Input on selection was provided by state and federal permitting agencies, as well as from Kerite and its insurance agency. The selected alternative was a retaining wall in place of the collapsed one. However, the wall needed to be of robust construction to withstand the high water velocity and scour potential. The design incorporated 24 micropiles spaced at 3 ft (0.9 m) on center. The tight spacing was intended to resist scour, in hopes that the dense soil matrix would provide a possible arching effect between the micropiles. The micropiles themselves consisted of 9.6 in (24.5 cm) diameter casing with 0.5 in (1.4 cm) thick walls, central Retaining Wall Construction GEI managed the construction of the retaining wall, and based on a competitive bid process, selected Waters Construction of Bridgeport, Conn., to be the project’s general contractor. Waters selected Moretrench as its geotechnical consultant for micropile and tieback installation. The first site operation by the general contractor was to construct a temporary work platform consisting of steel sheet piles driven to shallow depths, and supported laterally by the riprap and boulders used previously. Site workers used a steel plate bridge to access the platform from the other riverbank. Stacked concrete “mafia” blocks were used locally to prevent sloughing of soils beneath building foundations. While mostly providing work access for the drilling equipment, the temporary work platform also contained drill spoils, grout and other construction materials from entering the waterway. To facilitate construction below the Micropile installation Retaining Wall Construction An 18 ft (5.5 m) cast-in-place concrete retaining wall was chosen by the project team for the final wall type, designed with fairly non-typical proportions. The wall has a thick continuous-width 30 in (76 cm) stem to accommodate embedded micro- piles, which protruded approximately 5 ft (1.5 m) above riverbed. The heel length was limited to 5 ft (1.5 m) to avoid existing structures behind the wall, and the toe length was limited to 2 ft (0.6 m) to mini- mize stream encroachment. The base of the wall could not be significantly embedded into the riverbed, since that would require underpinning nearby foundations. The retaining wall design incorporated deep foundation elements. The project team chose vertical micropiles for a variety of reasons, particularly for supplementing the capacity of the oddly-proportioned wall, ease of drilling through dense glacial till and minimal disturbance to nearby structures. 76 • DEEP FOUNDATIONS • NOV/DEC 2013 #11-grade 75 thread bar, and 5,000 psi, (or 34.5 megapascal) grout. The micropiles were designed with a 5 ft (1.5 m) rock socket in bedrock, which lay approximately 15 ft (4.6 m) below riverbed elevation. To provide additional lateral capacity, the general contractor drilled the tiebacks approximately 7 ft (2.1 m) above riverbed elevation. The tiebacks were spaced approximately 9 ft (2.7 m) horizontally, and had to be located carefully to avoid the foundation and other structures behind the wall. The tiebacks consisted of a grout- bonded multiple corrosion protection anchors with an 1.75 in (4.5 cm) all-thread bar and 4,000 psi (27.6 MPa) grout. The wall backfill consisted of select granular fill, and compacted around site structures with vibratory walk-behind compactors. The final patio area was designed to be concrete with a chain-link fence. building foundations, the project team decided to use the micropiles to support the soil fill in the work platform down to the top of riverbed. Wooden lagging was placed between the micropiles, and later used as formwork during pouring of the retaining wall concrete. After the retaining wall concrete was poured, workers installed tieback anchors after partial backfilling with select granular fill. The engineers performed proof tests three days after grout placement, and lock off loads were achieved at 120 kips (534 kN). The pockets for the tieback anchors were then capped with concrete. Finally, after removing the sheet piles and temporary work platform, the riprap and boulders used for temporary repair were placed at the front of the wall. This not only provided additional scour protection, but also gave an appearance that mimicked the native site landscape. Structural Monitoring Active real-time survey monitoring of buildings was implemented to ensure safety of the Kerite workers and construction crews. GEI installed a robotic total station on a structure on the opposite river bank and established survey targets
