GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS · Page 5 of 15 . 23-1-01453-230_TM5. 23-1-01453-230 . slope stability model of the landslide. Because the slope has moved significantly,

SHANNON &WILSON, INC. GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS

June 6, 2017

Kadrmas, Lee & Jackson, Inc. 4585 Coleman Street Bismarck, North Dakota 58503-0431

Attn: Ms. Jen Turnbow and Mr. Troy Ripplinger

RE: PRELIMINARY GEOTECHNICAL DESIGN MEMORANDA TM-5, US-85-1-94 TO WATFORD CITY BYPASS, PROJECT NO. 9-085(085)075, PCN 20046, BILLINGS, MCKENZIE, AND STARK COUNTIES, NORTH DAKOTA

We are pleased to submit our preliminary geotechnical design recommendations for the

Horseshoe Bend (HSB) landslide. The enclosed Technical Memorandum TM-5 presents

geotechnical characterization of the HSB landslide and provides a conceptual-level discussion of

mitigation alternatives.

We appreciate the opportunity to be of service to you on this project. If you have any questions

or require further information, please contact me at 303-825-3800.

Sincerely,

SHANNON & WILSON, INC.

DAV:GRF/dav

Encl: Technical Memorandum TM-5, Horseshoe Bend Landslide Mitigation Considerations

1321 BANNOCK STREET, SUITE 200 DENVER, COLORADO 80204-4077 303-825-3800 FAX: 303-825-3801 TDD 1-800-833-6388

www.shannonwilson .com 23-1-01453-230

23-1-01453-230_TM5 23-1-01453-230

Shannon & Wilson, Inc. 1321 Bannock St., Suite 200 Denver, CO 80204

MEMORANDUM 303•825•3800 Fax: 303•825•3801 TO: Jen Turnbow and Troy Ripplinger (KLJ) FROM: Greg Fischer, David Vara DATE: June 6, 2017 RE: TECHNICAL MEMORANDUM TM-5, HORSESHOE BEND LANDSLIDE

MITIGATION CONSIDERATIONS, US-85 – I-94 TO WATFORD CITY BYPASS, PROJECT NO. 9-085(085)075, PCN 20046, MCKENZIE COUNTY, NORTH DAKOTA

This memorandum presents geotechnical characterization of the Horseshoe Bend (HSB) landslide and provides a conceptual-level discussion of potential mitigation alternatives. All stationing referenced herein refers to the existing alignment and stationing provided by KLJ in the file control_mckenzie.dgn.

BACKGROUND

Multiple landslides, which coalesce into a single large landslide, have been mapped1 in the HSB area (see Figure TM-5-1A and TM-5-1B), and as discussed in Technical Memorandum TM-22, chronic landslide activity has affected the roadway near HSB since at least the 1970s. Landslide activity resulted in a substantial realignment of the roadway in the 1980s (see Figure TM-5-1A. In 2011, an episode of movement adjacent to the roadway resulted in the North Dakota Department of Transportation (NDDOT) realigning the roadway further to east (see Figure TM-5-1A and B). Since realigning the roadway, landslide movement has continued in this area, as indicated by pavement distress and inclinometer monitoring.

1 Murphy, C.E., and Gonzalez, M.A., 2003, Surface Geology, Long X Divide, North Dakota Quadrangle, North Dakota Geological Survey, Scale 1: 24,000. 2 Shannon & Wilson, Inc., 2017, Preliminary Geotechnical Design Memoranda, US-85 – I-94 to Watford City Bypass, Project No. 9-085(085)075, PCN 20046, March 16.

Technical Memorandum TM-5 June 6, 2017 Page 2 of 15

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SITE CHARACTERIZATION

Regional Geology

Surficial deposits near HSB are mapped1 as Quaternary-age landslide deposits and fill (associated with the US 85 roadway). The mapped landslide deposits extend well beyond the project area, as discussed in the project Subsurface Characterization Report3 (SCR). Additionally, the large landslide mass is “unlikely to move as a single mass due to its present relatively gentle slope angle,” as discussed in the SCR. However, localized landslides and slope movement could occur within the larger landslide mass.

Reconnaissance

To assess landsliding affecting the roadway, engineers and geologists from Shannon & Wilson reconnoitered the site several times between October 2014 and October 2016. Details of these site visits are available in the project SCR. We completed an additional reconnaissance in April 2017. Observations from this reconnaissance will be presented in a forthcoming addendum to the project SCR.

Subsurface Explorations and Instrumentation

Shannon & Wilson completed several borings in the HSB area. The locations of the borings are shown in Figure TM-5-1B. Previous boring logs and laboratory test results for samples collected from the borings are provided in the project Geotechnical Data Report (GDR)4. Boring logs and laboratory data from recently completed borings will be subsequently provided in an addendum to the GDR.

Several of the borings were completed as inclinometers to monitor potential slope movement. The NDDOT also installed several inclinometers in this area in 2011, designated HSB-1 through HSB-4. Additionally, in April 2017 we installed two sondex devices at the site to monitor potential settlement. Data from the instrumentation are provided in the GDR and summarized below. Data from our recent installations, as well as instruments provided by the NDDOT, were

3 Shannon & Wilson, Inc., 2016, Subsurface Characterization Report, US-85 – I-94 to Watford City Bypass, Project No. 9-085(085)075, PCN 20046, Billings, McKenzie, and Stark Counties, North Dakota, November 14. 4 Shannon & Wilson, Inc., 2016, Geotechnical Data Report, US-85 – I-94 to Watford City Bypass, Project No. 9-085(085)075, PCN 20046, Billings, McKenzie, and Stark Counties, North Dakota, October 14.


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obtained after issuing the GDR (denoted by italicized inclinometer designations). Data from these instruments will be subsequently presented in an addendum to the GDR.

• RP-127-01: Since initialization in October 2015, this inclinometer has not indicated a distinct shear zone, but has shown slight tipping-type movement in the expected direction of landslide movement. This inclinometer may be located in the “active block” of the landslide (i.e. the upper wedge of the failure, above the horizontal portion of the failure surface).

• RP-127-01A: A nested sondex-inclinometer installation was completed in this borehole (an offset to boring RP-127-01) in April 2017 to facilitate long-term settlement monitoring by the NDDOT. This installation was recently initialized.

• RP-127-04: This inclinometer is located about 2,200 feet south of the primary HSB landslide area. This inclinometer has indicated about 0.05 inch of displacement at approximately elevation 2,055 feet since initialization in August 2016 (average displacement rate of about 0.1 inch per year).

• RP-127-05: Between August and November 2016, this inclinometer indicated about 0.75 inch of shear displacement near elevation 2,160 feet. The direction of movement is approximately 210 degrees, as shown in Figure TM-5-1B. The rate of movement over the monitoring period is about 3 inches per year.

• RP-127-07: A nested inclinometer-sondex installation was completed in this borehole in April 2017 to facilitate long-term slope stability and settlement monitoring by the NDDOT. This installation was recently initialized.

• HSB 1: This inclinometer was initialized by the NDDOT in April 2011 and monitored until September 2011. A single shear zone was observed at an elevation of approximately 2,165 feet (we estimated the top elevation of the inclinometer based on site topography, because this information was not available from the NDDOT). The inclinometer terminated at an elevation of about 2,135 feet. The rate of movement over the monitoring period was about 6 inches per year. Although the orientation of the inclinometer was not reported, assuming the Ao direction of the inclinometer was oriented parallel to the embankment, we estimate the direction of movement at 200 degrees.

• HSB 2: This inclinometer was initialized by the NDDOT in April 2011 and monitored until September 2011. The inclinometer indicated deformation consistent with settlement to a depth of 90 feet. It is unclear if the settlement is related to settlement of the inclinometer backfill or the ground itself. The data also indicated a


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potential (but not conclusive) shear zone at approximately elevation 2,130 feet (we estimated the top elevation of the inclinometer based on site topography, because this information was not available from the NDDOT). Movement on the potential shear zone was less than 0.2 inch over the monitoring period (less than 0.5 inch per year). The inclinometer terminated at an elevation of about 2,108 feet.

• HSB 3: The NDDOT attempted to install this inclinometer in 2011, but encountered refusal at a depth of 32 feet and abandoned the borehole without installing an inclinometer.

• HSB 4: This inclinometer was initialized by the NDDOT in October 2011 and sheared to failure (i.e. an inclinometer probe could no longer be passed to the bottom of the casing) in the summer of 2015. Two shear zones were observed, one between elevation 2,168 and 2,160 feet, and one between elevation 2,128 and 2,120 feet. Over the monitoring period, the average rate of movement for each shear zone was about 0.3 inch per year. The direction of movement was approximately 180 degrees for both shear zones (we measured the Ao direction of this inclinometer). This inclinometer was referred to as the Horseshoe inclinometer in the GDR.

Subsurface Conditions

Based on the available data, we prepared the four generalized subsurface profiles shown in Figures TM-5-2 through TM-5-5. We divided the subsurface profile into units based on engineering properties and geologic origin, as discussed in Technical Memorandum TM-42.

During drilling, groundwater was not encountered in any of the borings near HSB. However, a vibrating wire piezometer (VWP) installed in boring RP-127-01, in a coal seam near elevation 2,110 feet, indicated a groundwater elevation of approximately 2,150 feet.

Limits of Active Landslide Area

Our interpretation of the lateral limits of the active landslide area currently affecting the roadway at HSB is shown in Figure TM-5-1. During our reconnaissance, we were unable to identify, with certainty, the uphill and downhill extents of the landslide area affecting the roadway. Thus, only the lateral extents are shown on the figure.

BACK-ANALYSIS OF LANDSLIDE

We completed back-analyses of four representative cross-sections through the landslide mass, A-A’, B-B’, C-C’, and D-D’ (see Figure TM-5-1B). We then used these cross-sections to develop a


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slope stability model of the landslide. Because the slope has moved significantly, the factor of safety (FS) of the slope can be assumed to be 1.0 at the time of failure. Assuming an FS of 1.0, a stability model that is representative of conditions at the time of failure can then be developed and used as a baseline for comparison with various landslide mitigation alternatives. The results of the back-analysis to an FS of 1.0 are shown in Figures TM-5-2 through TM-5-5.

A general discussion of the procedures we used to complete slope stability analyses are provided in Technical Memorandum TM-2. Additional information specific to our analyses for the HSB area is provided below.

Soil/Rock Material Properties

We used the material properties described in Technical Memorandum TM-4. We assumed horizontal layers of weak material corresponding to the elevations of the shear zones indicated by the inclinometers. Similar to the previous analyses described in TM-4, we considered residual shear strengths on these weak layers, as movement in these layers has been documented in the inclinometer readings. We assumed the same residual shear strength for both the lower and upper slip surfaces and for all the cross-sections.

Slip Surface Geometry

Based on the inclinometer data, we considered two horizontal slip surfaces, one at elevation 2,160 feet (referred to as the upper slip surface) and one at elevation 2,120 feet (referred to as the lower slip surface.

Boring RP-127-01, which was completed between Sections A-A’ and B-B’, indicated that the base of the existing fill was located about 10 feet below the elevation of the upper slip surface. Additionally, the natural ground surface slopes towards the south. As such, it is our opinion that the ground surface before fill placement at Section A-A’ was below the upper slip surface elevation (see estimated fill-native contact shown in Figure TM-5-2). Therefore, we did not model the upper slip surface at this section.

Additionally, we forced the slip surface uphill far enough that the geometry and limits of the slip surface were consistent with the observed distress. For Sections B-B’, C-C’, and D-D’, where we considered both the upper and lower slip surfaces, the uphill slip surface limits were similar for both slip surfaces.


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Groundwater Conditions

At the outset of the back-analyses, we assumed a groundwater table based on the available subsurface data, which is relatively limited, and our judgment. We then adjusted the groundwater table to produce a factor of safety (FS) of 1.0 for both the upper and lower slip surface (or, in the case of Section A-A’, only the lower slip surface).

POTENTIAL LANDSLIDE MITIGATION ALTERNATIVES

As described in the following sections, we considered several potential alternatives to improve the stability of the active landslide area. These alternatives would only improve stability of the active landslide area and will not improve stability of the relatively large mapped landslide area. For analyses of the alternatives, we considered Sections B-B’ and C-C’, which are through the center of the landslide and representative of site conditions, based on the cross-section geometry.

For preliminary analyses of the mitigation alternatives, we assumed a target factor of safety (FS) of 1.3. However, it may not be possible to achieve an FS of 1.3 for some mitigation options. Therefore, we also considered designing to a lower FS in some cases, as discussed below. The FS for final design should be selected by NDDOT to reflect the uncertainty of the data, the criticality of the slope, cost implications, and the risk that the NDDOT is willing to accept.

Realignment

Stability of the landslide could be improved by flattening the existing fill slope on the west side of the roadway, which would be accomplished by shifting the roadway to the east. We understand a similar approach was attempted in 2011, when the alignment was shifted a maximum of about 110 feet to the east and the embankment slopes were flattened from 2 horizontal to 1 vertical (2H:1V) to 3.5H:1V. However, movement of the landslide has continued since then.

KLJ has indicated that it may be feasible to shift the alignment an additional 200 feet to the east, as shown in Figure TM-5-1B, and to further flatten the slopes on the west side of the alignment to about 6H:1V (the change in slope is less near the ends of the alternative alignment, where the shift is less). For cross-sections prepared by KLJ for this alternative alignment, our slope stability analyses indicate an FS of 1.2 to 1.3 (depending on the cross-section) could be achieved for the upper slip surface, and an FS of about 1.1 could be achieved for the lower slip surface (see Figures TM-5-6 and TM-5-7). While the FS for the lower slip surface is less than the value


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of 1.3 desired by the NDDOT, an FS of 1.1 may still be sufficient to stop or significantly decrease the rate of the landslide movement. These factors correspond to Sections B-B’ and C-C’, where the distance of the alignment shift would be near its maximum. A smaller FS would be obtained near the northern limit of the landslide, where the alignment shift would be smaller and the change in the slope angle would be less. A greater increase in FS could be obtained by further flattening the slopes, but doing so would require that grading extend beyond the existing right-of-way (ROW). A greater increase in FS could also be achieved by combining the realignment with one or more other stabilization options discussed herein (e.g. ground anchors).

In our opinion, if the alignment is shifted as proposed, there would still be risk of landslide movement continuing, particularly near the northern limit of the current roadway distress, where the shift in the alignment would be smallest. Additionally, because the alignment would be shifted closer to the debris flow/landslide above the roadway in this area, it would be critical to address the stability of the landslide above the roadway (see Technical Memorandum TM-2 for recommendations related to this landslide).

The most significant risk associated with this option is that the shifted alignment will still be located on mapped landslide material. Although this material appears to stable based on our reconnaissance, with the exception of some smaller localized failures, we have not installed inclinometers to confirm existing stability. There is risk that landslide movement could reactivate in this area of the realignment. This potential should be investigated during final design, if this approach is selected. Nevertheless, the proposed realignment would offer an improvement over existing conditions and may be less expensive than the other options discussed herein.

KLJ has indicated that this option will result in an additional waste volume of about 410,000 cubic yards compared to the initially proposed alignment. Based on typical unit costs for disposal of waste material ($7 per cubic yard), this represents a relative cost of about $3M, assuming the additional waste cannot be used as fill in another portion of the project. KLJ has indicated that the total cost of the realignment option may be on the order of $10M.

Lightweight Fill

Stability of the landslide area could be improved by reconstructing the existing roadway with lightweight fill. The upper portion of the embankment could be excavated and replaced with lightweight fill, thereby unloading the head of the landslide and reducing driving forces. Due to


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the magnitude of change in the driving forces required to stabilize the landslide, an appropriate lightweight fill material for this application would be expanded polystyrene (EPS), also referred to as geofoam, which has a unit weight of about 5 pounds per cubic foot (pcf). For comparison, other lightweight fill materials, such as cellular concrete and expanded shale, have unit weights of about 35 to 100 pcf. Due of the susceptibility of EPS to degradation when exposed to hydrocarbons (e.g. gasoline and diesel), design details, such as protective soil covers, are required to protect EPS from hydrocarbon exposure.

Stability analyses for lightweight fill options are provided in Figures TM-5-8 and TM-5-9. Assuming the upper 25 feet of the embankment is replaced with lightweight fill having a unit weight of 5 pcf, the FS for both the upper and lower slip surfaces would be about 1.1. The FS could be further increased by increasing the thickness of lightweight fill, but our analyses show about 40 feet of lightweight fill would be required to achieve an FS of 1.3. Replacing such a thickness of the embankment would present constructability challenges (e.g. dewatering, stability of temporary excavations).

Construction of this option would likely require a full closure of the roadway and detouring traffic on a temporary alignment. Additionally, geofoam is a relatively expensive option. Typical geofoam costs are on the order of $75 to $125 per cubic yard, but costs would probably be towards the upper end of this range because of the project site’s distance from geofoam suppliers. Assuming a replacement depth of 25 feet, the relative cost of geofoam reconstruction could be on the order $20M. Considering the relatively high cost of lightweight fill, geofoam may not be a cost-effective option, particularly if an FS of 1.3 is desired.

Toe Buttress

Stability could be improved by constructing a buttress near the toe of the landslide. However, because the active landslide is a small part of much larger inactive landslide, there is a high risk that placement of fill could activate movement below the currently active landslide area. Therefore, we do not recommend the use of a toe buttress to improve stability in the HSB area.

Shear Key

Stability could be improved by excavating a portion of the slip surface(s) and replacing the material with a granular backfill. This option would require relatively deep temporary


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excavations. Such excavations may destabilize the landslide and cause additional instability. Therefore, we do not recommend the use of a shear key to improve stability in the HSB area.

Drainage Improvements

Groundwater levels often play a significant role in slope instability. Although the inclusion of drainage features can significantly decrease the rate of landslide movement, drainage features alone are not typically sufficient to provide an FS of 1.3. Additionally, dewatering clayey soils/bedrock, such as those present at the landslide area, can be challenging, contributing to greater uncertainty in the effectiveness of drainage features compared other stabilization techniques. Nevertheless, drainage improvements are generally relatively low cost and have a favorable cost-benefit ratio.

Because of the depth of groundwater suggested by the available data and the depth of the interpreted slip surfaces, it will be challenging or infeasible to install typical subsurface drainage features, such has horizontal drains and trench drains, to a depth sufficient to intercept groundwater. Additionally, horizontal drains may have limited effectiveness in the clayey soil/rock at the site. Therefore, effective drainage improvements would likely be limited to surface drainage improvements, such as re-grading to seal cracks, eliminating ponding, maintaining/improving existing culverts, and directing runoff to a suitable discharge location. Regardless of the selected stabilization approach, surface drainage improvements should be implemented. Shallow trench drains could also be considered.

Ground Anchors

Stability of the landslide could be improved by installing multiple rows of ground anchors along the alignment shown in Figure TM-5-1B (similar to the US 2 landslide stabilization being completed by NDDOT). The bond zone of the ground anchors would be installed below the interpreted lower slip surface, and the heads of the anchors would react against precast concrete anchor blocks or a cast-in-place reaction structure. Typically, anchor blocks are approximately 8- to 10-feet square, and anchors are installed at a spacing slightly greater than the width of an individual block.

The results of stability analyses for the ground anchor option are shown in Figures TM-5-10 and TM-5-11. The analyses show that five to seven rows (depending on the cross-section) of 5-strand anchors with a center-to-center spacing of 12 feet would provide an FS o 1.3 for the upper


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slip surface. However, the same ground anchor configuration would only provide an FS of 1.1 for the lower slip surface. To provide an FS of 1.3 for the lower slip surface, our analyses show that approximately 15 rows of ground anchors would be required. (Many rows of ground anchors are required because the capacity of a single ground anchor is limited by the relatively weak bedrock at the site.) We are not aware of any other landslide stabilization projects that have been constructed with this many rows of ground anchors. In our opinion, it would not be feasible to use ground anchors alone to provide an FS of 1.3 for the lower slip surface, considering the cost and constructability of installing the number of rows of ground anchors required for this case. However, it may be feasible to combine ground anchors with one or more of the other options discussed herein to achieve an FS of 1.3 for the lower slip surface (e.g. lightweight fill or realignment).

Considering the orientation and extents of landslide movement, it is necessary for the ground anchor alignment to extend beyond the current ROW and into the adjacent Theodore Roosevelt National Park (TRNP). Because the direction of landslide movement is not perpendicular to the roadway alignment, it does not appear that it will be feasible to construct a stabilization structure parallel to the alignment.

For the five- to seven -row configuration of ground anchors (FS of 1.1 for the lower slip surface), we estimate a relative cost of about $9M.

Anchored Drilled Shaft Structure

An anchored drilled shaft structure (similar the structure constructed for the I-94 Painted Canyon Landslide project) could be utilized to improve stability of the landslide. A single row of drilled shafts would be installed along the alignments shown in Figure TM-5-1. The drilled shafts would be embedded a sufficient distance below the lower slip surface to obtain the necessary lateral resistance. Additionally, two rows of ground anchors, installed near the top of each shaft, would likely be required to provide additional stabilizing force to the landslide and to reduce the bending moment in the drilled shafts. A reinforced concrete cap beam would be installed atop the drilled shafts to tie the individual drilled shafts and ground anchors together and increase the stiffness of the structure.

The results of slope stability analyses for this option are included as Figure TM-5-12 and TM-5-13. The analyses show that a drilled shaft stabilizing force (i.e. the lateral load resisted by the drilled shafts) of 130 to 160 kips per linear foot (depending on the cross-section) and two rows of


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5-strand anchors with a center-to-center spacing of 10 feet would be required to provide a minimum FS of 1.3 for the upper and lower slip surfaces. Based on similar landslide stabilization projects we have completed, we anticipate that 125-foot long, 5- or 6-foot diameter drilled shafts, with a center-to-center spacing of approximately two shaft diameters could achieve the required drilled shaft stabilizing force.

The proposed alignment for this option is the same as for the ground anchor option (see Figure TM-5-1) and would require construction outside of the current ROW, as previously discussed for the ground anchor option.

For this option, we estimate a relative cost of $10M. For a reduced FS of 1.2 on the lower slip surface, we estimate a relative cost of about $8M, which is less than the relative cost of the ground anchor option which would provide an FS of 1.1 on the lower slip surface (i.e. the drilled shaft option provides a higher FS for a lower cost).

ADDITIONAL CONSIDERATIONS

Uncertainty and Risk

The stabilization options discussed herein are preliminary and were developed based on limited subsurface information. Considering the available data, we have identified several areas of uncertainty and risk, as discussed below.

Direction of Landslide Movement

Based on the inclinometer data, the direction of landslide movement varies, depending on location. Notably, inclinometer HSB 4 shows indicates that landslide movement near the toe of the roadway embankment is skewed relative to the anticipated direction of movement (perpendicular to the contours of the embankment). Other inclinometers in the area generally indicate movement in a southwest direction.

If a structural system (ground anchors or anchored drilled shafts) is selected for landslide stabilization, the location and orientation of the alignment of the stabilization structure will be governed by the direction of landslide movement, among other factors. The structure alignments shown in Figure TM-5-1 were developed to orient the structures generally perpendicular to the interpreted direction of landslide movement. However, if subsequent subsurface characterization


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indicates a different direction of movement, the alignments may need to be translated or rotated, which could alter ROW requirements and impacts on the adjacent TRNP.

Limits of Active Landslide

The limits of landslide stabilization discussed herein were developed to stabilize the portion of the active landslide currently affecting the roadway at the HSB area (approximately Station 6737+00 to 6747+00). As previously discussed, the extents of the active landslide movement extend beyond this area, and the limits of the inactive (mapped) landslide material extend even further beyond this area. Increased precipitation or site grading could destabilize currently stable portions of the landslide.

The proposed limits of landslide mitigation are based on our understanding of the portion of the currently active landslide area affecting the roadway. If the extents of the landslide affecting the roadway increase before final design or further study indicates a larger active landslide area, the extents of the landslide stabilization system may need to be increased to effectively improve the stability of the roadway.

Depth of Landslide Movement

Inclinometer HSB 4 indicated lower and upper slip surfaces (one at approximately elevation 2,120 and one at approximately elevation 2,160 feet). Nearby inclinometer RP-127-05 only indicated movement on the upper slip surface. (None of the other adjacent inclinometers indicated movement on the lower slip surface, but these inclinometers either did not indicate distinct movement at any depth or did not extend below the lower slip surface.) Additionally, the rate of movement indicated by the inclinometers varied between adjacent installations, ranging from about 0.3 to 6 inches per year.

In our opinion, the discrepancies in the depth and rate of slope movement between the inclinometers suggest that landslide movement is not constrained to a single coherent mass. Instead, the landslide mass is comprised of multiple failures that have coalesced into a large landslide area. The stabilization options presented herein are intended to address the two known failure surfaces. However, considering the complexity of landslide movement in this area, it is possible that additional failure surfaces exist. If present, failure surfaces deeper than those we considered will be challenging or possibly infeasible to stabilize. Additional slope monitoring


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and instrumentation will be necessary to further characterize the failure mechanisms of the landslide during final design.

Design Factor of Safety

We understand that the NDDOT typically prefers to design landslide mitigation projects to an FS of 1.3, based on the requirements of the AASHTO LRFD Specifications5. However, considering the magnitude of the landslide affecting the HSB area of the project, designing to an FS of 1.3 will be infeasible or relatively expensive, depending on the stabilization option. FS values less than 1.3 may be considered appropriate because the analyses of the landslide are based on a back-calculation to an FS of 1.0 for existing conditions (failure). A slope stability model based on back-analysis of an existing landslide has essentially been calibrated in its entirety (soil stratigraphy, shear strength parameters, groundwater conditions, etc.) to reflect actual conditions, i.e. an FS of 1.0 at the time of failure. As such, the back-calculated model can be considered more reliable than a typical forward-looking slope stability model based on data from borings and laboratory test results. Therefore, the NDDOT may consider accepting a lower design FS.

Additional Monitoring

Additional subsurface characterization will be necessary to support final design of landslide mitigation for the HSB area. As part of this characterization, we recommend that the NDDOT continue to obtain periodic readings from the VWPs, inclinometers, and sondex settlement systems installed at HSB. We also recommend that the NDDOT continue to document roadway distress and associated maintenance activities in the HSB area.

CONCLUSIONS

Several landslides have coalesced in the HSB area to form a large landslide area that is affecting the nearby roadway. Distress has been occurring for several decades, and despite the realignment of the roadway in 2011 to mitigate landslide distress, movement of the roadway has continued. Because of the scale of the landslide, mitigating the landslide will be challenging. The two most appropriate mitigation options are 1) realigning the roadway or 2) constructing an

5 American Association of State Highway and Transportation Officials, 2014, AASHTO LRFD bridge design specifications, Customary U.S. units, 7th edition: Washington, D.C., American Association of State Highway and Transportation Officials.


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anchored drilled shaft system. (An anchored drilled shaft structure is preferred over ground anchors alone because the former offers a greater FS than the latter for a similar cost.)

It is not feasible to realign the roadway onto ground that has not moved in the past, because of the scale of nearby landslide activity. As such, there is risk that landslide movement could occur in the area of the proposed realignment, although this area currently appears to be stable. Additionally, the proposed realignment would only achieve an FS of 1.1 for the lower slip surface, and the FS would be less near the ends of the realignment shift, where the magnitude of the shift is smaller.

An anchored drilled shaft system is a feasible option to improve stability of the existing alignment. This option will likely be similar in cost to realigning the roadway, but would have less risk of future landslide movement occurring. However, the use of a structural system would require construction outside of the existing ROW, which would cause impacts to the adjacent TRNP.

An anchored drilled shaft structure could be used to reduce the need for retaining walls along the roadway alignment. However, because the anchored drilled shaft structure is located near or below the toe of the existing embankment, doing so would require placing additional fill on a relatively large area of the slope. This fill would introduce additional driving weight that would need to be resisted by the structure. Constructing relatively small retaining walls along the roadway would likely have less of an impact on the loading applied to the anchored drilled shaft structure.

In terms of geotechnical risk, the anchored drilled shaft system is preferred over realignment, in our opinion, because:

1) A greater increase in the FS can be achieved with the anchored drilled shaft system.

2) Practically all of the ground near the area of roadway distress has experienced landslide movement to some degree. As such, it will not be possible to realign the roadway to avoid landslide material. Although some of the adjacent landslide material currently appears to be stable, the risk of reactivating previous landslides cannot be eliminated.

3) Compared to the realignment option, the structural option provides greater flexibility to address the landslide if additional subsurface characterization indicates that the


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mechanisms of landslide activity differ from our current understanding (e.g. the failure surface is deeper, extents of movement are larger, direction of movement is different).

However, we understand that other non-geotechnical issues must be considered by the NDDOT and KLJ in selecting an appropriate mitigation approach. These issues include cost, right-of-way requirements, impacts to the adjacent TRNP, environmental impacts, and risk tolerance.

Encl: Figure TM-5-1A Horseshoe Bend, Previous Alignments Figure TM-5-1B Horseshoe Bend, Site and Exploration Plan Figure TM-5-2 Section A-A’, Slope Stability Back-Analysis Figure TM-5-3 Section B-B’, Slope Stability Back-Analysis (2 sheets) Figure TM-5-4 Section C-C’, Slope Stability Back-Analysis (2 sheets) Figure TM-5-5 Section D-D’, Slope Stability Back-Analysis (2 sheets) Figure TM-5-6 Section B-B’, Alternative Alignment (2 sheets) Figure TM-5-7 Section C-C’, Alternative Alignment (2 sheets) Figure TM-5-8 Section B-B’, Lightweight Fill (2 sheets) Figure TM-5-9 Section C-C’, Lightweight Fill (2 sheets) Figure TM-5-10 Section B-B’, Ground Anchors (2 sheets) Figure TM-5-11 Section C-C’, Ground Anchors (2 sheets) Figure TM-5-12 Section B-B’, Anchored Drilled Shafts (2 sheets) Figure TM-5-13 Section C-C’, Anchored Drilled Shafts (2 sheets)

Scale in Feet

0

Geotechnical and Environmental Consultants

NC.IILSON, WHANNON & S

23-1-01453-230

FIG. TM-5-1A

McKenzie County, North Dakota

Project No. 9-085(085)075, PCN 20046

US-85 - I-94 to Watford City Bypass

NOTES

6735+0

0

6740

+00

6745+00

6720+0

0

6725+0

0

6730+00

.control_mckenzie.dgn

Stationing of existing alignment provided in KLJ file 2)

control_mckenzie.dgn.

and Photo_North_NDDOTPrelim_McKenzie.dgn

Site plan adapted from KLJ files 1)

400 800

LEGEND

Affecting Roadway

Interpreted Lateral Extents of Landslides

?

??

??

??

June 2017

Approximate Location of Pre 1980s Alignment

Alternative Alignment

Approximate Location of Pre 2011 Alignment

6750+00

6755+00

PREVIOUS ALIGNMENTS

HORSESHOE BEND

Scale in Feet

0

Geotechnical and Environmental Consultants

NC.IILSON, WHANNON & S

23-1-01453-230

FIG. TM-5-1B

McKenzie County, North Dakota

Project No. 9-085(085)075, PCN 20046

US-85 - I-94 to Watford City Bypass

NOTES6735+00

6740+00

6745

+00

6720+00

6725+00

6730+00

.control_mckenzie.dgn

Stationing of existing alignment provided in KLJ file 2)

control_mckenzie.dgn.

and Photo_North_NDDOTPrelim_McKenzie.dgn

Site plan adapted from KLJ files 1)

200 400

RP-126-04

RP-126-03

A

LEGEND

Location

Inclinometer Designation and Approximate

Location

Boring Designation and Approximate

and Approximate Location

Generalized Subsurface Profile Designation

RP-127-01

Inclinometer Rate of Movement (in./yr)0 3 6

HSB 4

RP-127-07

RP-127-07

RP-127-01A

RP-127-01A

HSB 2

HSB 1

HSB 3

Affecting Roadway

Interpreted Lateral Extents of Landslides ?

?

?

?

?

??

Inclinometer Movement

Approximate Direction and Rate of

SITE AND EXPLORATION PLAN

HORSESHOE BEND

Approximate Location of Pre 2011 Alignment

Alternative Alignment

Existing Right-of-Way

National Park Boundary

Approximate Location of Pre 1980s Alignment

RP-127-05

RP-127-02

RP-127-03

RP-127-04

A'

B'

D'

B

A

D

Structures

Proposed Alignment for Slope Stabilization

Approximate Location (Completed in 2017)

Nested Sondex/Inclinometer Designation and

Location (Completed in 2017)

Sondex Designation and Approximate

June 2017

C

C'

1.0

Distance (ft)-700 -650 -600 -550 -500 -450 -400 -350 -300 -250 -200 -150 -100 -50 0 50 100 150

Ele

vatio

n (ft

)

2,060

2,080

2,100

2,120

2,140

2,160

2,180

2,200

2,220

2,240

2,260

US-85 - I-94 to Watford City Bypass23-1-01453-210


Date: 5/10/2017File Name: HSB - STA 6737+50.gszName: Lower - BA

Color Name Model Unit Weight (pcf)

Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function

Fill 1 Mohr-Coulomb 125 100 25 1

Qcol Mohr-Coulomb 120 0 25 1

Tsb_Peak Mohr-Coulomb 125 1,500 22 1

Tsb_Resid_LL=175,CF = 50

Shear/Normal Fn. 125 1 Resid_LL=175,CF=50

HSB-4 (Proj. 270' S)

INC RP-127-05(Proj. 360' S)

INC RP-127-01(Proj. 160' S) US 85

DOT ROW(Jog in ROW at this section)

A A'

Figure TM-5-2Section A-A', Slope Stability Back-Analysis

1.0

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260




HSB-4(Proj. 10' N)INC RP-127-05

(Proj. 180' S)

INC RP-127-01(Proj. 150' N)

RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






US 85B B'

Figure TM-5-3Section B-B', Slope Stability Back-Analysis

(Sheet 1 of 2)

1.0

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260



Date: 5/9/2017File Name: HSB - STA 6741+00.gszName: Upper - BA


(Proj. 180' S)

INC RP-127-01(Proj. 150' N)

RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






US 85B B'

Figure TM-5-3Section B-B', Slope Stability Back-Analysis

(Sheet 2 of 2)

1.0

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260




HSB-4(Proj. 75' N)

RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






HSB-1(Proj. 30' N)

Exis

ting

US

85 A

lignm

ent

C C'

Figure TM-5-4Section C-C', Slope Stability Back-Analysis

(Sheet 1 of 2)

1.0

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260




HSB-4(Proj. 75' N)

RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






HSB-1(Proj. 30' N)

Exis

ting

US

85 A

lignm

ent

C C'

Figure TM-5-4Section C-C', Slope Stability Back-Analysis

(Sheet 2 of 2)

1.0

Distance-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200 300 400

Ele

vatio

n

2,050

2,100

2,150

2,200

2,250

2,300

2,350

2,400




Inc RP-127-05(Proj. 30' W)

US-85

HSB-4(Proj. 360' W)


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function





DO

T R

OWD D'

Figure TM-5-5Section D-D', Slope Stability Back-Analysis

(Sheet 1 of 2)

1.0

Distance-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200 300 400

Ele

vatio

n

2,050

2,100

2,150

2,200

2,250

2,300

2,350

2,400




Inc RP-127-05(Proj. 30' W)

US-85

HSB-4 (Proj. 360' W)


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function





DO

T R

OWD D'

Figure TM-5-5Section D-D', Slope Stability Back-Analysis

(Sheet 2 of 2)

1.1

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260



Date: 5/9/2017File Name: HSB - STA 6741+00.gszName: Lower - Alt Align 200


(Proj. 180' S)

INC RP-127-01(Proj. 150' N)

RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






Exis

ting

US

85 A

lignm

ent

B B'

Prop

osed

Alig

nmen

t

Figure TM-5-6Section B-B', Alternative Alignment

(Sheet 1 of 2)

1.3

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260



Date: 5/9/2017File Name: HSB - STA 6741+00.gszName: Upper - Alt Align 200


(Proj. 180' S)

INC RP-127-01(Proj. 150' N)

RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






Exis

ting

US

85 A

lignm

ent

B B'

Prop

osed

Alig

nmen

t

Figure TM-5-6Section B-B', Alternative Alignment

(Sheet 2 of 2)

1.1

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260



Date: 6/2/2017File Name: HSB - STA 6743+50.gszName: Lower - Alt Align

HSB-4(Proj. 75' N)

RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






HSB-1(Proj. 30' N)

Exis

ting

US

85 A

lignm

ent

C C'

Figure TM-5-7Section C-C', Alternative Alignment

(Sheet 1 of 2)

1.2

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260



Date: 6/2/2017File Name: HSB - STA 6743+50.gszName: Upper - Alt Align

HSB-4(Proj. 75' N)

RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






HSB-1(Proj. 30' N)

Exis

ting

US

85 A

lignm

ent

C C'

Figure TM-5-7Section C-C', Alternative Alignment

(Sheet 2 of 2)

1.1

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260



Date: 5/9/2017File Name: HSB - STA 6741+00.gszName: Lower - LW Fill 25'


(Proj. 180' S)

INC RP-127-01(Proj. 150' N)

RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






25' Lightweight Fill

Exis

ting

US

85 A

lignm

ent

B B'

Figure TM-5-8Section B-B', Lightweight Fill

(Sheet 1 of 2)

1.1

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260



Date: 5/9/2017File Name: HSB - STA 6741+00.gszName: Upper - LW Fill 25'


(Proj. 180' S)

INC RP-127-01(Proj. 150' N)

RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






25' Lightweight Fill

Exis

ting

US

85 A

lignm

ent

B B'

Figure TM-5-8Section B-B', Lightweight Fill

(Sheet 2 of 2)

1.1

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260



Date: 6/2/2017File Name: HSB - STA 6743+50.gszName: Lower - LW Fill 25'

HSB-4(Proj. 75' N)

RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






HSB-1(Proj. 30' N)

25' Lightweight FillExis

ting

US

85 A

lignm

ent

C C'

Figure TM-5-9Section C-C', Lightweight Fill

(Sheet 1 of 2)

1.1

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260



Date: 6/2/2017File Name: HSB - STA 6743+50.gszName: Upper - LW Fill 25'

HSB-4(Proj. 75' N)

RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






HSB-1(Proj. 30' N)

25' Lightweight FillExis

ting

US

85 A

lignm

ent

C C'

Figure TM-5-9Section C-C', Lightweight Fill

(Sheet 2 of 2)

1.1

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260



Date: 5/9/2017File Name: HSB - STA 6741+00.gszName: Lower - Anchors


(Proj. 180' S)

INC RP-127-01(Proj. 150' N)

Type: AnchorLength: 169.99996 ftDirection: 145 °Pullout Resistance: 4,000 psfResistance Reduction Factor: 2Bond Length: 40 ftBond Diameter: 0.67 ftAnchor Spacing: 12 ftTensile Capacity: 176,000 lbs (allowable)

RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






Exis

ting

US

85 A

lignm

ent

B B'

Figure TM-5-10Section B-B', Ground Anchors

(Sheet 1 of 2)

1.3

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260



Date: 5/9/2017File Name: HSB - STA 6741+00.gszName: Upper - Anchors


(Proj. 180' S)

INC RP-127-01(Proj. 150' N)


RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






Exis

ting

US

85 A

lignm

ent

B B'

Figure TM-5-10Section B-B', Ground Anchors

(Sheet 2 of 2)

1.1

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260



Date: 6/2/2017File Name: HSB - STA 6743+50.gszName: Lower - Ground Anchs

HSB-4(Proj. 75' N)


RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






HSB-1(Proj. 30' N)

Exis

ting

US

85 A

lignm

ent

C C'

Figure TM-5-11Section C-C', Ground Anchors

(Sheet 1 of 2)

1.3

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260



Date: 6/2/2017File Name: HSB - STA 6743+50.gszName: Upper - Ground Anchs

HSB-4(Proj. 75' N)


RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






HSB-1(Proj. 30' N)

Exis

ting

US

85 A

lignm

ent

C C'

Figure TM-5-11Section C-C', Ground Anchors

(Sheet 2 of 2)

1.3

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260



Date: 5/10/2017File Name: HSB - STA 6741+00.gszName: Lower - Shafts, Anchors


(Proj. 180' S)

INC RP-127-01(Proj. 150' N)


RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






Type: PileShear Force: 130,000 lbsPile Spacing: 1 ft

Exis

ting

US

85 A

lignm

ent

B B'

Figure TM-5-12Section B-B', Anchored Drilled Shafts

(Sheet 1 of 2)

1.9

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260



Date: 5/9/2017File Name: HSB - STA 6741+00.gszName: Upper - Shafts, Anchors


(Proj. 180' S)

INC RP-127-01(Proj. 150' N)


RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function







Exis

ting

US

85 A

lignm

ent

B B'

Figure TM-5-12Section B-B', Anchored Drilled Shafts

(Sheet 2 of 2)

1.3

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260



Date: 6/2/2017File Name: HSB - STA 6743+50.gszName: Lower - Shafts Anchs

HSB-4(Proj. 75' N)


RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function






HSB-1(Proj. 30' N)


Exis

ting

US

85 A

lignm

ent

C C'

Figure TM-5-13Section C-C', Anchored Drilled Shafts

(Sheet 1 of 2)

1.3

Distance (ft)-1,100 -1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 100 200

Ele

vatio

n (ft

)

2,0602,0802,1002,1202,1402,1602,1802,2002,2202,2402,260



Date: 6/2/2017File Name: HSB - STA 6743+50.gszName: Upper - Shafts Anchs

HSB-4(Proj. 75' N)


RO

W


Cohesion'(psf)

Phi' (°)

PiezometricLine

Strength Function







HSB-1(Proj. 30' N)

Exis

ting

US

85 A

lignm

ent

C C'

Figure TM-5-13Section C-C', Anchored Drilled Shafts

(Sheet 2 of 2)

Documents

GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS · Page 5 of 15 . 23-1-01453-230_TM5. 23-1-01453-230 . slope stability model of the landslide. Because the slope has moved significantly,