Combining Jet Grouting with Micropiling. Two of Hayward Baker’s recent jet grouting projects were renovation projects that combined jet grouting with micropiling. The first project, an addition of flue gas scrubbers to a fossil-fueled power plant, required new foundations. Originally specified as micropiling alone, a value engineered alternative included a jet- grouted solution with steel reinforcement to handle the large compression and tension loads associated with these elevated structures (see Figure 4). The second project was a bridge Figure 3. Plan view of jet grout columns for cut-off wall extension (Courtesy of Treviicos) concrete wall to bedrock through a zone consisting of till (cobble and boulder concerns). The schedule required three drilling and jetting setups, with every hole requiring verticality measurement, adding roughly 10% more holes to close possible gaps from drilling variation (see Figure 3). Testing Support Walls and Jet- Grouted Base Seal. In 2011, Layne GeoConstruction reported on the efficiency of dual-axis drill strings to increase productivity and reduce work schedule, all while assuring quality. For its Warm Springs Tunnel approach project for Bay Area Rapid Transit (BART) in San Francisco, Layne took a “system” approach to test the excavation support walls and the jet- grouted base seal. The walls and top of the jet-grouted base seal were fully exposed for visual inspection and testing at two loca- tions along the alignment. Fully instru- mented data acquisition enabled daily assurance of test section repeatability, sup- plemented by wet grab samples and cores. Horizontal Directionally Drilled Holes. Hiroshi Yoshida was the first to report jet grouting using horizontal directionally drilled holes in 2010. This was a pilot test where the columns could be excavated for inspection. With today’s drilling technologies, access was not the difficulty. The challenge was maintaining an open borehole and assuring spoil return with a spoil vent pipe. Lessons Learned Now that I’ve shared some success stories, every specialist jet grouter has a long list of lessons learned. Here are a few from my list: • Creating a bottom seal (for groundwater control) requires perfection, especially under high head, > 69 kPa (>10 psi), conditions. A small window (imperfection) can yield significant flows—enough to create a piping issue, and consequential ground losses that exacerbate the problem. • Open gravels and nested cobbles can pose unexpected problems, including loss of return spoil, air escape into the formation, unraveling of the formation and consequential lock-up of jet grout tooling. 88 • DEEP FOUNDATIONS • NOV/DEC 2012 • A high-quality Data Acquisition (DAQ) system, assures the work is done properly and offers information about procedural or equipment malfunctions. If used and reviewed daily, DAQ enables quick response to malfunctions. • Not all soil is erodible, and obstructions “shadow” the erosion. • Sewage in the groundwater, very low pH groundwater, or flowing groundwater can cause poor soilcrete quality. • As Yogi Berra once said, you can observe a lot just by watching. Spoil returns (density, viscosity, color) can tell you a lot about the erosion process in the ground. replacement for the New York Department of Transportation (NYDOT) that was to include driven steel piling to depths in excess of 40 m (131 ft). As a value engineered alternative, the contractor proposed a jet-grouted micropile using the micropile casing for spoil returns while creating a friction column in the Lake Albany Clays. After curing of the soilcrete, the column was post-drilled and rein- forced. Full-scale load testing offered confi- dence of 1,000 KN (225 kips) design loads, for an element only 17 m (56 ft) deep. Conclusion Jet grouting is the most complicated grouting technology available, but it can accomplish what no other technology can with the right equipment, good project planning and an experienced jet grouting contractor. The technology requires effective tooling, quality batching, efficient
