Modern Methods and Advances in Structural Engineering and Construction Yuschube, S. V., Samarin, D. G. (eds.) ISEC-6, Zürich, June 21–26, 2011 BORED PILES PRODUCED BY THE PULSED DISCHARGE TECHNOLOGY S.V. YUSCHUBE1 and D.G. SAMARIN2 1 Tomsk state university of architecture and civil engineering, Tomsk, Russia. E-mail: [email protected] 2 Tomsk state university of architecture and civil engineering, Tomsk, Russia. E-mail: [email protected] Application of pulsed discharge technology at the device of bored piles substantially allows to promote their bearing strength. It is connected with the forming of widening of barrel of bored pile and with the compression of near pile soil. Such widening of pile barrel is created due to hydraulic surges produced with electric discharges in material of pile barrel (concrete mixture) at its device. However, forming of widening of barrel of bored pile is a rather complex task. The cause is in briefness of hydraulic surges activity. Influence of energy parameters of electric discharge (tension U, kv, capacity of condenser C, mcF) is experimentally set on duration of mechanical action of hydraulic surges. Keywords: Bored pile, electric discharge technology, hydraulic surges. 1 Introduction Over recent years in Russian foundation engineering practice, bored piles, arranged with the use of pulsed discharge technology (pilesPDT) have become wide-spread (Evdokimov et al. 1991, Buhov 1997, Baholdin et al. 1998, Eremin et al. 2006). The researches, aimed at development of the method of device of bored piles with the use of pulsed discharge technology, are conducted at the Tomsk State University of Architecture and Civil Engineering. Essence of this method consists in the following: formed electric discharges (further el. discharges) in material of pile barrel provide occurrence of hydraulic surges. Such surges are able to accomplish mechanical work and due to it widening of the pile body, compression of near pile soil and, consequently, increase of bearing strength of pile happen. However, creation of electric discharges, able to be transformed in hydraulic surges with the identically effective mechanical operating on different soils, is a rather complex task. One of the determining factors is time of hydraulic 1 surges action (0,00002-0,00005 sec.), which appears not enough for any meaningful widening of barrel of pile, e.g., in slightly wet sandy or clay soils (Dzhantimirov C.А. et al. 2003, 2004). The object of this work was revealing possibility of increase of action duration of hydraulic surges, produced in material of pile barrel. Analysis of the researches results of hydraulic surges, produced with el. discharges in liquids, shows that time of hydraulic surges activity is largely determined by the energy parameters of el. charge such as tension (U) and capacity condenser (С). Varying of their correlation can increase duration of hydraulic surge’s action. It is known, that depending on correlation of tension and capacity of condenser it is possible to form two types of el. discharge (leader and thermal) in liquid. 2 Experimental researches At the thermal type (Fig.1, a) of el. discharge, its forming takes place in gas cover which appears due to heating of liquid. In this case Bored piles produced by the pulsed discharge technology Yuschube, S. V., Samarin, D. G. considerable part of energy of el. charge is expended on formation of gas cover and only its small part will be transformed in mechanical one. Severe losses of energy of el. charge result in reduction of time of hydraulic surges action. a. b. Figure 1. a–The thermal type of el. discharge; b– The leader type of el. discharge. 1 - electrodes; 2 –gas cover; 3 – electric discharge. At the leader type (Fig. 1, b) of el. discharge, its forming takes place in liquid itself. In this case considerable part of energy of el. discharge will be transformed in mechanical one. Transition from thermal type of el. discharge to leader one is possible due to level raising of tension. However, safety norms of piles- PDT production and durability of the applied equipment place restrictions on using very high tensions. By the carried out experimental researches (Samarin 2005) it is specified that minimum level of tension at which the leader type of el. discharge is yet saved, is within the limits of 30 kv. It provides possible level of safety norms of piles-PDT production 2 and reliability of equipment. Reduce of voltage less than 30 kv generates thermal type of el. discharge. The authors decided to analyze influence of energy W change of el. charge on time t of hydraulic surges activity at given level of tension - U=30kv. The researches were conducted on the experiment stand (Fig.2) that consists of working chamber (1 - 150150200 mm), filled by concrete mixture, with built-in electrodes (2). The carcass of chamber is fastened to the bed (3). Appearance of el. discharges in the chamber results in occurrence of hydraulic surges. The pressure, created here, is passed through a hard stamp (4) on a pressure monitor (5). The signal from a pressure monitor is passed on the receiving-measuring system (9). As a source of charges, the generator of impulsive currents was used (10). Creation of el. charges with set tension of U=30 kv took place at the different values of capacity of condenser C = 4, 6 and 10 mcF. The carried out researches showed that at the leader type of el. charges, the growth of energy results in intensive increase action of hydraulic surges time. So, at the capacity of condenser C = 4 mcF (energy of el. charge W = 1,8 kJ) time of hydraulic surge activity is 0,002 sec (Fig. 3, a). The increase of capacity of condenser to 10mcF (energy of charge W =4,5 kJ) brings to the increase of duration of hydraulic surges to 0,006 sec. (Fig. 3, b). The experiments of Dzhantimirov and others (2003, 2004) on the device of bored piles with the use of explosives showed that meaningful widening of barrel of bored pile was attained at the time of action of pressure, t ≥0,001 sec. The analysis of our researches shows that the required time of action of hydraulic surge can be attained by the selection of energy parameters of el. discharge. Further experiments were aimed at analyzing the influence of fitted energy parameters of el. discharge on forming of widening of bored piles, carried out with the help pulsed discharge technology. Modern Methods and Advances in Structural Engineering and Construction Cheung, S. O., Yazdani, S., Ghafoori, N., and Singh, A. (eds.) ISEC-6, Zürich, June 21–26, 2011 The researches were carried out in the experimental module (Fig. 4). The methodology of realization: working chamber (1) with diameter of 800 mm, height of 900 mm, filled with slightly wet sandy soil. After installing the basic elements of the experimental module on soil, pressure q = 100 kPa has been passed. Then the barrel of bored pile (11) has been formed. bore which was entered into soil through the central hole (5) in the hard stamp (2); setting in the mining hole of discharger (6), linked to the generator of impulsive currents ; replacement into the mining hole of concrete mixture and its treatment by el. charges (12). Figure. 3. The oscillograms of pressure p at energy of discharge W: a - at W = 1,8 kJ; b - at W = 4,5 kJ Figure. 2. The chart of the experiment stand: 1 - working chamber; 2 - electrodes; 3 - bed; 4 - hard stamp; 5 - pressure monitor; 6 - plates; 7 - metallic rods; 8 - rubber diaphragm; 9 - receiving-measuring system; 10 - generator of impulsive currents. The process of this pile forming: mining hole boring with diameter of 110 mm by a 3 During process of the pile producing the gel-forming substance was added into the concrete mixture for limitation of desiccation. Slightly wet sands were used in researches of the following characteristics: density γ = 16.5 – 16.8 kN/ m3; density of soil mineral particles γ s = 26.5 – 26.6 kN/ m3; coefficient of porosity e = 0,76 - 0,79; natural humidity w = 9 - Bored piles produced by the pulsed discharge technology Yuschube, S. V., Samarin, D. G. 11%; coefficient of water saturation Sr = 0,3 0,35; internal friction angle of soil = 23 25°; specific cohesion C = 1-2 kPa; module of deformation of E = 15 - 18 MPa. parameters - U = 30 kv, C = 10 mcf, result in substantial widening of barrel of bored pile. The diameter of the extended part of pile barrel exceeds her primary one practically twice. The primary diameter of the pile (D = 110 mm) arised to 200 mm after the series of hydraulic surges. It is known, that discharge energy for a pile expansion by hydraulic surges in a concrete mixture is equal 100 kJ. The experiment shows that revealing possibility of increasing duration of hydraulic surges allows to improve effectives of the pulsed discharge technology using by producing the bored piles in different soils. In this case discharge energy for the pile expansion is decreasing considerably. References Figure. 4. The chart of the experimental module: 1 - working chamber; 2 - hard stamp; 3 - bed; 4 – ribs; 5 - central opening; 6 - discharger; 7 - rubber plate; 8 - pressure, transferred from a compressor; 9 – rubber chamber; 10 - manometer; 11 - barrel of bored pile; 12 - el. discharge; 13 formed widening of pile. Figure. 5. Dependence of diameter of pile D on the amount N of hydraulic surges : U = 30 kv, C = 10 mcf (W = 4,5 kJ); U is tension; C is a capacity of condenser; W is energy of discharge. 3 Conclusions The results of our researches are presented on Fig. 5. It is defined, that hydraulic surges, at energy of el. charge W = 4,5 kJ and energy 4 Eremin V.Y., Budanov A.A., Deformation of sands by organization pile produced of the electric discharge technique, Vestnik VGSU, 1, 150163, May, 2006 (in Russian). Evdokimov A.L., Egorov V.I, Borisenkov V.I., Bored piles, made on pulsed discharge technology, Design and engineering researches, 2, 17 – 19, Sept., 1991 (in Russian). Baholdin B.V., Dzhantimirov C.A., New pulsed discharge technology in construction, Bases, foundations and mechanics of soils, 1998, 4, 5, 47 – 52, May, 1998 (in Russian). Buhov V.M., Pulsed discharge technology in construction, Transport construction, 10, 32 - 33, Jan, 1997 (in Russian). Samarin D.G. Making barrel of the bored piles with the use of electric discharges, Izvestiya vuzov. Construction, 3, 120-124, March, 2005 (in Russian). Dzhantimirov C.A., Kryuchkov, P.V., Geotechnical technologies on the basis of electrochemical explosion in Reconstruction of historical cities and geotechnical construction, Volume 2, 91 – 94, Publishers: АСВ St. Petersburg, 2003 (in Russian). Dzhantimirov C.А., Kryuchkov, P.V., Smirnov P. V., Geotechnical technologies on the basis of electrochemical explosion in Proceedings of the international conference, Volume 2, 104 – 109, Publishers: АСВ St. Petersburg, 2004 (in Russian).