Cutter Soil Mixing Excavation and Shoring in Seattle s Pioneer Square District

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1 Cutter Soil Mixing Excavation and Shoring in Seattle s Pioneer Square District Doug Lindquist 1, Ben Upsall 1, and Garry Horvitz 1 1 Hart Crowser, 1700 Westlake Avenue North, Suite 200, Seattle, WA 98109; ; Doug.Lindquist@hartcrowser.com, Ben.Upsall@hartcrowser.com, Garry.Horvitz@hartcrowser.com ABSTRACT In order to make a 13.1 m (43 ft) deep excavation, a 20.7 m (68 ft) Cutter Soil Mixing (CSM) groundwater cutoff shoring system was constructed in Seattle. The wall alignment was pre-trenched to remove obstructions and the wall was constructed using cement-bentonite slurry technology. The lower aquifer was depressurized to stabilize the base of the excavation while the upper aquifer was monitored closely for unexpected drawdown, which could result in unacceptable settlement of adjacent structures. Underpinning micropiles were installed below the perimeter footing of an adjacent 8-story building. The southern portion of the new building is supported on drilled shafts where the site s triangular shape made excavation less efficient. Permanent tie-downs were installed to resist hydrostatic uplift forces after the dewatering wells are shut off because the water pressure on the base of the foundation is greater than the building s weight. INTRODUCTION This paper presents a case history of the shoring system for the 505 First Avenue Building using Cutter Soil Mixing (CSM) technology in Seattle s historic Pioneer Square district. CSM is a relatively new technology developed in by combining the technology from two European firms, Soletanche Bachy and Bauer (Mathieu et al. 2006). This shoring system was selected for an excavation 13.1 m (43 ft) below the ground surface and 11 m (36 ft) below the groundwater table. Challenging subsurface conditions included very soft soil containing wood debris, abandoned timber piling, and numerous other obstructions. Also, a lower aquifer required depressurization to avoid blowout of the excavation base. There have been a number of excavations performed to depths greater than 25 m (82 ft) in downtown Seattle using soldier pile and tieback shoring methods with timber lagging, and numerous excavations performed to shallower depths using soil nail shoring methods. This project, however, is located south of downtown Seattle on a portion of reclaimed land where buildings are typically constructed with no more than one level below grade because of the high groundwater table. The site also had challenges: the new structure required anchoring to the subgrade soils to counteract hydrostatic uplift pressure on its foundation, an adjacent 8-story ER2010 Lindquist et al. Page 1 of 8

2 building required underpinning, new tiebacks had to be installed between and through the adjacent building s existing piles and the presence of a nearby pile supported onramp to the settlement-sensitive Alaskan Way Viaduct. A discussion of each challenge and its solution is addressed in this paper along with a description of the shoring system s performance. SITE AND SUBSURFACE CONDITIONS Site. The site is located in an area south of downtown Seattle that has been reclaimed from Elliott Bay as part of multiple historical regrading projects. The property was initially developed as a wharf on pilings for timber mill-related businesses. Fill material was deposited in the late 1880s and early 1890s and included sawdust from adjacent sawmills, wood planks and pilings, ship ballast, and burn debris from the Great Seattle Fire of Figure 1 shows the site s proximity to the historical shoreline and low tide. Figure 1. Vicinity Map Illustrating Historical Shoreline The project site is bounded to the south by the historic, 3-story Triangle Pub building and to the north by the 8-story 83 King Street building (Figure 2). Three structures were demolished prior to site development; two historic building facades were preserved. The historic west facade was removed and rebuilt in kind, while the east façade was stabilized and kept in place during construction. Soil. Subsurface conditions were based on 12 geotechnical borings extending into the bearing layer and two test pits performed within the fill. The soil conditions were generalized as 7.3 to 10.4 m (24 to 34 ft) of fill consisting of an upper crust of silty sand over wood debris, brick, silt and sand (upper aquifer) over marine silts and sands (historical beach deposits), over dense silts and silty sands (aquitard) over outwash sands (lower aquifer). Groundwater. Groundwater conditions were evaluated based on seven shallow wells and two deep wells. Prior to construction, the lower and upper aquifers had ER2010 Lindquist et al. Page 2 of 8

3 piezometric head depths averaging 1.3 m (4.2 ft) and 2.3 m (7.5 ft) below the ground surface, respectively (i.e., the lower aquifer was under pressure). Figure 2. Site Plan Illustrating Tieback Layout CSM SHORING SYSTEM AND CONTRACTOR SELECTION At the time of design, secant piles and ground freezing were the primary methods of shoring below the water table in Seattle. To coordinate details of the shoring system, underpinning, and dewatering these options were submitted to local shoring contractors to develop costs and select a shoring contractor early in the design process. The secant pile system was less costly than the soil freeze system; however, the estimates excluded the cost of delays for obstructions, which were known to exist. The contractor was selected for their alternate design-build approach consisting of cement-bentonite slurry pre-trenching and CSM cutoff wall (Parmantier and Giwosky 2009). This alternative was selected because the pre-trenching would remove the obstructions and potential delay costs. The CSM shoring system was considered superior to a secant pile wall system because of the reduced number of joints in CSM panels compared to overlapping secant piles (Brunner et al. 2006). The contractor performed cement-bentonite slurry pre-trenching with an excavator to a depth of 10.4 m (34 ft) to remove wood debris. The CSM shoring consisted of 108 overlapping panels 0.8 m thick by 2.8 m wide (2.6 ft by 9.2 ft) installed to a depth of 20.7 m (68 ft) around the 263 m (861 ft) site perimeter. While the cement/grout was still wet, soldier piles were placed through the panels on 1.7 m (5.4 ft) centers to a ER2010 Lindquist et al. Page 3 of 8

4 depth of 18.3 m (60 ft). During excavation, the panels were chipped away to expose the soldier piles and tiebacks were installed through sockets within the soldier piles. The overlapping CSM panels created a virtually water-tight shoring system. DESIGN AND CONSTRUCTION CHALLENGES Some of the design and construction challenges on this project are described here. Design Earth Pressures. The challenging subsurface conditions described previously led to the unique pre-trenching and CSM shoring. The design earth pressure diagram used to develop the tieback layout for the CSM shoring system is shown in Figure 3 along with a schematic of the soil profile. Figure 3. Design Earth Pressure Diagram and Soil Profile Dewatering and Cutoff Wall. The project required the excavation to extend 11 m (36 ft) below the static water table with limited drawdown allowed in the upper aquifer. The designers estimated that a drawdown of the lower aquifer of approximately 12.5 m (41 ft) would be required to avoid blowing out the base of the excavation and that a maximum drawdown of 1.5 m (5 ft) of the upper aquifer would be acceptable to avoid settlement of adjacent structures. Recharging the upper aquifer from the dewatering wells was considered in case the low permeability aquitard was found to be discontinuous during construction dewatering. ER2010 Lindquist et al. Page 4 of 8

5 Excavated soils had to be dewatered to facilitate transport off site. Six shallow interior wells were installed in the upper aquifer to dewater the excavated soils and two deep dewatering wells were installed outside of the site to depressurize the deep aquifer (Figure 2). One deep well was located interior to the site but was decommissioned early in construction because it was obstructing the excavation. Dewatering monitoring results are described in the Construction Monitoring section. Hydrostatic Uplift Pressure. Upon recharge of the lower aquifer, the building, which was designed to be essentially water-tight, has an unbalanced hydrostatic uplift force in excess of the building weight acting on the foundation. This required the installation of 360 tiedown micropiles to hold the building in place. Adjacent Structures. The adjacent 8-story 83 King Street building required micropile underpinning of its perimeter footing to replace existing pile support that encroached on the subject property. Piles located below the 83 King Street building and the Alaskan Way Viaduct on-ramp required accurate installation of tiebacks around existing timber foundations (Figure 2). The nearby on-ramp required tiebacks as long as 41 m (135 ft) to limit changes in soil stress near those piles. CONSTRUCTION MONITORING Construction monitoring is critical to verify that below ground construction and conditions conform to the design assumptions and that the performance is as anticipated. Site monitoring included measurement of site movement (i.e., inclinometers and optical surveys) and groundwater levels within the two aquifers. Quality assurance and quality control of the CSM wall included visual inspection of the panels during excavation as well as strength testing of the CSM soil-cement mix. Inclinometers. Three inclinometers were used to measure the lateral displacement of the CSM shoring system. The inclinometers were installed on three soldier piles at the locations shown on Figure 2. Deflection readings are shown on Figure 4, with positive deflection indicating movement into the excavation. The results indicate initial movements into the excavation followed by movements back into the soil after the top row of tiebacks were stressed. Upon further excavation, the movement went back toward the excavation. In general, these lateral movements were less than 25 mm (1 in) in either direction and were considered acceptable. Optical Monitoring. The optical monitoring plan consisted of over 200 survey points located near the top of every other soldier pile, on the buildings adjacent to the site, on the sidewalk next to the excavation, in the street, and on the adjacent Alaskan Way Viaduct on-ramp columns. The largest recorded lateral movement was approximately 4 cm (1.5 in) of movement into the soil on a pile located on the west side of the excavation. When lateral deflections of greater than about 25 mm (1 in) into the excavation were observed for any given monitoring point, the project team discussed the exceedance, visually inspected the shoring and adjacent CSM panels, and continued to monitor those deflections very closely. Deflections larger than about 50 mm (2 in) or visual signs of CSM panel or shoring system distress would ER2010 Lindquist et al. Page 5 of 8

6 likely have called for a more thorough review accompanied by corrective actions such as installation of whaler beams with additional tiebacks. There was negligible movement of many points including those on the pile-supported Alaskan Way Viaduct on-ramp columns. 0 2 Deflection (mm) Deflection (mm) Deflection (mm) Depth (m) Depth (ft) Pile E35 Pile W10 Pile W Deflection (in) Deflection (in) Deflection (in) Figure 4. Inclinometer Results Visual Inspection of CSM Panels. During excavation the condition of the CSM wall was observed visually and probed with a 13 mm (0.5 in) diameter T-probe to identify cracks, seeps, and voids or pockets of weaker-strength materials. Tieback pockets, seeps and cracks were generally filled with expanding epoxy material. The largest voids and pockets of weaker-strength material were encapsulated with a steel plate spanning between adjacent soldier piles and then backfilled. In general, the CSM wall had increased quality and consistency with depth. CSM Strength Testing. The design called for 1.4 MPa (200 psi) 28-day compressive strength of the CSM panels. Wet samples were collected from the panels following installation for laboratory testing. Approximately 95 percent of the samples exceeded the design strength. Low strength test results required one of the CSM panels to have additional grouting on the outside of the wall to improve performance in that area. Groundwater Monitoring. Groundwater monitoring was performed beginning 8 months prior to the start of depressurization of the deep aquifer, which began in February 2008, until the dewatering system was shut off in September 2009 following construction of the building to its full height and placement of exterior brick cladding. Monitoring was observed using two deep wells within the lower aquifer and seven shallow wells within the upper aquifer. Steady state pumping of 75 lpm (20 gpm) at the site resulted in a depressurization of approximately 16 m (52 ft) of water in the lower aquifer while the upper aquifer showed only 0.3 to 0.7 m (1 to 2 ft) of variation in the 7 monitoring wells, which was well within the criteria of 1.5 m (5 ft) established during design to minimize impacts to adjacent structures and ER2010 Lindquist et al. Page 6 of 8

7 utilities. Figure 5 illustrates the monitoring well data including the depressurization and recharge of the deep aquifer Groundwater Depth (Meters) Shallow Aquifer (7 wells) Deep Aquifer (2 wells) Localized Shallow Dewatering for Vault Installation Groundwater Depth (Feet) Jun-07 Aug-07 Oct-07 Dec-07 Feb-08 Apr-08 Jun-08 Aug-08 Oct-08 Dec-08 Jan-09 Mar-09 May-09 Jul-09 Sep-09 Date Figure 5. Dewatering Performance as Illustrated by Monitoring Wells DISCUSSION ON LATERAL SHORING MOVEMENTS The largest lateral movement of the shoring system was into the soil, which is not common. This is attributable to the shoring designer using total anchor design loads approximately 25 percent higher than would be inferred from the design earth pressure diagram to control intermediate stages of construction with only three tieback rows (Parmantier et al. 2009). Portions of the fill (e.g., the wood debris) also likely had a lower earth pressure than assumed in design. SHORING CONSTRUCTION COSTS The project consisted of approximately 11,300 m 2 (37,000 ft 2 ) of exposed shoring wall. The shoring contractor s design and construction costs (e.g., CSM design and installation, soldier piles, tiebacks, dewatering, micropile underpinning of north wall, facade support, etc.) were approximately $8.5 million, or $750/m 2 ($230/ft 2 ). These were the contractor s costs and do not include the owner s design team, who were the designers of record with the city and provided design and construction support. CONCLUSIONS This paper presents a case history of the successful use of CSM technology to perform a 13.1 m (43 ft) deep excavation in an area with a high water table and ER2010 Lindquist et al. Page 7 of 8

8 challenging subsurface conditions. It also demonstrates the value of retaining a specialty shoring contractor early in the design process for a unique excavation solution. The contractor-proposed pre-trenching and CSM shoring performed well and reduced risk for obstruction-related delays compared to secant pile shoring methods used in Seattle in these difficult soils. Figure 6 is a picture of the project site near the end of excavation including the temporarily supported historic east facade. Figure 6. Photo Looking North Following Final Tieback Row Installation REFERENCES Brunner, W.G., Fiorotto, R., Stötzer, E., and Schöpf, M. (2006). The Innovative CSM-Cutter Soil Mixing for Constructing Retaining and Cut-Off Walls. GeoCongress Geotechnical Engineering in the Information Technology Age, ASCE. Mathieu, F, Borel, S., and Lefebvre, L. (2006). CSM: An Innovative Solution for Mixed-In-Situ Retaining Walls, Cut-Off Walls and Soil Improvement. 10th International Conference on Piling and Deep Foundations, DFI/EFFC. Parmantier, D. and Giwosky, D. (2009). Cutter Soil Mixing Comes to Seattle. Foundation Drilling, ADSC, February 2009, Parmantier, D.M., Stow, R.F.P., and Byrne, R.J. (2009). Case Study: Cement- Bentonite Pre-Trenching and Cutter Soil Mixing (CSM) for Temporary Shoring and Groundwater Cutoff. Contemporary Topics in Ground Modification, Problem Soils, and Geo-Support, ASCE, GSP 187, ER2010 Lindquist et al. Page 8 of 8