The International Information Center for Geotechnical Engineers

Deep Soil Mixing for Retention of Excavations - Conclusion

 

Conclusion

 

The sections above analyzethe applications, construction aspects, design considerations, and lessons learned from four case studies involving DMM for excavation support.   Each project involved difficult construction conditions and high performance expectations.  In particular, most of the projects involved construction in dense urban areas with many unique conflicts.  Surface roads, overhead power lines, utilities, buried sludge lines, and other underground utilities each presented unique obstacles to these projects.  The SMW method was used to create hardened retaining walls in each case.  The key points of each case study are summarized in Table 2.

 

Table 2. Summary of presented case studies

Case Study

Summary of Key Points

Lake Parkway,

Milwaukee, WI

Construction in dense urban area involved excavation trench 9 m below grade to extend Interstate 794.

Very Low Hydraulic conductivity specified (10 m/sec).  Depth of mixing from 12-18 m. Scheduling and noise pollution not an issue using SMW construction. Walers and tieback earth anchors used to perform construction in stages (Fig. 1.2). 

Site consisted of sand and over consolidated silty clay at depths of 12-18 m from the surface.  The water table was within 1m from the surface.

Mixing carried out at batch plant.  Used high w/c content slurry for improved uniformity.  Construction involved primary auger passes and re-augering of previous column lines.  While in fresh state, soldier beams were placed through templates (Fig. 1.3).

Results included 1-2 MPa UCS, hydraulic conductivities as low as 10-10 m/sec. Post-construction inclinometer readings all below 25 mm (Fig 1.4).  Exposed layers experienced surficial crumbling and shotcrete was used for repair of any degradations.

Boston Central Artery

Bird Island Flats (BIF)

DMM was used to create SMW with earth anchored tiebacks for          support of the excavation for the cut and cover tunnel through BIF (Fig. 2.1).

 

During excavation of the east wall, large lateral deformations were observed when the excavation was 13.4 m deep (Fig. 2.4). The excavation was partially backfilled to protect against deep rotational failure.

 

After backfilling, the base was reinforced with DMM buttresses with jet grouting adjacent to the SMW (Fig. 2.5).

 

The reinforced base was composed of three components (Figs. 2.6 and 2.7): 1) A series of SMW buttresses that were parallel to each other on 2.4 m center spacing that were installed as a single row of interlocking DMM columns. 2) An expansion of each individual buttress to three rows to create a “hammer head” at the end nearest the wall. 3) Three pairs of jet grout columns to stabilize the area between the hammer head and the wall.

 

The buttresses of the east wall penetrate into the glacial deposits underneath the clay.

 

The west wall was reinforced in the same manner to protect against deep rotational failure. DMM and jet grouting was incorporated when the excavation was 8.1 to 10.9 m (Fig. 2.9).

 

The buttresses of the west wall do not penetrate into the glacial deposits but “floats” in the base clay (Fig. 2.9). This was done to avoid abrupt changes in improved soil on till to thick clay, which in turn reduced the potential of differential settlement.

 

A cross section of the west wall with instrumentation and displacements for excavations stages 3, 4, and 5 is shown in Figures 2.10(a) – 2.10(d). The figures include the Porter Street Combined Sewer. The stages of excavation are explained in Table 1.

 

Hydrostatic pore pressure was assumed in the design of the walls and the SMW had approximately the same hydraulic conductivity as the horizontal conductivity of the marine clay. Taking in account weep holes created by the tiebacks and thin sand and silt layers, the water pressures behind the walls were significantly less than hydrostatic.

 

Deep rotational analysis was performed on the west wall. The analysis demonstrated that the stabilized soil combined with the tiebacks provided sufficient shear resistance to prevent failure.

 

Pennsylvania DOT

Excavation Support

Construction involved creating a highway tunnel underpass to extend State Route 54 through Danville, business district (Fig. 3.1). 

Site consisted of historic brittle mansions and construction needed to limit potentially damaging vibrations.  Noise pollution was also a primary concern for project success. 

Site conditions consisted of Dense sands, gravels, and silts.  Deep groundwater table.  Depths of SMW wall panel reached 6.7 – 9.4 m

Modeled off continuous walls, sheet pile walls, and segmental walls.  The wall had to support both the geostatic earth pressures and the surcharge loads from the mansion foundations within 1.5 m from the face of the wall (Fig 3.2).   

Extensive use of structural support including W18x40 Soldier Beams, use of pre stressed horizontal braces, and struts used to control surcharge loading (Fig 3.2)

Design used Rankine Rectangular pressure diagram with a safety factor of 1.4 to account for uncertainty.  Used previous case studies and literature to place an upper bound of 10 mm on post-construction settlement (Fig. 3.3).

Results included excellent crack control and no differential settlement at the location of the brittle mansions.  6 mm of lateral movement was recorded using inclinometers (Fig 3.4).  UCS reached as high as 3.8 – 4.2 MPa.

Museum of Fine Arts

Boston

A new addition was built within the courtyard of the existing museum campus at a depth up to 9.1 m below the lowest slab of the existing buildings.

 

Earth retention systems composed of SMW walls and jet grouting was used to support the excavation, control movements of adjacent structures, and act as a cutoff wall for a perched groundwater table in the alluvial sands and confined aquifer (Fig. 4.1).

 

SMW was used to support the entire 1,200 ft perimeter of the excavation. Jet grouting was also used to underpin sections of the existing structures, act as cutoff barrier for groundwater, and improve subgrade for a mat foundation. The SMW was supported by struts, corner diagonals, and tiebacks.

 

The subgrade of the site consisted of fill that covered  a thin discontinuous layer of organic silt. The soft clay layer became very soft with depth. The elevation of the groundwater in the borings was about +3 m and perched on top of the organic silt layer. The silt layer with the clay was a confined aquifer (Fig. 3.2).

 

Groundwater cutoff in the fill and alluvial sands was requires as well as permanent cutoff of the confine aquifer.

 

No significant movement of the existing structures was observed during construction.

 

 

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