1. Drilling Operations: The operation of the construction equipment is a highly important element in terms of productivity. Indeed, a qualified operator with skill and expertise will produce a higher level of efficiency, safety and quality, as well as a reduction in equipment maintenance (not including normal and scheduled servicing) and daily wear-and-tear. Additionally, operator training must be emphasized in the organization, which is outside of the scope of this discussion. 2. Drill Tooling: Selection of the drilling tools depends directly on the soil and/or rock to be excavated. In Chapter 5, “Construction Procedures,” of the Sociedad Mexicana de Ingeniería Geotécnica (SMIG) Deep Foundations Engineering Manual (in Spanish), Paniagua (2016) presents a variety of choices, depending on different stratigraphic conditions. 3. Equipment Selection: Considering elements 1 and 2, a practical graph, as shown below, can be used to select the required equipment while considering the following: • Pile diameter is plotted along the horizontal axis. • Different soil and rock conditions are plotted along the vertical axis. The most unfavorable soil or rock condition within the stratigraphic profile at the worksite should be used when selecting the drilling equipment. • The required drilling equipment (“capacity”) is chosen based on the plotted location of the pile diameter and soil/rock condition (intersection of the horizontal and vertical lines). The “capacity” is defined in terms of the rated torque of the drilling equipment. As an example, the project required that drilled shafts be installed each with a diameter of 130 cm (52 in). The drilled shafts were to be installed in a heterogeneous stratigraphy with the presence of boulders and rock, which had a variable unconfined 70 • DEEP FOUNDATIONS • MAR/APR 2018 Equipment selection based on pile diameter and soil/rock characteristics Practical Examples of Equipment Selection Example 1: Drilled shafts with a diameter of 1.5 m (59 in) were to be constructed in an old open-air mine that was filled without proper control. The groundwater table was not found during the exploratory drilling to the maximum depth of about 30 m (98 ft). • Drill Selection • Pile diameter = 1.5 m (59 in) • Most unfavorable soil/rock conditions = Boulders, ≤1.5 m diameter, sandy silt matrix • Drilling Tools: For Stratum A, a soil auger; for stratum II, rock augers and buckets, and possibly use a chisel and/or hammer grab Casing placement for Example 1 compressive strength (UCS) up to 20 MPa (2,900 psi). The required drill rig capacity was selected as follows: • Pile diameter = 130 cm (52 in) • Most unfavorable soil/rock conditions = boulders and rock, UCS ≤ 20 MPa (2,900 psi) • Dr i l l ing equipment capaci ty = 200 kN-m (147,512 ft-lb) It is worth noting that it is also possible to drill using different stages, with a smaller diameter first and increasing the size gradually in two or three steps. Clearly, this sequence would significantly reduce productivity, but it is possible to excavate the borehole using smaller and lower capacity drilling equipment. • Drilling equipment capacity = 300 kN-m (221,268 ft-lb) • Drilling Stability: For Strata A and B, the drilling requires the aid of steel casing, which may be installed in the following manner: • Placing the casing in a previously excavated or drilled borehole, whenever possible • Using a casing oscillator • Alternating drilling and advancing the casing – drill with a stabilizing drilling fluid to a given depth, advance the casing to that depth, and then continue to the next depth increment (repeating the process until the desired depth is achieved) • Using telescopic casings a) Casing in previous borehole b) Casing oscillator
