Drilling Of Geotechnical And Other Exploratory Boreholes

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Geotechnical boreholes must satisfy two goals: provide reliable information about the geological structure of the site and give sufficient data on the physical and mechanical properties of the soils. To solve these tasks, the technique and technology of drilling geotechnical boreholes must allow obtaining soil cores along the entire height of the borehole, ensure the ability to extract soil samples from the borehole with a structure close to the natural state, and ensure the possibility of conducting work to study the filtration properties of soils. These conditions are most fully met by vibrational borehole drilling technology, which represents a cyclical process including the penetration of a probe into the borehole bottom, its vibrational (Fig. 79a) or percussion-vibrational (Fig. 79b) driving into the rock, and subsequent extraction of the tool with the core filling it to the surface. Unloading of the soil as needed can be carried out by turning on the vibration exciter, extracting the soil from the longitudinal cutout, or pushing the core out using a special gripping device moving along the cutout.

In the initial period of drilling geotechnical boreholes, vibration-drilling was mainly carried out by the BT-9 vibrator (NIIOSP), arranged according to the scheme of the simplest type of vibro-driver, spring vibro-hammers of the VMG type (NIIOSP) with a rigid attachment of the headpiece to the drilling tool, and vibrators VPM-1 and VPM-2 (VNIIGS), made in the form of vibro-drivers with a spring-loaded weight (O. A. Savinov, A. Ya. Luskin, M. G. Tseitlin, S. V. Plekhanova, 1954).

At the present time, wide application in the practice of geotechnical surveys has been found by the technology of borehole installation according to the diagram in Fig. 79b with a percussion-vibrational springless driver (B. M. Rebrik, 1979). For this purpose, they successfully use the mass-produced vibro-drilling rig AVB-2M, which is equipped with a springless vibro-hammer VB-7 with an impact piston. The power of the drive electric motor of the vibro-hammer is 7 kW, the static moment of the eccentric weights is 200 kg·cm, the frequency of oscillations is 21 Hz, the maximum driving force is 35 kN, and the weight is 340 kg. The energy source was an electrical generator with a capacity of 20 kW, driven by the base vehicle’s engine, which serves to power the vibrator. The discussed equipment solves the task of drilling exploration wells to a depth of up to 20 m in all categories of sandy and clayey soils, either with casing pipes or without them.

Expanding the application of vibro-drilling for constructing various technical wells in hard rocks to a depth of up to 30 m is achieved via the scheme shown in Fig. 79b, which illustrates the use of a surface vibro-hammer for rotary-vibratory drilling with flushing. Combining the rotation of the drilling tool, which ensures drilling both full-face and core-face, with its vibration allows for increasing the rotary drilling speed by 20–30%. It also improves the efficiency of the drilling method and eliminates accidents associated with the tool seizing at the bottom of the hole. This scheme was practically implemented in the AVP-3 rotary-vibratory drilling rig. It is mounted on the chassis of a ZIL-131 truck. As a source of power for its mechanisms, it features a rotor, a mud pump, and a BB-7 spring-loaded vibrator. The latter, compared to the VB-7 vibrator, features higher drive electric motor power (9.4 kW), higher static moment of unbalance masses (240 kg·cm), maximum driving force amplitude (60 kN), and a total mass of 600 kg.

Vibratory and impact-vibratory drilling for engineering exploration wells allows for a progress rate of up to 50 m per shift. Rotary-vibratory drilling possesses the positive qualities of vibratory and impact-vibratory drilling and extends its applicability from category I–IV rocks in terms of drillability to category V–VI, while simultaneously increasing the depth of the wells.

In terms of technological parameters, rotary-vibratory drilling is similar to the vibratory technology developed by “Vibrogidroset”. A specific parameter of the drilling operations under consideration is the round-trip length. B.M. Rebrik (1979) established that the optimal round-trip length ensuring maximum drilling speed can be achieved using existing vibratory equipment.

In light soil conditions, the optimal round-trip length is 2–7 m, in medium conditions 1.5–3.5 m, and in hard soils it varies from 0.3 to 3 m. Within a single operational cycle, the operating time of the vibro-driver or vibrator ranges from 3 to 10 minutes, which, as a rule, constitutes no more than 25% of the total cycle duration, which also includes time spent on lowering, lifting, and auxiliary operations.

The rotary-vibratory drilling method is recommended for use at well depths greater than 15 m, as well as when drilling frozen soils, boulder-pebble deposits, and layers of hard rock.

The parameters of combined action on the drilling tool during rotary-vibratory drilling are as follows: tool rotation speed is 40–80 rpm, vibration frequency (impacts) of the vibro-mechanism is about 20 Hz. The length of the trip using this method is 0.5–2 m.

A further improvement in rotary-vibratory drilling is being developed by the French firm “Foraco”. This drilling rig’s distinct feature is the use of a double drill string. It combines the rotation of the rock-breaking tool with its impact-vibratory driven penetration using a spring-loaded vibro-hammer capable of delivering downward as well as upward impacts. Compressed air is used to lift the crushed rock and cool the drill bit. The double drill string makes it possible to adjust the volume of the annular space by altering the diameters of the pipes. This allows for achieving the required lifting speed for the crushed rock regardless of the well depth, given the specified parameters of the installed compressor. This feature, combined with the possibility of combining various types of dynamic action, ensures high rock penetration speeds (from 1.6 m/min in sands to 0.1 m/min in granites) over a wide range of drilling diameters (from 100 to 600 mm).

When performing work to freeze soils, casing strings—used to place refrigerant circulation pipes in them—are usually lowered into wells drilled by rotary rigs. Vibratory technology for constructing freezing wells avoids turning rocks into slurry and lifting it to the surface by switching to another type of operation—effective subsurface driving. When freezing ground, bottom-sealed casing pipes must be positioned (as a rule, 114 mm in diameter for lengths up to 25 m).

The vibratory technology for constructing freezing wells can be practically realized using the OIT-1 rig (see Table 16), built by Glavtonnelmetrostroi (Ministry of Transport Construction of the USSR) with the participation of VNIIGS. In this rig, driving bottom-sealed casing pipes is carried out according to the scheme in Fig. 79d, which uses a free-floating spring-loaded vibrator with adjustable tension on its working springs as the tool sinks deeper. The use of a vibro-mechanism in such a device is necessitated by the need to ensure minimum time expenditures during auxiliary operations to accelerate the installation rate of casing pipes not only in soft water-saturated soils, but also in clay interlayer soils encountered along the driving path.

This same unit can also be used for the vibration extraction of pipes from wells after completing freezing operations. The OIT-1 unit is a self-propelled unit on a crawler chassis with mechanism drives powered by the construction site’s electrical network. The unit (Fig. 80) includes a tubular mast, four winches, a device for heating freezing columns during their extraction, pipe clamps, a pipe breaker, and containers for freezing brine. The main operating mechanism of the unit is the BBS-1 spring-loaded vibro-hammer designed by VNIIGS.

The shift production capacity of the unit includes: driving four column strings with a diameter of 114 mm to a depth of 25 m; extracting five freezing columns from a depth of up to 30 m. Operation of the OIT-1 unit has shown that vibration impact allows strings to be extracted from freezing wells in a number of cases without thawing them out, thereby significantly speeding up the construction work.

Figure 79e shows the hydro-vibration method for constructing wells up to 25 m deep, intended for construction dewatering. With this method, the immersion of the casing pipe, which is closed at the bottom by a hydro-nozzle, is carried out by a VPP-2 type vibro-sinker (B-401).

The destruction of the soil and its removal from the well is carried out by a high-pressure water jet supplied through the nozzle to the bottom of the well. To intensify the lifting of sludge to the surface, aeration can be performed using an upward flow of air forced into a ring sprayer located above the hydro-nozzle. Constructing a hydro-vibrated dewatering well with a diameter of 400–600 mm and a depth of 25 m, including filter installation and gravel packing, takes one to two shifts. Vibrating the pipe allows for effectively overcoming clay interlayers and weak sandstones encountered along the well-driving path.

When conducting blasting operations to place explosives in the ground, wells are drilled, pits are dug, and shafts and charge chambers are set up. Various drilling methods are used to construct wells: impact-rope, rotary, and thermal. Experience has been gained in using vibration and vibration-rotary methods for drilling blast holes, carried out according to the scheme in Fig. 79, using a specialized VVB-25A unit. This unit allows for drilling wells in hard rocks with a diameter of 146 mm to a depth of 25 m using conventional roller cone bits. The vibro-rotary units operate at an impact frequency of 25 Hz, while the drilling tool rotation speed is 55 rpm, and the flushing fluid flow rate is 180 l/min.

When drilling blast holes using the vibration-rotary method, penetration rates equal to the drilling speed of similar rock types by conventional rotary actions are achieved. The effect of applying vibration consists of the fact that an identical penetration speed is provided by equipment that weighs 3–4 times less than conventional rotary units. Another important result obtained by vibration-rotary drilling is the increased wear resistance of the bits.

Increasing the efficiency of constructing pits during blasting operations in cohesive, stable soils can be achieved by using a vibro-grab with longitudinal-rotary action, the operational technology of which does not differ from the technology of driving wells using a vibro-grab in unstable soils. Vibro-grabs can also be effectively used for driving various technical wells with a diameter of 500 mm or more, up to a depth of 30 m.