SW—Soil and Water: Construction of an Artificial Perched Watertable, Part 2: Soil Cavity Production by Air pressure

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<ul><li><p>Biosystems Engineering (2002) 82 (1), 107114w</p><p>a</p><p>;</p><p>h</p><p>cdiameter of 3m and a height of 03m, at a depth of about 08m from the soil surface in the elds, it was</p><p>envisaged that the runo caused in the summer could preferably be held in this layer and, hence, the excessmoisture loss could be prevented. The water in the articial soil cavity could be used as capillary water forplants germinating in the spring. The soil failure experiments by the injection of high-pressure air wereconducted to form the articial perched watertable. In this paper, the size of the underground soil cavityproduced by the high-pressure air was determined. The gas energy and gas pressure required for the propersize of the soil cavity were also evaluated.The results show that the energy required for soil-cavity production signicantly increased with the larger</p><p>radius of the soil cavity, the energy of the soil-cavity production being 100 kJ for a cavity radius of 1mcompared with 1000 kJ for a cavity radius of 2m. When the design point of the soil-cavity radius is 15m (thediameter of the soil cavity is 30m), the required energy is about 300 kJ. The height of the underground soilcavity produced by the high-pressure air injection was always 03m regardless of the working depth of theinjector. The radius of the soil cavity produced was 15m on average when the working depth of the injectorwas more than 06m. When a soil cavity is produced at a depth of 08m in the eld of the meadow soil orplanosol in China, the tensile strength of 20 kPa is more than the earth pressure of 14 kPa and hence, the airpressure in the charge tank should be at least more than 20 kPa. # 2002 Silsoe Research Institute. Published by ElsevierScience Ltd. All rights reserved</p><p>1. Introduction</p><p>The Three-river Plain of the Black Dragon provinceof the Peoples Republic of China near the border withRussia is a huge delta produced by three big rivers: theAmur, Wusuri and Pine Flower rivers. This plain is at,wide and one of most important grain-growing areas inthe world. Soils in the Three-river Plain consists of402% meadow soil and 254% planosol, these two soilsoccupying a major portion of the total available soils.</p><p>July and August, and there is almost no rainfall in thewinter and spring seasons. Plants often suer due toexcess moisture during the growing season in the summerand due to drought during the sowing season in thespring. If heavy rain occurs in the summer season, a partof the runo ows over the soil surface and gathers atthe lowest place in the eld. Hence, the lowest placebecomes a pond during every rainfall and the plantsthere cannot avoid excess moisture in the summer.By constructing an articial perched watertable whichdoi:10.1006/bioe.2001.0041, available online at http://wwSW}Soil and Water</p><p>Construction of an Articial Perched Wby Air p</p><p>K. Araya</p><p>Environmental Science Laboratory, Senshu Ue-mail of corresponding aut</p><p>(Received 21 January 2001; accept</p><p>By constructing an articial perched watertable whiBoth soils produce poor crop yields. The meadow soil isclay in all horizons and impermeable. The planosol hasan Aw horizon at a depth of 200mm, that is extremelyhard and quite impermeable.The annual precipitation in the Three-river Plain is</p><p>only 500600mm and, besides, the annual distribution ofrainfall is uneven; 6070% of the precipitation occurs in</p><p>1537-5110/02/$35.00 10.idealibrary.com on</p><p>tertable, Part 2: Soil Cavity Productionressure</p><p>G. Guo</p><p>niversity, Bibai, Hokkaido 079-0197, Japan;or: araya@senshu-hc.ac.jp</p><p>ed in revised form 8 January 2002)</p><p>h is a horizontal and cylindrical soil cavity with ais a horizontal and cylindrical soil cavity with a diameterof 3m and a height of 03m, at a depth of about08m from the soil surface in the meadow soil andplanosol elds, it was envisaged that the runo caused inthe summer could preferably be held in this soil cavityand hence, the excess moisture loss could be prevented.The water in the soil cavity could be used as capillary</p><p>7 # 2002 Silsoe Research Institute. Published byElsevier Science Ltd. All rights reserved</p></li><li><p>paper (Araya &amp; Guo, 2002). The sand valve on the sandthe machine on the soil surface injecting sand. Hence,the horizontal soil cavity and the vertical soil pillar lled</p><p>Fig. 1. Schematic diagram of a machine which constructs anarticial perched watertable</p><p>K. ARAYA; G. GUO108tank is then closed at this time. When a suitable soilcavity is obtained, the air valve is closed and the high-pressure air is charged again into the charge tank. Next,the sand valve on the sand tank is opened and sand ischarged into the nozzle of the injector. The air valve isthen opened again, and the sand at the nozzle is blowno by the high-pressure air and lls the soil cavity. Byslowly turning the injector, the entire soil cavity could belled by sand. When the cavity is perfectly lled with thesand, the three-point linkage is operated slowly to raise</p><p>with sand could be obtained.The excess runo occurring in the summer season</p><p>could be held in the underground vertical and horizontalsand spaces.</p><p>3. Analysis of soil-cavity production</p><p>In Fig. 2, in order to rupture the soil layer with thesoil depth of h, the air pressure pg in the charge tankwater for plants germinating in the spring. For thispurpose, a machine to construct the articial perchedwatertable was envisaged.The meadow soil and planosol have a natural ground</p><p>water level below more than 10m (Zhao et al., 1992), somaking an articial perched watertable at a depth of lessthan 1m below the soil surface would not cause anywater contamination.In this paper, the size of the underground soil cavity</p><p>produced by the high-pressure air was determined. Thegas energy and gas pressure required for the proper sizeof the soil cavity were also evaluated.</p><p>2. Principle of construction of an articial perchedwatertable</p><p>Figure 1 shows a schematic diagram of the envisagedmachine which can produce an articial perched water-table suitable for attaching to a tractor by the three-point linkage. The vibrator operates and the injectorpenetrates to a depth of about 08m. At that level, thereis a Cg1 horizon in the meadow soil elds (Zhang &amp;Araya, 2001) and B horizon in the planosol elds (Arayaet al., 1996). Both horizons are clay and quite imperme-able.Next, the air valve is closed and the air compressor is</p><p>operated and high-pressure air is charged into thecharge tank. The air valve is then quickly opened, anda large, horizontal and cylindrical soil cavity (crack) asshown in Fig. 2 would be produced by the static airpressure if the soil has a poor air permeability of lessthan 101m2 s1MPa1 as described in Part 1 of this</p><p>Nota</p><p>E energy for production of soil cavity, JEg gas energy in charge tank, JE1 energy for upheaval, JE2 energy for tensile failure, JE3 energy for soil expansion, Jg acceleration due to gravity, m s2</p><p>h soil working depth, mtion</p><p>hc height of soil cavity produced, mpinf produced pressure, Papg gas pressure in charge tank, Parc Radius of soil cavity produced, my soil water content, % d.b.rs Soil bulk density, kgm</p><p>3</p><p>st Tensile strength of soil, Pa</p></li><li><p>(expanding energy E3). The solution of Eqn (3) is</p><p>E prsghhcr2c psthcr</p><p>2c 4p</p><p>2hsthcrc 4</p><p>When the proper radius rc and the height hc of the soilcavity are given, the energy required for the soil-cavityproduction can be predicted by Eqn (4). The gas energyin the charge tank Eg in Fig. 1 should be greater than Ein Eqn (4).</p><p>4. Experimental details</p><p>Laboratory tests were conducted at an indoorexperimental eld in Japan. The soil of the experimentaleld was pseudogley soil. The pseudogley soil was thetypical Japanese heavy clay, in place of the meadow soilor planosol, because it was not feasible to transport asucient amount of soil from China. The soil watercontent was controlled at about 30% d.b., slightly lessthan the liquid limit. The pseudogley soil with such a soilwater content had a horizontal and cylindrical soil</p><p>ARTIFICIAL PERCHED WATERTABLE 109should be greater than the earth pressure, namely</p><p>pg &gt; rsgh 1</p><p>where rs is the bulk density of the soil and g is theaccerelation due to gravity.The impermeable underground soil failed by ruptur-</p><p>ing due to injection of the high-pressure air (Araya &amp;Kawanishi, 1984) producing a tensile stress in the soil atthe top of the cavity. Hence, the air pressure pg in thecharge tank should be greater than the tensile strengthof the soil st and is</p><p>pg &gt; st 2</p><p>The charge tank should have at least the pressure pgwhich satises Eqns (1) and (2).Next, when a soil cavity is produced as shown in</p><p>Fig. 2. Schematic diagram of horizontal soil cavity formed; h,soil working depth; hc, height of soil cavity formed; rc, radius of</p><p>soil cavity formedFig. 2, the energy required for the soil-cavity productionE is obtained as</p><p>E Z hc0</p><p>Z rc0</p><p>2prcrsgh drc dhc Z hc0</p><p>Z rc0</p><p>2prchcst drc dhc</p><p>Z 2prchc2prc</p><p>2prchst drc 3</p><p>where rc is the radius of the soil cavity, and hc is theheight of the soil cavity produced.The rst term in Eqn (3) is the energy required for</p><p>heaving the cylindrical soil with a radius of rc and adepth of h up to a height of hc (heaving energy E1). Thesecond term is the energy required for a production of acavity with a radius of rc and a height of hc against thetensile strength of the soil st (energy of tensile failureE2). The third term is the energy required for expandingthe soil with a circle of 2prc and a depth of h against thetensile strength of the soil st up to a circle of 2 p(rc+hc)cavity produced by the high-pressure air injection asdescribed in Part 1 of this paper (Araya &amp; Guo, 2002).The eld was perfectly dug at a depth of 1m with a</p><p>radius of 25m using a backhoe. The soil water contentof the dug soil was adjusted to about 30% d.b. andreplaced. The soil surface was compressed by a rollerand the experimental eld with uniform soil penetrationresistance was prepared.In Fig. 3, air was charged into the charge tank by an</p><p>electric air compressor. The injector in Fig. 4 with a</p><p>Fig. 3. Schematic diagram for producing an articial perchedwatertable</p></li><li><p>t the tip and a pressure sensor in the middle</p><p>K. ARAYA; G. GUO11015m length was pushed into the soil. The workingdepth of the injector was varied over four ranges: 04,06, 08 and 10m. The injector had a nozzle with a50mm hole at the tip of the injector from whichthe high-pressure air was injected into the undergroundlayer. The pressure in the charge tank was variedover six ranges: 100, 200, 400, 600, 800 and 1000 kPa.When the air valve was opened, the operating pressurepinf to produce the soil cavity was monitored at apressure sensor on the injector tube. The radius of thesoil cavity produced rc was determined by setting pegs atsix points along the circular upheaval at the soil surface</p><p>Fig. 4. Experimental injector, has a nozzle aaround the injector formed by the air injection. A scalewas set on the head of the injector before the airinjection and then, when the injector was heaved up bythe air injection, the height of the soil cavity hc wasdetermined.Table 1 shows the mechanical properties of three soils</p><p>which were determined in previous reports (Zhang et al.,2001; Jia et al., 1998). The soil water content of thesubsoils in the meadow soil and planosol elds wasabout 30% d.b., the bulk density was about 1500 kgm3</p><p>and the tensile strength was 20 kPa.</p><p>Table 1Mechanical properties of soils</p><p>Soil Soil watercontent</p><p>Bulk densityrs, kgm</p><p>3Tensilestrength</p><p>y, % d.b. st, kPa</p><p>Pseudogley 30 (soil bin) 1380 10Meadow (Cg1) 30 (eld) 1590 20Planosol (B) 28 (eld) 1530 205. Results and discussion</p><p>5.1. Energy required for soil-cavity production</p><p>Figure 5 shows the predicted energy for the soil-cavityproduction using Eqn (4). In Table 1, the soilbulk density rs for an average value of 1500 kgm</p><p>3</p><p>and a tensile strength st of 20 kPa of the Cg1 horizonof the meadow soil and the B horizon of the planosolwere used for the calculations. The working depth hwas 08m and the height of the soil cavity hcwas 03m.Fig. 5. Energy required for production of soil cavity as afunction of radius of soil cavity; E1, energy for upheaving; E2,energy for tensile failure; E3, energy for soil expansion; E, sumof E1, E2 and E3; soil bulk density rs=1500 kgm</p><p>3; workingdepth h=08m; soil tensile strength st=20 kPa; height of soil</p><p>cavity hc=03m</p></li><li><p>In Fig. 5, the energy for the soil expansion E3 is thelargest, followed by the energy for the tensile failure E2and the energy for upheaving E1. The total energy E ofthese energies signicantly increased with the largerradius of the soil cavity that produced rc. Namely, theenergy of the soil-cavity production E is 100 kJ for asoil-cavity radius of 1m but the value of E becomes1000 kJ for a soil-cavity radius of 2m. When the designpoint of the soil-cavity radius rc is 15m (the diameter ofthe soil cavity is 3.0m), the required energy E is about300 kJ. The maximum pressure produced by thecompressor in Fig. 3 was 15MPa and hence, the sizeof the charge tank in Fig. 3 should be 02m3. A chargetank with a radius of 550mm and the length of 860mmwas designed and built by rolling a steel plate with the</p><p>thickness 6mm. A steel pipe with the diameter 70mmwas welded to the charge tank as a central pillar totolerate the high pressure.</p><p>5.2. The radius and height of the produced soil cavity</p><p>Two examples of soil failure caused by the injection ofhigh-pressure air in the experimental eld are shown inFig. 6. The pressure in the charge tank pg was 1MPa.Figure 6(a) was when the working depth h was 04m andan eruption of the soil surface occurred because theworking depth of the injector was shallow. When theworking depth of the injector was more than 06m, theeruption of the soil surface did not occur but a</p><p>id</p><p>ARTIFICIAL PERCHED WATERTABLE 111Fig. 6. States of soil surface; (a) working depth h was 04m, sowas circularly heavel surface has erupted; (b) working depth h was 08m, soil surfaceup but not erupted</p></li><li><p>horizontal circular cavity around the injector wasformed underground and the soil surface heaved as</p><p>of the injector was varied. The height of the soil cavitywas always 03m regardless of the working depth. Theradius of the soil cavity was 15m on average when theworking depth was more than 06m. When the workingdepth was 04m, the radius of the soil cavity was about10m because the eruption of the soil surface took placeand the gas energy did not work well in the underground.Figure 8 shows the measured radius and height of</p><p>the soil cavity produced when the pressure pg in thecharge tank, namely, the gas energy Eg was varied.Figure 8 corresponds to the measured values ofFig. 5. When the air pressure in the charge tank wasless than 200 kPa, a soil cavity was not formed under-ground and hence, there was no change in the soilsurface. Here, the cavity radius and cavity height wereall zero. When the air pressure was more than 400 kPa, asoil cavity was formed. The cavity height hc increased inproportion to the gas pressure. The cavity radius alsoincreased with the greater gas pressure but the increas-ing rate was small. This is because a large increase in thecavity radius cannot be obtai...</p></li></ul>