Small Diameter Wells Drilled by Jet‐Percussion Methoda

Abstract

AbstractObservation wells and access holes for neutron probe use were drilled by a jet‐percussion drill rig in coarse alluvial material near Tucson, Arizona. The method combines the jetting action which effectively removes loose materials with the percussion necessary to break up tighter formations and large particles. A unique feature of the equipment is the provision for simultaneous drilling and driving the casing to keep the hole open in loose formations. Washed samples of the material being drilled can be obtained from the recirculating water system. Drilling rates were from seven to ten feet per hour, and costs including casing were less than $1.50 per foot. Maximum depth is about 100 feet.Hydrologic study of a ground‐water reservoir requires access to the aquifer. Observations are made where the water table intersects the land surface, such as at springs or seeps, or through existing wells. However, wells are not always found in desired locations or are pumped so heavily that their usefulness as observation wells is impaired. Measurements are also made in the unsaturated zone with neutron moisture measuring equipment, which requires a small diameter cased access hole for the neutron probe.An observation well needs a diameter only large enough for access by measuring devices and must be immediately responsive to changes in aquifer water level. For neutron probe use, the required diameter is about two inches, and the well must have no significant effect on moisture movement in the unsaturated zone. To meet these requirements the drilling process should have no lasting effect on the formation surrounding the well. Finally, since the wells have no economic utility, construction cost must be low.A network of observation wells and neutron probe access holes was drilled as part of a project to evaluate the natural recharge from Rillito Creek near Tucson, Arizona. The formation along the creek bed is a coarse alluvial outwash containing almost no clay, considerable running sand, and some large gravel and boulders. Commercial churn (percussion) and hydraulic rotary drills, commonly used for drilling in this area, were first tried but encountered much difficulty in the loose materials.Previous investigators used jetting equipment drill small diameter holes in a variety of alluvi formations (Cederstrom and Tibbitts, 1961; Bowma: 1911; and Pillsbury and Christiansen, 1947). Howeve in most cases, either open hole drilling was used because the fine material content and the limited numb. of boulders or large gravel particles made this methc possible, or drilling mud was used to maintain th hole. The jetting method uses water pumped unde pressure through hollow rods and a drill bit to loose the material and carry it to the surface. The jettin water receives little aid from the drill in loosenin the material to be removed. Usually the drill cutting are settled out in a tank or basin, and the water i recirculated.