This paper deals with drilling results through a weathered sequence of rocks using two different drilling methods: down-the-hole hammer and diamond core drilling. Rate of penetration is log-related with rock physical parameters(density; Vickers microhardness; Ultrasonic longitudinal velocity) and rock mass quality (R.Q.D.). Calculated specific energy also shows the same variations in the uppermost part of the drilling profile as the indicated physical parameters. It is this same portion of the hole the major difficult part for the drilling operation.
Dans le travail on presente des resultats de forabilite dans des sequences de roche alterees en utilisant deux methodes de forages: marteaux en fond de trou et carottage rotatif avec outil diamantee. La vitesse de forage presente des relations diagraphiques avec des caracteristiques physiques des roches (densite; microdurete Vickers; vitesse lalgitudinal des ultrasons) et la qualite du massif de roche (R.Q.D.). L'energie specifique de forage calculee presente des variations importantes dans la partie superieur du puit comme les paramètres physyques. Cette même partie du puit c'est nonnallement la plus difficile à traversee pour les foreurs.
Dieses referat befabt sich mit bohrergebnissen durch eine vezwitterbe gestensfolge, unter benutzung zweier verschiedener bohrmethodem: sinkbohrloch hammer und diamantkernbohrung. Die bohrgeschwindigkeit ist mit den gesteinsphyskalischen parametern(dichte; vickers-rnikroharte;ultraschalllongitudinalgeschwindigkeit) und der gebirgsqualitat(R.Q.D.) log-verbunden. Die berechnete spezifische energie zeigt im obersten teil des bohrprofils auch die gleiche variation wie die angegebenenphvsikalischen parameter. Es ist genau dieser abschnitt des bohrloches, der den schwierigsten teil fur den bohrablaufdarstellt.
Drilling through weathered rocks is always a concern for the drilling engineer. Not only intact core samples are difficult to obtain from weathered materials but also when drilling through weathered rocks the sudden and sometimes unexpected changes in drillability requires special attention by the operators. Two types of drilling operations (waterwell drilling using down-hole hammers and diamond coring for geotechnical site investigation) supplied a large amount of data which can be considered of interest for drillability characterization of weathered materials. In a previous paper (MELLO MENDES & MOURAZ MIRANDA (1979)was shown how physical parameters varied along drilled cores on different grades of weathered greywacke.
Down-hole drilling is a common practice by water well industry in granite and metassedimentary schist-greywacke areas in Portugal (MOURAZ MIRANDA,1982) Drilling diameters are commonly 215 mm and 165 mm. The larger diameter is used for the first part of the hole to 10–15m depth. Rotary diamond core drilling is normally conducted with 86 mm for the beginning of the role and 76 mm for the rest of the drilling, for geotechnical site investigation. Using data from drilling recorders mounted on rigs it was possible to use as a field drilling parameter the instantaneous rate-of-penetration (R.O.P.) Collected cuttings from down-the-hole drilling enabled same physical characterization of drilled rock Vickers microhardness (VHN)and density (d). The diamond cored samples enabled the use of one more physical parameter:longitudinal ultrasonic velocity (VL). A typical section through weathered granite is shown in Fig.1 Diamond drilling was conducted close to a down-hole drilled hole. The intermediate top part of the weathered profile shows the most irregular variation of all the parameters (R.O.P.; large dispersion of VHN). Using data gathered from several drilled holes (±50) in different rock types since top-soil to sand rock it was found that R.O.P. decreased from top to bottom of the hole with large variations in intermediate part of the profile. Calculating the specific energy of drilling, using BULLOCK (1976) formulae as in MOURAZ MIRANDA & MELLO MENDES(1983), it is possible to observe some correlation with the measured physical parameters. Fig.2 indicates the calculated specific energy for the weathered profile of Fig.1 with reduction of the specific energy in the beginning of the hole followed by a steady increase with depth. Trying to understand the small variations on the production parameter(R.O.P.) in terms of geological variations the RQD was also noted (Fig.2). The upper part of the profile can be considered almost a soil and the intermediate upper portion a mixture of soil and rock.
As per our knowledge it does not exist the universal optimum drilling method for a weathered sequence. The upper part of the profile where drilling parameters show an irregular variation and to an unknown depth. Per drillers practice the selection of the drilling method is done empirically, with economic consequences, whim could be optimised if the distribution of the various weathering degrees within a profile could be predicted in advance, particularly for the most weathered part. Our aim of investigation was to predict the correction of drilling operation, optimizing it through the control of actual drilling parameters.
