of the landslide. For a thorough description of the general landslide characteristics, which have not significantly changed since 2014, please refer to the previous site assessment conditions contained in the WFL Geotechnical Memorandum (GM 09-14) dated March 27, 2014.

Test Borings and Instrumentation

Two test borings were advanced within the landslide mass. Locations of the test borings are shown in Figure 6, the topographic site survey map and within Figure 7, the geologic cross-section.

The lower test boring (B16-01si+vwp) was advanced 85 feet below the ground surface (bgs), encountering four interpreted episodes of landslide deposits overlying a thick sequence of shale from approximately 67 to 85 feet bgs. The upper (December 2014) landslide deposit is demarked by the 2014 reconstructed paved road surface at about 15 feet bgs. The second landslide deposit (January 2012) is demarked by an organic layer with preserved leaves at approximately 22 feet bgs. Two more interpreted prehistoric landslide deposits lie between approximately 22 and 67 feet (bgs).

These prehistoric landslide deposits clearly change around 45 feet to clayey sand with cobbles and boulders that appears older with more soil development than the overlying prehistoric landslide deposit from about 22 to 45 feet bgs. From 60 to 67 feet bgs, a landslide shear zone is present with broken splinters of shale rock. A slope inclinometer and a vibrating wire piezometer were installed in the lower test boring to measure landslide movement and groundwater. The slope inclinometer does not show subsurface landslide movement from February 22 to April 15, 2016 and groundwater is approximately 6 feet bgs and fluctuates approximately 0.5 feet during precipitation events greater than or equal to 1 inch in 24 hours.

The upper test boring (B16-02si+vwp) was advanced 121 feet bgs, encountering a mixed landslide deposit overlying a sequence of sandstone and shale below 34 feet bgs. In the upper test boring, different landslide events/deposits are not discernable in proximity to the landslide headscarp. A slope inclinometer and a vibrating wire piezometer were installed in the upper test boring to measure landslide movement and groundwater. The slope inclinometer does not show subsurface landslide movement from February 22 to April 15, 2016 and groundwater is approximately 33 feet bgs and fluctuates approximately 0.5 feet during precipitation events greater than or equal to 1 inch in 24 hours.

The test boring logs and instrumentation data will be presented in the final geotechnical memorandum for this project following selection of a preferred risk reduction conceptual mitigation for the landslide.

LANDSLIDE ANALYSIS

The landslide model was developed based on the test borings, instrumentation monitoring data, and site observations. The model is well represented in the upper half of the model where the test borings were advanced, but the lack of subsurface investigation within the landslide mass below the lower test boring results in less confidence in the model for the lower portion of the landslide mass.

Therefore, the impacts of loading the lower portion of the landslide mass is not well understood south of the 2014 road alignment, where we propose the new roadway be located to limit disturbance to the upper portion of the landslide mass.

The stability model indicates the lower portion of the landslide is currently failing and we know this is not true based on our field observations over the last year. So, without additional geotechnical data to refine our subsurface geometry and understanding of the landslide mechanics below the road better, we have evaluated the risk of an embankment failure as the difference in factor of safety between the different types of embankment materials presented in the next section. The result of this relative risk evaluation of having a bearing capacity failure indicates that there is a 5 to 10% reduction in embankment stability when using traditional fill materials; such as, soil and rock.

Minimizing the load of the embankment with the unknown landslide geometry and mechanics downslope would be a better approach to reduce the risk of embankment failure. The following bullets provide the information learned during the stability analyses and evaluation of risk reduction conceptual mitigation options explored. Note that no structural elements; such as, tieback retaining walls, shear-pin/piles, or stone columns, etc. were considered due to the scope of the work outlined in the Damaged Survey Report dated July 30, 2015:

• Referring to the Figure 8 landslide model, the back-analysis of the landslide shows that the landslide likely failed before the large rock debris mass failed from the cliff face.

• It appears that the active landslide mass is primarily contained to the upper two-thirds of the landslide feature and the lower third may be more of a landslide debris run out zone, as witnessed during the last two recent landslide events. This might explain the difficulties in modeling the lower portion of the landslide deposit for embankment loading bearing capacity failures.

• The presence of boulders and cobbles in a silty sand or sandy clay matrix soil of the landslide debris resulted in higher internal angles of friction than laboratory test and empirical shear strength correlations results indicate.

• The 2014 road reconstruction removed resisting landslide debris from the toe of the landslide mass, effectively debuttressing the landslide mass and making it less stable for winter groundwater conditions. Therefore, the existing landslide mass at the previous road relocation should not be removed. It will result in near-term failure and continued retrogression of the landslide mass to the north and west, as occurred during the last landslide event in December 2014.

• Reducing groundwater elevations within the upper landslide mass provides the greatest slope stability improvement, but near-surface (within 15 feet) conventional cutoff trenches are not reasonably constructible to reach the groundwater in the upper landslide mass above the previous roadway alignment. Therefore, horizontal drains will be an integral part of the risk reduction solution for establishing a roadway across this landslide.

Raising the road grade to match the existing landslide deposit elevation near the road alignment is critical in maintaining existing marginal stability of the landslide mass and reducing the likelihood that the landslide will reactivate on a larger scale. Whatever the final selected alternative is, it is important to, at a minimum, raise the road grade and reduce the groundwater levels in the upper landslide mass.

RISK REDUCTION CONCEPTUAL LANDSLIDE MITIGATION ALTERNATIVES

Several risk reduction conceptual landslide mitigations were evaluated that involved drainage improvements and earthwork adjustments only. None of the risk reduction conceptual landslide mitigations presented meets the typical factor of safety design standard of 1.25 to 1.30 for highways when mitigating landslides. The alternative presented below focused on a 10% to 20% stability improvement to reduce the risk of future landslide events. The pros and cons for each of the risk reduction conceptual alternatives presented are provided in order of apparent lowest to highest improvement in stability:

1. No drainage improvements and raise the road grade a minimum of 25 feet above the 2014 road elevation (Figure 9): Approximately a 10% stability improvement, but doesn’t account for winter high groundwater conditions since we don’t have any groundwater data to support this modeling yet. For this reason, this is likely less stable than modeled with currently available information. Furthermore, raising grade 25 feet doesn’t seem feasible for loaded log-haul traffic and safety when approaching the sharp road curve off the landslide feature to the west.

2. Install horizontal drains and raise the road grade a minimum of 15 feet above the 2014 road elevation within the landslide deposit (Figure 10): Approximately a 15% stability improvement, but requires horizontal drains (1595 lineal feet) to (hopefully) reduce the groundwater levels nearly 30 feet in the upper landslide mass. There may not be enough horizontal drains scoped for the project to achieve this reduction in groundwater elevations.

Furthermore, removal of the existing landslide debris to achieve a new road bed may decrease the stability of the slope in steeper ground and/or thinner landslide deposit areas that require more aggressive excavation to establish the roadway again. A disadvantage when utilizing horizontal drains is they will require periodic maintenance inspection every year or two and cleaning to maintain flow rates and remove biological build up usually every 5 to 10 years (Figure 11). This maintenance work can be contracted or a horizontal drain “rat” can possibly be borrowed from Oregon DoT (ODOT). It is our understanding that they own such a horizontal drain cleaning system for many of their horizontal drain installations along Highway 101. The Washington State DoT (WSDOT) also owns similar equipment that resides in the Olympic Region with maintenance crews. Horizontal drains installed in soil for this proposed quantity are typically on the order of $30 to 35 per lineal foot (collector pipe and horizontal drill pads are not included).

3. Install horizontal drains and raise the road grade a minimum of 15 feet above the 2014 road elevation in front of the existing landslide deposit: Approximately a 20% stability improvement and requires a modest reduction in groundwater elevations, about a 5 foot reduction at the lower test boring, and a significant draw down of the upper landslide area to realize full stability improvement. The full stability improvement will not be known until the horizontal drains are installed and groundwater is measured in the upper vibrating wire piezometer. If the modeled groundwater reduction is not realized, some of the stability improvement will be eroded, but likely no more than 10%. Two types of embankments were evaluated for the fills in front of the existing landslide debris. They are described as follows:

a. Reinforced Soil Slope (RSS) Fill containing aggregate with adequately spaced geosynthetic reinforcement to hold a steeper embankment face (ranges from 1V:1.5H with a turf mat to a 1:1 wrapped face) (Figure 12). The only disadvantage with this concept is it imposes a larger load on the lower landslide materials that may cause a rapid-to-slow bearing capacity failure over time in the underlying landslide debris. The relative reduction in stability for a bearing capacity failure below the new embankment section, which is different from the failure plane shown in the active landslide failure upslope in Figure 8, is between approximately 5 to 10%. Due to this modest decrease in stability with a more traditional reinforced soil slope concept, we believe it is prudent to minimize the loading effect of the embankment materials.

b. Lightweight Fill reduces the load of the embankment section between 66 and 90% of the aggregate (RSS) fill option, minimizing the potential for a bearing capacity failure from the embankment loading in the underlying landslide debris.

i. Wood Chip fill can also hold a steeper embankment face (ranges from

1V:1.5H with a turf mat to a 1:1 wrapped face with adequately spaced geosynthetic reinforcement. The disadvantages are wild fire susceptibility; although, less likely on the west side of the Coast Range of Oregon than in other Regions, and inclusion of a cutoff trench in the uphill ditch will be necessary to keep the wood chips from becoming saturated and accelerating decay of the wood chips. A permanent erosion control mat should be pinned to the exposed face of the 1:1 wrapped face embankments (RSS and Wood Chip) to mitigate ultraviolet decay of the geosynthetic materials and to promote rapid vegetation growth. Typical costs for a geotextile wrapped embankment face per square foot of face are very similar to aggregate. They cost of this item will more likely be controlled by haul distance for this remote project location. We estimate that the wood chip or aggregate (RSS) fill material will be on the order of $40 to $50 per square foot of finished embankment face (cutoff trench not included).

Some earlier literature by the US-Forest Service, Region 6 - Oregon and Washington, discusses a shorter design life for wood chip fills at 20 to 25 years, but further review of this paper and contacting the Oregon Department of Transportation (ODOT) and Washington State Department of Transportations (WSDOT) has resulted in the following findings:

• They have not removed any lightweight wood chip, sawdust, or hogs fuel fills in their collective memory over the last 30 years for life cycle deterioration reasons.

• WSDOT exhumed a 1972 sawdust fill on Highway 101 south of Aberdeen, Washington after 15 years in-place to evaluate the condition of the fill. They found the outer 1 to 2 feet of sawdust material had deteriorated, but the material underneath was still as fresh as it was when installed 15 years earlier.

• The originating article of unknown publication date by the USFS does not include an uphill cutoff trench as part of their guide for design of lightweight fills, which would increase the decay rate of the wood chip fill from repeated wetting and drying cycles.

• They have seen some settlement at the transitions between the wood chip fill and the heavier soils that must be maintained over time on a paved surface, such as a highway.

ii. EPS-Geofoam fill can also hold a steep faced embankment and is even lighter than the wood chip fill; however, the EPS foam must be protected from petroleum products with a full geomembrane wrap to mitigate dissolution of the EPS foam (by petroleum products) into the environment. The EPS geofoam must also have a cutoff trench to capture groundwater in the uphill ditch line to alleviate buoyance of the blocks by groundwater uplift pressure. The other disadvantage is the potential cost of the materials and the need to typically contain the geomembrane wrapped EPS foam with a wall structure. A rigid wall structure is not appropriate for this potentially active and creeping landslide site. A flexible wall face would be required to confine the EPS blocks and this would result in added expense to the already expensive geofoam materials. The cost for EPS geofoam is approximately $95 to $105 per cubic yard and the cost for the flexible wall face to contain the geofoam blocks would be an additional $45 to $55 per square foot.

PREFERRED RISK REDUCTION CONCEPTUAL LANDSLIDE MITIGATION

Based on the risk reduction conceptual mitigations evaluated and the complexity of the large landslide mechanics, we believe installing horizontal drains is the most important risk reduction measure followed by raising the road grade a minimum of 15 feet above the 2014 road elevation in front of the existing landslide deposit with lightweight, wood chip fill with a geotextile wrapped face in front of the recent landslide mass and across the larger, prehistoric landslide mass (Figure 13).

The landslide stability advantages and apparently lower cost than the EPS-Geofoam option appear to outweigh the disadvantages for this preferred risk reduction landslide mitigation. The grade raise for the road will require steeper grades and longer reconstruction lengths to tie in to the current roadway section. Wood chip fill for the embankment section, with the geotextile wrapped face, is only required when within the limits of the landslide mass.

CONTRACTING CONSIDERATIONS

If there is an opportunity with the project timeline, we would suggest installing horizontal drains, as shown in the topographic site survey map at both locations and monitoring the reduction of groundwater levels in the landslide. This will allow us to refine our landslide stability model and predicted stability improvements for the landslide.

CONCLUSION

This geotechnical memorandum documents the information and methods used for evaluating landslide mitigation options with a moderate level of geotechnical data to arrive at a preferred risk reduction conceptual landslide mitigation for the Wells Creek Landslide. Further refinement of the lower landslide and embankment failure analysis for the different types of embankment materials could be done with one additional test boring in the lower portion of the landslide mass.

If you have any questions concerning the information contained in this geotechnical memorandum, please contact Douglas A. Anderson at 360-619-7958 or at douglas.a.anderson@dot.gov.

REFERENCES

A.R. Niem, W.A. Niem, and E.M. Baldwin, 1989, Geology and Oil, Gas, and Coal Resources, Southern Tyee Basin, Southern Coast Range, Oregon, State of Oregon Department of Geology and Mineral Industries, O-89-03.

Anderson, D.A., 2014, Geotechnical Reconnaissance with Design and Construction

Considerations for the Wells Creek Road Landslide and Associated Waste Sites, BLM ERFO 121-3301, FHWA – WFL Geotechnical Memorandum (GM 09-14).

Steward, J., unknown publication date, Guide for Design of Lightweight (sawdust) fills, USFS

– Region 6, Engineering Soils and Materials Group Engineering Technical Services Guide.

mailto:douglas.a.anderson@dot.gov

Figure 1. A topographic map showing the vicinity of the Wells Creek Road Landslide project. The landslide is denoted by a red flag.

Umpqua River

C.3

STATE PROJECT

NUMBER

SHEET

I n t_ ft

D c m y fi le s p w p r o d u c ti o n d I n t_ ft

D .d g n

OR BLM ERFO 151-3301

A

M

J u n e

C h e c k e d b y

D e s ig n e d b y

CASED IN SOIL

HORIZONTAL DRAINS

above horizontal installation angle

Horizontal drain varies

Angle

5'-0"

HORIZONTAL DRAINS-CASED IN SOIL

Coupler

Concrete pack

Horizontal drain manifold system

Cleanout plug polyethylene drain pipe pipe - 8" corrugated

Horizontal drain collector

(min.)

6"

NO SCALE

PVC wye coupler

2" Schedule 80 polypropylene pipe

2" Flexible solid PVC pipe

2" Schedule 80 slotted PVC pipe

2" Schedule 80 matthew.hinshaw Typewritten Text

FIGURE 11

P

J u ly

U

S S u r ft m y fi le s p w p r o d u c ti o n d o r b lm c b .d g n

STATE PROJECT

NUMBER

SHEET

C.2

OR

BLM ERFO 151-3301

C h e c k e d b y

D e s ig n e d b y

FOOTNOTE:

Construct slopes as shown in the Staking Report.[2] the plan and profile curve data.

Superelevate roadway on curves at the rate 'e' as indicated on[1]

TYPICAL SECTION

RSS FILL

24+00 to 28+00

WELLS CREEK ROAD

1:

Traveled way

9'

LC

Traveled way

9'

Profile grade

1:2

C le a r in g l im it

12-inch compacted depth

Roadway aggregate, method 2, Class 1, Type D

Geotextile, Separation Stabilization-

Existing ground

[2

1:

1.

[1] 2% Crown

:1

12" compacted depth

Select borrow, C le a r in g l im it

( 12" Layers)

Geogrid, Stabilization

Wrapped FaceSee Detail, Sheet F.2

Underdrain System

Turf establishment

Turf estab lishment

(RECP)

Erosion Control Product

Pinned Permanent

Geogrid, Stabilization

Select Granular Backfill embankment construction

Minor excavation for matthew.hinshaw Typewritten Text

FIGURE 12

1:

1:2

LC

C le a r in g l im it

C le a r in g l im it

Turf establishment

Existing ground

12-inch compacted depth

Roadway aggregate, method 2, WELLS CREEK ROAD

20+00 to 24+00

28+00 to 30+00

P

J u ly

U

S S u r ft m y fi le s p w p r o d u c ti o n d o r b lm c a .d g n

STATE PROJECT

NUMBER

SHEET

C.1

OR

BLM ERFO 151-3301

C h e c k e d b y

D e s ig n e d b y

FOOTNOTE:

1:1.5

12" compacted depth

Select borrow, Class 1, Type D

Geotextile, Separation Stabilization-

Profile grade

Unclassified Borrow

Traveled way

9'

Traveled way

9'

[2

1:

1.

Construct slopes as shown in the Staking Report.[2] the plan and profile curve data.

Superelevate roadway on curves at the rate 'e' as indicated on[1]

[1] 2% Crown

24+00 to 28+00

1:

Traveled way

9'

LC

Traveled way

9'

Profile grade

1:2

C le a r in g l im it

12-inch compacted depth

Roadway aggregate, method 2, Class 1, Type D

Geotextile, Separation Stabilization-

Existing ground

1:

1.

[1] 2% Crown

12" compacted depth

Select borrow, C le a r in g l im it

(12" Layers)

Geogrid, Stabilization

Wrapped FaceSee Detail, Sheet F.2

Underdrain System

Turf establishment

Turf estab lishment

(RECP)

Erosion Control Product

Pinned Permanent

Geogrid, Stabilization Wood Chip Fill

TYPICAL SECTION

(WOOD CHIP FILL)

WELLS CREEK ROAD

Turf estab lishment embankment construction

Minor excavation for

FIGURE 13