Geotech_RPT_Appendix_G_-_FLAC3D_FINAL.pdf

JULY 2017

B3ECGCLM

Rectangle

APPENDIX C – FLAC3D Results

Lower Wood River Slurry Cutoff Trench

General

A deep slurry trench cutoff wall to eliminate excess seepage during high river stages is planned to be constructed using panel construction in the Wood River Drainage and Levee District (WRD&LD). The WRD&LD owns, operates, and maintains three independent levee systems: Upper Wood River Levee, East and West Fork Levee, and Lower Wood River. In the Lower Wood River, panel construction was chosen due its ability of providing additional stability to an open cut slurry trench during construction. Two-dimensional (2D) analyses have been conducted by the St. Louis District, MVS, however, these analyses have shown to be very conservative because the slope stability analyses utilized assumes the wall is a continuous infinitly long feature. Results of the analyses have produced factors of safety well below Corps criteria of l.3 and even below the stable threshold of 1 thus revealing a major cause for concern.

Three-dimensional (3D) applications and formulas have been developed by well known researchers such as Azzouz and Fox. Their methods are applicable under very limited scenarios and cannot accurately represent true field conditions at the Lower Wood River location. Moreover, Azzouz’s 3D calibration to turn a 2D model into a 3D approximation is based on a failure zone being circular. When implementing Azzouz’s method to a 2D model factors of safety almost double in some instances. However, the majority of the 2D stability models produce a wedge type failure (Figure 1) and thus the usability of Azzouz’s method is not always best suited for each individual model due to the development of nontraditional circular slip surface failures at these locations. In order to model field conditions as closely as possible and represent the physical mechanism of a failure more realistically a FLAC3D (Fast Lagrangian Analysis of Continua) v 5.0 finite difference code has been conducted.

FLAC3D analyses for two distinct cross sections (Figure 2) at Lower Wood River levee stationing 171+00 and 175+20 have been conducted to determine if a deep slurry trench cutoff wall with primary and secondary panels of 20 feet in length would affect the integrity of the adjacent levee. Stability during panel construction is a highly studied concept.

Not only are the panel lengths crucial for stability other factors such as in-situ soil weights and strengths, the hydrostatic distribution of the slurry, ground water elevation, filter cake formation, duration of excavation, construction surcharges, and soil arching are all critically important to the total performance of maintaining stability.

The design methodology behind panel construction is that stress distribution that develops within the cross section will be supported by the side surfaces of the failure zone which will provide a higher lateral resistance in the adjacent sides of the trench and during failure the resulting stress will arch around the panel creating higher factors of safety within the cross section. Arching will occur since there is a difference in the material stiffness between the slurry trench and the surrounding soil. Since the trench is less stiff than the soil the load arches away and around the trench.

The shearing resistance tends to keep the yielding mass in its original position which reduces the pressures on the yielding part of the support and increases the pressure on the adjoining part of soil (Terzaghi, 1943).

The Itasca Consulting Group, Inc. (Itasca) has developed for MVS a FLAC3D Slurry Wall template to analyze the stability of a levee during the construction of a panel-type slurry wall trench in the vicinity of the levee toe. Itasca was asked to create a template to generate a FLAC3D model for a typical cross-section of the Lower Wood River levee to analyze the three-dimensional (3D) stability with an open slurry panel. The 3D model geometry is produced by geometrical extrusion of a representative two-dimensional (2D) cross-section of the levee. The template provides flexibility to modify the ground parameters, ground surface, subsurface profiles and panel length to adjust to the conditions at a particular cross-section location. A Mohr-Coulomb constitutive model is used for the soil layers. The template is set up to perform an effective stress analysis, assuming that the pore pressure distribution is known and remains constant in the creation of model’s initial stress conditions. A factor of safety calculation, based on the strength reduction technique, is included as part of the template functionality. This technical model of geometry creation section below summarizes the template.

Figure 1: Approximation of the rupture body, estimate of supporting shear stress in the triangular side surfaces of the rupture body (German DIN 4126).

Figure 2: Analyzed Reach for Lower Wood River

Model Geometry Creation

Typical cross-sections have been created in GeoStudio’s Slope/W and converted to DXF format via MicroStation so that the cross section can easily be imported into FLAC3D. The material properties utilized within the program are unit weight, cohesion, friction angle and elastic moduli (Young’s modulus and Poisson’s ratio) of each soil layer.

The Cartesian coordinate system is configured for the model so that the cross-section is in X-Z plane, the Z-axis positive direction is vertically upward,Y-axis is in the out-of-plane direction and X-axis is determined by the right-hand rule. FLAC3D has a built-in grid generation tool for models that can be represented as extruded sections.

The DXF file of interest is imported into FLAC3D as part of the template, for use with the extrusion tool. The imported section contains geometric and stratigraphic information, rig surcharge load position, water table elevation, trench position and elevations. This file has been scaled so that the bottom most point is at an elevation of 0.0. This coordinate system was adjusted so that the FLAC3D extruder could more accurately import the DXF file.

FLAC3D has a concept of grouping model items (zones, faces, edges) into named containers (known as groups).

Multiple group names can be assigned to the same area of the model (group names are assigned into slot 1, slot 2, etc.). In the extrusion tool, a group name was assigned to each soil layer; the name is the same as that used by MVS’s Slope/W models (e.g., “Clay 1”). The trench excavation increments (“Trench01”, “Trench02”, etc.) are assigned names in slot 2. These group names are used to easily reference selected regions in the model. Some face groups, which will be useful to apply boundary condition and loads, are also set. The face groups include the side and bottom surfaces, rig load position, water pressure surfaces and trench sides. In the direction of extrusion of the template, the extrusion dimension (out-of-plane dimension) is set to 20 feet. The extrusion dimension is divided into two sections and each section is 10 feet. Symmetry is taken advantage of to save calculation time, since either the middle plane of the trench or the middle plane of the offset can be considered a symmetry plane, so that it is sufficient to consider a half-trench and a half-offset in the model. Additionally, the out-of-plane dimensions are adjusted accordingly to keep symmetry when longer panel lengths are analyzed. Discretization parameters (e.g., zone size) can be specified both in the cross-section and extrusion direction. During interactive model creation in the extruder, the equivalent underlying commands are recorded. These commands were saved to a data file called “grid_20ft.f3dat.” The user can directly modify the data (e.g., extrusion dimension, discretization parameters and group names) in the data file.

Boundary Conditions and Pore Pressure

The model side surfaces are restricted to roller movements and the bottom surface is fixed. The water table elevation is specified in the FLAC3D template by the normal direction of the water table and a representative point on the water table; the pore pressure in each zone is calculated automatically by the code, using the specified water table elevation and the unit weight of water. The pore pressures are kept constant in the model. At the top surfaces of the model located below the water table, constant hydrostatic water pressures are applied. The constitutive laws in FLAC3D (e.g., the Mohr Coulomb model), are formulated in terms of effective stress.

Initial Stress Setup

Since excavation will be performed in Mohr-Coulomb material in the model, the initial stress is of importance. Itasca used the following steps to set the initial stress before excavation.

(a) Prior to the embankment placement, the vertical stress is estimated directly from the gravity equilibrium by cycling the model with the soils as linear elastic materials. The horizontal effective stress then is estimated by multiplication of the vertical effective stress and the coefficients. The total horizontal stress is derived (zone pore pressure is known) and initialized in the model. The coefficients can be specified by the user and currently are estimated by the friction angle of the soil. After the setup of coefficients, the soils are changed to an elastic-plastic model and the model is cycled to equilibrium, and thus, FLAC3D is able to obtain the stress state prior to the emplacement of the embankment.

(b) After placing the embankment, plastic material properties are assigned to the soil layers in the embankment, and FLAC3D cycles to reach equilibrium again to obtain the initial/in-situ stress. The drained material properties for each layer are used for stress initialization

Surcharge Load

Before applying the trench-excavating rig surcharge load, the drained Poisson’s ratios are switched to undrained conditions and are utilized for the cohesive soil layers near the surface.

The trench-excavating rig surcharge load is applied on an area of 24 feet wide (X-Dir.) and 22 feet long (Y-Dir., or out-of-plane direction) with an assumed unit weight of 100 pounds per cubic foot (pcf) that ranges from an elevation of 7.5 feet to 3.8 feet. This three dimensional layout equates to a crane that weights roughly 300,000 lbs. Currently, a trapezoidal distribution is assumed that starts 5 feet away from the trench. The load position was specified during the model geometry creation stage based on the surcharge position specified in the DXF file.

Surcharge=− ((7.5−3.8) �X−(X Coordinate 5 ft from Edge of Trench) 24+3.8) �Yrig

Trench Excavation

There are 25-27 trench excavation increments at approximately 5 feet intervals in each cross section. After each excavation, the applied pressure to the trench walls is assumed to be the hydrostatic distribution of the slurry with a unit weight of 72 pcf. The model is brought to equilibrium after each excavation increment is executed and the corresponding slurry pressure is applied.

Factor of Safety Calculation

A factor of safety (FoS) calculation, based on the shear strength reduction technique (Itasca, 2012), is performed after the slurry wall reaches the final depth and the slurry pressure is applied to the trench walls. In these analyses the cohesion and friction strength properties are factored by a strength reduction-factor (SRF) until the model is essentially unstable. The SRF associated with the point of instability is called the FoS.

DATA FILES

The data files needed to run the FLAC3D model are in text format. All data files are run from a single data file called “master.f3dat”. For convenience, all user specified variables, model parameters, including the information of the model geometry, water table, trench, surcharge load and material properties are placed in a data file called “global.f3dat.” Descriptions of all data files are summarized in Table 1.

To support these analyses in-depth laboratory and in-situ field testing were performed so that the most representative data can be used within the models.

To develop the elastic parameters for the FLAC3D model the calculated constrained modulus (MDMT) from the in-situ dilatometer testing was utilized. This value was obtained in drained conditions so thus a drained Poisson's ratio and known elastic relationships were used to calculate a Young's Modulus. A drained Poisson's ratio calculated from Ko

& a drained Young's Modulus were used to calculate a drained Bulk Modulus. Then a drained Poisson's ratio & a drained Young's Modulus were used to calculate a drained Shear Modulus. A drained Shear Modulus and an undrained Shear Modulus equal each other since it is independent of drainage conditions and water has no shear strength. Taking into account the incompressibility of water an undrained Bulk Modulus for materials under the water table was calculated. Lastly the undrained Shear and Bulk Modulus coupled together were used to calculate an Undrained Young's Modulus and Poisson's ratio.

This appendix presents results from factor of safety analyses performed for stationing 171+00, and 175+20. A total of 2 FLAC3D models were performed along the alignment which included locations along the main line levee, the bend at the bridge, and the Wood River Creek flank levee. The model with the lowest factory of safety at each of these two locations was chosen to be included in this report. The locations, geometrics and foundation conditions for all reaches are further elaborated on within the DDR. All elevations are presented in NAVD88 unless otherwise stated. Figure 3 shows the development of a 3D model with an excavated trench and adjacent soils.

Figure 3: Representative 3D model of Lower Wood River

Data File Functions and notes Required files and files to be saved master.f3dat Organizes all other data files.

The main data file called by FLAC3D.

grid_20ft.f3dat

Generates 3D grid according to the selected DXF format cross-section.

Created by the extrusion tool in FLAC3D.

File saved (after called in “master.f3dat”):

grid.f3sav global.f3dat

Generates 3D grid according to the selected DXF format cross-section.

Created by the extrusion tool in FLAC3D.

ini1.f3dat

Estimates the vertical stress prior to the embankment.

Files required:

grid.f3sav global.f3dat File to be saved:

ini_stress1.f3sav ini2.f3dat

Adjusts the horizontal stresses prior to the embankment based on the specified Ko coefficient.

File required:

ini_stress1.f3sav ini_stress2.f3dav

Ini3.f3dat

Calculates the stresses distribution after placing the embankment.

File required:

ini_stress2.f3sav ini_stress3.f3dav

Change2undrained.f3dat

Changes the Poisson’s ratio to undrained properties at some selected soil layers.

Called by “surcharge.f3dat.”

Surcharge.f3dat

Applies the rig surcharge load. Files required:

ini_stress3.f3sav change2undrained.f3dat surcharge.f3dav

Excavation.f3dat

Excavates the trench in user-defined increments, apply the slurry pressure and bring equilibrium after each excavation stage.

File required:

surcharge.f3sav excavation.f3sav fos.f3dat

Performs factor of safety (FOS) analysis when the slurry wall reaches the final stage.

File required:

excavation.f3sav Files to be saved:

FOSInitial.f3sav FOSStable.f3sav FOSUnstable.f3sav

Table G-1: Data File Descriptions

STATION 171+00

Introduction

The design section includes a clay capped levee embankment with the levee crown at EL 450.3 feet, underlain by a hydraulically filled sandy deposit between EL 438 feet to EL 426 feet. The hydraulic fill is underlain by a clay stratum between EL 430 feet and 424 feet which mantles a lower sand deposit located between EL 424 feet and 300 feet at which bedrock is encountered.

Figure 4: Geo-Studio model for LWR sta. 171+00

EL. 300

Foundation Parameter

The FLAC3D analyses are based on simple Mohr-Coulomb constitutive models (elastic-perfectly plastic soil behavior). Unit weight and shear strength parameters for the various soil layers were based on the information located in the table G-2. The strength values used were the same from the Phase I cutoff wall for Sta. 170.

Soil modulus values were determined or selected assuming the soils are linearly elastic and isotropic with Poisson’s ratio based on expected drainage conditions during loading. The soil modulus values used in the FLAC3D model were primarily based on dilatometer testing. Depending upon the layer, through modeling experience sometimes the min or the average Young's modulus was used from the different dilatometer results that were obtained from in-situ testing. The shear modulus and bulk modulus values were determined using the known elastic relationships below with a Young’s Modulus value and a calculated Poisson’s ratio. Drained and undrained modulus values were both utilized in the model depending on the material type and stress sate. The at-rest earth pressure coefficients are computed only to initialize stresses during model development so equilibrium can be reached in fewer computational steps. A drained Poisson's ratio was used to calculate drained parameters and an undrained Poisson's ratio was used to calculate undrained parameters. The Poisson's ratio was based on friction angle correlations

The slurry level specified within the model is located at a distance 2 feet below the bottom of the work platform. By maintaining the slurry at this level higher hydrostatic pressures will develop within the trench and aid in stability.

Additionally the ground water is set at a specified distance of 8 feet below the bottom of the work platform. This distance is the maximum height the water will be allowed to rise to before construction operations are forced to stop.

Cross Section 171+00 - REACH 3

Layer Name Su (psf) φundrained φdrained γ (pcf) Ko n(porosity)

Clay Cap 600 0 23 120 0.609 0.297

Work Platform 600 0 23 120 0.609 0.297

Hydraulic Fill 0 30 30 125 0.500 0.516

Clay 1 850 0 28 120 0.531 0.297

Sand 1 0 30 30 125 0.500 0.516

Sand 2 0 34 34 125 0.441 0.516

Sand 3 0 32 32 125 0.470 0.516

Till 2600 0 26 135 0.562 0.287

Layer Name νundrained νdrained Edrained (psf) Eundrained (psf) Kdrained (psf) Kundrained (psf) Gundrained (psf) Gdrained (psf)

Clay Cap 0.494 0.38 1.10E+05 1.10E+05 1.51E+05 1.51E+05 3.99E+04 3.99E+04

Work Platform 0.494 0.38 1.10E+05 1.10E+05 1.51E+05 1.51E+05 3.99E+04 3.99E+04

Hydraulic Fill 0.33 0.33 4.80E+05 4.80E+05 4.80E+05 4.80E+05 1.80E+05 1.80E+05

Clay 1 0.4999 0.35 8.00E+04 8.91E+04 8.70E+04 1.41E+08 2.97E+04 2.97E+04

Sand 1 0.33 0.33 1.80E+05 2.02E+05 1.80E+05 8.11E+07 6.75E+04 6.75E+04

Sand 2 0.31 0.31 3.20E+05 3.67E+05 2.75E+05 8.12E+07 1.23E+05 1.23E+05

Sand 3 0.32 0.32 6.70E+05 7.61E+05 6.20E+05 8.15E+07 2.54E+05 2.54E+05

Till 0.4998 0.36 1.40E+05 1.54E+05 1.66E+05 1.46E+08 5.15E+04 5.15E+04

Table G-2: Summary of Soil Parameters

The automated c-Φ reduction technique in FLAC was used for all factor of safety (FoS) calculations. Figure 5 through 11 represents the computed factors of safety of 1.72 for a 20 ft panel at a specified water elevation of 8 feet below the top of the work platform. The most critical slip surface develops on the land side slope of the levee and extends deeply to the bottom of the sand layer number 2 at approximate EL 355 feet. This deep failure is indicative of the higher shear strength materials found within the upper portion of the stratigraphy as it has pushed the higher stress strain relationship deeper into the subsurface layers. Figure 11 represents the horizontal displacement when the trench is excavated at full depth. Horizontal displacements of 1 to 1.4 inches were calculated in the levee slope and around the face of the trench. With the modulus values being moderately variable and with the uncertainly of not having true calibration data from measured movements in the field the projected movements are only an estimate and may not truly be representative of field conditions. With the nature of the soils at these locations and utilizing panel construction movements less than 1 inch are expected.

Figure 5: The maximum shear strain increment distribution trench side throughout the cross section.

Figure 6: The maximum shear strain increment distribution trench side at the failure zone.

Figure 7: The maximum shear strain increment distribution rotated 180° throughout the entire cross section.

Figure 8: The maximum shear strain increment distribution rotated 180° at the failure zone.

Figure 9: The maximum shear strain increment distribution trench side at the failure zone.

Figure 10: The maximum shear strain increment distribution rotated 180° at the failure zone.

Figure 11: Horizontal displacement after the final stage of trench excavation

STATION 175+20

Introduction

The design section includes a clay capped levee embankment with the levee crown at EL 450.3 feet, underlain by a hydraulically filled sandy deposit between EL 438 feet to EL 426 feet. The hydraulic fill is underlain by a clay stratum between EL 430 feet and 424 feet which mantles a lower clay seam and sand deposit located between EL 424 feet and 296 feet at which bedrock is encountered.

Figure 12: Geo-Studio model for LWR sta. 175+20

Clay Blanket

Hydraulic Fill

Work Platform

Sand 1

Clay 1

Sand 2

Till

Bedrock

Foundation Parameter

The FLAC3D analyses are based on simple Mohr-Coulomb constitutive models (elastic-perfectly plastic soil behavior). Unit weight and shear strength parameters for the various soil layers were based on the information located in the table G-3 that were developed from the shearlines for Reach I.

Soil modulus values were determined or selected assuming the soils are linearly elastic and isotropic with Poisson’s ratio based on expected drainage conditions during loading. The soil modulus values used in the FLAC3D model were primarily based on dilatometer testing. The shear modulus and bulk modulus values were determined using the known elastic relationships below with a Young’s Modulus value and a calculated Poisson’s ratio. Drained and undrained modulus values were both utilized in the model with undrained values being used below the water table.

The at-rest earth pressure coefficients are computed only to initialize stresses during model development so equilibrium can be reached in fewer computational steps.

The slurry level specified within the model is located at a distance 2 feet below the bottom of the work platform. By maintaining the slurry at this level higher hydrostatic pressures will develop within the trench and aid in stability.

Additionally the ground water is set at a specified distance of 8 feet below the bottom of the work platform. This distance is the maximum height the water will be allowed to rise to before construction operations are forced to stop.

Table G-3: Summary of Soil Parameters

Layer Name Su (psf) φundra ine d φdra ine d γ (pcf)

Clay Cap 600 0 23 120

Work Platform 600 0 23 120

Hydraulic Fill 0 30 30 125

Clay Blanket 1000 0 28 120

Sand 1 0 33 33 125

Clay 1 500 0 28 120

Sand 2 0 31 34 125

Till 3000 0 26 135

Layer Name νundra ine d νdra ine d Edra ine d (psf) Eundra ine d (psf) Kdra ine d (psf) Kundra ine d (psf) Gundra ine d (psf) Gdra ine d (psf)

Clay Cap 0.494 0.38 1.10E+05 1.10E+05 1.51E+05 1.51E+05 3.99E+04 3.99E+04

Work Platform 0.494 0.38 1.10E+05 1.10E+05 1.51E+05 1.51E+05 3.99E+04 3.99E+04

Hydraulic Fill 0.33 0.33 4.80E+05 4.80E+05 4.80E+05 4.80E+05 1.80E+05 1.80E+05

Clay Blanket 0.4999 0.35 8.00E+04 8.91E+04 8.70E+04 1.41E+08 2.97E+04 2.97E+04

Sand 1 0.33 0.33 1.80E+05 2.02E+05 1.80E+05 8.11E+07 6.75E+04 6.75E+04

Clay 1 0.4999 0.35 9.20+05 9.20E+05 8.70E+04 1.41E+08 2.97E+04 2.97E+04

Sand 2 0.32 0.32 6.70E+05 7.61E+05 6.20E+05 8.15E+07 2.54E+05 2.54E+05

Till 0.4998 0.36 1.40E+05 1.54E+05 1.66E+05 1.46E+08 5.15E+04 5.15E+04

Cross Section 175+20

Ko n(po ro s ity)

0.609 0.297

0.609 0.297

0.5 0.516

0.47 0.516

0.562 0.287

0.531 0.297

0.5 0.516

0.531 0.297

The automated c-Φ reduction technique in FLAC was used for all factor of safety (FoS) calculations. Figure 13 through 19 represents the computed factors of safety of 1.62 for a 20 ft panel at a specified water elevation of 8 feet below the top of the work platform. The most critical slip surface develops on the land side slope of the levee and extends deeply to the bottom of the clay layer number 1 at approximate EL 400 feet. This deep failure is indicative of the higher shear strength materials found within the upper portion of the stratigraphy as it has pushed the higher stress strain relationship deeper into the subsurface layers. Figure 19 represents the horizontal displacement when the trench is excavated at full depth. Displacements of 1.0-1.2 inches were calculated in the levee slope and around the face of the trench. With the modulus values being moderately variable and with the uncertainly of not having true calibration data from measured movements in the field the projected movements are only an estimate and may not truly be representative of field conditions. With the nature of the soils at these locations and utilizing panel construction movements less than 1 inch are expected.

Figure 13: The maximum shear strain increment distribution trench side throughout the cross section.

Figure 14: The maximum shear strain increment distribution trench side at the failure zone.

Figure 15: The maximum shear strain increment distribution rotated 180° throughout the entire cross section.

Figure 16: The maximum shear strain increment distribution rotated 180° at the failure zone.

Figure 17: The maximum shear strain increment distribution trench side at the failure zone.

Figure 18: The maximum shear strain increment distribution rotated 180° at the failure zone.

Figure 19: Horizontal displacement after the final stage of trench excavation

DESIGN VALIDATION:

Previous work by Fredlund & Krahn (1977) compared 2D FLAC analyses with known slope stability methods to see the similarities or differences in the calculated factors of safety. From their work a relationship of similarly was found between the methods analyzed proving FLAC’s strength reduction technique gives similar results when compared to well known limit equilibrium analyses. To compare the two design tools used in the slope stability analysis for Lower Wood River a comparative model was developed in FLAC3D to simulate a 2D solution. Using the previously built FLAC3D model at sta. 171+00, alterations were made to the data file to reflect a continuous trench. The model was initiated but once the data file reached excavation sequence 10 the model would never converge to reach equilibrium.

The model would continually cycle proving that the trench had begun to fail. Once this discovery was made the excavation sequence was limited to nine stages which ended at the sand 1 and sand 2 interface. After the excavation was complete the model was ran to determine the corresponding factor of safety which resulted in a value of 1.04.

Using GeoStudio’s Slope/W Sta. 171+00 a fully specified non-optimized slip surface was created based on the failure surface developed within FLAC3D. By creating a model in this manner the FLAC3D results can be directly compare to the Slope/W 2D analyses to evaluate the similarities in how the material parameters are being utilized within the model. It was found that the factors of safety in the two models were very similar proving the input parameters in FLAC are being properly utilized. By knowing the relationship between the models is valid, taking the FLAC3D model into the third dimension brings more confidence to the design team and proves that the 2D analysis is over-conservative. It has been found that a project of this nature should ultimately be designed utilizing the added benefit of a third dimension to acquire higher and more reasonable factors of safety.

Results from Fredlund & Krahn (1977) compared to FLAC

Case SBM Spencer JRM MPM FLAC 1 2.08 2.07 2.01 2.08 2.02 2 1.38 1.37 1.43 1.38 1.36 3 1.77 1.76 1.71 1.77 1.71 4 1.12 1.12 1.16 1.12 1.16 5 1.83 1.83 1.78 1.83 1.80 6 1.25 1.25 1.30 1.25 1.30

Case No. Example Problem 1 Simple 2:1 slope, 40 ft high, φ’=20°, c’=600 psf 2 Same as 1 with think, weak layer with φ’=10°, c’=0 psf 3 Same as 1 except ru=0.25 4 Same as 2 except with ru=0.25 for both materials 5 Same as 1 except with a piezometric line 6 Same as 2 with a piezometric line for both materials

SBM Simplified Bishop Method Spencer Spencer’s Method JRM Janbu’s rigorous method MPM Morgenstern-Price Metho

1.00

-0.1 0.0 0.1

DISTANCE IN FEET

Figure 20: Slope/W model for LWR sta. 171+00 with tension removed from the model

1.06

-0.1 0.0 0.1

DISTANCE IN FEET

Figure 21: Slope/W model for LWR sta. 171+00 without tension removed from the model

Figure 22: FLAC3D model for LWR sta. 171+00 analyzed as a 2D model

Figure 23: FLAC3D model for LWR sta. 171+00 analyzed as a 2D model displaying the maximum shear strain increment distribution.

Results from MVS GeoStudio’s Slope/W slope stability analysis for Lower Wood River Sta. 171+00

Case Spencer FLAC Δ

1 (No Tension) utilized Tension Cracks 1.00 1.04 0.04 2 (Tension is still present within the model) 1.06 1.04 0.02

The FLAC3D models that have been created are very complex models. To ensure the coded data files are properly functioning within the model validation checks at various locations throughout model Sta. 171+00 were performed. Since slurry pressures are critical in the stability of the trench Gridpoints at various locations along the face of the trench were checked to see how the hydrostatic pressure distribution developed. Hydrostatic pressures were calculated down the trench starting 2 feet below the top of the work platform by taking the unit weight of the slurry multiplied by the height at which the slurry was acting. Since FLAC3D calculates a force, the square footage the hydrostatic pressure was acting on was multiplied by the hydrostatic pressure to give the appropriate applied force. It was found that FLAC3D’s calculated applied force was almost exact to the hand calculated applied force which signifies the slurry pressure is developing correctly. Additionally, vertical and lateral stresses which affect the entire performance of the model were hand calculated and compared to FLAC3D’s output. It was found that the stress values were very comparable. To be conservative in the design the tension limit was set to zero in all of the data files. In FLAC3D the tension limit will be automatically reduced to cohesion/tan (friction) for sand and since sand has zero cohesion the tension limit is negligible. However, the clay material will be affected and if it cannot resist tension in any principal stress direction the factor of safety will decrease and the analysis will be more conservative. Thus the tension limit was set to zero and double checked with the plot contour of maximum principle stress to ensure all the tension had been removed and the clay materials were acting in compression. Moreover, horizontal (Fig 11 & 19) and vertical displacements were evaluated in the model. It was found that displacements within the levee cross section were minimal and the development of the displacement vectors were indicative of the slip surface that developed within the levee section. Based on the results from the 2D to 3D calibration model, the validation tests performed, and the anticipated outcome based on engineering experience and judgment, the model is executing as intended and the factor of safety results are deemed representative.

Figure 24: FLAC3D input Checks

DESIGN OPTIMIZATION:

The slurry trench between stationing 170+00 to 188+60 may be constructed with panel lengths up to 40 feet based on the results found in Figure 25. Using the models created for station 171+00 and 175+20, the trench and adjacent landmass was appropriately extended to reflect panel lengths up to 50 feet. It was found that the factor of safety trend between differing panel lengths was highly linear and that just after 48 ft the factors of safety dropped below the target factor of safety of 1.30. Since the slurry trench equipment is designed in 10 feet lengths building the trench in four 10 foot panels equaling a panel length of 40 feet would be the most practical approach. The slip surfaces that developed from the lengthened panel analysis displays the critical failure to be on the river side of the trench. The findings from this analysis prove that the contactor must take all precautions to distribute the loading from the equipment so as to not induce a failure into the trench. Additionally, the sensitivity with regards to differing unit weights was analyzed. Unit weights of 65 pcf and 68 pcf were analyzed with a 20-ft panel model for Sta. 175+20 and the factors of safety that were calculated were 1.34 and 1.47 respectively.

Moreover, Displacements ranging from 1 inch up to 6 inches were calculated in the levee slope, around the face of the trench, and/or within the subsurface clay layers throughout these sensitivity analyses. With the modulus values being moderately variable and with the uncertainly of not having true calibration data from measured movements in the field the projected movements are only an estimate and may not truly be representative of field conditions. With the nature of the soils at these locations and utilizing panel construction movements less than 1 inch are expected.

Acceptable movement for the inclinometer threshold was set by subject matter experts within USACE and the results from the Phase I Deep Slurry Wall Project just immediately upstream from this site. Results from all inclinometers typically showed less than 0.5 inches of movement with a 23-ft wide panel. The largest subsurface displacements measured generally correspond to cobble layers. Inclinometer displacement in these layers typically ranged from approximately 0.25 to 0.35 inches (towards the cutoff wall), with the largest displacement of nearly 0.5 inches. The contractor shall take all precautions as to not induce excess movements. Embankments are sufficiently flexible and can with stand minimal movements however excess movements can initiate the development of cracks within the soils of the embankment. If potential problems are indicated, surface movements data must be obtained daily and continuous plots maintained so that the development of excessive movement can be detected early and corrective actions taken.

Currently the specs were written based on acceptable Factors of Safety and not strain since the movement in FLAC is less accurate. Historically in Corps designs the factor of safety has controlled over the calculated displacement values.

Site specific data with inclinometers in the test section will be the best way to inform USACE on tolerable movement like the intentions set forth in the specification for Phase I. This site will be monitored with automated inclinometers that will give real time data during construction. If unacceptable displacements over 1 inches are encountered remedial measures shall be taken such as decreasing panel length, increasing slurry density or increasing the height of the work platform to develop higher hydrostatic pressures.

Per the specifications:

Perform and submit stability analysis if there any revisions of variations/deviations from the cross sections shown on the plans. Variations/deviations include but are not limited to changes in the levee cross-section, alterations from the proposed minimum platform, changes in alignment and location of the cut-off wall and changes in the maximum length of panels. Stability analysis shall meet Factors of Safety of at least 1.3 using a 3D finite difference analysis.

The slurry shall be assumed to have no shear strength and no more than 72 pcf unit weight unless it can be justified with mix design results. Panel lengths up to 40 ft have shown to meet the minimum factor of safety, however, excess deformation has been observed in the finite difference model. If the contractor deviates from the specified 25 ft foot panel they shall do so in the test section and at their own risk. If undesirable performance or problems are found such as excess movement outside the limitations and provisions within the specification the contractor shall repair any and all damages at the their own cost.

Figure 25: Comparison between Panel length and the corresponding factor of safety.

1.72

1.51

1.31

1.17

1.62

1.33

1.47

1.34

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

15 20 25 30 35 40 45 50 55

Fa ct o r o f Sa fe ty

Panel Length (ft)

Panel Length "Factor of Safety"

STA 171

STA 175

68 pcf

65 pcf

Section A

Data Files Lower Wood River Cutoff Wall Phase II new set fish auto off call grid_20ft.f3dat save grid call ini1.f3dat call ini2.f3dat call ini3.f3dat call surcharge call excavation call fos extrude select Sta175 geometry import "WRL_B-SC0001_175+00_FLAC.dxf" format dxf set

WRL_B-SC0001_175+00_FLAC

extrude metadata set GEODATASET

"Loaded0GeoSetNameWRL_B-SC0001_175+00_FLACFilePathWRL_B-SC0001_175+00_FLAC.dxfScale1 Rotation0Position00Transparency0.25Color2021560MapAxes0MapOrigin000MapXVector100MapY

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