The Deep Mixing Method (DMM) is an in-situ modification technique which is performed to improve strength, reduce liquefaction potential, lower permeability, reduce deformations and allow construction in difficult areas. In particular, DMM has been used to create wall type geometries which function as excavation support or hydraulic cutoff walls. These techniques are classified based on geometry, construction techniques, type of stabilizers, state of stabilizers, and the method of mixing (Bruce, 2000). The primary goal is to provide retention and reduce seepage in a cost effective manner. This is generally achieved by mixing slurry containing combinations of ordinary Portland cement, bentonite, fly ash, set retarders, or super plasticizer with the in-situ soils by using a series of overlapping auger flights (figure 1). In the U.S., this method for construction is known as the Soil Mixing Wall technique (SMW) as a trade name.
Figure 1. Auger mix paddles used in the SMW method (Bauer, 2012)
The main aspects of construction of SMW walls are summarized in Figure 2. As shown, construction is separated into two distinct categories by operation. Operation II is accomplished by using a multiple axis auger mixing tool and shaft like the one shown in Figure 1 which is drilled into the ground at a specified rate by a drill rig (Proboha, 2001). Drilling power can be focused on one single shaft to penetrate problematic layers and different auger shafts can be used to accommodate cohesive soils, sands, and gravels (Taki and Yang, 1991). Before mixing and injection, a guide trench is excavated to handle the spoils produced during the process.
Figure 2. Construction aspects of DMM (Proboha, 1998)
Operation I consists of a batch mixing plant which mixes the cement slurry with bentonite or other binders and provides the mixture to the drill rig by means of a pressurized delivery pump (Figure 3). To ensure that the mixture is homogenous, an agitator re-mixes the final product before delivery to the rig. The plant also typically contains a computer to monitor flow and batching scales for quality control (Taki and Yang, 1991). Quality control during construction is monitored at the batch plant. According to FHWA, there are 3 levels of increasing complexity which categorize quality control. Level 1, the most basic level, is often specified in projects with very simple construction and predictable site conditions where only a single user is needed to monitor results (Bruce, 2000). In larger more sophisticated projects, higher levels are specified to fully automate the process.
Figure 3. Construction operations of DMM (Bauer, 2012)
Sites that have low tolerance to vibration and noise disturbance, high groundwater tables, and in-situ soils that are ideally suited to form either low permeability or high strength and durability mixed columns are well-suited for this technique (Proboha, 1998). One of the main advantages of DMM over many of its competitors is that it generates moderately low noise pollution and very low vibration (Figure 4).
Figure 4. A Comparison of the environmental impacts of various techniques (Proboha, 1998)
The SMW method is often in competition with various other ground improvement techniques including permeation grouting, soil nailing, and jet grouting (Rutherford, 2004). Alternative wall type construction techniques include sheet pile walls, lagging walls, secant/tangent walls, and slurry walls (diaphragm walls). The main advantages of DMM that make it a competitive and viable ground improvement technique are that it produces fewer spoils than slurry wall applications, creates low noise and vibration disturbances, can be adapted to handle complex fills and obstructions, can be used for near-surface groundwater tables, and can be used with tieback anchors and struts to provide structural support (FHWA, 2000). The disadvantages of the technique are:
This paper focuses on the application, construction aspects, design considerations, and lessons learned from case studies involving DMM for excavation support and hydraulic cutoff walls. The goal will be to present some of the fundamental concepts of deep soil mixing and analyze the trends among the projects that have used it successfully.
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.
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.
Figure 1.1. Layout Map of Lake Parkway Milwaukee, WI project (Anderson, 1998; Bahner and Naguib, 1998)
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.
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.
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).
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.
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.
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.
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
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)
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.
Figure 2.3. Soil profile of the BIF east wall (McGinn and O’Rourke, 2000)
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.
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)
Figure 2.6 Plan view of the reinforcing buttress used for base stabilization of the BIF excavation (McGinn and O’Rourke, 2000)
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).
Figure 2.7. Plan of BIF base stabilization (O’Rourke and O’Donnell, 1997b)
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.
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)
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).
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)
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 (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.
Figure 2.11. Critical slip circle of excavation at station 160 + 30 (O’Rourke and O’Donnell, 1997b)
The primary goal of the project was to connect a newly constructed bridge crossing the Susquehanna River to State Route 54 through the Danville, business district in Pennsylvania. This was achieved by creating a highway tunnel underpass that replaced an old street, Factory Street, and extended below another Street, Market Street. The site location consisted of historic mansions, which were as close as 0.9 to 1.2 m to the proposed construction (McMahon, 2001). These structures were problematic to many design options because they were three stories high and were constructed from brittle materials such as brick. Therefore, the difficulty of construction in an urban area was only worsened by the conditions of the structures in the direct vicinity of the project. Limiting wall movements and vibrations became an absolute necessity for successfully completing the project. The excavation reached depths of 6.7-9.4 m. The Project Location relative to the Susquehanna River and the historical buildings is shown below in Figure 3.1.
Figure 3.1 Connector (Underpass) and Excavation Support Location (McMahon, 2001)
To achieve low wall movements and to minimize potentially damaging vibrations associated with other ground improvement methods, the excavation wall was constructed using a DMM wall. The method used in-situ soil mixing with a cement stabilizer that was mixed with the soil using rotary energy to create an overlapping, continuous wall type configuration. The minimum allowable compressive strength for design was specified as 1.38 MPa (McMahon 2001). Borehole tests for the site showed loose to dense sands, gravels, and silts with some boulders. Unlike some of the previous case studies mentioned in this paper, the groundwater table was not shallow for this project. In fact, the groundwater elevation fell below the bottom of the DMM wall.
The design was complicated by the difficulty in modeling the wall. The behavior of the DMM wall in terms of lateral movement, stiffness, and embedment could only be approximated by research on continuous walls, sheet-pile walls, and segmented walls. The method employed in design was to identify which assumptions could be made in terms of behavior of the wall and how to use theory to predict lateral movements. For instance, above the subgrade of the excavation, the wall could be approximated by a segmental wall for design purposes due to the modular ratio of the steel soldier beam/soil cement structure. Since the soldier beams are much stiffer than the soil cement, the soldier beams need to have capacity to carry the surcharge load at the surface (McMahon, 2001). These loads resulted from the fact that the foundations of the historic building were close enough to the excavation wall to impose additional loads above the geostatic earth pressures assumed in design. Below the subgrade, the wall was modeled as continuous due to the high stiffness of the DMM wall relative to the soil.
The design also called for cross lot bracing struts connected using W18X40 soldier piles which were anchored to the DMM wall. These would further reduce lateral movements and improve the performance of the entire structure. These horizontal braces were stressed to 100% of the design load to ensure that loads could effectively be transferred between the two walls and that they did not lose contact (McMahon, 2001). The maximum vertical spacing between horizontal bracing struts was 3.0 meters. The top two struts are close to the surface to help control surcharge loading. Figure 3.2 below shows a cross section view of the structural elements mentioned above.
Figure 3.2. Cross sectional view of excavation and rigid frame DMM Wall (McMahon, 2001)
Depending on the level of the bracing system and the assumptions about design, different earth pressure diagrams could be used to model the forces acting on the wall. The rectangular pressure diagram presented by Terzaghi and Peck (1967) was used where there were multiple bracing levels. It would be applicable to use active Rankine pressures for single bracing areas. After summing the areas of the pressure diagrams mentioned above, the force per unit length acting on the wall can be determined. The surcharge loading must also be included as mentioned earlier. However, to ensure a conservative design, the lateral earth pressures calculated above were also multiplied by a safety factor (which was 1.4 in this case).
To determine whether or not the design met some allowable standards in terms of settlement, previous case studies and other literature sources were used to create a rough estimate of the expected vertical movements. Figure 3.3 shows two plots for predicting vertical movement. The first one is a normalized plot of the distance from the wall vs the expected settlement (Peck, 1969). The second plot uses the same idea but presents actual deflections from various other case studies (O’Rourke, 1989).
Figure 3.3 Estimation of settlements away from DMM wall (Left: Peck, 1967 and Right: O’Rourke, 1989)
Considering both plots and the simplifying assumptions above, it was found that the left figure predicted settlements of 102 mm (which is clearly unacceptable) and the right figure predicted settlements from 28 mm - 10mm which is about 1% of the excavation height. Therefore, for this project, deflections of more than 10 mm would not be acceptable. Anything higher than 10mm settlement could result in cracking and differential settlement of the basement walls of the historical mansions next to the excavation.
The construction called for a total of 372 m of soil mixed wall (SMW). The mixing tools used rotation and a high amount of torque to mechanically agitate and mix the soil with the stabilizing agent. In this case, the stabilizing agent consisted of a mixture of cement grout in a wet state injected through the end of a hollow shaft with rotary mixing. The water/cement ratio by weight was 1.5:1. Cutting heads at the end of the rotating rods ensured that the soil came into contact with the mixing blades to be uniformly mixed. Four mixing augers were grouped together and penetrated the soil at the proper locations. Upon the withdrawal of the tools, cement grout was mixed with the soil. Consecutive passes would then be made with the rig by spacing less than 4 auger diameters. Then on later passes, the curing columns would be re-augered and overlapped to create the final product.
The unconfined compressive strength (UCS) was measured for the wet soilcrete by taking in-situ samples. The 7 day compressive strength reached 3.02 MPa which already exceeded the design specified value of 1.38 MPa. The 28 day strength reached as high as 4.2 MPa (McMahon, 2001). Coring samples were taken from various locations on the finished wall and UCS on the samples indicated strengths of about 3.8 MPa . The results of the construction were monitored using inclinometers and tape extensometers. A plot of the horizontal movements is shown in figure 3.4 below.
Figure 3.4. Lateral deflection vs. depth for the wall (McMahon, 2001)
Both lateral and vertical displacements were acceptable and within the predicted range above. As shown in figure 3.4, the maximum lateral movement of 6 mm occurred at around depths of 7 m and the final settlement was 10 mm (the lower bound of O’Rourke). Although these values are effectively within the ranges of other case studies, the success of this project was in its effective implementation of proper construction practices with DMM, bracing struts, and anchorage using reinforcing soldier beams. The effects of poor construction would outweigh even the soundest design. The combination of careful consideration of literature and case studies coupled with good construction practice and knowledge of site conditions leads to the best results in ground improvement, even in the most challenging and limiting situations.
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).
Figure 4.1. Plan of excavation support system, underpinning, ground improvement, and secant wall for the new addition (Weatherby and Zywicki, 2012)
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.
Figure 3.2. Composite soil boring of the MFA site (Weatherby and Zywicki, 2012)
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.
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).
Figure 4.3. Section of the structural SMW (Weatherby and Zywicki, 2012)
The sections above analyze the 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
Summary of Key Points
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.
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
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.