appendix g geotechnical engineering

Appendix G

Geotechnical Engineering

Souris River Basin Flood Risk Management Draft Feasibility Report With Integrated Environmental Assessment; Bottineau, McHenry, Renville, and Ward Counties, North Dakota

USACE | Geotechnical Engineering Appendix G G-2

SOURIS RIVER BASIN FLOOD RISK MANAGEMENT FEASIBILITY STUDY AND INTEGRATED ENVIRONMENTAL ASSESSEMENT

WARD COUNTY, NORTH DAKOTA

APPENDIX G Geotechnical Engineering



Contents

1. Introduction ............................................................................................................................ 5

2. Regional Geology ..................................................................................................................... 5

2.1 Topography ...................................................................................................................... 5

2.2 Geology ............................................................................................................................. 6

2.3 Seismic Risk and Earthquake History ..............................................................................7

3. Subsurface Investigation ..........................................................................................................7

3.1 Groundwater Data Review ................................................................................................7

4. Site Specific Geology ................................................................................................................ 8

5. Selection of Design Parameters ............................................................................................... 9

5.1 Permeability ..................................................................................................................... 9

5.2 Shear Strength Parameters .............................................................................................. 9

5.3 Compressibility ............................................................................................................... 12

6. Bearing Capacity and Settlement Analysis ............................................................................. 12

6.1 Bearing Capacity Analysis for River Closure Structures ................................................. 12

6.2 Settlement Analysis for River Closure Structures ........................................................... 14

6.3 Levee Settlement Analysis............................................................................................... 14

6.4 Roadway and Railroad Closure Structures ..................................................................... 15

7. Maple Diversion Seepage and Stability Analysis.................................................................... 15

7.1 Modeling Method ............................................................................................................ 15

7.2 Model Cross-sections ...................................................................................................... 16

7.3 Seepage Analysis ............................................................................................................. 17

7.3.1 Seepage Model Properties ........................................................................................... 17

7.3.2 Boundary Conditions ................................................................................................... 18

7.3.3 Seepage Results ........................................................................................................... 18

Seepage Mitigation .................................................................................................. 19

Seepage Collection System Filter Design ................................................................ 22

7.4 Slope Stability Analyses ................................................................................................. 23

7.4.1 Slope Stability Model Properties ................................................................................ 24

7.4.2 Setback Analysis ......................................................................................................... 25

7.4.3 Levee Stability Results ................................................................................................ 25

8. Exploration Trench ................................................................................................................ 27



9. West Tieback Levee Geotechnical Assessment ..................................................................... 27

10. Erosion Protection ............................................................................................................. 28

11. Cofferdam Design .............................................................................................................. 28

12. Sources of Construction Materials ..................................................................................... 29

13. Phase I Environmental Site Assessment ........................................................................... 29

14. References .......................................................................................................................... 29



Geotechnical Engineering Appendix G

1. Introduction This appendix provides the geotechnical engineering in support of the Recommended Plan for the feasibility study for the Souris River in Minot, ND. The report was based on developing sufficient geotechnical engineering and design to enable refinement of the project features, prepare the baseline cost estimate, develop a construction schedule, and allow detailed design on the Recommend Plan to begin immediately following receipt of Preconstruction Engineering and Design (PED) funds.

The Recommended Plan is the Maple Diversion with the West Tieback levee. The main features consists of the following: levees, floodwalls, river control structures, and a high-flow bypass channel. The geotechnical components of the design and analysis consists of channel slope stability, levee slope stability, seepage stability, and evaluation of settlement and bearing capacity of the upstream and downstream closure structures. Section 3.9 of the Main Report provides a detailed summary of the Recommended Plan and operation.

The majority of the design and analysis that was used to complete the appendix was completed by Barr Engineering.

2. Regional Geology This section specifically addresses the topography and geology in the Souris River Valley taken from the USACE 1972 Design Memorandum No. 1 Flood Control Souris River at Minot, ND.

2.1 Topography

The project is located near the western border of the Central Lowlands Physiographic Province and within a few miles of the adjoining Great Northern Plains Physiographic Province. Except for the slopes of the Souris (Mouse) River Valley, the maximum relief of the adjacent uplands is about 100 feet and the total relief is about 250 feet. The valley walls are relatively steep, having an average slope of about 15 percent. The valley walls have been eroded by a series of shallow, round bottomed steeply sloping parallel depressions, the bottoms of which are usually brush covered. In additional, there are occasional stream terraces, and deposits from Glacial waters, such as that which is being mined immediately upstream from Minot.

The Souris River flood plain ranges from one to two miles wide and has a relief of about 25 feet including the depth of the river channel. An unusually well-developed and intricate meander pattern exist along with numerous abandoned channels which, like the meanders, extend literally in all directions. Some of the abandoned channels are but a few feet deep and represent either temporary high water channels or deep channels that have been filled with silt, sand and organic materials. A large number of the abandoned channels extend the full depth of the present Souris stream channel and contain deep ponds. In some areas, abandoned channels have been filled by uncontrolled artificial random fills. The area adjoining the stream channel, and abandoned channels, supports a heavy growth of native deciduous trees of several types as well as light growth of underbrush.



2.2 Geology

The region is covered by glacial drift of late Wisconsin age except for small areas where the soft shales of the Fort Union Formation (Tertiary) outcrop. No shale outcrops are present in the Souris River flood plain. Glacial drift includes all glacial ice deposits, as well as river, stream and lake deposits, that were directly associated with the continental glaciation. Till was deposited directly by glacial ice and consists of a heterogeneous mixture of clay, silt, sand, gravel, and boulders in which silt and clay predominate. Glaciofluvial material was deposited by glacial meltwaters and consists predominately of sand and gravel. Alluvial material consists of cobbles, gravel, sand, silt and clay deposited by running water that was not directly derived from, or otherwise associated with, the glacier. A wealth of additional information is contained in “Geological Survey in Cooperation with the North Dakota State Water Commission and the City of Minot in 1965”. It contains the results of many years’ study of the drawdown and recharge of the Minot aquifer which underlies a portion of the flood plain under consideration.

The intermittent melting of the continental glacial ice released enormous quantities of water. In addition, large floods often occurred due to meltwater accumulating behind a barrier, such as a terminal moraine, until it caused a failure due to overtopping. This meltwater transported great quantities of sand, gravel, cobbles and large boulders and even greater quantities of these abrasive materials were transported by the floodwaters. At the proposed Burlington dam site (about one-half mile north of the confluence of the Souris and Des Lacs Rivers), five borings indicated that the alluvial fill extended to about 130 feet below the present level of the flood plain. Therefore, the gorge would have extended about 280 feet below the bordering uplands. The gradient of the bottom of the gorge was much steeper than the present gradient of the Souris River; due to the great quantity of stream flow, the abrasive bedload and the fact that the oceans were about 300 feet below their present level (due to the storage of water in the continental glaciers). It may therefore be concluded that the ancestral Souris River was about 150 feet below present stream level in the Minot area. During the erosion of the gorge, occasional terrace deposits of sand and gravel were left on the inner side of the bends, adjacent to the walls of the gorge. Also, there are indications of glaciofluvial material at sand and gravel mining operations in the area upstream from Minot.

Due to the gradual decrease in the quantity of glacial meltwaters over a long period of time, the river ceased to transport its former large bedload of coarse material. This coarse material was deposited and then over-lain, by slope wash and streams, with finer materials that had been included in the glacial ice. The elevation of the flood plain increased rather rapidly at first and then gradually slowed due to the lessening of the gradient and precipitation. When the gradient became quite flat, meanders began to develop. Natural levees formed due to silt and fine sand accumulating from the slack water during flood stage in the brush bordering the stream channel. As mentioned previously, cutoff meanders and abandoned channels are very common in the present flood plain grade from intermixed lenses of clays, silts, and sands to coarser intermixed materials at depth. Except for some slight preconsolidation of clay lenses by desiccation, the materials have a loose to medium compact condition.

At least eighteen wells have been developed in the Souris River flood plain to provide the water



supply for the City of Minot. GSWSP 1844 states that the water level has declined more than 70 feet since the first city well began pumping in 1916. The City now practices artificial recharge of the aquifer.

2.3 Seismic Risk and Earthquake History

The project is located in a low risk seismic zone. A seismic evaluation was completed for the City of Devils Lake Embankments project, approximately 90 miles away, resulting in a conclusion that design earthquake and ground motions were below values requiring additional analysis. Due to the close proximity, it is assumed that design earthquake and ground motions are going to be below values requiring additional analysis. Earthquake and ground motions should be furthered evaluated during PED to validate this assumption.

3. Subsurface Investigation No subsurface investigation was completed for the feasibility study. Therefore the study relied on available soils data in the area. A review of available information concerning historic soil explorations, groundwater levels, and testing documents in the area near the project site was completed. A few borings from the original USACE project (1975) were determined to be located within the study area, located along the existing river channel. However, the most recent soil investigation is from the Mouse River Enhanced Flood Projection Project (MREFPP), see Section 2.3.5 of main report for a description of the MREFPP. Geotechnical information was obtained from the previous reports including MRFEPP Phase 1, MRFEPP Phase 2/3, Proposed Storm Sewer 6th Street Southwest Underpass Proposed Pump Station 6th Street and 2nd Avenue Southwest, Minot Water Treatment Plant, and design documents from several bridge construction projects. All the borings generally indicate alluvium deposits with varying amounts of sands and silts with some gravelly deposits. Attachment G-1 provides all the available soil data nearby the study area.

Data closest to the project site was reviewed thoroughly and incorporated into the design when appropriate. Data from locations farther from the project were considered as part of the review of available data used for comparison purposes of data closer to project site. Subsurface investigation and testing will be developed during PED and will be used to conduct geotechnical analysis.

3.1 Groundwater Data Review

Groundwater data was obtained from records taken at the time the borings in the project area were being conducted and from piezometers installed as part of the MRFEPP Phase 1 and MRFEPP Phase 2/3 projects. In the case of soil boring groundwater readings, permeability of the soil surrounding dictates the rate at which water will flow into the boring and fill the boring to a static value equal to the groundwater table. Soil borings are typically backfilled soon after the final sample is collected, thereby creating a situation in which groundwater has not reached a static condition before a final groundwater reading is taken. Conversely, piezometers are established to monitor groundwater conditions for greater period of time. Therefore, piezometer readings are preferred to evaluate long-term groundwater levels for analysis. Groundwater



levels used to define the feasibility groundwater levels for design are provided in Table 1.

Table 1: Apparent Groundwater Levels nearby the project area.

Location ID Source Reading Type Total

Head (ft) B-36-15 MRFEPP Phase 2/3 Piezometer 1545 B-41-15 MRFEPP Phase 2/3 During/Following Soil Boring 1544 B-42-15 MRFEPP Phase 2/3 During/Following Soil Boring 1545.3 B-43-15 MRFEPP Phase 2/3 During/Following Soil Boring 1543 B-49-15 MRFEPP Phase 2/3 During/Following Soil Boring 1544.7 B-51-15 MRFEPP Phase 2/3 During/Following Soil Boring 1540.1 B-53-15 MRFEPP Phase 2/3 During/Following Soil Boring 1540.9 B-54-15 MRFEPP Phase 2/3 During/Following Soil Boring 1541

#1 Proposed Pump Station 6th Street and 2nd Avenue Southwest During/Following Soil Boring 1538.4

B6 Proposed Storm Sewer 6th Street Southwest Underpass During/Following Soil Boring 1542.6








ST-1 MRFEPP Phase 1 Piezometer 1541.5 ST-3 MRFEPP Phase 1 Piezometer 1541

4. Site Specific Geology The main soil types encountered from subsurface investigation consist of the following:

• Low-fines-content Sand Soils (SP and SP-SM) • Higher-fines-content Silty/Clayey Sand Soils (SM and SC) • Mixed Silty/Sandy/Lean Clay Soils (CL, CL-ML, and ML and visually classified as

CL/CH) • Fat Clay Soil (CH)

Review of the deeper historical borings in the project area indicated soils with high SPT N-values and descriptions of laminated/thinly-layered clay soils (possibly weathered shales of the underlying bedrock based on historic soil descriptions). The elevation of the transition to this harder stratum was estimated to range from 1,475 to 1,480ft, approximately 65-75ft below the ground surface. The “hard pan” layer was assumed to have high strength and low compressibility.



5. Selection of Design Parameters Seepage and shear strength parameters were developed for the Recommended Plan based primarily on the values reported in the MRFEPP Phase 1 and MRFEPP Phase 2/3 reports. A comparison of the modeling parameters developed for the MRFEPP Phase 1 and MRFEPP Phase 2/3 Projects was completed for the current feasibility phase for the Recommended Plan and parameters were generally found to be comparable between the two phases. Final design parameterization of the site will be determined during PED through site specific soil exploration and testing.

5.1 Permeability

Material properties used for the seepage analysis are summarized in Table 2. Seepage parameters are based on values in MRFEPP Phase 2/3 and are generally comparable to MRFEPP Phase 1 parameters. The calculated values for the Phase 1 and 2/3 were developed using site specific piezometer data and correlations. The seepage parameters will be further developed during PED with site specific exploration and testing.

Table 2: Summary of Permeabilities used in Seepage Analysis.

Material kh kv kv/kh

ratio (ft/sec) (ft/sec) Fat Clay 5.58E-10 5.58E-10 0

Lean Clay Mix 2.76E-07 7.65E-09 0.03 Sand/Silt 1.21E-06 1.21E-07 0.1

Clean Sand 1.30E-04 1.30E-04 1 Levee Fill 1.90E-09 1.90E-09 1

Cohesive Fill(1) 7.65E-09 7.65E-09 1 Granular Fill(1) 1.21E-07 1.21E-07 1

Riprap(1) 3.28E-04 3.28E-04 1 Hard Pan(1) 8.47E-09 8.47E-09 1

(1) Assumed based on typical values and engineering judgment.

5.2 Shear Strength Parameters

Additional review of the drainage shear strengths for fat clay and clay mix materials was completed to develop representative strengths for the materials based on Consolidated Undrained Triaxial data presented in MRFEPP Phase 1 and MRFEPP Phase 2/3. Drained shear strength data CL, CL-ML, and ML and visually classified as CL/CH (Clay Mix) materials is presented in Figure 1. Using a 1/3-2/3 rule to divide the data set where approximately 1/3 of data points are below a straight line approximation of the shear strength envelop and approximately 2/3 of data points are above the envelope results in a shear strength angle of 32 degrees. Similar for the CH (Fat Clay) material, applying the 1/3-2/3 rule to the data set results in a shear strength angle of 32 degrees, as presented in Figure 2. Figure 3 shows the drained levee borrow shear strength envelope and data.



Figure 1: Lean Clay Mix Drained Shear Strength Data.

Figure 2: Fat Clay Mix Drained Shear Strength Data.



Figure 3: Borrow Drained Shear Strength Envelope, Maximum Deviator Stress Failure Criterion.

Figure 4: Borrow Undrained Shear Strength Envelope, Maximum Deviator Stress Failure Criterion.

Material properties used for the slope stability analysis are summarized in Table 3. Unit weights are based on values in MREFPP Phase 2/3. The parameters defining levee fill materials are based on values found in MREFFP Phase 2/3, which reflect the Highway 2 borrow site which is assumed to be the borrow site for this project. The Highway 2 borrow site is characterized as clayey glacial till material which is anticipated to be the general fill material for the levee. The excavation of the diversion is not thought to be suitable as levee fill material. Future soil exploration and testing will evaluate the diversion channel for suitability as levee fill, this will be



completed during PED. Unit Weights are based on values presented in MRFEPP Phase 2/3.

Table 3: Summary of Unit Weights and Shear Strength Parameters used in Slope Stability Analysis.

Material Unit Weight ESSA - Drained USSA - Undrained

Moist (pcf) Sat (pcf) c’ (psf) φ’ (degrees) c (psf) φ (degrees) Fat Clay 115 116 0 32.0o 1,100 0

Lean Clay Mix 123 126 0 32.0o 1,300 0

Sand/Silt 120 124 0 33.5o - - Clean Sand 119 123 0 33.5 - -

Levee Fill 122 128 c’ = 0, φ’ = 44o; at σ’ = 510psf, φ’ = 26.5o

100 21.5o

Cohesive Fill(1) 120 125 0 33.5o 1,500 -

Granular Fill(1) 120 125 0 33.5o - -

Riprap(1) 130 135 0 45o - - Hard Pan Impenetrable

(1) Thin surficial layers of existing materials or riprap. Assumed based on typical values and engineering judgment.

5.3 Compressibility

Material properties used for the settlement analysis are summarized in Table 4. Settlement parameters are based on values in MRFEPP Phase 2/3. These parameters were used as an initial check and future soil exploration and testing will be used to further define the compressibility parameters.

Table 4: Summary of Consolidation Parameters used in Settlement Analysis.

Material OCR Cc Cr eo CH 1 0.29 0.06 0.979

CL Mix 2.5 0.17 0.03 0.807

6. Bearing Capacity and Settlement Analysis The bearing capacity and settlement analyses have been completed for the upstream and downstream closure structures. Due to limited soil information along the levee profile, a settlement analysis for this specific levee section was not completed at this time. Settlement analyses for portions of the levee and for various closure structures will be evaluated during PED.

6.1 Bearing Capacity Analysis for River Closure Structures

The bearing capacity analysis for the river closure structures were performed using soil parameters provided in Table 5. The bearing capacity analysis was completed assuming the closure structure would be founded on a fat clay material, which is the weakest general material type defined for current feasibility phase of the Recommend Plan. Because the upstream and downstream closure structures have similar configurations and applied forces, the analysis was



completed in terms of a typical closure structure founded on fat clay. The Meyerhoff method, presented in EM 1110-1-1905 was used for bearing capacity analyses of the river closure structures. Bearing capacity analysis was performed for both peak undrained and drained shear strengths.

The following parameters have been used for the analysis based preliminary structural analysis and design.

Table 5: Summary of Foundation Parameters for Closure Structures.

Structure

Foundation Footprint

Depth (ft)

Applied Bearing

Pressures lbf/ft(1)

Eccentricity(2)

B (ft) L (ft) Usual Case (ft)

Unusual Case (ft)

Extreme Case (ft)

Upstream & Downstream River Closure

22.5 100 6 22,163 0.01 2.40 4.60

(1) The buoyant portion of applied surcharge weight was neglected, such that the structure analyzed as completely non-submerged.

(2) Eccentricity measured from center of wall with negative being upstream.

The upstream river closure structure will be located in the river between borings B-54-15 and B-55-15. The downstream river closure structure will be located in the river near boring ST-1. Based on the boring logs the structure foundations materials could range from clean sand to fat clay. The fat clay has the lowest shear strength for drained and undrained conditions relative to the shear strength properties of the other materials developed. A typical river closure structure was analyzed, because the upstream and downstream river closure structures have similar dimensions, weights, and applied forces. The required Factor of Safety and calculated Factor of Safety are provided in Table 6.

The allowable bearing capacity is governed by the undrained condition. The detailed bearing capacity calculations are provided in Attachment G-2. Using the soil parameters presented, the soil will be able to support the applied bearing stresses of the structure. All calculated Factor of Safeties are above the required Factor of Safety. Uplift pressures will be evaluated during PED to evaluate the stability of the structures. The bearing capacity analysis will be analyzed to account for seepage under the closure and transition structures to evaluate the impact of seepage on the stability.



Table 6: Summary Factor of Safety for Bearing Capacity for River Closure Structures.

Foundation Material

Eccentricity Case Usual Unusual Extreme

Required FoS FoS Required

FoS FoS Required FoS FoS

Fat Clay (CH) – Undrained 3.5 7 3.0 5 2.0 4

Fat Clay (CH) – Drained 3.5 45 3.0 30 2.0 18

6.2 Settlement Analysis for River Closure Structures

EM 1110-1-1904 was used as a guidance document of settlement calculations. Granular soils are considered to undergo immediate settlement and will not contribute to long-term consolidation settlement.

Construction of the river closure structures has the potential to settle due to underlying clay soils, particularly the normally consolidated fat clays. Therefore, the settlement of the river closure structures were analyzed to evaluate the design, construction method, construction sequence, schedule, and long-term operation and maintenance of the river closure structures.

The upstream river closure structures will be located in the river between borings B-54-15 and B-55-15. The downstream river closure structure will be located in the river near boring ST-1.

It is considered that the earthen levee are located at a sufficient distance from the river closure structure as to not induce additional settlement at the closure structures. The preliminary structural design indicates a normal applied bearing stress of 986 psf (from the structure under usual conditions), a footprint of 22.5ft by 100ft, and a depth of 6ft below grade. Consolidation settlement from unusual loading cases is not considered to apply loads for durations long enough to cause long-term consolidation settlement.

Settlement for the upstream and downstream river closure structures are estimated to be around 0.37in and 0.46in respectively. Detailed settlement calculation are provided in Attachment G-3. Settlement of the filled transition walls will be required to determine if differential settlement will be a concern. This analysis will be completed during PED when site specific soil exploration and testing has been completed.

6.3 Levee Settlement Analysis

Levee settlement analysis was not completed for the current feasibility phase, due to limited site specific data. Therefore, a review of available boring logs in the project area was used to generally match stratigraphy that was consistent with other area pf the MRFEPP that has been designed. MRFEPP Phase 1 indicated a levee settlement of approximately 3 to 4in. Levee settlement calculations for MRFEPP Phase 2/3 indicate a settlement of approximately 0.5 to 14in. Due to the unknown site specific geology, a conservative settlement of 14in was used for the feasibility design. Additional analysis will be completed during PED to further evaluate the



settlement.

6.4 Roadway and Railroad Closure Structures

Bearing capacity and settlement analysis was not completed for the feasibility design of the roadway and railroad closure structures. It was assumed that a shallow spread footing foundation would be used for the structures which is based on similar structures for MRFEPP Phase 2/3. Further evaluation will be completed during PED when site specific soil data is known.

7. Maple Diversion Seepage and Stability Analysis Seepage and stability analyses were performed for the proposed levee and diversion configuration under the following scenarios:

• A qualitative assessment of levee setback was performed to determine the adequacy of the distance between the levee and river channel (proper) or diversion channel.

• A stability analysis was performed, which examined levee stability under low river flow (or dry when applicable) conditions. Both landside and diversion-side analyses were performed using drained and undrained soil conditions as well as sudden drawdown scenarios.

o For these analyses Landside refers to the area to the northwest of the levee, dry-side. Diversion-side refers to the wet-side of the levee, which is to the southeast of the levee.

• Landside stability was analyzed during flooding events using the Design Water Surface Elevation (DWSE) and Maximum Water Surface Elevation (MWSE). Stability was evaluated using drained soil conditions for both DWSE and MWSE and undrained soil conditions for DWSE.

• An underseepage stability analysis was also performed. Piping/erosion and heave factors of safety were computed for the regions of upward gradient at the landside toe and topographic low lying areas on the landside of the levee for both the DWSE and MWSE events.

The following section of this report discusses the methodology, assumptions, and results of the modeling.

7.1 Modeling Method

The seepage conditions and slope stability of the levee embankment, foundation, and riverbanks were analyzed with software created by GEO-SLOPE International Ltd. The integrated software suite is called GeoStudio 2016 and includes SEEP/W and SLOPE/W. SEEP/W is a finite element program that analyzes groundwater seepage within porous material like rock and soil for traditional steady-state flow. The computed pore-water pressures and groundwater surface can then be imported into SLOPE/W analyses, allowing the program to analyze complex saturated/unsaturated or transient conditions. SLOPE/W uses limit equilibrium methods to perform slope stability analyses.



7.2 Model Cross-sections

A total of three cross-sections were analyzed. Figure 5 shows the plan view of the site and the relative location of the geotechnical cross-sections.

The typical configuration of the diversion channel consists of a 1V:3.5H cut into existing soil to a specified depth up to a maximum depth of approximately 20 feet in the downstream direction to the northeast. The modeled diversion has an upper width of 250 feet and a base width of approximately 150 feet. Upstream of the weir, the diversion channel will include drainage to the middle of the channel which will then be sloped back the Souris River for drainage. The diversion channel downstream of the weir, will be constructed with a flat bottom due to backwater for the Souris River. However, as a conservative measure, a low flow channel was included in the model cross-sections. The low flow channel was modeled as a small drainage ditch approximately 2 feet deep located within the middle of the diversion channel. The low flow drainage channel could also be a temporary feature during construction to maintain a dry channel. The requirement for a low flow channel will be evaluated during PED and it will be determined then if it is appropriate to include in the modeling.

The typical configuration of the levee embankment prism consists of a Diversion-side slope of 1V:3H slope and a Landside (dry side to the northwest of the levee) slope of 1V:3H. The levee crest is flat with a width of 15 feet. The elevation of the crest varies along alignment based on the water levels flowing downstream and a difference in anticipated settlement along the levee alignment.



Figure 5: Location of cross-sections analyzed.

7.3 Seepage Analysis

Seepage modeling was conducted to gain a better understanding of the groundwater conditions at each cross-section and to provide inputs for slope stability analyses. The seepage analyses estimated the levee under-seepage rate and evaluated the upward gradient at the Landside levee toe (piping/erosion and heave evaluation) and at a distance of approximately 50 feet from the levee Landside toe or in other low lying areas adjacent to the levee prism. Heave calculations were completed to verify that total stress of soils overlying high permeable units connected to the river were able to resist upward water pressures induced during flooding events. Piping/erosion and heave calculations assume that no additional fill (buttress or seepage blanket) is placed against the Landside side of the levee.

7.3.1 Seepage Model Properties

The main parameter to seepage analyses is hydraulic conductivity or permeability. Additionally, boundary conditions and stratigraphy characterization play a large role in seepage analyses. Hydraulic conductivity is a measure of the resistance of flow through saturated soil media based on a known total hydraulic head differential. This parameter is used to evaluate the potential for groundwater movement through soils below the levee and fill materials anticipated for use within the levee.

Permeability values used in the seepage analysis are provided in Table 2. Other parameters



defining hydraulic conductivity function and volumetric water content function, which apply to the unsaturated region of the model above the phreatic surface, were based on values used in MRFEPP Phase 2/3.

Due to the uncertainty of the thickness and character of diversion channel sediments (silt) over time, the presence of these were omitted from the analysis and the soils within the stratigraphic layers are assumed to be in direct contact with the diversion channel.

7.3.2 Boundary Conditions

Boundary conditions were varied depending on the analysis performed. A summary of the total head boundary condition is provided in Table 7. Boundary conditions consistent with MWSE and DWSE water levels were applied along the ground surface through the diversion bottom to the Diversion side crest of the levee. The normal flow river elevation (NFE) and low flow river elevation (LFE) water surfaces were applied to the diversion channel. In the cases the NFE and LFE were below the base of the diversion, the NFE and LFE were applied as boundary conditions on the Diversion side edge of the model as a far-field condition.

Table 7: Summary of Hydraulic Boundary Conditions.

Cross-section

Maximum Water Surface

Elevation (MWSE)(1)

Design Water Surface

Elevation (DWSE)

Normal Flow Elevation

(NFE)

Low Flow Elevation

(LFE)

Far-field Total Head

(ft) (ft) (ft) (ft) (ft) 1 1570.1 1565.9 1544.2 1544.0 1545.0 2 1569.0 1564.8 1541.4 1541.3 1542.5 3 1568.3 1564.1 1541.4 1541.3 1541.4

(1) Top of constructed levee.

7.3.3 Seepage Results

Steady-state seepage analyses were performed to determine pore-water pressures for stability modeling and also evaluate the potential for piping or heave issues along the toe of the levee, at a distance of approximately 50 feet from the levee Landside toe, and/or in any other low lying areas. The results of the seepage modeling were directly imported into the Slope/W stability module in GeoStudio software package. Seepage analyses can be view in Attachment G-4 and Attachment G-5.

The minimum required seepage factors of safety against piping/erosion and heave are 1.6 for the DWSE and 1.3 for MWSE. The factor of safety for piping/erosion is estimated by dividing the critical gradient (buoyant soil unit weight divided by unit weight of water) by the exit gradient (change in total head divided by the distance between measured total heads). The exit gradient was calculated between the Landside toe of the levee and 2 feet below the Landside toe of the levee when the levee is founded on homogeneous materials. Alternatively, if the levee is founded on low-permeability materials, which are underlain by higher permeability materials, calculations are performed across the entire thickness of the uppermost low permeability layer.



The piping/erosion factor of safety is only applied at cross-sections where groundwater is passing through the ground surface at or near the Landside toe of the levee. When ground water is not passing through the ground surface (considered to be impermeable) at or near the Landside toe of the levee, only the heave factor of safety was calculated. The heave factor of safety is determined by dividing total vertical stress by pore-water pressure at the interface between a high-permeability material overlain by a low-permeability material. Water above the ground surface was accounted in the heave calculation by subtracting the pore-water pressure at the ground surface from the total vertical stress and pore-water pressure at the interface between the high and low permeability material.

Due to the high permeability content of near surface materials (shown as fill on boring logs with high granular content) and near surface sandy alluvial deposits, a high pore-water pressure head through the strata underlying the levee was calculated in cross-section 2 and 3. The high pressure heads transmitted through the landside toe of the levee, resulted in slope stability factors of safety that were insufficient. Variations in the surficial materials through the Maple Diversion may lead to further reduction of seepage and slope stability factors of safety should additional sandy layers be identified during future soil exploration. To address the high seepage rates calculated using the information currently available, a seepage mitigation system will be required along at least this portion of the Maple Diversion Project.

The pore-water pressures and slope stability factors of safety met or exceeded the targeted values at cross-sections 1 and 2. However, without a defined geotechnical investigation for the project, a limited understanding of the site stratigraphy creates several unknowns for the geotechnical assessment. It is possible other currently unidentified problematic areas, such as high permeability sand or gravel zones could be present beneath the levee. Therefore, a continuous seepage mitigation option along the levee alignment was included to reduce the risk of a failure by intercepting any sand/gravel zones. Seepage mitigation measures were required for other phase of the MRFEPP.

7.3.4 Seepage Mitigation

As discussed in the section above, a portion of the piping/erosion and heave factors of safety did not meet the required factors of safety. There are three general types of seepage mitigation;

1. Overburden/raised grade features (seepage berms) 2. Seepage cutoffs (slurry trenches and sheet piling) 3. Pressure relief structures (relief wells or relief trenches)

A number of these seepage mitigation options were considered to control seepage near the Landside levee toe. Table 8 provides brief descriptions of the mitigation options.



Table 8: Evaluation of Seepage Mitigation Options.

Option Pro Con

No Additional Seepage

Mitigation Minimal additional costs and work

Does not reduce seepage or increase stability Does not improve low factors of safety at toe Would require a waiver for reduced factor of safety requirements

Seepage Berm

Lower costs to construct Improve stability at toe Would not require a specialty contractor

Could not be performed in easement Does not reduce under-seepage

Zoned Embankment

Lower cost Would not require a specialty contractor

Possibly more seepage Does not improve under-seepage Does not improve low factors of safety at toe when embankment is on flat ground

Relief Wells

Lower cost than cutoff walls or sheet pile Relief of pressure at toe Improves stability at toe Minimal space required Likely does not require specialty contractor

Does not reduce seepage, so must collect and remove water Maintenance intensive Would require many wells to adequately relieve pressure May not be as effective with the possible variable soil conditions

Partially Penetrating

Pressure Relief Trench (Seepage

Collection Trench) –

Recommended Option

Lower cost Relief of pressure at the toe Improved stability at toe Minimal space required

Does not reduce seepage, so must collect and remove water Reliance upon graded filter design Long-term maintenance will be required

Deep Slurry Cutoff

Reduces seepage Improves stability at the toe Can be performed within the easement

Higher cost Wide, low permeability cutoff works most effective Requires specialty contractor

Shallow Slurry Cutoff Wall

Somewhat reduces seepage Marginally improves stability

Not as effective May not meet required factor of safeties

Sheet Pile Cutoff (steel)

Reduces seepage Improves stability at the toe Can be installed within easement

Higher cost May need a specialty contractor

Sheet Pile Cutoff (plastic)

Lower cost Reduces seepage Can be installed in easement

Specialty Contractor Depth of sheeting penetration may be limited due to hard soils

Based on the evaluation and analyses of the MRFEPP Phase 2/3 Project, a seepage collection trench was determined to be best suited to mitigate the pore-water pressures at the levee toe and in underlying sandy seams rather than other mitigation options. A seepage collection trench is being recommend over the other options due to costs, available space, and impact on existing groundwater flow.



The upper portion of the partially penetrating pressure relief trench (seepage collection trench) will provide a collection system for groundwater flow passing beneath the levee, thus lowering the phreatic surface near the Landside toe. The lower portion of the trench will be extended into the underlying high permeability sand/gravel units extending to the approximate base elevation of the diversion. The seepage collection trench wall will allow for controlled relief of high pore pressures in granular units underlying the levee. By collecting near-surface groundwater below the levee and relieving pressure in the underlying high permeability soil units, seepage collection trench will increase the seepage and slope stability factors of safeties to meet the requirements.

A summary of factors of safety against piping/erosion and heave can be found in Table 9. Seepage rates associated with this configuration are provided in Table 10. Due to the potential for variable soil layers, the seepage collection trench should be placed along the entire portion of the alignment from river railroad crossing to the east end of the project. The depth to the top of the lowest sandy unit which may potentially be directly connected to the river and may be substantially impacted by changes in diversion channel water levels was typically on the order of 11 to 20 feet. An onsite geotechnical engineer or specialist shall determine the required termination depth between 11 and 20ft. The depth of the relief trench should be extended to the top of the uppermost sand layer or a depth of 20 feet representing the depth from the ground surface at the base of levee to the top of the lowest sandy unit potentially connected to the diversion channel.



Table 9: Summary of heave/piping erosion factors of safety.

Model Required Factor of

Safety

Section 1 (1)

Section 2 Section 3

No Drain With Drain No Drain With

Drain Steady-State Seepage – Proposed – Case III - DSWE

Toe of Levee (Heave) 1.6 2.45 1.81 2.66 1.74 2.58 Toe of Levee + 50 feet (Heave) 1.6 - 1.55 2.31 1.98 2.41 Low Areas or River bottom (Heave) 1.6 1.86 - - - -

Toe of Levee (Piping) 1.6 Note 2 4.01 Note 2 2.23 Note 2 Low Areas or River bottom (Piping) 1.6 19.74 - - - -

Steady-State Seepage – Proposed – Case III – MSWE Toe of Levee (Heave) 1.3 2.42 1.66 2.63 1.56 2.51 Toe of Levee + 50 feet (Heave) 1.3 - 1.41 2.29 1.82 2.51 Low Areas or River bottom (Heave) 1.3 1.82 - - - -

Toe of Levee (Piping) 1.3 Note 2 2.51 Note 2 1.54 Note 2 Low Areas or River bottom (Piping) 1.3 19.74 - - - -

(1) Section 1 did not require a seepage collection trench. (2) Groundwater is below the ground surface elevation.

Table 10: Summary of seepage rates.

Model Section 1 Section 2 Section 3 Steady-State Seepage – Proposed – Case III - DSWE

Ground Surface Flux Line [ft3/sec] 8.61E-06 7.76E-10 3.07E-09 Levee Flux Line [ft3/sec] 2.49E-10 7.72E-10 3.07E-09 Seepage Perforated Drain Pipe Flux Line [ft3/sec] - 8.68E-06 1.91E-06 Cumulative (All Flux Lines) [ft3/sec] 8.61E-06 8.68E-06 1.92E-06 Cumulative (All Flux Lines) [gpm] 3.86E-03 3.89E-03 8.63E-04

Steady-State Seepage – Proposed – Case III – MSWE Ground Surface Flux Line [ft3/sec] 1.02E-05 1.21E-09 5.17E-09 Levee Flux Line [ft3/sec] 3.26E-10 1.19E-09 5.17E-09 Seepage Perforated Drain Pipe Flux Line [ft3/sec] - 1.15E-05 2.58E-06 Cumulative (All Flux Lines) [ft3/sec] 1.02E-05 1.15E-05 2.59E-06 Cumulative (All Flux Lines) [gpm] 4.58E-03 5.18E-03 1.16E03

7.3.5 Seepage Collection System Filter Design

The upper portion of the seepage collection trench will consist of a shallow filtered trapezoidal trench drain with a 6 inch diameter perforated pipe centered horizontally in the trapezoid. The sides of the trapezoid will be sloped at 1V:1H and the bottom of the trapezoid will be 3 feet wide. The lower portion of the seepage collection trench will consist of a 3 foot wide vertical trench extending through the bottom of the trapezoid and backfilled with a filter material.

A complete filter design will be performed during PED following site specific field and laboratory investigation. However, it is anticipated the filter design of the upper trapezoidal



section of the seepage collection trench will consist of a perforated pipe surrounded by a two stage filter. The first stage filter material (Filter A) will be designed to be placed in the lower extension of the seepage collection trench and the outer edge of the upper trapezoidal section. Filter A will likely be a fine to coarse sand. The second stage filter (Filter B) will likely be a gravel to provide a separation of materials between the perforated drainage pipe and Filter A. Figure 6 provides a typical seepage collection trench detail.

The pipe sizing, efficiency losses will be further evaluated during PED. The trench drain discharge will be to the Broadway pump station which should have adequate capacity for the seepage trench discharge. The seepage trench discharge is not anticipated to be significant

Figure 6: Example of the two stage filter for the seepage collection trench.

7.4 Slope Stability Analyses



The main objective of the slope stability analysis was to evaluate the stability of the levees and diversion channel under static hydraulic conditions and sudden drawdown of the diversion. Two types of stability analyses are typically performed for slopes: the Undrained Strength Stability Analysis (USSA) and the Effective Stress Stability Analysis (ESSA).

The USSA is performed to analyze the case in which loading or unloading is applied rapidly and excess pore-water pressures do not have sufficient time to dissipate during shearing. This scenario typically applies to loading from, for example, embankment construction where the loading takes place quickly relative to the permeability of the soils. It is often referred to as the “end of construction” case.

The ESSA is performed at account for much slower loading or unloading, or no external loading, in which the drained shear strength of the materials is mobilized and no excess pore-water pressures are allowed to develop. For example, a slowly moving landside is best analyzed using the ESSA method. For this reason, the ESSA is often referred to as the “long-term” case.

The stability of a slope is often reported using a factor of safety value. The factor of safety is the ratio of the summation of forces and moments that are resisting slope movement to the summation of forces and moments that cause slope movement. These forces and moments could result from increased loading or decreased resistance, which may be caused by variation in pore-water pressure and the buttressing effect induced by changes in river levels.

Both the USSA and ESSA were performed as part of the slope stability analysis for this project. The factor of safety was completed by incorporating the results of the seepage analysis under steady-state conditions. Incorporating the groundwater pore pressures from the limit equilibrium calculations captures the effect of fluid/soil interaction of the factor of safety calculations. In this manner, emphasis was placed on evaluating the impact of groundwater flow on stability.

Minimum factors of safety were determined in the models by two slip surface search methods. The first method, entry exit, was used to review the slip surfaces passing through the levee. The second method, grid-radius, was used to review slip surfaces associated with complex river embankment slopes and diversion excavated slopes. The grid-radius approach determines the lowest factor of safety based on a grid of points; the entry-exit methods limits the entry and exit ranges of the slip surface over defined ranges in the model. The entry-exit method was used for the levee stability analyses (Case I, II, IIIa, IIIb, IIIc). Entry and exit ranges were chosen based on preliminary runs and engineering judgment, focusing on failures below the levee prism. A grid-radius analysis was completed to verify the minimum factor of safety was contained in the entry-exit ranges for models with complex river embankment slopes and diversion excavated slope models.

7.4.1 Slope Stability Model Properties

The key parameter associated with levee stability is shear strength. Material properties used for the slope stability analysis are summarized in Table 3. Modeling parameters for the proposed materials and existing soil conditions were determined from a review of available geotechnical



information, specifically the MRFEPP Phase 1 and MRFEPP Phase 2/3 reports.

7.4.2 Setback Analysis

Setback analyses are required to ensure slope stability of the levee with respect to the configuration of the diversion slope or the river embankment.

A setback analysis was not performed for the Diversion side of the levee, the diversion channel cut was analyzed in conjunction with the levee embankment to provide the required overall stability factor of safety for the levee and diversion slope geometry combined.

A setback analysis at cross-section 1 was incorporated into the general slope stability assessment of the levee. Using a grid and radius critical surface search method to analyze the levee configuration with respect to distance from the Souris River (cross-section 1), a minimum factor of safety for the entire slope geometry was determined.

7.4.3 Levee Stability Results

The levee prism was analyzed for the designated Case I, Case II, Case III as described in EM 1110-2-1913 and in Table 11. In addition, the levee was analyzed for the Case IIIb and Case IIIc conditions, which are associated with the MWSE and the DWSE. This is consistent with the developed Mouse River Design Guidelines that was developed for the overall MREFPP. A summary of factors of safety can be found in Table 12. All of the modeled slope stability results met the required factor of safeties. The slope stability results can be view in Attachment G-6.



Table 11: Factors of Safety for Stability Analysis.

Case Name Factor

of Safety

Levee Slope Description Shear Strength Parameters

1 End of Construction 1.3

Wetside and

dryside

End of Construction Case using steady-state seepage condition of the

natural riverbank.

Low-permeability soils – undrained; free draining soils –

effective stress

2 Sudden Drawdown 1.0 Wetside

Sudden or rapid drawdown condition where the DWSE saturates a portion of the slope and stays partially saturated

Low-permeability soils – 3-stage; free draining soils – effective stress

3a Steady-State

Seepage DWSE

1.4 Dryside

Steady-state seepage conditions that develop from a full DWSE. The DWSE

needs to remain long enough for steady-state seepage to develop.

Effective Stress

3b Steady-State Seepage at

MWSE 1.3 Dryside

Steady-state seepage conditions that develop from a full MWSE. The

MWSE needs to remain long enough for steady-state seepage to develop.

Effective Stress

3c Steady-State Seepage at

DWSE 1.3 Dryside

Steady-state seepage conditions that develop from a full DWSE. The DWSE

needs to remain long enough for steady-state seepage to develop.

Low permeability soft soils – undrained strengths based on pre-flood effective stress conditions for

soils with OCR < 2 to 4; Free draining soils and low permeability stiff soils (OCR > 2 to 4) – effective

stress

Table 12: Summary of Slope Stability Factors of Safety.

Model Required Factor of

Safety Section 1

Section 2 Section 3 No

Drain With Drain

No Drain

With Drain

Steady-State Seepage, Proposed (Case I) Slope Stability, Proposed (ESSA – Landside) 1.3 2.27 - 2.62 - 2.66

Slope Stability, Proposed (ESSA – Riverside) 1.3 2.52 - 2.12 - 2.15

Slope Stability, Proposed (Case I – USSA - Landside) 1.3 1.86 - 2.15 - 2.02

Slope Stability, Proposed (Case I – USSA - Riverside) 1.3 1.96 - 2.02 - 1.98

Slope Stability, Proposed (Case II – Sudden Drawdown – Riverside) 1 1.3 - 1.05 - 1.09

Slope Stability, Proposed (Case IIIc – USSA – Landside) Steady-State Seepage, Proposed (Case III – MWSE) 1.4 2.23 1.95 2.61 1.12 2.24

Slope Stability, Proposed (Case IIIb – ESSA – Landside) 1.3 1.86 1.71 2.15 1.46 1.95

Steady-State Seepage, Proposed (Case III – MWSE) Slope Stability, Proposed (Case IIIb – ESSA – Landside) 1.3 2.22 1.66 2.55 0.65 2.03



8. Exploration Trench An exploration trench will be required to examine the levee subgrade at or next to the footprint of the levee prior to placement of levee fill. The exploration trenches will be used to identify any utilities or drainage pipes which may be present through the developed areas of the project as well as any unsuitable fill materials which may have been previously placed. It is anticipated that the exploration trench may be 10 feet deep or more in the area of previous houses or other basement foundations and utilities. In undeveloped areas, trenches 6 feet deep are considered adequate.

The landside seepage collection trench can be used in lieu of a separate exploration trench as long as the trench is excavated to at least to the required depths. Therefore, the seepage collection trench will be used as the exploration trench where feasible.

Following visual inspection of the trench, the trench will be deepened and be incorporated into seepage collection system. Should the inspector need access to the trench, the side slopes of the trench will be excavated to meet OSHA guidelines.

9. West Tieback Levee Geotechnical Assessment A detailed geotechnical assessment of the west tieback levee was not completed for the feasibility study. The feasibility study relied on previous analysis of a tieback levee that is located adjacent to the proposed west tieback levee and it was assumed that the results from this analysis would be applicable to the proposed west tieback levee. The previous analysis was completed on another tieback levee prism having a typical section of 1V:3H side slopes and a top width of 10 feet with 2 foot shoulders. The previous analysis levee height is approximately 6 feet from the levee toe to crest with a crest elevation of approximately 1566 feet. The Recommend Plan West Tieback levee is similar to the previous levee. The West Tieback levee typical section consist of 1V:3H slopes, top width of 13 feet, and a height of approximately 10 feet. Therefore, with the similarities between the levees, the results from the previous analysis were considered adequate to use for the feasibility design. The previous analysis indicated that seepage and slope stability were substantially above the required factors of safeties. The previous analysis can be reviewed in Attachment G-8. Additionally, a seepage mitigation system is not included for the west tieback levee. This is mostly due to the higher ground elevation and the levee will be setback away from the river. Further, the Phase 2/3 levee that the west tieback levee ties into did not require seepage or stability mitigation. One small section of the levee of the Phase 2/3 did require relief wells but this section was located through an old river oxbow resulting in a lower elevation than the surround levee.

The West Tieback Levee ties into the U.S. Hwy 83 Bypass Embankment. The levee at the tie-in location is approximately 4-5ft in height which should results in minimal loading. Additionally, the roadway embankment is sufficiently oversized in height and width. Tying into the road embankment is more efficient and economical than extending to natural height ground. During the design of MREFPP Phase MI-2, an analysis was completed which found the highway embankment suitable as a flood risk reduction feature. This analysis was completed at the roadway location approximately 1,500ft south of the proposed tie-in which results in large hydraulic loading on the roadway embankment. Final configuration, soil exploration, and design



will be completed during PED.

10. Erosion Protection Erosion protection is being proposed in areas where there is the potential for high velocities and turbulent flows. The following are the main areas for erosion protection: Upstream River Closure, Diversion Weir, 6th Street Diversion Crossing, Downstream River Closure, and at the Diversion Outlet. The Hydrology and Hydraulic engineers, provided the minimum W50 requirements. This was used in determining what gradations to use. The riprap and bedding gradations are presented in Table 13 and Table 14.

Table 13: Riprap Gradations

Riprap R80 R140 R270 Thickness,

inches 20-30 24-36 30-45

Bedding B2 B2 B3

% Finer Weight, lbs. Weight, lbs. Weight, lbs.

Max Min Max Min Max Min

100 400 160 690 280 1350 550 50 170 80 290 140 570 270 15 80 25 150 45 260 85 5 65 15 130 25 220 50

Table 14: Bedding Gradations.

Sieve Size Grain Size (mm)

B2 Bedding B3 Bedding Percent Passing By

Weight Percent Passing

By Weight 9in thickness 12in thickness

Max Min Max Min 8” 203.2 - - 100 - 6" 152.4 100 - 79 100 4" 101.6 79 100 69 86 3" 76.2 71 88 62 79

1.5" 38.1 54 71 44 61 3/4" 19 38 54 27 43 3/8" 9.5 22 38 11 27 #4 4.75 7 22 0 11 #10 2 0 5 - 5

#20 0.85 - - - -

11. Cofferdam Design Concept level design and discussion is included in the Structural Appendix M. A detailed soil



exploration and design will be completed during PED.

12. Sources of Construction Materials Sources of construction materials are considered to be available locally. These included riprap, bedding stone, and impervious fill which should be available locally for the contractor.

13. Phase I Environmental Site Assessment A Phase I ESA was conducted by USACE covering the footprint of the Maple Diversion and West Tie-Back. For further details on the Phase I Environmental Site Assessments refer to Appendix K.

14. References

[1] Barr Engineering Co., Ackerman-Estvold Engineering, Moore Engineering and CPS, Ltd, Mouse River Enhanced Flood Protection Plan – Preliminary Engineering Report, 2012.

[2] U.S. Army Corps of Engineers, Draft Programmatic Environmental Impact Statement

Mouse River Enhanced Flood Protection Project Ward County, North Dakota, November 2016.

[3] North Dakota State Water Commission, The Mouse River Enhanced Flood Protection Plan, May 2013.

[4] Task Order 3 Mouse River Enhanced Flood Protection Alternatives Assessment (Work in Kind) Engineer's Opinion of Probable Cost (OPC)

[5] BARR Engineering, Co., Adele Braun, Mouse River Enhanced Flood Protection Project Value Based Design Workshop Technical Memorandum Draft, June 15, 2017

[6] Scott Sobiech and Mark Kretschmer, Maple Diversion Entrance Alternatives Memorandum, June 30, 2017

[7] U.S. Army Corps of Engineers, Management Measures Digital Library (MMDL) Floodwalls, Levees, and Dams, http://library.water-resources.us/docs/MMDL/FLD/

[8] U.S. Army Corp of Engineers, Engineering and Design Manual EM 1110-2-1913: Design and Construction of Levees, Washington: U.S. Army Corp of Engineers, 2000.

[9] Barr Engineering Co., Ackerman-Estvold Engineering, Mouse River Enhanced Flood

Protection Project – Phase MI-2 and MI-3 Basis of Design Report – 100% Draft Design Submittal, 2016.

http://library.water-resources.us/docs/MMDL/FLD/


USACE | Geotechnical Engineering Appendix G Attachment G-1

Attachment G-1: Available Soil Exploration Data

• Excerpts from 1975 As-builts for the Flood Control Project at Minot, ND

• Excerpts from the Mouse River Enhanced Flood Protection Project – Phases MI1, MI-2 and MI-3 Basis of Design Report – 100% Draft Design Submittal

• Excerpts from the Proposed Storm Sewer 6th Street Southwest Underpass Minot, ND

• Excerpts from the Report of Geotechnical Exploration for the Proposed Pump Station 6th Street and 2nd Avenue Southwest Minot, ND



Attachment G-2: River Closure Structures Bearing Capacity Analysis



Attachment G-3: River Closure Structures Settlement Analysis



Attachment G-4: Insufficient Factors of Safety without Seepage Mitigation



Attachment G-5: Seepage Model Output Results



Attachment G-6: Slope Stability Model Results



Attachment G-7: Nearby Levee Analyses that is near the West Tieback

appendix g geotechnical engineering

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