Biosystems Engineering (2002) 82 (1), 107–114doi:10.1006/bioe.2001.0041, available online at http://www.idealibrary.com onSW}Soil and Water
Construction of an Artificial Perched Watertable, Part 2: Soil Cavity Productionby Air pressure
K. Araya; G. Guo
Environmental Science Laboratory, Senshu University, Bibai, Hokkaido 079-0197, Japan;e-mail of corresponding author: [email protected]
(Received 21 January 2001; accepted in revised form 8 January 2002)
By constructing an artificial perched watertable which is a horizontal and cylindrical soil cavity with adiameter of 3m and a height of 0�3m, at a depth of about 0�8m from the soil surface in the fields, it wasenvisaged that the runoff caused in the summer could preferably be held in this layer and, hence, the excessmoisture loss could be prevented. The water in the artificial 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 artificial 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 significantly increased with the larger
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 1�5m (thediameter of the soil cavity is 3�0m), the required energy is about 300 kJ. The height of the underground soilcavity produced by the high-pressure air injection was always 0�3m regardless of the working depth of theinjector. The radius of the soil cavity produced was 1�5m on average when the working depth of the injectorwas more than 0�6m. When a soil cavity is produced at a depth of 0�8m in the field 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
1. Introduction
The Three-river Plain of the Black Dragon provinceof the People’s 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 flat,wide and one of most important grain-growing areas inthe world. Soils in the Three-river Plain consists of40�2% meadow soil and 25�4% planosol, these two soilsoccupying a major portion of the total available soils.Both 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
only 500–600mm and, besides, the annual distribution ofrainfall is uneven; 60–70% of the precipitation occurs in
1537-5110/02/$35.00 10
July and August, and there is almost no rainfall in thewinter and spring seasons. Plants often suffer 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 runoff flows over the soil surface and gathers atthe lowest place in the field. Hence, the lowest placebecomes a pond during every rainfall and the plantsthere cannot avoid excess moisture in the summer.By constructing an artificial perched watertable which
is a horizontal and cylindrical soil cavity with a diameterof 3m and a height of 0�3m, at a depth of about0�8m from the soil surface in the meadow soil andplanosol fields, it was envisaged that the runoff 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
7 # 2002 Silsoe Research Institute. Published by
Elsevier Science Ltd. All rights reserved
K. ARAYA; G. GUO108
Notation
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 s�2
h soil working depth, m
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�3
st Tensile strength of soil, Pa
Fig. 1. Schematic diagram of a machine which constructs anartificial perched watertable
water for plants germinating in the spring. For thispurpose, a machine to construct the artificial perchedwatertable was envisaged.The meadow soil and planosol have a natural ground
water level below more than 10m (Zhao et al., 1992), somaking an artificial 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
produced by the high-pressure air was determined. Thegas energy and gas pressure required for the proper sizeof the soil cavity were also evaluated.
2. Principle of construction of an artificial perchedwatertable
Figure 1 shows a schematic diagram of the envisagedmachine which can produce an artificial 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 0�8m. At that level, thereis a Cg1 horizon in the meadow soil fields (Zhang &Araya, 2001) and B horizon in the planosol fields (Arayaet al., 1996). Both horizons are clay and quite imperme-able.Next, the air valve is closed and the air compressor is
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 10�1m2 s�1MPa�1 as described in Part 1 of thispaper (Araya & Guo, 2002). The sand valve on the sandtank 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 blownoff by the high-pressure air and fills the soil cavity. Byslowly turning the injector, the entire soil cavity could befilled by sand. When the cavity is perfectly filled with thesand, the three-point linkage is operated slowly to raise
the machine on the soil surface injecting sand. Hence,the horizontal soil cavity and the vertical soil pillar filledwith sand could be obtained.The excess runoff occurring in the summer season
could be held in the underground vertical and horizontalsand spaces.
3. Analysis of soil-cavity production
In Fig. 2, in order to rupture the soil layer with thesoil depth of h, the air pressure pg in the charge tank
Fig. 2. Schematic diagram of horizontal soil cavity formed; h,soil working depth; hc, height of soil cavity formed; rc, radius of
soil cavity formed
Fig. 3. Schematic diagram for producing an artificial perchedwatertable
ARTIFICIAL PERCHED WATERTABLE 109
should be greater than the earth pressure, namely
pg > rsgh ð1Þ
where rs is the bulk density of the soil and g is theaccerelation due to gravity.The impermeable underground soil failed by ruptur-
ing due to injection of the high-pressure air (Araya &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
pg > st ð2Þ
The charge tank should have at least the pressure pgwhich satisfies Eqns (1) and (2).Next, when a soil cavity is produced as shown in
Fig. 2, the energy required for the soil-cavity productionE is obtained as
E ¼Z hc
0
Z rc
0
2prcrsgh drc dhc þZ hc
0
Z rc
0
2prchcst drc dhc
þZ 2pðrcþhcÞ
2prc2prchst drc ð3Þ
where rc is the radius of the soil cavity, and hc is theheight of the soil cavity produced.The first term in Eqn (3) is the energy required for
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)
(expanding energy E3). The solution of Eqn (3) is
E ¼ prsghhcr2c þ psthcr2c þ 4p
2hsthcrc ð4Þ
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).
4. Experimental details
Laboratory tests were conducted at an indoorexperimental field in Japan. The soil of the experimentalfield 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 asufficient 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 soilcavity produced by the high-pressure air injection asdescribed in Part 1 of this paper (Araya & Guo, 2002).The field was perfectly dug at a depth of 1m with a
radius of 2�5m 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 field with uniform soil penetrationresistance was prepared.In Fig. 3, air was charged into the charge tank by an
electric air compressor. The injector in Fig. 4 with a
Fig. 4. Experimental injector, has a nozzle at the tip and a pressure sensor in the middle
K. ARAYA; G. GUO110
1�5m length was pushed into the soil. The workingdepth of the injector was varied over four ranges: 0�4,0�6, 0�8 and 1�0m. 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 surfacearound 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
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 fields wasabout 30% d.b., the bulk density was about 1500 kgm�3
and the tensile strength was 20 kPa.
Table 1
Mechanical properties of soils
Soil Soil watercontent
Bulk densityrs, kgm
�3Tensilestrength
y, % d.b. st, kPa
Pseudogley 30 (soil bin) 1380 10Meadow (Cg1) 30 (field) 1590 20Planosol (B) 28 (field) 1530 20
5. Results and discussion
5.1. Energy required for soil-cavity production
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
�3
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 0�8m and the height of the soil cavity hcwas 0�3m.
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�3; workingdepth h=0�8m; soil tensile strength st=20 kPa; height of soil
cavity hc=0�3m
ARTIFICIAL PERCHED WATERTABLE 111
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 significantly 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 1�5m (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 1�5MPa and hence, the sizeof the charge tank in Fig. 3 should be 0�2m3. A chargetank with a radius of 550mm and the length of 860mmwas designed and built by rolling a steel plate with the
Fig. 6. States of soil surface; (a) working depth h was 0�4m, soiwas circularly heaved
thickness 6mm. A steel pipe with the diameter 70mmwas welded to the charge tank as a central pillar totolerate the high pressure.
5.2. The radius and height of the produced soil cavity
Two examples of soil failure caused by the injection ofhigh-pressure air in the experimental field are shown inFig. 6. The pressure in the charge tank pg was 1MPa.Figure 6(a) was when the working depth h was 0�4m andan eruption of the soil surface occurred because theworking depth of the injector was shallow. When theworking depth of the injector was more than 0�6m, theeruption of the soil surface did not occur but a
l surface has erupted; (b) working depth h was 0�8m, soil surfaceup but not erupted
Fig. 7. Measured cavity radius and cavity height produced as afunction of working depth of injector; pseudogley soil; gaspressure in charge tank pg=1MPa; soil water content y=30%
d.b.; , cavity radius rc; , cavity height hc
K. ARAYA; G. GUO112
horizontal circular cavity around the injector wasformed underground and the soil surface heaved asshown in Fig. 6(b).
Figure 7 shows the measured radius rc and the heighthc of the soil cavity produced when the working depth h
Fig. 8. Measured soil-cavity radius and soil-cavity heightproduced as a function of gas pressure or gas energy in chargetank; soil water content y=30%d.b.; working depth of injector
h=0�8m; , cavity radius rc; , cavity height hc
of the injector was varied. The height of the soil cavitywas always 0�3m regardless of the working depth. Theradius of the soil cavity was 1�5m on average when theworking depth was more than 0�6m. When the workingdepth was 0�4m, the radius of the soil cavity was about1�0m 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 ofthe 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 obtained by an increase in theenergy of only 100–200 kJ as shown in Fig. 5. In Fig. 8,when the air pressure was 1MPa, namely, the gas energy
Fig. 9. Predicted earth pressure, tensile strength of soils andmeasured pressure produced as a function of soil working depthof injector; gas pressure in charge tank pg=1MPa: , measuredpressure pinf; earth pressure; , tensile strength st ofmeadow soil and planosol; , tensile strength st of pseudogley
soil
ARTIFICIAL PERCHED WATERTABLE 113
was 200 kJ, a 1�5m cavity radius was obtained. In Fig. 5,a gas energy of 300 kJ was required for a the cavityradius of 1�5m. This is because of the prediction inFig. 5, a tensile strength st of 20 kPa for the Chinesesoils was used for the calculation. However, in the fieldexperiments in Fig. 8, the pseudogley soil in Japan inwhich a tensile strength of 10 kPa was used and hence, acavity radius of 1�5m could be obtained using a smallergas energy.When the soil-cavity radius 1�5m and the soil-cavity
height 0�3m were obtained, the volume of the soilcavity is 2�12m3 and the volume of sand required toconstruct the artificial perched watertable is about2�0m3 per unit. About 150mm of water could be newlystored in this soil cavity because the porosity of the sandis about 0�5.
5.3. The pressure required for soil cavity
Figure 9 shows the predicted earth pressure of Eqn (1)using the values in Table 1. The soil bulk density rs1500 kgm�3 which is the average value in Table 1 wasused. The earth pressure at a depth of 0�8m is about14kPa. Figure 9 also shows the tensile strength of themeadow soil, planosol and pseudogley soil in Table 1. InFig. 9, when a soil cavity is produced at a depth of 0�8min the field of the meadow soil or planosol in China, atensile strength of 20 kPa [Eqn (2)] is more than the earthpressure of 14 kPa [Eqn (1)] and hence, the air pressure inthe charge tank should be at least more than 20 kPa.
Figure 9 also shows the pressure pinf measured by thepressure sensor mounted on the injector. The measured
Fig. 10. Profiles of soil penetration resistance, MPa (cone peneoperation; (b) after operation; working depth wa
pressure pinf was always 30–40 kPa regardless of theworking depth. This is because the tensile strength inEqn (2) is more effective than the earth pressure of Eqn(1) within the variation of the working depth from 0�4 to1�0m. The measured pressure pinf was slightly largerthan a tensile strength of 10 kPa for the pseudogley soil.This is because the measured pressure also contained thefriction resistance of the nozzle.
5.4. Soil penetration resistance
Figure 10 shows the typical soil penetration resistancemeasured by a cone penetrometer (308 cone angle, 16mmbase diameter). Figure 10(a) is that before the operation.Here, the soil penetration resistance was 0–2MPa up to adepth of 1m and a uniform field was obtained.
Figure 10(b) shows that after the operation in whichthe working depth of the injector was 0�8m. The airpressure in the charge tank pg was 1MPa. Figure 10(b)corresponds to Fig. 6(b). A soil cavity was formed at adepth of 0�65–0�85m. The cavity height hc was about0�2m which was smaller than 0�3m in Fig. 7. This isbecause a value of 0�2m was measured when theair injection had already stopped after the cavityproduction. The next goal is how to fill the formed cavitywith sand.
6. Conclusions
(1) The energy required for soil-cavity productionsignificantly increased with the larger radius of the
trometer, 308cone angle and 16mm base diameter); (a) befores 0�8m. , 0–1; , 1–2; , 2–3; , 3–4; , 4–5.
K. ARAYA; G. GUO114
soil cavity that is produced. Namely, the energy ofthe soil-cavity production is 100 kJ for soil-cavityradius of 1m but the value of the energy becomes1000 kJ for soil-cavity radius of 2m. When thedesign point of the soil-cavity radius is 1�5m (thediameter of the soil cavity is 3�0m), the requiredenergy is about 300 kJ.
(2) The height of the underground soil cavity pro-duced by the high-pressure air injection, was always0�3m regardless of the working depth of theinjector.
(3) The radius of the soil cavity produced was 1�5m onaverage when the working depth of the injector wasmore than 0�6m.
(4) When a soil cavity is produced at a depth of 0�8min the field of the meadow soil or planosol in China,a tensile strength of 20 kPa is more than theearth pressure 14 kPa and hence, the air pressurein the charge tank should be at least more than20 kPa.
(5) The measured produced pressure was always 30–40 kPa regardless of the working depth of theinjector.
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