A stable borehole is essential for enabling spoils return to the surface and for controlling the jet grouting process. If you cannot successfully stabilize the bor- ehole—often the case with cohesionless, gravelly, and cobbly soil—then you will need to install a temporary casing. identify a scope of treatment that meets the objective, paired with a product char- acter. The design, project objectives and nearby elements (structures, utilities, substructures) can offer valuable guidance in selecting the jet grouting system most appropriate for the project. • The tooling choices depend on which fluid delivery system you select. Single, double and triple fluid systems have selec- tions exclusive to their respective systems. - Tooling choices include swivels, rods, monitors (and nozzle sizes) and bit configuration and bit nozzle sizes. Figure 1. Super-sized 7 m (23 ft) diameter soil improvement accomplished with a 90 mm (3.5 in) diameter tool (Photo Credit: Hiroshi Yoshida) For a good drilling technique: • Match the speed of penetration with the flow rate of flushing fluid • Use a bit type that can dislodge the overburden with fine particles that will travel up to the surface with the flushing fluid • Select a bit diameter in concert with the above decisions so that up-hole fluid velocity will remove the dislodged soil • Choose a flushing fluid, again in concert with the above decisions, capable of carrying the soil and stabilizing the borehole The drilling fluids most used are water, polymer mud, hydrated bentonite or cement slurry. Deciding which fluid to use depends on the subsurface materials and stability of the borehole, both during drilling and jetting. Objective 2: Erode the Soil. Jet grouting has been known as a mixing or replacement process. It’s always a mixing process, but never 100% replacement; however, depending on procedures used, jet grouting can replace more than 50% of the soil. This erosion process depends on many aspects of the technology. Let’s review the aspects within our control. • Before selecting the jet grouting system (single, double or triple fluid), study the preliminary design. Designs generally - Fluid delivery choices include the means and methods of batching the cementitious slurry, drilling fluid (if different) and the pumps that generate the energy necessary to erode and mix the soil. • Another important choice is the drill. A great variety of drills, masts or leads hydraulically control the drilling head. These controls require special hydraulic valving to smoothly and consistently rotate and lift the jetting monitor. You can set these controls manually, but most are computer- controlled with feedback loops to constantly adjust for temperature and weight variations. Objective 3: Predict the Product. Predicting product quality and geometry is one of the more difficult aspects of jet grouting. We have control over accessing the treatment zone and eroding the soil, but the ground we work in dictates our intervention with it. This is why pre- production test sections are important for successful jet grouting. The ground can vary considerably in all three dimensions, and this impacts both the geometry and the quality of the jet grouting work. A perfect understanding of material type can assist with product prediction, but even the same material can vary in its physical character, influencing erodability, and subsequently, quality. This quality variation should be part of the design. Case Studies of Recent Innovations Jet grouting technology, with its flexibility and variabi l i ty, provides myriad opportunities for innovations. The following are some examples. Cut-off Wall Extension in the Arctic. In 2006 Treviicos completed a long and deep groundwater cut-off extension to a plastic concrete wall, up to 41 m (135 ft) deep, 210 km (130 mi) south of the Arctic Circle (see Figure 2). Weather conditions were extremely challenging, with -31°C (-24°F) average January temperatures. The treatment zone connected a plastic Figure 2. Jet grouting working procedure for cut-off wall extension (Courtesy of Treviicos) DEEP FOUNDATIONS • NOV/DEC 2012 • 87
