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Deep Soil Mixing for Retention of Excavations - Case Study 2: Boston Central Artery Bird Island Flats

 

Case Study 2: Boston Central Artery Bird Islands Flats

The Ted Williams Tunnel is an extension of Interstate I-90 underneath the Boston Harbor to Logan Airport. Cut and cover techniques used to create the portion of the tunnel through Bird Island Flats (BIF). The location of the BIF project is shown in Figure 2.1. Excavation at this site was performed between 1992 and 1995. The BIF tunnel is approximately 915 m of double barrel reinforced concrete highway next to Logan Airport. The depth of constructed highway ranges from 12.5 to 25.9 m with a width that ranges from about 24.4 to 53.3 m.

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Figure 2.1 Location of the Bird Island Flats Project (McGinn and O’Rourke, 2000)

Deep Mixing Methods (DMM) were used to create soil mixed walls (SMW) with earth-anchored tiebacks for temporary support of the excavation. The SMW were installed by means of a triple auger deep mixing rig equipped with 860 mm overlapping soil mixed columns (O’Rourke and O’Donnell, 1997b). These walls were the largest and the deepest of their kind in North America at the time of construction. The total area covered by these walls was 37,180 m2. The wall was structurally reinforced with W21 X 50 steel sections with 1.22 m spacing.

 Extensive instrumentation was used for the BIF project, which included inclinometers, extensometers, settlement points, and water observation wells. A plan view of the locations of instrumentation in relation to the excavation is shown in Figure 2.2

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Figure 2.2 Plan view of the BIF project site  cross sections A-A’ and B-B’ are the east and west wall respectively (O’Rourke and O’Donnell, 1997a)

 BIF East Wall

The soil profile of the east wall is shown in Figure 2.3. Excavation depths ranged from 17.2 to 19.4 m. During the BIF excavation through thick marine clay deposits (cross-section A-A’ Figure 2.2), large lateral and some vertical deformation was observed when the excavation was ~ 13.4 m deep (O’Rourke and O’Donnell, 1997a). Figure 2.4 shows the cross-section with the ground conditions, support system, and the ground movements. The excavation was partially backfilled to remediate against deep rotational movements in the soil due to unbalanced vertical pressure acting on the base of the excavation (O’Rourke and O’Donnell, 1997a). Afterwards the base was reinforced with DMM buttresses with jet grouting adjacent to the wall at an excavation elevation of 8.1-10.9 m below the ground surface.

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Figure 2.3. Soil profile of the BIF east wall (McGinn and O’Rourke, 2000)

 

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Figure 2.4. Cross section A-A’ showing the soil strata, support system, and ground movements of the east wall at the BIF site (McGinn and O’Rourke, 2000)

The DMM and jet grouted base of cross section A-A’ is shown in Fgure 2.5. Three components make up the reinforced area. The main component is a series of SMW buttress that are parallel to each other with center spacing of 2.4 m. Each buttress was installed as a single row of interlocking DMM columns using the same techniques that were used for the excavation support wall. except no steel reinforcement was installed. For the end nearest the wall, each individual buttress was expanded to three rows to create what O’Rourke and McGinn (2004) refer to as a “hammer head.” A plan of the buttress is shown in Figures 2.6 and 2.7. The area between the wall and the hammer head was stabilized with three pairs of jet grout columns, which served as a means to transfer load. A double jet grouting system was used where the high pressure grout is dispersed within an envelope of compressed air that erodes the soil more efficiently. The column pairs next to the hammer head were installed vertically while the pairs next to the wall were installed at an angle to supplement vertical support. The buttresses penetrate the glacial deposits underneath the clay.

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Figure 2.5. Cross section of the base stabilization of the east wall: 1) DMM buttress; 2) Jet grouting; 3) Final subgrade and tiebacks (McGinn and O’Rourke, 2000)

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Figure 2.6 Plan view of the reinforcing buttress used for base stabilization of the BIF excavation (McGinn and O’Rourke, 2000)

BIF West Wall

Cross-section B-B’ from Figure 2.7 represents the west wall. The soil profile is shown in Figure 2.8. This was also reinforced for protection against deep rotational failure. Excavation depths along this wall varied from 13.7 to 15.9 m. DMM and jet grouting were incorporated when the excavation was 8.1 to 10.9 m. The base was stabilized using the same buttress pattern as the east wall (Figure 2.7).

 

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Figure 2.7. Plan of BIF base stabilization (O’Rourke and O’Donnell, 1997b)

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Figure 2.8. Soil profile of the BIF west wall (McGinn and O’Rourke, 2000)

 Figure 2.9 shows the DMM and jet grouting of cross-section B-B. The difference from the east wall is that here the buttress does not penetrate into the glacial deposits but “floats” in the base clay. Due to shallower excavation depth of the west wall, the thickness of the marine clay below subgrade was the greatest. The floating wall was used to avoid an abrupt change in improved soil on till to thick clay, which in turn reduced the potential of differential settlement of the highway (O’Rourke and McGinn, 2006). The jet grout columns were also placed vertically and at an angle to provide vertical and lateral wall support.

 

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Figure 2.9. Cross section of the base stabilization of the west wall: 1) DMM buttress; 2) Jet grouting; 3) Final subgrade and tiebacks (McGinn and O’Rourke, 2000)

 West Wall Excavation Performance

A plan of the instrumentation is shown in Figure 2.2. For cross section B-B’ (station 160 + 30) three inclinometer/probe extension meters (IPE) of interest were installed at distances 0.6, 3.66, and 6.71 m behind the wall. An IPE can measure settlements at depth using extensometer magnets.

The array of IPEs for cross section B-B’ with locations of deformation measurement points (DMP) is shown in Figure 2.10(a). The figure includes the Porter Street Combined Sewer (PSCS), which is a posttensioned reinforced concrete box culvert that is supported by drilled shafts on 18.3 m centers (Figure 2.8).

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Figure 2.10. Cross section of instrumentation and displacements for major excavation stages: (a) instrumentation; (b) stage 3; (c) stage 4; (d) stage 5 (O’Rourke and O’Donnell, 1997b)

 Excavation of the east wall was performed in 5 major stages as described by O’Rourke and O’Donnell (1996) in Table 1. Figures 2.10(b)-2.10(d) shown the cumulative ground movements for stages 3 through 5. Brief descriptions of the ground movements in figures 2.10(b)-2.10(d) are as follows:

Stage 3: Figure 2.10(b) is after the installation of  SMW buttress. After installation and before curing the buttress did not demonstrate enough strength to resist the lateral earth pressures at depth. The results were lateral wall movements up to 50 mm. The SMW settled 18 mm at this stage. An interesting observation is that 40 mm of settlement was recorded next to the PSCS but none took place directly above it (O’Rourke and O’Donnell 1997b).

Stage 4: The third - level tiebacks were installed and the jet grouting was completed (Figure 2.10(c)). Significant movements of the wall continued and most occurred during jet grouting. Cumulative lateral wall movement increased to 153 mm. Also total vertical SMW settlement increased to 69 mm.

Stage 5: The excavation reached the final subgrade depth of 15.25 m and the final three levels of tiebacks were installed. The lowest tieback (sixth level) was anchored in the glacial deposits (Figure 10(d)). At this stage there was only a modest increase in lateral displacement near the top of the SMW. An average cumulative lateral displacement recorded by the three IPEs was 159 mm.

 Table 1. Stages of excavation at station 160 + 30 (O’Rourke and O’Donnell, 1996)

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 Pore Water Pressure

Hydrostatic pore pressure was assumed in the design of walls. To determine if this assumption was valid, pore water pressure was directly measured in the soil next to the wall with four vibrating piezometers (VWPs) and an open standpipe observation well.

Soil mix walls typically have hydraulic conductivities of 1x10-6 to 1x10-7 cm/s (Taki and Yang, 1991) which are approximately the same as the horizontal conductivity of the marine clay. Taking in account the weep holes created by the large number of tiebacks combined with thin sand a silt layers, the water pressures measured behind the wall were significantly less than hydrostatic (O’Rourke and McGinn, 2006). This was observed in both the east and west walls.

Deep Rotational Stability

Deep rotational stability (DRS) analysis was performed on the west wall. Critical slip circles were evaluated using Bishop’s method. Figure 2.11 shows the critical slip circle before the final level of tiebacks anchored in the glacial deposits were installed. The DRS analysis demonstrated that the stabilized soil with the combination of the tiebacks resulted in critical circles in the deepest part of the marine clay at the final stage of excavation (O’Rourke and O’Donnell, 1997b). Along these surfaces, there was sufficient shear resistance to prevent a deep rotational failure.

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Figure 2.11. Critical slip circle of excavation at station 160 + 30 (O’Rourke and O’Donnell, 1997b)

 

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