Bender Park Slope Stabilization Demonstration Project
Milwaukee County Parks Department
Basin Program Funds:
Significant bluff slope erosion is occurring on the entire south half of the Bender Park Lake Michigan shoreline.
Stability of soil slopes generally depends on gravitational forces of the slope geometry, soil characteristics, and subsurface water. Improving the stability of soil slopes require reducing the gravitational forces, or increasing the strength of the soil or lowering the water level. Bluff slope instability has been a continuing problem along the west shoreline of Lake Michigan in Wisconsin. Public and private studies have found that the instability is caused by two sources: Lake Michigan wave erosion on the bluff toe, and water internally within the bluff soil profile.
Bender Park is a public access park in the Milwaukee County Park system. The bluff are in question is along the Lake Michigan shoreline in the southern portion of the park between Fitzsimmons Road, and Oakwood Road. This portion of the park is about one half mile long and is currently unimproved, except that a shoreline riprap stone revetment is present. Tall grass and brush generally cover the top of the bluff. Two isolated and environmentally sensitive areas of trees also exist at the bluff crest. The bluff face is steep, with immature brush and grass vegetation in some areas, and barren, eroded, and slump soil in most areas of the face.
The bluff height above Lake Michigan is about 116 feet. Surface topography west of the bluff varies gradually by about 20 feet. No evidence of recent bluff stability problems is present in the ground surface at the top of the bluff. No cracks and no depressions indicative of bluff soil sliding were present in the soil surface. Areas of past water seepage and soil flow and erosion are evident from the top of the bluff. Scarps from shallow depth slope failures are generally present just below the bluff crest. Several large volume scraps from probable deep seated rotational slope failures are also present. Water is saturating a layer of sand which is below 30 feet of surface clay on the site. The slope is continuing to erode until the slope is graded to a lower angle and drained or, the slope reaches a stabilized angle of repose naturally.
The proposed project consists of the installation of approximately 3,000 linear feet of wick drains 30 to 40 feet below the existing top of the Lake Michigan shoreline bluff between Oakwood Road and Fitzsimmons Road.
Project implementation will consist of the following tasks:
- Soil boring logs and peziometers
- Drilling equipment set up and operational on site
- Drill pilot and directional shafts for boring equipment
- Install wick drains and outlets
- Clean up construction debris
- Monitor bluff and water discharge from the drains
- Project supervision and monitoring conducted by the Parks Department and the Milwaukee County DPW professional engineers
By installing the wick drains, saturated soil pressure will be significantly reduced. Removing the water from the bluff reduces soil pressure which will allow the steep bluffs to stabilize and become vegetated. This project will demonstrate the use of this type of drainage system as compared to re-grading the slope to a stabilized angle of repose at an estimated cost of $2.5 million.
Four soil borings and the installation of peziometers at two sites in the park were completed in October 2003 and preliminary soil testing to determine the level of saturation and the level to install the wick drains had been completed. A final soil analysis was done and the final design for wick drain installation was done. Additional wick drains based upon the volume of water being released from the drain system were installed where needed after additional soil borings to determine location of sand/clay interface were done. The soil variability determined the location for additional drains.
After installation, groundwater levels were recorded and a slope stability analysis was done and reported in the engineering paper included in this report. Indications are that wick drainage of subsurface water proved bluff stability. A successful lowering of the water levels with the wick drains is occurring within the bluff sites in Bender Park. Wick installation with directional drilling techniques at the top of the bluff is considered to have been more successful than installations bottom of the bluff. This may have been due to the accumulated soils at the bottom of the bluff from past slope failures.
Contact: Mr. Jim Ciha, (414) 257-4887