DDR_Appendix_C_-_Geotechnical_Report_FINAL.pdf

Geotechnical Soils Report

For the Lower Wood River Cement-Bentonite Cutoff Wall Phase II: Wood River Drainage and Levee District

July 7th, 2017 i

Table of Contents

1. Introduction 1

1.1 Project Description and Design Features 2

1.2 Ongoing Projects 2

2. Geology and Soil Descriptions and Shearlines 3

2.1 General 3

2.1.1 Historical Documentation 3

2.1.2 Levee Design Section Selection 3

2.1.3 Field Survey 3

2.1.4 Subsurface Investigation and Laboratory Testing Data 4

2.2 Regional Geology 5

2.3 Shearlines 5

2.3.1 Section 170+00 to 183+00 Specific Geology and Shearline 7

2.3.2 Section 183+00 to 197+00 Specific Geology and Shearline 8

2.4 Permeability 8

3. Geotechnical Design Criteria and Results of Analyses 11

3.1 General 11

3.2 Design Grade and Hydraulic Criteria 11

3.3 Seepage Analysis Summary 12

3.4 Stability Analysis Summary 12

3.5 Proposed Cutoff Wall Requirements 13

3.6 Instrumentation Plan 14

3.6.1 Inclinometers 15

3.6.2 Open system piezometers 16

4. Conclusions and Recommendations 16 ii

Table of Contents

Tables

Table 1. Reach 1 Design Values 7

Table 2. Reach 2 Design Values 8

Appendices

A Section Location Plan, Boring Locations, and Boring Logs

B Geologic Profile and Stratigraphy

C Empirical Strength Calculations

D Mel Price/Wood River Under Seepage Project Permeability Report

E Permeability Calculations

F GROUND WATER Model Report

G FLAC 3D Model Report

H Dilatometer (DMT) Data Reduction and Reference Documents

For the Lower Wood River Cement- Bentonite Cutoff Wall Phase II:

Wood River Drainage and Levee District

1. Introduction

This Design Documentation Report has been developed in support of preparation of Plans and Specifications (P&S) for the Lower Wood River cement-bentonite slurry trench cutoff wall phase II. This cutoff wall is one feature of the underseepage controls recommended in the 2011 Limited Reevaluation Report (LRR) that addressed the need to correct design deficiencies in the Wood River Levee system associated with underseepage. The Wood River Drainage and Levee District lies in southwestern Illinois, on the left bank of the Mississippi River, within Madison County, Illinois, between river miles 195 and 203 above the Ohio River. The levee system is split into Upper Wood River, Wood River East/ West, and Lower Wood River, which are separated by Wood River Creek.

To correct the underseepage design deficiencies, the 2011 LRR recommended a combination of 94 new relief wells; 3 new pump stations; 815 linear feet of seepage berm; 1,010 linear feet of landside clay fill; 3,970 linear feet of deep slurry trench cutoff wall at the riverside levee toe to bedrock; and 2,875 linear feet of shallow slurry trench cutoff wall at the riverside levee toe. As detailed design has progressed some of these features have changed. However, the deep cutoff wall has not changed.

The first phase of the deep cutoff wall (100 to 135 ft deep) extends from levee station 152+00 to 170+00. Construction of this cement-bentonite wall began in the fall of 2015 and has been completed. The Southwestern Illinois Flood Prevention District Council awarded the $13,991,000 contract to Treviicos South Corporation from Charlestown, Massachusetts.

As the second phase of this project, this DDR addresses only the design of the deep (approx. 120 to 150 ft deep) cutoff wall extending from levee station 169+95 to station 188+50 in Lower Wood River. This portion of the wall will tie 5 feet into the recently constructed deep cutoff wall that ends at station 170+00. The project was broken into phases because of previous funding constraints.

The cutoff wall extends from levee stationing 169+95 to 188+50. The stationing along the centerline of the cutoff wall is 0+00 to 18+60. The stationing along the cutoff wall does not match up with the levee stationing because the cutoff wall is located at the toe of the existing levee (see plan sheet C-101).

For the Lower Wood River Cement- Bentonite Cutoff Wall Phase II:

1.1 Project Description and Design Features

This report is being submitted to present the geotechnical design for a cement bentonite cutoff wall. It includes the following items:

• Brief description of existing levee protection features.

• Subsurface information and laboratory test data used in analyses, and shearlines developed to represent subsurface conditions.

• Evaluation of the levee section stability using FLAC 3D.

• Evaluation of the underseepage factor of safety using GMS.

• Provides conclusions and recommendations from analyses.

The engineering studies for this report incorporate the results of the investigations, analyses, and computations in the areas of surveys and site reconnaissance, and geotechnical evaluations.

1.2 Ongoing Projects

There are two ongoing projects in the vicinity of the Lower Wood River Cement- Bentonite Cutoff Wall, one by USACE and one by the SWIFPD. The ongoing USACE project is implementing the under seepage design deficiency corrections identified in the 2011 LRR. The corrective measures referenced in the Introduction are for the entire Wood River Drainage and Levee District. This report covers the cutoff wall and associated relief wells design between stations 169+95 and 188+50. Any corrective measures required outside of this stationing are outside the scope of this report.

The SWIFPD has an ongoing project relating to FEMA certification. This project covers the entirety of the Wood River Levee and Drainage District. Of particular interest are four remaining relief wells to be installed between stations 152+00 to 188+50. These relief wells are included in the 3D groundwater model as they affect the landside uplift pressures both upstream and downstream of the existing cutoff wall as well as groundwater flow through the cutoff wall window.

For the Lower Wood River Cement- Bentonite Cutoff Wall Phase II:

2. Geology and Soil Descriptions and Shearlines

2.1 General

2.1.1 Historical Documentation

The “Wood River Levee System Limited Reevaluation Report Design Deficiency Corrections” report dated August 2011 was referenced during the design for historical information. The report documents the study of deficiencies in the underseepage design and describes the feasibility level basis for recommending implementation of a design deficiency correction project. In addition the A/E firm AMEC provided survey data, utility information, soil borings and soil testing information which was utilized in the design process.

2.1.2 Levee Design Section Selection

Two Design Sections (DS) was developed for stability analysis and design of the levees along the phase II reach (Station 170+00 to Station 197+00). The current top of levee is approximately elevation 445. The phase II design soil reaches selected were based on the results of the various laboratory testing and the detailed boring logs. The results from the various borings were compared and, where appropriate, the borings were grouped into reaches of similar soil stratigraphy and strength values. The two reaches identified by levee stationing were reach 1 (Station 170+00 to Station 183+00) and reach 2 (Station 183+00 to Station 197+00). Within each soil reach the surface geometry was evaluated and the critical design cross section at station 171+00 and 175+20 was selected to reflect both typical site geometry and significant deviations in geometry. For the 1960 ft of wall two cross sections for FLAC 3D Modeling represented the main geological features found within this site. Both sections (171+00 and 175+20) were analyzed and deemed representative for the conditions that would be encountered during the Phase II Construction. Some subtle deviances do exist within this reach as any geological feature or reach will. However, MVS felt the sections analyzed are a worthy depiction of expected conditions to be encountered.

2.1.3 Field Survey

Topographical survey data was obtained using Light Detection and Ranging (LIDAR) technology as well as information shown on existing as-built drawings. LIDAR was obtained from Madison County, IL Information Technology Department.

Utility survey data was contracted to Hanson Professional Services and field work was completed in July and August of 2014. Additional spot surveys were obtained by USACE, as necessary, throughout the design process.

For the Lower Wood River Cement- Bentonite Cutoff Wall Phase II:

The horizontal datum used for this project was the North American Datum of 1983 (NAD 83). The vertical datum used was the North American Vertical Datum of 1988 (NAVD 88). All horizontal coordinates shown on the plans reference the State Plane Coordinate System, Illinois West Zone.

2.1.4 Subsurface Investigation and Laboratory Testing Data

The boring and laboratory data was collected in four primary drilling and testing programs. The first drilling and testing program was completed by USACE in 2008 and 2009 as part of the Limited Reevaluation Report (LRR). The second drilling and testing program was completed by the A/E firm AMEC in 2012 and provided to USACE for use in the design. The third drilling and testing program was completed by USACE in 2013 as part of this design effort. The final drilling and testing program was completed by USACE in 2014 as part of this design effort. The entire testing program consisted of machine borings, sonic borings, dilatometer (DMT) and Cone Penetrometer Test (CPT) soundings. Data from the field and laboratory testing program completed along the levee is stored in a gINT data base and is available upon request. In addition, in 2013 USACE obtained DMT soundings for use in the FLAC 3D model.

The 2014 drilling and testing program consisted of 10 machine borings with undisturbed sampling in cohesive soils and SPTs in cohesionless soils, as well as 6 DMT soundings. The 2013 drilling and testing program consisted of 22 machine borings with undisturbed sampling in cohesive soils and SPTs in cohesionless soils, as well as 6 DMT soundings. The 2012 AMEC borings consisted of 8 sonic borings and 8 machine borings. The 2008-2009 LRR borings consisted of 11 disturbed borings and 7 CPTs. The three drilling programs resulted in borings approximately every 200 feet along the river side levee toe. Locations of the borings are shown on drawing B-101 in Appendix A. Copies of the boring logs are also presented in Appendix A. Borings along the river side toe of the levee and along the levee centerline extended to bedrock. Borings on the landside of the levee generally did not go as deep.

Laboratory testing included parameters such as unit weight, shear strength, moisture content, unconfined compressive strength, Atterberg limits, grain size analysis, and soil classification.

For the Lower Wood River Cement- Bentonite Cutoff Wall Phase II:

2.2 Regional Geology

The geologic profiles for the phase II Lower Wood River Cement-Bentonite Cutoff Wall are included in Appendix B. Geologic profiles were created for the centerline of the levee and the riverside toe of the levee.

Along the riverside toe of the levee the subsurface generally consists of surficial clays extending between 1 and 15 deep, underlain by a deep course grained aquifer.

Between station 152+00 and 162+00 the course grain aquifer is underlain by approximately 30 feet of clayey glacial till. Between station 162+00 and 170+00 the course grain aquifer extends to bedrock at an approximate elevation of 300. Between station 170+00 and 188+50 the course grain aquifer extends to a silt at approximate elevation 310. Bedrock is at an approximate elevation of 300 to elevation 290.

Along the centerline of the levee, the levee consists of a clay cap and a hydro fill core.

The hydrofill core is underlain by either the course grained aquifer or the naturally occurring clay blanket. The center line borings show the course grained aquifer to be interspersed with deposits of clays and silts. Between station 152+00 and 162+00 the course grain aquifer is underlain by approximately 30 feet of clayey glacial till.

Between station 162+00 and 170+00 the course grain aquifer extends to bedrock at an approximate elevation of 300. Between station 170+00 and 188+50 the course grain aquifer extends to a silt or till at approximate elevation 310. Bedrock is at an approximate elevation of 300 to elevation 290.

The land side borings are primarily used for the ground water model. However, landside borings near the design sections were used to characterize the landside clay blanket. Generally, landside of the levee the subsurface consist of a thin clay blanket underlain by the course grained aquifer.

2.3 Shearlines

Using the field and laboratory test data from 2012, 2013 and 2014, shearlines were developed as a basis for the geotechnical analyses. Development of the shearlines consisted of tabulating and plotting test data on a boring-by-boring basis.

The data included shear strength from unconfined compression (UCT), saturated unit weight, water content, soil type, grain size, and DMT and CPT shear strength correlations. Data values considered to be outliers were not used in the generation of the shearlines and, in general, the shearlines were drawn conservatively and simplified whenever possible. Drained Strengths (φ’) were calculated using the plasticity index

For the Lower Wood River Cement- Bentonite Cutoff Wall Phase II:

(PI) and Figure 3-2 in EM 1110-2-1913, 30 April 2000. Undrained strengths (c) were determined using unconfined compression tests (UCT). The undrained shear strength of clays was also estimated using two methods (Robertson & Campenella, 1984 and Larsson, 1980) however the strength values from these approximations were not used in design calculations. The correlations were only used as a check against the UCT testing. MVS felt the UCT testing was more accurate than empirical correlations. The equation for Robertson and Campenella is shown in equation 1 below. The equation for Larsson is shown in equation 2 below. In sands the Friction Angle (φ’ and φ) was calculated based on SPT-N values using Peck, Hansen, and Thornburn, 1974 as approximated by Wolff, 1989 and is shown in equation 3. The calculation sheets for determining the empirically obtained strength values are presented in Appendix C following the shear lines.

𝐶𝐶 = (0.11 + 0.0037 ∗ 𝑃𝑃𝑃𝑃) ∗ 𝜎𝜎′ (1)

𝐶𝐶 = 0.45∗𝐿𝐿𝐿𝐿∗𝜎𝜎′

(2)

∅′° = 27.1 + (0.3𝑁𝑁1)60 − 0.00054(𝑁𝑁1)602 (3)

The shear lines were determined using the riverside borings. These borings were used because the primary concern of the design is the stability of the slurry trench during construction which is located at the riverside toe of the levee. The soil layers in the shear lines were determined using both the geologic profile/boring logs and laboratory testing information. The clays were broken into layers with similar undrained strengths and Atterberg limits. The sands were divided into layers based primarily on the SPT-N value correlations. When determining the strength parameters for a given layer the 1/3 - 2/3 rule was used as the starting point and then the strength parameters were adjusted based off of engineering judgment, index tests, and the boring logs.

In the absence of testing, the strength parameters for the clay cap and work platform are based off of conservative assumptions of the strength of compacted clay. The critical failure surfaces in the stability analyses were deeper seated failures so further refinement of the clay cap and work platform strengths was not conducted. The hydrofill is a material dredged from the Mississippi River is primarily made up of silty sands (SM). This material was assigned strength parameters consistent with the sand layers found to have the lowest friction angle for conservatism.

For the Lower Wood River Cement- Bentonite Cutoff Wall Phase II:

The shearlines were broken into two design reaches and are included in Appendix B.

Reach 1 extends from station 170+00 to 183+00. Reach 2 extends from station 183+00 to 197+00. One shear line was developed for the clayey glacial till.

2.3.1 Section 170+00 to 183+00 Specific Geology and Shearline

Design soil reach 170+00 to 183+00 falls within the Reach 1 shear line. Also the critical design section 175+20 falls within this reach The six borings and one DMT sounding from the 2012-2014 subsurface investigation programs and one boring and four CPTs from the 2008-2009 subsurface investigation program were selected to develop the Reach 1 shearlines. These borings and CPTs include: 13-LWRCW-108R, 13-LWRCW-109R, 13-LWRCW-110R, 13-LWRCW-111R, 14-LWRCW-112R, 14-

LWRCW-113R, WRL-RS-S2092, WRLCPT-653-08, WRLCPT-654-08, WRLCPT-655-

08,WRLCPT-656-08, WRLR-655-08. UCT values from borings 13-LWRCW-108R, 13- LWRCW-109R and 13-LWRCW-111R were used in developing the surficial clay blanket strengths. Only the UU values from boring 14-LWRCW-112 were used in developing the glacial till. The shearlines developed for this section are presented in Appendix B. The 1/3 to 2/3 rule as well as engineering judgement were used to develop the strengths in Table 1. The design soil parameters for Reach 1 are shown in Table 1.

Table 1. Reach 1 Design Values

Soil Layer

Reach 1 (172+00 to 183+00) Undrained Drained ɸ (°) c (psf) ɸ (°) c (psf) γs (pcf) Clay Cap 0 600 23 0 120

Work Platform 0 600 23 0 120

Hydraulic Fill 30 0 30 0 125 Clay 1 0 1000 28 0 120 Sand 1 30 0 31 0 125 Sand 2 33 0 33 0 125 Sand 3 31 0 31 0 125

Till 0 3000 26 0 135

For the Lower Wood River Cement- Bentonite Cutoff Wall Phase II:

2.3.2 Section 183+00 to 197+00 Specific Geology and Shearline

Design Section 183+00 to 197+00 falls within the Reach 2 shear line. Four borings from the 2013-2014 subsurface investigation program and one boring and four CPTs from the 2008-2009 subsurface investigation program were selected to develop the Reach 2 shearlines. These borings and CPTs include: 14-LWRCW-114R, 14-LWRCW-

115R, 14-LWRCW-116R, 14-LWRCW-117R, WRLR-659-08, WRLCPT-657-08,

WRLCPT-658-08, WRLCPT-659-08 and WRLCPT-660-08. UCT values from borings 14-LWRCW-116R and 14-LWRCW-117R were used in developing the surficial clay blanket strengths. The shearlines developed for this section are presented in Appendix B. The shearlines developed for this section are presented in Appendix B.

The 1/3 to 2/3 rule as well as engineering judgement were used to develop the strengths in Table 2. The design soil parameters for Reach 2 are shown in Table 2.

Table 2. Reach 2 Design Values

Soil Layer

Reach 1 (183+00 to 197+00) Undrained Drained ɸ (°) c (psf) ɸ (°) c (psf) γs (pcf) Clay Cap 0 600 23 0 120

Work Platform 0 600 23 0 120

Hydraulic Fill 30 0 30 0 125 Clay 1 0 400 28 0 120 Sand 1 31 0 31 0 125 Sand 3 31 0 31 0 125

Till 0 3000 26 0 135

2.4 Permeability

The permeability values for the course grained aquifer (sands) are based off of D10 correlations and pump tests performed at the Mel Price/Wood River Under seepage project. The Mel Price/Wood River Under seepage project is located immediately upstream of the Upper Wood River Cutoff Wall and is an area in which under seepage has been studied extensively. Given the close proximity of the projects and similar depositional environments it was determined to use the wealth of information available at the Mel Price/Wood River Under seepage project as the foundation for the permeability’s used on this project The D10 correlation was developed for the

For the Lower Wood River Cement- Bentonite Cutoff Wall Phase II:

Mississippi River Basin and is shown in EM 1110-2-1913 in Figure 3-5. A reproduction of that relationship is shown below in Figure 1.

Figure 1. Effective Grain size, D10 versus Coefficient of Permeability, kh (from EM 1110-2- 1913, Figure 3-5.

A total of 5 pump tests were performed in the vicinity of the Mel Price project.

Analyses of the pump tests compared to the D10 correlated permeability data has shown that the measured aquifer permeability could be on the order of as little as 1.5 times higher than the correlated permeability. Further details on the pump test and permeability analyses at the Mel Price project can be found in Appendix D.

Once the D10 correlated permeability values were plotted for the analyzed reaches a lower bounds, average and upper bounds was determined. These bounds were then increased 1.5 times and were input into the Lower Wood River ground water model.

The permeability values were then adjusted based on course calibrations of the

For the Lower Wood River Cement- Bentonite Cutoff Wall Phase II:

groundwater model using the highest and lowest river/groundwater levels available with piezometric data during the period when ponding depth data was also available.

Permeability calculations using the D10 correlation and 1.5 factor are presented in Appendix E. Permeably values for fine grained soils are based on published values for clays and silts and were then adjusted based on course calibrations of the ground water model described above.

Appendix B contains figures with the stratigraphy used in the groundwater model superimposed on the boring logs.

For the Lower Wood River Cement- Bentonite Cutoff Wall Phase II:

3. Geotechnical Design Criteria and Results of Analyses

3.1 General

The geotechnical analyses were completed to determine constructability of the cutoff wall and design sections. This section documents the design criteria and analyses used to determine the recommendations of this report. Geotechnical analyses included underseepage, and slope stability.

• As previously mentioned, all elevations in this report refer to NAVD88, unless otherwise noted.

The following design criteria serve as the geotechnical design basis for this report:

• EM 1110-2-1902, Slope Stability, October 2003.

• EM 1110-2-1913, Design and Construction of Levees, April 2000.

• EM 1110-2-1901, Seepage Analysis and Control for Dams, April 1993.

• EM 110-2-1914 Design, Construction and Maintenance of Relief Wells,

• “Guidelines for Using the CPT, CPTU and Marchetti DMT for Geotechnical Design”, Volume 3 (of 5) – DMT Test Methods and Data Reductions, Report Number FHWA _PA-87-024+84-24 (attached for reference)

• “The Flat Dilatometer Test (DMT) in Soil Investigations, Report of the ISSMGE Committee TC-16, 2001 (attached for reference)

Computer software used:

• FLAC3D Version 5.01.141 by Itasca Consulting Group, Inc for 3-D slope stability

• GMS Version 8.3 by Aquaveo for 3-D seepage modeling

3.2 Design Grade and Hydraulic Criteria

The authorized project flood is elevation 443 for the Lower Wood River Cement- Bentonite Cutoff Wall. The existing levee crown is approximately elevation 445.

For the Lower Wood River Cement- Bentonite Cutoff Wall Phase II:

3.3 Seepage Analysis Summary

Given the complex geometry and the desire to develop the final authorized solution a 3D ground water model was developed. A detailed report on the 3D ground water modeling and the results of the modeling is included in Appendix F.

Steady-state seepage analyses were used to model seepage with a 2 foot thick cutoff wall in place with a permeability of 1.0x10-6 centimeters per second. Seepage analyses were run using the authorized project water surface elevation 443. The interior ponding elevation for the analyses was 412. This was chosen as it is a conservative estimate of the interior ponding elevation, which can naturally get as high as elevation 417.

More details on the permeability values can be found in Appendix E and F.

The model was used to evaluate the sensitivity of the groundwater system to a range of hydraulic conductivities and the Mel Price boundary condition. Various project conditions were also analyzed including the current wall conditions for the project (Station 152+00 to 170+00), and with wall conditions for the authorized project (Station 152+00 to 188+50). Also, all existing wells and wells currently under construction were added to the configuration. These wells were installed under the 2010 USACE relief well MATOC contract, and the SWIFPD bid package 03 and 7b contracts.

3.4 Stability Analysis Summary

The stability of the deep panel type construction was evaluated using FLAC 3D for the during construction case. Two representative cross sections at Sta. 171 and 175+20 were evaluated to inform the USACE team how slurry filled panels will perform during excavation. The cutoff wall is located at the levee toe and analyses showed that factors of safety and levee deformation are within the governing USACE criteria. A detailed FLAC 3D report is included in Appendix G.

The analysis ran utilized a slurry unit weight of 72 pounds per cubic foot to maintain stability of the trench. The slurry level in the trench analyzed was 2 feet below the top of the work platform with a minimum work platform elevation of El. 426.0. Also the ground water analyzed was elevation 418.0. Lastly, a surcharge load equivalent to a 300,000 pounds surcharge with no vehicles on the levee slope nor the levee crown was used.

For the Lower Wood River Cement- Bentonite Cutoff Wall Phase II:

Design Section 171+00 and 175+20 represents the typical levee Reach I between stations 170+00 and 183+00. Reach II located between 183+00 to 197+00 was the second reach section broken out for this project. After the reaches were broken out it was determined through engineering judgement that Reach I was more critical and thus the FLAC3D analyses focused on Reach I. The lower weak clay seam around El.

400 ft in reach I was more susceptible to movement and failure due to it’s lower strength and resistance to shear stress leaving it more prone to failure.

The during construction stability of the cutoff wall was evaluated with FLAC3D; failures originating riverside of the cutoff wall and failures originating levee side of the cutoff wall were evaluated within the search routine. It was found that at Stations 171+00 and 175+20 with a 20 ft panel the governing factory of safety was 1.7 and 1.6, respectively which is well above design criteria factor of safety. Furthermore, panel lengths up to 40 ft also met the design criteria factor of safety of 1.3. The model estimated displacements upwards of 1.0 inches to 1.5 inches in the levee slope. Given the nature of the soils at these locations and utilizing panel construction displacements from phase I construction, displacements less than 1 inch are expected. The results of the stability analyses are presented in Appendix G.

3.5 Proposed Cutoff Wall Requirements

A number of requirements for the proposed cutoff wall were developed as a result of the GMS modeling results, FLAC 3D modeling results, and the constructed phase I cutoff wall characteristics.

The modeling analysis ran utilized the below cutoff wall characteristics and were used to develop specification requirements.

• Slurry unit weight, minimum, 72 pounds per cubic foot to maintain stability of the trench and is the minimum unit weight of the slurry material.

• The cutoff wall permeability will be 1.0x10-6 centimeters per second or lower.

• The average unconfined compressive strength of 50 psi at 28-day cure.

• Slurry level to be maintained during construction the trench analyzed was 2 feet maximum below the top of the work platform with a minimum work platform elevation of El. 426.0.

• The ground water elevation used in analysis was elevation 418.0.

Specifications will require excavations of a panel to stop if Mississippi River elevations are at or predicted to go above elevation 416.5. 2 new open

For the Lower Wood River Cement- Bentonite Cutoff Wall Phase II:

system piezometers will be installed in addition to 4 existing open system piezometers to monitor ground water elevations during construction.

• A surcharge load equivalent to a 300,000 pounds surcharge with no vehicles on the levee slope nor the levee crown was used.

• Maximum panel width is 40 feet and no adjacent panels can be open immediately upstream nor downstream of panel to be excavated.

• Displacements less than 1.0 inch are expected during construction, 6 inclinometers are being utilized to monitor trench instability during the construction of the test section.

The proposed phase II wall will tie into the phase I wall which was built to the requirements below:

• Effective panel width of 2 feet, the contractor utilized equipment that excavated panel bites that were 9.2 feet by 3 feet.

• The verticality requirements was 0.05% of the measured depth of the constructed panel in both the in plane direction and transverse direction.

• The panel overlap required was 20 inches.

The phase II cutoff wall will have the following additional geometric requirements:

• The verticality requirement will be 1.0% of the measured depth of the constructed panel in the transverse to wall alignment direction.

• The minimum effective panel width will be 2.0 feet.

• The minimum effective panel overlap will be 3.0 feet.

• The minimum effective bite overlap will be 1.0 feet.

• The minimum overlap of the proposed wall to the existing wall will be 5 feet.

The contractor will be required to locate and excavate down to the top of the existing phase I wall and field locate the downstream most portion of the wall before determining the 5 foot overlap. The overlap will be excavating through the middle of the existing wall utilizing the verticality as-built information from the phase I wall.

3.6 Instrumentation Plan

Biaxial inclinometers and open system piezometers will be utilized within the contract to allow design assumptions to be adjusted based on real-time data gathered from the field and to help monitor and potentially mitigate excessive movements. The Contractor will be required to submit a monitoring plan to detail the means and methods the Contractor will utilize to gather and report the instrumentation readings.

For the Lower Wood River Cement- Bentonite Cutoff Wall Phase II:

The Contractor shall provide a secure web based instrumentation data viewer including but not limited to real-time data plots, and thresholds for alarms.

3.6.1 Inclinometers

There will be both manually read and in-place inclinometers to monitor movement during excavation of the test section panels. A total of 9 inclinometers will be installed as shown in the drawings. Two rows of three inclinometers will be installed in the test section at separate primary panels. Two automated inclinometers will be installed 5 feet both sides of a test section panel perpendicular to the cutoff wall alignment. One manually read inclinometer will be placed mod-slope on the riverside. All inclinometers will be installed and anchored 15 feet into bedrock. Inclinometers in the work platform will require a manhole accessible at the surface and able to withstand the weight of cutoff wall construction equipment.

In general for the in-place inclinometers installation, inclinometer sensors will be spaced close together at the surface and spacing increased with depth. Once inclinometers are installed and duplicate manual base readings have been taken the Contractor shall read all inclinometers once per week both automated and manual.

Once excavation starts manually read inclinometers shall be read hourly and automated inclinometers shall be read every 15 minutes and shall continue reading the inclinometers until the panel and adjacent panels have reached an unconfined compression strength of 15 psi. The Contractor shall analyze the inclinometer data continuously throughout the monitoring period so that if any deviations from the baseline readings are encountered.

During construction if the Contractor discovers inclinometer readings differing 0.5 inches from the baseline readings the Contractor shall immediately notify the contracting officer representative so that he or she is aware of potential problems. If the Contractor discovers inclinometer readings differing 1.0 inches from the baseline readings or any other sign of potential problems that would indicated a slope failure including surface cracking the Contractor shall cease work immediately and notify the contracting officer representative and implement the remedial action as submitted in the approved plan of action at the discretion of the contracting officer representative.

Inclinometers installed by the Contractor's instrumentation personnel within close proximity of the slurry trench construction shall be abandoned in place by being backfilled/ sealed in place with grout at the end of construction.

For the Lower Wood River Cement- Bentonite Cutoff Wall Phase II:

3.6.2 Open system piezometers

Well point open system piezometers will be utilized to monitor ground water elevations during construction. All piezometers will be automated. There are four existing open system piezometers that will be utilized to monitor ground water elevations within the aquifer during construction. Once the automated piezometers are installed and manual base readings have been taken, the Contractor shall read the piezometers once per week. Once excavation starts the piezometers shall be read once per shift until the project is complete. Two new riverside open system piezometers will be installed at the end of the construction. All piezometers will remain in operation following the end of the contract and remain as part of the project instrumentation.

4. Conclusions and Recommendations

Based on the results of the geotechnical investigation and subsequent analysis, the following conclusions and recommendations are made:

• Panel Construction is required to maintain stability of the cutoff wall during construction. Panel lengths of 25 ft are deemed adequate for the construction of this project. From station 169+95 to 188+50 panel widths up to 40ft will be allowed if the contractor can prove their means and methods will meet all the required allowances per the specifications in the test section.

• The proposed deep cutoff wall that will extend the existing wall to Station 188+50 effectively reduces uplift pressures and protects a large reach of the Lower Wood River mainline levee with the applied authorized flood (500-year).

• Between Station 169+95 and 172+00 the cutoff wall will tie 5ft into glacial till classified at CL or CH.

• Between station 172+00 and 188+50 the cutoff wall will extend to bedrock and included a 2ft rock socket into bedrock.

2.4 Permeability

4. Conclusions and Recommendations