the reinforcement of the structure. The resulting SPTC (Soldier Pile Tremie Concrete) wall has become a model for other transit work projects in the country. Equipment Improvements Equipment had improved dramatically from the days of the WTC “bathtub” to the beginning of the “Big Dig.” Crane operated mechanical and hydraulic clamshells became heavier and more productive and the introduction of hydromill equipment, utilizing reverse circulation to excavate soil and particularly rock, reduced construction times and consequently costs. (See p. 79) The previous landmark projects have shown how slurry wall techniques facilitated the construction of complex urban projects, but slurry walls have also been extensively used to provide a watertight barrier in dam and levee construction as well and open excavation and landfills. Providing a concrete cut-off for the construction of a new dam has a long history even in North America, starting with the previously mentioned Kinzua Dam and the two cut-offs built by Hydro Quebec for dams on the Manicouagan River, but a new challenge arose when a cut-off had to be installed through an existing earthen dam with the reservoir impounded. The successful installation at the Wolf Creek Dam in the mid-70s of a 320 ft (97.5 m) deep cut-off in those conditions opened new possibilities for dam remediation, a very important subject given the large inventory in the U.S. of aging structures in varying degree of disrepair. The construction technique used at Wolf Creek was a hybrid of rotary drilling, clamshell and percussion excavation, and it introduced, for the first time in the U.S., large-scale reverse circulation drilling in dam construction, a technique subse- quently utilized by the Corps in both the Walter F. George Dam and in the new cut- off at Wolf Creek Dam. Shor t ly af terward, hydromi l l technology was used in dam repair, first at Mud Mountain Dam and subsequently in a score of other dams. The increased productivity of the equipment, its capacity to penetrate large amounts of rock economically, and its continuing improvement in drilling ever more hard formations has resulted in making possible repairs that would have been unthinkable half a century ago. The continuous slurry trench instal- lation is the other area of slurry wall construction that has been used in the U.S. more widely than any other and it is a truly American development of the technique. While some shallow trenches were built in California in the 1940s, the technique came to the fore in the early 1970s with the installation of an 80 ft (24.4 m) deep cut-off, filled with a blend of sand, gravel, clay and bentonite to allow for the open cut excavation of the Huxtable pumping plant in Arkansas, on a tributary of the Mississippi River. The astounding success of the cut-off, which maintained the excavation dry with nominal pumping even when the site was totally flooded, spawned an industry which today has installed several million square feet of such trenches, to depths well in excess of 100 ft (30.5 m), backfilled with soil-bentonite, soil-cement-bentonite, self- hardening cement-slag-bentonite and excavated by clamshells, long stick backhoes and, recently, by hydromills. This technique is particularly important for the remediation of the large inventory of levees, both public and private, that do not measure up to the modern safety criteria. Construction of the old barrier wall at the Wolf Creek Dam by clamshell, percussion and reverse circulation, circa 1970s DEEP FOUNDATIONS • NOV/DEC 2012 • 61
