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Deep Soil Mixing for Retention of Excavations - Case Study 1: Lake Parkway, Milwaukee, WI

 

Case Study 1: Lake Parkway, Milwaukee, WI

Project Overview

The project involved an extension of a new freeway into a dense urban area characterized by nearby railroad lines, surface roads, overhead power lines, utilities, buried sludge lines, and other underground utilities.  The freeway was an extension of Interstate 794 which would connect Hoan Bridge to Layton Avenue.  These conflicts would be addressed by re-routing the development of other projects and careful, deliberate scheduling. The extension for the freeway was 1000m long and was constructed in a trench 9 m below grade (Bahner and Naguib, 1998).  Initially, the project sought to elevate portions of the freeway to avoid excavations but, due to public outcry, the design was modified to be depressed, below grade (Anderson, 1998; Bahner and Naguib, 1998).  The winning bid for the construction called for a design based on deep soil mix wall (SMW) technology.  The main advantages of using deep soil mixing to create the retention wall for this project were that the wall could easily be made essentially impermeable, scheduling with other developing urban projects would not be an issue, noise pollution would not be significant, and the spoils generated would be far less than in slurry-wall applications.  Deep soil mixing would also generate relatively low noise pollution due to disturbances and vibration.  The height of the DMM wall was 12 to 18 m (Bahner and Naguib, 1998).  Walers and tieback earth anchors were used to provide lateral stability.  The wall needed to support the entire lateral load at the bridge abutments. 

Site Conditions

From boring tests, it was determined that the soil stratigraphy at the site consisted of layers of sand, silt, clay, and very stiff clay till sloping down towards the North.  The clay was highly overconsolidated and it was located within about 6-12 m from the surface.  Limestone bedrock was located at depth so 30 m North and South of the proposed construction.  The site layout is shown in Figure 1.1 below.  The water table for the project was very close to the surface.  Water depths were less than 1 meter from the surface in some cases.  The high groundwater table is one of the main reasons DMM walls were so attractive to the project. 

Alex C1.1

Figure 1.1. Layout Map of Lake Parkway Milwaukee, WI project (Anderson, 1998; Bahner and Naguib, 1998)

DMM Wall Design and Construction

Towards the north and south end of the depressed roadway the primary objective was to minimize seepage and create an impermeable cutoff wall.  This was achieved by constructing non-structural DMM walls and locking them within the stiff, over-consolidated clay where lateral loads did not need to be transferred to the wall.  The hydraulic conductivity of the wall reached as low as 10-9 m/sec. 

The design of the SMW wall consisted of mixing the soil within 12-18 m of the surface using the wet method with rotary mixing.  The additive consisted of cement-bentonite and it was in a wet, slurry state during injection (Bahner and Naguib, 1998).  The additive was mixed on site at a batch plant.  The structural components of the wall are shown in Figure 1.2 below.  It is interesting to note that higher water to cement ratios were used in mixing.  The fact that lower strength soil-cement columns result from higher w/c is not necessarily true. Increased strength can result from higher w/c ratios due to better mixing and greater homogeneity (O’Rourke and McGinn, 2006).  This would allow for a very uniform soil mixture and facilitate easier installation of the structural reinforcing soldier beams.  There is not a clear tradeoff between strength and w/c content in deep soil mixing applications.  Better strengths result from good construction practice and more homogenous mixtures.  This can be achieved in part by using higher w/c content grout.

  Alex C1.2

Figure 1.2. DMM wall section design from (Anderson, 1998; Bahner and Naguib, 1998)

To enhance the flexural and tensile capacity of the wall, soldier beams were added into the freshly mixed cement-bentonite wall.  This reinforcement along with the use of anchorage by studs, a reinforced concrete face, walers, and tiebacks created a very stiff wall which would allow minimal displacements.

To mix the soil in-situ with the cement-bentonite stabilizer mentioned above, a 136 ton crane rig with rotary mixing was employed (Bahner and Naguib, 1998).  The individual augers were   0.9 m in diameter and they were designed to mix the soil on a downward sweep.  Then from the bottom up, cement-bentonite stabilizer was injected through the end of the rotary mixing tool.  The augers were grouped together and would form overlapping wall type columns.  This was achieved by making several passes and re-augering and mixing over previously drilled column lines.  This process and the equipment used are shown below in Figure 1.3.Alex C1.3

Figure 1.3. Configuration of rotary elements to form DMM wall pattern (FHWA, 2000 and GeoCon, 2014) 

As shown, different passes were used and the initial passes were not continuous.  The second, third, and fourth passes would overlap with the first few.  This would ensure that the wall was continuous and that it extended to the proper depths. 

After mixing, steel soldier beams were placed in the fresh concrete by using templates.  As mentioned above, the desired center to center spacing of the elements was 1.37 m.  The templates were the primary method used to achieve this spacing.  These templates were anchored above the fresh, curing soilcrete .  The beams were lowered through these templates to meet the desired tolerances (Bahner and Naguib, 1998).

Results and Performance

The design unconfined compressive strength (UCS) for the wall was 0.50 MPa.  UCS tests were performed on samples of the hardening soilcrete mixture at 3, 7, 14, and 28 days during curing.  It was found that the actual field strength was 2-4 times the design strength of 0.5 MPa (Bahner and Naguib, 1998).  The hydraulic conductivity was also measured in the laboratory and was well below the 10-7  m/s design value.  In fact, in the areas where structural support was not needed and the wall functioned as a cutoff wall, hydraulic conductivities reached as low as 10-10  m/s.  As the tiebacks were completed and the wall was sealed off, the water levels also varied and showed a consistent decrease.  Measurements were taken at various locations using piezometers to quantify this.   

Inclinometers were used to determine whether or not the lateral displacements of the wall were within the allowable range developed by design.  Inclinometer readings were taken a three different locations and times to compare to the design allowable displacement of 25 mm.  The data for a few of the inclinometers used in the study is shown below in Figure 1.4.Alex C1.4

Figure 1.4. Wall movements and inclinometer readings at different locations (Bahner and Naguib, 1998)

As shown in Figure 1.4, the inclinometer measurements at each location and time are significantly less that the 25 mm allowable design value.  Therefore the structural components of the design were a complete success.   Both the unconfined compressive strength and the lateral movements were well within the allowable range. The permanent wall was excavated at some locations and became exposed to the environment after hardening.  It was exposed to freezing and thawing which was not addressed in design in terms of modifications of the stabilizer.  Surficial crumbling resulted at a few locations.  During the service life of the wall, any location that deteriorated significantly was repaired with a new layer of shotcrete. 

 

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