evaluation & preliminary design

EVALUATION & PRELIMINARY DESIGN

For

Repairs of the Montello Dam

Prepared For The

Wisconsin Department of Natural

Resources

AUGUST 27, 2009

McM. No. W0890-990342.08 CCS:car

EVALUATION AND PRELIMINARY DESIGN

For Repairs of the Montello Dam

Prepared For The

Wisconsin Department of Natural Resources

Prepared By McMAHON

Neenah, Wisconsin

August 27, 2009 McM. No. W0890-990342.08

Table Of Contents

I. HISTORY II. AUTHORIZATION III. SITE CONDITIONS IV. SUBSURFACE EXPLORATION V. SUBSURFACE SOILS VI. LABORATORY TESTING VII. LABORATORY TESTING RESULTS VIII. SLOPE STABILITY ANALYSIS IX. SETTLEMENT ANALYSIS X. PRELIMINARY DESIGN ANALYSIS XI. CONCLUSIONS

List of Attachments Attachment #1 Riprap Plan Attachment #2 Dam Survey Report (1985) Attachment #3 Wisconsin DNR Memorandum Concerning Dam Condition (1986) Attachment #4 Dam Safety Inspection Report (1997) Attachment #5 Wisconsin DNR Memorandum Concerning Dam Condition (2009) List of Appendices Appendix I Geotechnical Report Appendix II Slope Stability Analysis Appendix III Remedial Repair Analysis

EVALUATION & PRELIMINARY DESIGN For Repairs of the Montello Dam Prepared For The WISCONSIN DEPARTMENT OF NATURAL RESOURCES - Page 1

EVALUATION AND PRELIMINARY DESIGN

For Repairs of the Montello Dam

Prepared For The

Wisconsin Department of Natural Resources

Prepared By McMAHON

Neenah, Wisconsin

August 27, 2009 McM. No. W0890-990342.08

I. HISTORY

The Montello Dam and levee were constructed utilizing a mid-1800’s United States Army Corps of Engineers (USACE) design commonly referred to as a dredge bank. The original dam was constructed in 1855 and the current dam was constructed in 1933 by the USACE. It is believed the levee (referred to as the left dike in the dam reports) was constructed by excavating material from what is now Buffalo Lake to create the embankment. The method of construction and material in the levee are unknown. A diagram from 1982 shows a plan to install riprap along the levee. This was to control erosion along the lakeshore. A copy of this plan is attached. A dam survey report from 1985 (attached) states the concrete dam structures are in good condition but the embankment areas have uncontrolled vegetation (brush and trees) growing out of them that should be removed. This is reiterated in a memo from 1986 (attached) which also states this vegetation should be removed “as soon as reasonably practicable”. The dam survey report states riprap had not been placed at the time of the inspection and that riprap should be placed after the tree removal. A dam safety inspection report from 1997 (attached) recommended repair of the left masonry abutment wall of the spillway and restated the need to remove the trees on the levee. Two flooding events occurred in 2004 and 2008 causing traffic along STH ‘22’ to be rerouted to the road on top of the levee. Many local sinkholes developed and were subsequently filled and paved after each of these events.


The trees and brush were cut down in the spring of 2009 by the City of Montello. The stumps were not removed as recommended in several preceding reports.

II. AUTHORIZATION

The City of Montello experienced flooding at the dam that resulted in STH ‘22’ being flooded and traffic rerouted onto the levee for a period of 10 days in 2008. In addition, ground settlement has occurred at or near the dam and the dam must be repaired or removed by December 31, 2012 per Wisconsin Department of Natural Resources (DNR) order. This order is attached. In February 2009 the Wisconsin DNR solicited proposals to assist in evaluating the current dam conditions, evaluate alternatives, and make a recommendation of what remedial action, if any, should be take at this site. The Wisconsin DNR and McMAHON entered into an agreement to provide these services in April 2009.

III. SITE CONDITIONS

The current levee cross section is generally an 18 foot to 24 foot wide top with side slopes ranging from approximately 2:1 to 4:1. There is an asphalt pavement access road that ranges from 12 feet to 19 feet wide. The waterside of the slope includes riprap as recommended in previous reports. The land side has a grass vegetative cover. Several sinkholes are apparent on the landside of the levee (especially in the old river bed area) and there is an area on the east-west portion of the levee that exhibits signs of general settlement. All of the trees have been cut down but the stumps remain in place.

IV. SUBSURFACE EXPLORATION

Our subconsultant, River Valley Testing Corporation, conducted a total of 20 soil borings. The borings were drilled on April 9, 2009 through April 16, 2009. Boring depths were extended to 40 feet below existing grade. Piezometers were then installed to a depth of 25 feet below grade. Copies of the boring logs are included in Appendix I. Groundwater was generally detected in the 7 feet to 14 feet range during drilling in the embankment section. Borings on the island generally encountered water in the 6 to 8 foot range during drilling. The groundwater level observations as recorded in the test boring logs is included in Appendix I.


The bore holes were converted to piezometers using 1-inch diameter PVC and were screened at the 5 foot or 10 foot level as directed by Wisconsin DNR personnel. Flush mount covers were installed over these. Disclaimers for the interpretation of subsurface conditions are standard ASFE conditions language. These disclaimers are printed in Appendix I and are considered an integral part of this report.

V. SUBSURFACE SOILS

The plan-profile sheets included in this report provide a visual representation of the soil strata through the embankment. It should be noted the profile showing the strata was developed using the information from the soil borings (in the cases of two borings across from each other, the one on the water side was used) and interpolating the information in between the boring locations. In some instances, the materials changed between borings. This is indicated on the drawings as “Transition Location Unknown”. The majority of the soils encountered are sand and silt or some combination of the two. Organic content ran as high as 12% with the majority of the borings having less than 5%. Unconfined compressive strength ranged from 1,110 to 4,200 pounds per cubic foot.

A. Sand

Sand was present in all of the borings. In general, this layer started around 10 feet below ground level and extended to depths ranging from 23 to 33 feet.

B. Silty Sand

Silty sand was also present in all of the borings. In most borings it was part of the upper fill layer within a few feet of the ground surface. Borings B2, B3, B4, B5, B8, B9, B10, & B19 also showed a layer of silty sand varying between the 23 and 28 feet deep range.

C. Sandy Silt

Sandy silt was present in borings B2, B7, B8, B9, B12, B13, B14, B15, B18, & B20. These were typically very deep, occurring at the 25 to 30 feet deep levels.

D. Silt

Silt was present in borings B4, B5, B7, B11, B13, B14, B18, B19, & B20. Generally, the silt layer was in the 10 to 12 feet deep range. However, borings 19 and 20 showed silt at the 38 to 40 feet deep level.


E. Lean Clay

Lean clay was present in borings B1, B2, B3, B4, B5, B6, B7, B8, B9, B12, B13, B14, B15, B17, B18, B19, & B20. Generally, this was encountered at the lower levels of the borings past 20 feet deep.

F. Organic Clay

Organic Clay was present in borings B1, B2, B8, B16, B17, & B20. This was encountered closer to the surface in the 5 to 15 feet deep range.

In addition, small amounts of sandy lean clay, sandy organic silt, organic silt, and peat were encountered in various boring samples.

VI. LABORATORY TESTING

Thin-walled Shelby tube samples were extracted at ten of the boring locations. Continuous samples were collected to 15 feet and then one every 5 feet in accordance with ASTM D-1586. Visual classifications of soils were conducted according to the Unified Soil Classification System, ASTM D-2488.

Water Content Determinations were conducted on each sample conducted.

Pocket Penetrometer Testing was performed.

Mechanical analysis of samples (ASTM C-117-90 & C-136-92) were performed on

13 samples.

Atterberg Limits Testing was performed on six samples.

Dry Unit Weight Determinations and Unconfined Compressive Strength Tests were performed on 6 samples.


VII. LABORATORY TESTING RESULTS

The results of the laboratory testing results are summarized below and are also included in the Report of Geotechnical Exploration (Appendix I).

Boring

Number Moisture Content

(%)

Dry Density

(pcf)

Unconfined Compressive Strength (pcf)

Organic Content (%)

Atterberg Limits L.L.

1-09 15 - 71% 58 1,100 12% 26

2-09 15 - 67% 7.2% 21

3-09 18 - 43% 123 3,800 3.5% 22

4-09 5 - 32% 4.7%

7-09 19 - 22% 117 3,100 2.6% 20

9-09 21 - 46% 110 2,800 5.9% 28

10-09 21 - 22%

11-09 40% 2.9%

12-09 21 - 23% 109 4,200

13-09 15% 2.1%

14-09 4 - 46% 105 5.6%

17-09 19 - 30%

18-09 21 - 22% 32

The testing results indicate quite a variation in soil properties throughout the embankment. One item of note that was fairly consistent was a layer of sand or silty sand in the 10 to 16 feet deep level that exhibited a low blow count and was generally below the groundwater level detected during drilling.

VIII. SLOPE STABILITY ANALYSIS

All earth dams have seepage resulting from water permeating slowly through the dam and its foundation. Seepage must be controlled in both velocity and quantity. If uncontrolled, it can progressively erode soil from the embankment or its foundation, resulting in rapid failure of the dam. Erosion of the soil begins at the downstream side of the embankment, either in the dam proper or the foundation, progressively works toward the reservoir, and eventually develops a direct connection to the reservoir. This phenomenon is known as ‘piping’. Piping action can be recognized by an increased seepage flow rate, the discharge of muddy or discolored water, sinkholes on or near the embankment, or a whirlpool in the reservoir. Once a whirlpool (eddy) is observed on the reservoir surface, complete failure of the dam will probably follow in a matter of minutes. Fully developed piping is virtually impossible to control and will likely cause failure. Seepage can cause slope failure by creating high pressures in the soil pores or by saturating the slope. The pressure of seepage within an embankment is difficult to


determine without proper instrumentation. A slope which becomes saturated and develops slides may be showing signs of excessive seepage pressure. River Valley Testing performed a slope stability analysis at the following locations: Section 1 is located in the northern portion of the embankment near boring 2-09.

Section 2 is located south of the boat launch between borings 4-09 and 5-09.

Section 3 is located in the east-west portion of the levee near borings 16-09 and 17-

09.

Section 4 is located in the east-west portion of the levee near borings 10-09 and 11-09.

All of the locations were analyzed for the following four conditions: Condition 1 is normal water level with traffic loading.

Condition 2 is normal water level with no traffic.

Condition 3 is high water level without seepage.

Condition 4 is high water level with seepage.

The traffic loadings used were based on the Wisconsin Department of Transportation Bridge Manual. Slope stability was computed using the Morgenstern-Price full equilibrium method to calculate the factor of safety. A synopsis of the results is as follows:

A. Condition 1: Normal Water Level with Traffic Loading

Section 1: Factor of safety ranges from 0.85 to 1.29.





B. Condition 2: Normal Water Level without Traffic Loading





C. Condition 3: High Water Level without Seepage





D. Condition 4: High Water Level with Seepage





For comparison purposes, the minimum factor of safety recommended by the US Army Corps of Engineers in Engineer Manual EM 1110-2-1913 Design and Construction of Levees is 1.4 for existing levees in the long-term (steady seepage) condition. This is the minimum factor of safety that we would recommend for this facility in this analysis. It is noted in the analysis the two most critical factors in the analysis are the seepage water level and the soil strength of the fill material. The soil strength values of the swamp deposits and the outwash sands had a minimal effect of the factor of safety.

The full report including soil strength parameters used and typical critical failure planes is included as Appendix II.


IX. SETTLEMENT ANALYSIS

Testing results indicate no anticipated further settlement through the embankment. The majority of the embankment has settled at a consistent rate. The only area that has exhibited additional settlement is the area where the old river bed crosses the embankment. This area has settled an additional 6 to 8-inches over the rest of the embankment.

X. PRELIMINARY DESIGN ANALYSIS

The information gathered indicates that slope stability is of greater concern than settlement for this embankment. Seepage through the embankment and low soil strength within the embankment are the two greatest factors contributing to the low factors of safety for the slope stability analysis. The foundation materials appear to be adequate. Any rehabilitation work should be able to utilize these soils without exhaustive removal and replacement.

Options to increase the factor of safety by addressing seepage include widening the embankment, installation of toe drains on the landside slope, riprap on the landslide slope, increasing the soil strength or combinations of these. Increasing the soil strength will involve removal of portions of or the entire embankment. In particular, we were requested to comment on the following three options:

A. Do Nothing

In reviewing the calculated factors of safety for the existing conditions, there is no consistent F.S. of 1.4 or greater at any location or under any of the analyzed conditions. Therefore, McMAHON cannot recommend leaving the levee in its current condition.

B. Rehabilitate with No Traffic Allowed

Since the factor of safety is inconsistent through the embankment, McMAHON recommends any rehabilitation effort include the entire embankment. This will allow control of the materials and methods of construction to assure a completed project with an adequate factor of safety. McMAHON analyzed three different methods of rehabilitation for the no traffic allowed condition that would achieve the required factor of safety. The “no traffic allowed” condition does include a 12 foot wide maintenance road on top of the


embankment that would not be open to the public. These options are discussed in further detail here:

1. Construct Additional Landward Slope with No Additional Drainage Facilities

This option consists of flattening out the landward slope using structural fill to provide a maximum side slope of 4:1. This would involve stripping away existing vegetation and topsoil in the work area, placing and compacting structural fill, and restoring the work area with vegetative cover. This work would extend beyond the current limits of Wisconsin DNR property. A typical section showing this option is included at the end of this narrative. The advantage of this option is the existing embankment can remain in place while this work is completed. The disadvantages are preliminary indications show possible historical sites in the work area, wetlands are present in the work area, and property or permanent easement acquisition will be required.

An estimated project cost for this scenario is $650,000.

2. Construct Additional Landward Slope with Additional Drainage Facilities

This option consists of flattening out the landward slope using structural fill to provide a maximum side slope of 3:1 and provided additional drainage facilities to address seepage through the embankment. This would involve stripping away existing vegetation and topsoil in the work area, placing and compacting structural fill, installing a pervious toe trench and collector pipe, and restoring the work area with vegetative cover. This work would extend beyond the current limits of Wisconsin DNR property. A typical section showing this option is included at the end of this narrative. The advantage of this option is the existing embankment can remain in place while this work is completed. The disadvantages are preliminary indications show possible historical sites in the work area, wetlands are present in the work area, and property or permanent easement acquisition will be required. An estimated project cost for this scenario is $615,000.

3. Reconstruct Entire Embankment with Additional Drainage Facilities


This option consists of reconstructing the entire embankment using structural fill to provide maximum side slopes of 2-1/2:1 and provide additional drainage facilities to address seepage through the embankment. This would involve stripping away existing vegetation and topsoil in the work area, removing the existing embankment and overexcavating to an elevation at least 2 feet into the existing outwash soil, placing and compacting structural fill, installing a pervious toe trench and collector pipe on the landward side, placing riprap on the water side, and restoring the work area with vegetative cover. This work would be completed the current limits of Wisconsin DNR property. A typical section showing this option is included at the end of this narrative. The advantages of this option are the work could be completed within existing Wisconsin DNR property, no historical sites or wetlands would be disturbed. The disadvantages are cost, temporary easements may be required to perform the work, and the work may involve temporary lowering of the Buffalo Lake water level to complete. An estimated project cost for this scenario is $1,500,000.

C. Rehabilitate with Traffic Allowed

This option consists of reconstructing the entire embankment using structural fill to provide maximum side slopes of 2-1/2:1 and provide additional drainage facilities to address seepage through the embankment. This would involve stripping away existing vegetation and topsoil in the work area, removing the existing embankment and overexcavating to an elevation at least 2 feet into the existing outwash soil, placing and compacting structural fill, installing a pervious toe trench and collector pipe on the landward side, placing riprap on the water side, and restoring the work area with vegetative cover. This work would be completed the current limits of Wisconsin DNR property. A typical section showing this option is included at the end of this narrative. The advantages of this option are the work could be completed within existing Wisconsin DNR property, no historical sites or wetlands would be disturbed. The disadvantages are cost, temporary easements may be required to perform the work, and the work may involve temporary lowering of the Buffalo Lake water level to complete. An estimated project cost for this scenario is $1,790,000.

XI. CONCLUSION


As previously stated, the current factor of safety based on our analysis is less than the recommended amount. Therefore, we recommend a minimum of some rehabilitation take place at this location. The embankment is so inconsistent throughout in the material that was encountered; it would be difficult to only reconstruct areas that don’t meet the minimum factor of safety requirements. Therefore, all of the scenarios presented include work along the entire embankment. The rehabilitation methods utilizing the existing embankment include other considerations such as permitting and land acquisition that would need to be evaluated prior to proceeding. The full reconstruction option could be done either with or without traffic allowed on the top of the embankment. For a full reconstruction, there is not a lot of difference (other than top of embankment width) between the designs that would be considered adequate with or without traffic loading. The cost to construct a road capable of handling two-way traffic during an emergency adds about $290,000 to the total project cost. This cost includes the additional embankment width and road width. At the very least, a maintenance road should be constructed to allow continued inspection and monitoring of the facilities.

If nothing is done, we recommend continuing to monitor the water levels in the embankment so we can add information on pore water pressures and possibly refine our analysis, and consider lowering the normal water elevation level in Buffalo Lake to relieve some of the pressure against the embankment.

I.D.\REPORT\W0890\990342\Montello Dam Report-CCS.doc (car)

APPENDIX I

Geotechnical Report

River Valley Testing Corporation June 4, 2009

APPENDIX II

Slope Stability Analysis River Valley Testing Corporation

July 9, 2009

APPENDIX III

Remedial Repair Analysis

River Valley Testing Corporation August 21, 2009

evaluation & preliminary design

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