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Deep Soil Mixing for Retention of Excavations - Case Study 4: MFA Boston

     

Case Study 4: Museum of Fine Arts Boston

 The Museum of Fine Arts (MFA) Boston set forth the very ambitions undertaking of building a new addition within the courtyard of the existing museum campus. This new addition was at a depth of 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 (Weatherby and Zywicki, 2012). The SMW was a total of 66,000 ft2 and the jet grouting totaled 150 cyd (Schnabel, 2014). SMW was used to support the entire 1,200 ft perimeter of the excavation (Schnabel, 2014). 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 buildings next to the proposed excavation were supported by shallow spread footings and low capacity piles in alluvial sands, and caissons on the alluvial sands of the desiccated crust of the Boston Blue Clay (Weatherby and Zywicki, 2012). Support for the new addition was large spread footings bearing in the desiccated crust of the Boston Blue Clay.

Figure 4.1 shows a plan of the museum addition with the respect to the SMW, jet grout underpinning and cutoff, jet grout subgrade improvement, and a jet grout secant wall. Discussion of the jet grouting will be limited here. The MFA addition has a “T” shaped footprint. The SMW was supported by struts, corner diagonals, and tiebacks (Weatherby and Zywicki, 2012).

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Figure 4.1. Plan of excavation support system, underpinning, ground improvement, and secant wall for the new addition (Weatherby and Zywicki, 2012)

 Soil and Subsurface Conditions

A composite boring with average properties is shown in Figure 4.2 (Weatherby and Zywicki, 2012). Shear strength was estimated using isotropically consolidated undrained triaxial compression (CICU) tests (Weatherby and Zywicki, 2012). 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. 

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Figure 3.2. Composite soil boring of the MFA site (Weatherby and Zywicki, 2012)

Excavation Support and Ground Improvement

Project specification required that a maximum displacement of 19 mm was allowed for adjacent buildings. Other requirements included groundwater cutoff in the fill and alluvial sands, permanent cutoff of the confined aquifer, and to prevent bottom heave of the excavation. No pile driving was allowed on this project.

Structural SMW Cutoff Wall

The SMW was reinforced with W 18 X 50 or W 24 X 146 soldier beams spaced on 1.37 m  centers. The SMW extended  a minimum of 1.5 m below the confined aquifer. A section of the retained excavation is shown in Figure 4.3. No significant movement of the existing structures was observed during construction (Weatherby and Zywicki, 2012).

SB C2.4Figure 4.3. Section of the structural SMW (Weatherby and Zywicki, 2012)

 

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