sw—soil and water: construction of an artificial perched watertable, part 1: air permeability of...

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Biosystems Engineering (2002) 81 (3), 333–345 doi:10.1006/bioe.2001.0030, available online at http://www.idealibrary.com on SW}Soil and Water Construction of an Artificial Perched Watertable, Part 1: Air Permeability of Soils and Aspects of Soil Failure K. Araya; G. Guo Environmental Science Laboratory, Senshu University, Bibai, Hokkaido 079-0197, Japan (Received 21 January 2001; accepted in revised form 13 December 2001) The annual precipitation in the Three-river Plain of the People’s Republic of China is only 500–600 mm and, besides, the rainfall is uneven; 60–70% of the annual precipitation occurs in July and August and there is almost no rainfall in the winter and spring seasons. Experiments were conducted to form the artificial perched watertable where the runoff caused in summer could preferably be held, by injecting high-pressure air into the soil. A horizontal soil cavity is required, so this paper deals with the determination of the air permeability of soils and the difference in the soil failure mechanisms due to the different air permeabilities. The results show that the air permeability k of 10 m 2 s 1 MPa 1 defined the situation between the fluidization and the V-shaped soil failure. When the value of k was from 10 to 01m 2 s 1 MPa 1 , the V-shaped soil failure took place and when it was less than 01m 2 s 1 MPa 1 , the soil cavity production took place. In order to produce the soil cavity in the B horizon of the planosol or Cg1 horizon of the meadow soil by air injection, the value of k for these soils should be less than 01m 2 s 1 MPa 1 and the soil water content of these soils should be more than 30% d.b. # 2002 Silsoe Research Institute. Published by Elsevier Science Ltd. All rights reserved 1. Introduction The Three-river Plain of the Black Dragon province of the People’s Republic of China near the border with Russia is a huge delta produced by three big rivers: the Amur, Wusuri and Pine Flower rivers. This plain is flat, wide and one of the most important cereal-growing areas in the world. Soils in the Three-river Plain consist of 254% planosol in Fig. 1(a) and 402% meadow soil in Fig. 1(b) and hence, these two soils occupy a major portion of the total available soils. All horizons of the meadow soil are clays and impermeable. The author has developed a special plough, a three-stage soil layer mixing plough (Zhang & Araya, 2001a, 2001b; Zhang et al., 2001), for improvement of the meadow soil and the subsoil of 500 mm thickness with reasonable permeability could be obtained. The planosol has an Aw horizon, at a depth of 200 mm, that is extremely hard and quite impermeable. The author also developed a special three-stage subsoil mixing plough (Araya et al., 1996a, 1996b, 1996c; Jia et al., 1998a, 1998b, 1998c; Liu et al., 1998) for improvement of the planosol. By using this plough, the Aw horizon was mixed with the soft and clayey B horizon which is below the Aw horizon in a one to one ratio and the Aw horizon completely disappeared. Hence, 600 mm thick subsoil with reasonable perme- ability was obtained. The annual precipitation in the Three-river Plain is only 500–600 mm and, besides, the rainfall is uneven; 60–70% of the annual precipitation occurs in July and August and there is almost no rainfall in the winter and spring seasons. Plants often suffer due to excess moisture during the growing season in the summer and drought during the seeding season in the spring. If heavy rain occurs in the summer season, a part of the runoff flows on the soil surface and gathers at the lowest place in the field and a part of the runoff also gathers at the lowest place through the subsoil because the permeability of the subsoil was improved. The lowest place becomes a pond during every rainfall and the plants cannot avoid excess moisture in the summer season. By constructing an artificial perched watertable at about 800 mm depth from the soil surface in the meadow soil and planosol fields, it was envisaged that the runoff caused in the summer could preferably be held in this artificial watertable and hence, the excess moisture loss could be prevented. The water in the 1537-5110/02/$35.00/0 333 # 2002 Silsoe Research Institute. Published by Elsevier Science Ltd. All rights reserved

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Page 1: SW—Soil and Water: Construction of an Artificial Perched Watertable, Part 1: Air Permeability of Soils and Aspects of Soil Failure

Biosystems Engineering (2002) 81 (3), 333–345doi:10.1006/bioe.2001.0030, available online at http://www.idealibrary.com onSW}Soil and Water

Construction of an Artificial Perched Watertable, Part 1: Air Permeability of Soilsand Aspects of Soil Failure

K. Araya; G. Guo

Environmental Science Laboratory, Senshu University, Bibai, Hokkaido 079-0197, Japan

(Received 21 January 2001; accepted in revised form 13 December 2001)

The annual precipitation in the Three-river Plain of the People’s Republic of China is only 500–600mm and,besides, the rainfall is uneven; 60–70% of the annual precipitation occurs in July and August and there isalmost no rainfall in the winter and spring seasons. Experiments were conducted to form the artificial perchedwatertable where the runoff caused in summer could preferably be held, by injecting high-pressure air into thesoil. A horizontal soil cavity is required, so this paper deals with the determination of the air permeability ofsoils and the difference in the soil failure mechanisms due to the different air permeabilities.The results show that the air permeability k of 10m2 s�1MPa�1 defined the situation between the

fluidization and the V-shaped soil failure. When the value of k was from 10 to 0�1m2 s�1MPa�1, the V-shapedsoil failure took place and when it was less than 0�1m2 s�1MPa�1, the soil cavity production took place. Inorder to produce the soil cavity in the B horizon of the planosol or Cg1 horizon of the meadow soil by airinjection, the value of k for these soils should be less than 0�1m2 s�1MPa�1 and the soil water content of thesesoils should be more than 30% d.b. # 2002 Silsoe Research Institute. Published by Elsevier Science 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 the most important cereal-growingareas in the world. Soils in the Three-river Plain consistof 25�4% planosol in Fig. 1(a) and 40�2% meadow soilin Fig. 1(b) and hence, these two soils occupy a majorportion of the total available soils. All horizons of themeadow soil are clays and impermeable. The author hasdeveloped a special plough, a three-stage soil layermixing plough (Zhang & Araya, 2001a, 2001b; Zhanget al., 2001), for improvement of the meadow soil andthe subsoil of 500mm thickness with reasonablepermeability could be obtained.The planosol has an Aw horizon, at a depth of

200mm, that is extremely hard and quite impermeable.The author also developed a special three-stage subsoilmixing plough (Araya et al., 1996a, 1996b, 1996c; Jiaet al., 1998a, 1998b, 1998c; Liu et al., 1998) forimprovement of the planosol. By using this plough,the Aw horizon was mixed with the soft and clayey B

1537-5110/02/$35.00/0 33

horizon which is below the Aw horizon in a one to oneratio and the Aw horizon completely disappeared.Hence, 600mm thick subsoil with reasonable perme-ability was obtained.The annual precipitation in the Three-river Plain is

only 500–600mm and, besides, the rainfall is uneven;60–70% of the annual precipitation occurs in July andAugust and there is almost no rainfall in the winter andspring seasons. Plants often suffer due to excess moistureduring the growing season in the summer and droughtduring the seeding season in the spring. If heavy rainoccurs in the summer season, a part of the runoff flowson the soil surface and gathers at the lowest place in thefield and a part of the runoff also gathers at the lowestplace through the subsoil because the permeability of thesubsoil was improved. The lowest place becomes a pondduring every rainfall and the plants cannot avoid excessmoisture in the summer season.By constructing an artificial perched watertable at

about 800mm depth from the soil surface in themeadow soil and planosol fields, it was envisaged thatthe runoff caused in the summer could preferably beheld in this artificial watertable and hence, the excessmoisture loss could be prevented. The water in the

3 # 2002 Silsoe Research Institute. Published by

Elsevier Science Ltd. All rights reserved

Page 2: SW—Soil and Water: Construction of an Artificial Perched Watertable, Part 1: Air Permeability of Soils and Aspects of Soil Failure

K. ARAYA; G. GUO334

Notation

Ac sectional area of soil cell, m2

Av sectional area of venturi, m2

Ap sectional area of port, m2

d diameter of object, mGt air mass flow rate, kg s�1

k air permeability, m2 s�1MPa�1

l thickness of soil cell, mM total mass of soil in soil cell or soil sampler, kgMs mass of solid phase in soil cell or soil sampler, kgMw mass of liquid phase in soil cell or soil sampler,

kgp0 absolute atmospheric pressure, 1�013� 105 Pap1 gauge pressure at inlet of soil cell, Papf resistance gauge pressure caused by soil in soil

cell, Papm resistance gauge pressure caused by apparatus, Papp gauge pressure at port, PaDp differential pressure of venturi, PaR gas constant of air, 286�8Nmkg�1K�1

Re Reynolds numberRes Reynolds number modified for soilrp hydraulic radius, mSv surface area per unit volume of soil particles

with moisture (specific surface area), m�1

s degree of saturation

t air temperature, 8CV total volume of soil cell or soil sampler, m3

Vc volume of porosity phase in soil cell or soilsampler, m3

Vj actual volume in soil cell or soil sampler,m3

Vw volume of liquid phase in soil cell or soilsampler, m3

w air velocity when soil is not charged, m s�1

w0 actual air velocity among soil particles (verticaldirection), m s�1

wv air velocity at venturi, m s�1

e porosityea air porosityy soil water content, d.b.l friction factorm coefficient of viscosity, N sm�2

v kinematic coefficient of viscosity, m2 s�1

r air density, kgm�3

r0 atmospheric air density, kgm�3

r1 air density at inlet of soil cell, kgm�3

r2 air density at outlet of soil cell, kgm�3

rp air density at port, kgm�3

rs density of solid phase, kgm�3

c flow coefficient (0�98)

artificially permeable layer dam could be used for plantsin the sowing season of spring as capillary water. Forthis purpose, a machine to construct the artificialperched watertable 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 less than 1mbelow the soil surface would not cause any watercontamination.The depth of water that can be held in the existing soil

profiles of the meadow soil and planosol is about100mm because only the Ap horizon with 200mmthickness can hold water and the subsoil below the Aphorizon is impermeable. When the maximum designdepth of the artificial perched watertable is 0�3m, waterof about 150mm depth could be newly stored becausesand is introduced and its porosity is about 0�5.The soil failure experiments by the injection of high-

pressure air were conducted to form the artificialperched watertable. In this process, the soils havingpoor air permeability were broken down with a largehorizontal soil cavity or crack caused by tensile stress.However, the soils having comparatively good airpermeability were broken down with V-shaped soilfailure caused by shear stress (Araya & Kawanishi,1984). In order to form the artificial perched watertable,

a horizontal soil cavity is required. This paper deals withthe determination of the air permeability of soils and thedifference in the soil failure mechanisms due to thedifferent air permeabilities.

2. Definition of air permeability of soil

When fluid flows into a particle layer in onedimension, the relation between the flow rate and theresistance pressure produced is obtained by a capillarytube model (Shirai, 1975). However, this model is for asaturated flow such as when water flows in soilssaturated by water.When air flows in soils as in this study, the soil

particles always have moisture on their surface andhence, an unsaturated flow takes place. In this paper, acorrection coefficient Sv is newly defined for theinduction process of the capillary tube model and hence,the modified capillary tube model could also be adaptedto the unsaturated flow (Araya, 1982).The friction of flow is given by Poiseuille (Shirai,

1975) as

pf ¼llrw2

0

3rpð1Þ

Page 3: SW—Soil and Water: Construction of an Artificial Perched Watertable, Part 1: Air Permeability of Soils and Aspects of Soil Failure

Fig. 1. Soils producing poor crop yields in Black Dragon Province of People’s Republic of China: (a) typical planosol at farmnumber 853; Ap horizon, humic soil and suitable for plant growth; Aw horizon, lessivage soil and dense and quite impermeable; Bhorizon, diluvial heavy clay; (b) typical meadow soil in Baoqing county; Ap horizon, humic soil with abundant organic matter andgood structure; A horizon, impermeable soil with less organic matter; Cg1 horizon, impermeable gleyed parent material with ferrous

oxide; and Cg2 horizon, impermeable gleyed soil

ARTIFICIAL PERCHED WATERTABLE, PART 1: AIR PERMEABILITY OF SOILS AND ASPECTS OF SOIL 335

where l is the thickness of soil cell, w0 is the actual airvelocity through the soil pores, rp is the hydraulic radius,r is the air density and l is the friction factor and is inproportion to the flow velocity for laminar flow and inproportion to the square of flow velocity for turbulentflow. When gas flows into the soil as defined in Fig. 2,the hydraulic radius should be considered as the totalvolume of the gas phase divided by the total surface areacontacted:

rp ¼Vc � Vw

Vð1� eaÞSv

¼ea

1� eað ÞSv

¼e 1� sð Þ

1� eþ esð ÞSv

ð2Þ

where V is the total volume of the soil cell, Vc isthe volume of the porosity phase in the soil cell, Vw isthe volume of the liquid phase in the soil cell, e is theporosity and ea is the air porosity and is given by

ea ¼Vc � Vw

V¼ e 1� sð Þ ð3Þ

and the degree of saturation s is given by

s ¼Vw

Vc¼ 1�

ea

eð4Þ

The correction coefficient Sv is newly defined here forthe unsaturated flow as the surface area divided by the

unit volume of soil particles having soil water (specificsurface area). A greater value of Sv means that thesurface of the soil particles is rougher and air cannot bepassed easily. The value of Sv is a function of the soilwater content and each soil possesses a unique relation-ship between Sv and soil water content. The values of Sv

were experimentally determined as described later.The velocity w0 in Eqn (1) is the actual air velocity

among the soil particles. If the air velocity w when soil isnot charged is used instead of w0:

w0 ¼w

ea¼

w

e 1� sð Þð5Þ

Substituting Eqns (2) and (5) into Eqn (1):

pf ¼lrw2lSv

3

1� eþ esð Þ

e3 1� sð Þ3ð6Þ

Equation (6) agrees with the Carman–Kozeny equation(Shirai, 1975) when the degree of saturation s is zero.Next, the Reynolds number, which shows the state of

flow produced when air flows among the soil particles, isconsidered. The definition of the Reynolds number Re is

Page 4: SW—Soil and Water: Construction of an Artificial Perched Watertable, Part 1: Air Permeability of Soils and Aspects of Soil Failure

Fig. 2. A model in which air flows into soil particles with soil moisture in one-dimensional direction; p0, absolute atmosphericpressure; p1, gauge pressure at inlet of soil cell; M, total mass of soil in soil cell or soil sampler; Ms, mass of solid phase in soil cell orsoil sampler; Mw, mass of liquid phase in soil cell or soil sampler; V, total volume of soil cell or soil sampler; Vc, volume of porosityphase in soil cell or soil sampler; Vj, actual volume in soil cell or soil sampler; Vw, volume of liquid phase in soil cell or soil sampler

K. ARAYA; G. GUO336

generally given by

Re ¼w0d

nð7Þ

where d is the diameter of the object and v is thekinematic coefficient of viscosity.Substituting Eqns (2) and (5) into Eqn (7), the

modified Reynolds number for the soil Res is expressedas

Res ¼6w

1� eþ esð ÞSvnð8Þ

In Eqn (8), the coefficient of 6 is included because of thedefinition method of the Reynolds number by Leva(1948). The friction factor l of Eqn (1) is given by Leva(1948) in a laminar flow zone

l ¼100

Reð9Þ

If Eqn (9) can be adopted for moist soils, Eqn (6) is

pf ¼50S2

vnrlw

9

ð1� eþ esÞ2

e3ð1� sÞ3ð10Þ

The kinematic coefficient of viscosity v is affected bypressure and is not easily used and is replaced by thecoefficient of viscosity m:

n ¼mr

ð11Þ

where r is the air density.

The coefficient of viscosity m is a function only oftemperature (Fujimoto, 1963) and is given by

m ¼ 1 � 723� 10�5380

t þ 380

t þ 273

273

� �3=2

ð12Þ

Substituting Eqn (11) into Eqn (10),

pf ¼50S2

vmlw

9

1� eþ esð Þ2

e3 1� sð Þ3ð13Þ

The mass flow rate Gt can be obtained from Eqn (24),

Gt ¼ Acwr ð14Þ

where Ac is the sectional area of the soil cell.Substituting Eqn (14) into Eqn (13),

pf ¼50S2

vmlGt

9Acr1� eþ esð Þ2

e3 1� sð Þ3ð15Þ

or

Sv ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi9pf Acr50mlGt

e3 1� sð Þ3

1� eþ esð Þ2

sð16Þ

The value of Sv can be determined from Eqn (16) bysubstituting the value of the pressure pf obtained whenair is slowly pumped into the soil under the laminar flowwhose physical properties (porosity e and the degree ofsaturation s) are known.Substituting Eqns (11) and (14) into Eqn (8), the

modified Reynolds number for the soil is

Res ¼6Gt

1� eþ esð ÞSvmAcð17Þ

Page 5: SW—Soil and Water: Construction of an Artificial Perched Watertable, Part 1: Air Permeability of Soils and Aspects of Soil Failure

Fig. 3. Schematic diagram of an apparatus which provides airflow into soil cell under pressure

ARTIFICIAL PERCHED WATERTABLE, PART 1: AIR PERMEABILITY OF SOILS AND ASPECTS OF SOIL 337

Equations (15) and (16) have the air density r andhence, the compressibility of air should be considered. Itis assumed that the average value of the air density atthe inlet r1 and the outlet r2 in Fig. 2 is correct. The airdensity is obtained from the characteristic equation for aperfect gas as

r ¼r1 þ r0

p1 þ p0

2R 273þ tð Þð18Þ

where r0 is the atmospheric air density, p0 is the absoluteatmospheric pressure, p1 is the gauge pressure at theinlet of the soil cell, R is the gas constant of air and t isthe air temperature.On the other hand, the saturated water permeability

obtained when water is pumped into a soil, is given byDarcy,s law (Akai, 1966):

pf ¼wl

kð19Þ

where k is the permeability and has the dimension ofm2 s�1 Pa�1. With Eqns (13) and (19):

k ¼9

50mS2v

e3 1� sð Þ3

1� eþ esð Þ2ð20Þ

The smaller values of Eqn (20) mean that air cannotbe easily pumped into the soil layer and a high pressureis produced. The values of Sv [Eqn (16)] and k [Eqn (20)]were experimentally determined for the soils in thisstudy. These values are a function of the soil watercontent y.The soil water content y is defined as in Fig. 2:

y ¼Mw

Msð21Þ

where Ms is the mass of the solid phase and Mw is themass of the liquid phase.

3. Experimental details

3.1. Apparatus to provide air into soil

Figure 3 shows a schematic diagram of the apparatusused to measure the air permeability of soils by pumpingair into the soil in the soil cell under pressure. The aircompressor first charges air into the charge tank. Thepressure in the charge tank was about 1MPa. Byadjusting the air valve, the flow rate to the soil cell wascontrolled and the resistance pressure produced in thesoil cell was determined. The airflow rate was measuredby the venturi in Fig. 3. The differential pressurebetween the port and the venturi was monitored by adifferential pressure sensor. The static pressure pro-duced at the port was monitored by a pressure sensor

mounted on the port. The resistance pressure producedin the soil cell was monitored by another pressure sensormounted on the soil cell.The prototype soil cell is shown in Fig. 4. The soil cell

consisted of acrylic resin plates with a hole and had afilter at the bottom to retain the charge of soil. The soilcell was held together by eight bolts through rubberpacking. The air entered from the upper part.In the apparatus in Fig. 3, the flow velocity wv at the

venturi is given by Bernoulli’s law:

wv ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2Dp=rp

1� cAv=Ap

� �2vuut ð22Þ

where Dp is the differential pressure between the venturiand the port which is monitored by the differentialpressure sensor, rp is the air density at the port, Av is thesectional area of the venturi, Ap is the sectional area ofthe port and c is the flow coefficient.The air density at the port rp is a function of the

pressure and temperature and is obtained from thecharacteristic equation for a perfect gas as

rp ¼pp þ p0

R 273þ tð Þð23Þ

where pp is the gauge pressure at the port. Hence, themass flow rate of air Gt is

Gt ¼ Avwvcrp ð24Þ

As the soil cell in Fig. 4 has a filter to retain the chargeof the soil, the pressure p1 measured by the pressuresensor mounted on the soil cell is the sum of the pressureresistance of the soil pf and the pressure resistance of thefilter pm. Hence, pf is

pf ¼ p1 � pm ð25Þ

Page 6: SW—Soil and Water: Construction of an Artificial Perched Watertable, Part 1: Air Permeability of Soils and Aspects of Soil Failure

Fig. 4. Soil cell to measure resistance pressure of airflow to the soil

K. ARAYA; G. GUO338

The pressure resistance of the filter pm is obtainedwhen air is pumped through the empty soil cell.

3.2. Soils and physical factors

For comparison, 1mm dia. glass beads, the sand andthe pseudogley soil which was a typical Japanese heavyclay, were tested. The Chinese soils in this study were thefirst (Ap), second (Aw) and third (B) horizons of theplanosol and the first (Ap), second (A) and third (Cg1)horizons of the meadow soil. Microscopic photographsof the soil particles used in this study are shown inFig. 5. The glass beads in Fig. 5(a) have a very smoothsurface. The sand in Fig. 5(b) has a smoother surfacethan the pseudogley soil in Fig. 5(c). The Ap horizons ofthe planosol in Fig. 5(d) and the meadow soil in Fig. 5(g)have organic matter such as plant roots. The Awhorizon of the planosol in Fig. 5(e) is a binary mixtureof soil particles where silt forms the frame structureand clay fills the pore spaces, and it is extremelyhard and impermeable (Araya, 1991). The B horizon ofthe planosol in Fig. 5( f ), the A horizon of themeadow soil in Fig. 5(h) and the Cg1 horizon of themeadow soil in Fig. 5(i) are all heavy clays with fine soilparticles.The porosity e of the soils was determined using the

actual volumetric method (Society of Soil Physics,1987). The soil water content y was measured by aninfrared moisture meter (Society of Soil Dynamics,1990). The soil was filled in a soil sampler and the mass

of the soil M was determined by a balance. Thesoil sampler was then set in the actual volumetric meterand the actual volume Vj (see Fig. 2) was determined.With these values, the actual density of solid phase rs

is given by

rs ¼M=ðyþ 1Þ

Vj � My=ðyþ 1Þ � 10�3ð26Þ

In Fig. 2, the porosity e is given by

e ¼Vc

V � Vj þ My=ðyþ 1Þ � 10�3

Vð27Þ

The degree of saturation s is given by

s ¼Vw

Vc¼

My=ðyþ 1Þ � 10�3

V � Vj � My=ðyþ 1Þ � 10�3� � ð28Þ

This soil in the soil sampler was placed in the soil cellin Fig. 3 and by using flowing air, the pressure resistancepf was determined.In this study, the air pressure p1 was given artificially

by pumping into the soil as shown in Figs 2 and 3 andthe soil moisture among the soil particles in Fig. 2moved along the airflow which is quite different fromthe natural situation. Hence, even when the soils arenearly at saturation and the degree of saturation s ofEqn (28) is nearly 1, the measured value of pf of Eqn(15) did not become infinity and the measured values ofSv of Eqn (16) and k of Eqn (20) did not become zero.These values were determined when the modifiedReynolds number Res of Eqn (17) was less than 10and, namely, laminar flow took place (Araya, 1982).

Page 7: SW—Soil and Water: Construction of an Artificial Perched Watertable, Part 1: Air Permeability of Soils and Aspects of Soil Failure

Fig. 5. Microscopic photographs; (a) 1 mm glass beads, magnification was � 10; (b) sand, � 50; (c) pseudogley soil, � 50; (d) Aphorizon of planosol, � 50; (e) Aw horizon of planosol, � 50; (f) B horizon of planosol, � 50; (g) Ap horizon of meadow soil, � 10;

(h) A horizon of meadow soil, � 10; (i) Cg1 horizon of meadow soil, � 10

ARTIFICIAL PERCHED WATERTABLE, PART 1: AIR PERMEABILITY OF SOILS AND ASPECTS OF SOIL 339

3.3. Soil bin experiments

A simplified injector was prepared. The injectorwas a pipe with a diameter of 150mm and a length of1�5m and had a hole of 50mm at one end. The injectorwas vertically set in the soil bin and then soil wasfilled into the soil bin. The soil in the soil bin was

Japanese pseudogley soil, in place of the planosol ormeadow soil because it was not feasible to transportsufficient soil amounts from China. The soil watercontent was varied at 6�39, 14�8, 19�0, 28�2 and 46�0%d.b. For comparison, sand was used. The soil watercontent of the sand was varied at 7�53, 9�90, 12�5, 15�0,18�3 and 20�5% d.b. The soil bin had a transparent side

Page 8: SW—Soil and Water: Construction of an Artificial Perched Watertable, Part 1: Air Permeability of Soils and Aspects of Soil Failure

Fig. 5} (continued ).

Fig. 6. Porosity e of soils in soil sampler; , glass beads; ,sand; , pseudogley soil; , planosol Ap; , planosol Aw; ,

planosol B; , meadow Ap; , meadow A; , meadow Cg1

K. ARAYA; G. GUO340

and the state of the soil failure was recorded by acamera.

4. Results and discussion

4.1. Soil porosity

Figure 6 shows the measured porosity of the soilsdefined by Eqn (27). The soil was filled and compacted

in the soil sampler by hand in order to avoid any deadspace in the soil sampler.From Fig. 6, the porosity of the glass beads was the

smallest at 0�39. The porosity of the other soils was0�45–0�60 and the average value was 0�52 regardless ofthe soil water content. Hence, the soil porosity was notaffected by the soil water content.

4.2. Specific surface area of soils

Figure 7 shows the measured specific surface area Sv

defined by Eqn (16). Figure 7(a) shows the values of Sv

for the glass beads, sand and pseudogley soil. The valueof Sv for the glass beads was 4� 103m�1. The value ofSv for the sand was 104m�1 and that for the pseudogleysoil was 3� 104m�1 when the soil water content was lessthan 10% d.b. Here, the soils were dry and the surface ofthe soil particles was rough as shown in the microscopicphotographs in Figs 5(b) and (c). However, in the case ofthe sand, when the soil water content became 20% d.b.where the sand was saturated, the value of Sv for thesand became the same as that of the glass beads. This isbecause, as shown in Fig. 2, the soil moisture coveredthe surface of the soil particles and made it smootherand hence, air passed with a lower resistance. The valueof Sv for the pseudogley soil decreased to 8� 102m�1

for the soil water content of 40% d.b. when the soil wassaturated. Here, the soil moisture would perfectly coverthe soil particles and there is no roughness on thesurface of the soil particles and it becomes smootherthan the surface of the glass beads.

Page 9: SW—Soil and Water: Construction of an Artificial Perched Watertable, Part 1: Air Permeability of Soils and Aspects of Soil Failure

Fig. 7. Specific surface area Sv of soils: (a) , glass beads; , sand; , pseudogley soil; (b) , planosol Ap; , planosol Aw; ,planosol B; (c) , meadow Ap; , meadow A; , meadow Cg1

ARTIFICIAL PERCHED WATERTABLE, PART 1: AIR PERMEABILITY OF SOILS AND ASPECTS OF SOIL 341

Figure 7(b) shows the measured values of Sv for theplanosol. The value of Sv for the Ap horizon did noteasily decrease with higher soil water contents, but whenthe soil water content became about 50% d.b., the soilwas saturated and the value of Sv decreased. This isbecause the Ap horizon contains organic matter asshown in Fig. 5(d ). The trends in the values of Sv for theAw and B horizons were similar and the soil particlesformed soil clods as shown in Figs 5(e) and ( f ) and thesurface of the soil was rough up to the soil water contentof 20% d.b. However, when the soil water contentbecame 30% d.b., both soils were saturated and thevalue of Sv significantly decreased. The minimum valueof Sv for the Ap, Aw and B horizons of the planosol was2� 102m�1 which would be a proper value for theplanosol.

Figure 7(c) shows the value of Sv for the meadow soil.The trend in the Ap horizon was similar to the Aphorizon of the planosol in Fig. 7(b). Namely, at a soilwater content of about 50% d.b., the soil was saturatedand the value of Sv decreased because of the organicmatter as shown in Fig. 5(g). The A horizon wassaturated at a soil water content of 35% d.b. and thevalue of Sv decreased. The Cg1 horizon was saturated ata soil water content of 30% d.b. and the value of Sv

decreased. This is because the A horizon consists of finersoil particles than the Cg1 horizon as shown in Figs 5(h)and (i) (Zhang & Araya, 2001a). The minimum values of

Sv for the three horizons were all 2� 102m�1 whichwould be a proper value of the meadow soil.In Figs 7(a)–(c), the curve of the Sv shifted to the right

side when the soil consisted of finer soil particles orgreater amounts of organic matter.

4.3. Air permeability of soils

Figures 8(a)–(c) show the measured air permeability kdefined by Eqn (20). The value of Sv in Fig. 7 shows theresistance of airflow produced by the roughnessof the soil particles on a microbasis. On the otherhand, the value of k in Fig. 8 shows the flow resistanceof the entire soil layer, namely, the total flow resi-stance on a macrobasis. In order to obtain the soilcavity in the ground, the value of k plays a significantrole.

Figure 8(a) shows the measured value of k for theglass beads, sand and pseudogley soil. The value of k forthe glass beads was 100m2 s�1MPa�1. The value of kfor the sand and pseudogley soil decreased with thegreater soil water content because air passed with moreresistance through the soil. The minimum value of k atsand saturation was 1m2 s�1MPa�1 and that for thepseudogley soil was 0�05m2 s�1MPa�1.

Figure 8(b) shows the measured value of k for theplanosol. The value of k for the three horizons decreased

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Fig. 8. Air permeability k of soils: (a) , glass beads; , sand; , pseudogley soil; (b) , planosol Ap; , planosol Aw; , planosol B;(c) , meadow Ap; , meadow A; , meadow Cg1

K. ARAYA; G. GUO342

with the greater soil water content because of the higherresistance to airflow through the soil. However, the flowthrough the Ap horizon was hardly affected by thegreater soil water content. The value of k for the Awhorizon was comparatively small even at a low soilwater content and hence, the Aw horizon is the originalimpermeable soil (Araya et al., 1996a). The value of kfor the B horizon decreased suddenly at a soil watercontent of 30% d.b. and the airflow became poor. Thisis a characteristic of the clayey soil.

Figure 8(c) shows the measured value of k for themeadow soil. The flow through the Ap horizon hardlychanged with the greater soil water content similar tothe Ap horizon of the planosol.The A and Cg1 horizons were all clay and the trend in

the value of k was similar to the B horizon of planosoland the airflow suddenly decreased at a soil watercontent of 30% d.b.As shown in Fig. 8, the curve of the value of k shifted

more to the left side when the soil particles were smallerbecause of the poorer airflow and it shifted to the rightside when the soil contained a greater amount of organicmatter because of the better airflow.In Fig. 8, the air permeability of the sand was

1m2 s�1MPa�1 and that of the clay was0�02m2 s�1MPa�1. These values are much larger thanthe water permeability, namely, 0�1m2 s�1MPa�1 forthe sand and 10�6m2 s�1MPa�1 of the clay producedwhen water flows into soils under saturation (Akai,

1966). Hence, air can flow much more easily in the soilsthan water.

4.4. Aspects of soil failure

Figure 9 shows the different aspects of soil failure withdifferent air permeabilities when tests were conducted inthe soil bin. Figure 9(a) shows the soil failure of the drysand with a soil water content of 7�53% d.b. Here,fluidization (Shirai, 1975) took place where each soilparticle moved independently along the airflow. Whenthe soil water content of the sand was greater than7�53% d.b., the fluidization did not increase and the soilparticles did not move independently but a V-shaped soilfailure took place. Hence, the value of k of10m2 s�1MPa�1 in Fig. 8(a) is a boundary value betweenfluidization and the V-shaped soil failure mechanism.

Figure 9(b) shows the soil failure of the pseudogleysoil with a soil water content of 14�8% d.b. The V-shaped soil failure occurred. Fluidization was notapparent in pseudogley soil even if the soil water contentwas low.

Figure 9(c) shows the soil failure of the pseudogleywith a soil water content of 28�2% d.b. A soil cavity wasproduced. Hence, in Fig. 8(a), when the value of kranged from 10 to 0�1m2 s�1MPa�1, the V-shaped soilfailure took place and when it was less than1�0�7m2 s�1MPa�1, the soil cavity production took

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Fig. 9. Aspects of soil failure in a glass-sided soil bin: (a) fluidization in sand, soil water content, 7�53% d.b.; (b) V-shaped failure inpseudogley soil, soil water content, 14�8% d.b.; (c) horizontal cavity in pseudogley soil, soil water content, 28�2% d.b.

ARTIFICIAL PERCHED WATERTABLE, PART 1: AIR PERMEABILITY OF SOILS AND ASPECTS OF SOIL 343

place. The value of k of 0�1m2 s�1MPa�1 is a boundaryvalue.In Figs 8(b) and (c), in order to produce the soil cavity

in the B horizon of the planosol or Cg1 horizon of themeadow soil by air injection, the value of k for thesesoils should be less than 0�1m2 s�1MPa�1 and hence,

the soil water content of these soils should be more than30% d.b. The commonly occurring soil water content ofthe B horizon of the planosol field was 25–32% d.b.(Jia et al., 1998a) and that of the Cg1 horizon of themeadow soil field was 26–33% d.b. (Zhang & Araya,2001a).

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K. ARAYA; G. GUO344

5. Conclusions

(1) The specific surface area Sv for the glass beads was4� 103m�1. The value of Sv for the sand was104m�1 and that for the pseudogley soil was3� 104m�1 when the soil water content was lessthan 10% d.b. Here, the soils were dry and thesurface of the soil particles was rough. However, inthe case of the sand, when the soil water contentbecame 20% d.b. where the sand was saturated, thevalue of Sv for the sand became the same as that forthe glass beads.

(2) The value of Sv for the pseudogley soil decreased to2� 102m�1 for a soil water content of 40% d.b.when the soil was saturated. Here, the soil moisturewould perfectly cover the soil particles, effectivelymaking them smoother than the surface of the glassbeads.

(3) The minimum value of Sv for the Ap, Aw and Bhorizons of the planosol was 2� 102m�1 whichwould be a proper value for the planosol.

(4) The minimum value of Sv for the Ap, A and Cg1horizons of the meadow soil was also 2� 102m�1

which would be a proper value of the meadowsoil.

(5) The air permeability k for the glass beads was100m2 s�1MPa�1. The values of k for the sand andpseudogley soil decreased with the greater soilwater content because air passed with moreresistance through the soil. The minimum valueof k at sand saturation was 1m2 s�1MPa�1 andthat for the pseudogley soil was 0�05m2 s�1MPa�1.

(6) The value of k for the three horizons of theplanosol decreased with the greater soil watercontent because of greater resistance to airflowthrough the soil. However, the flow through the Aphorizon was hardly affected by the greater soilwater content. The value of k for the Aw horizonwas comparatively small even at a low soil watercontent and hence, the Aw horizon is the originalimpermeable soil. The value of k for the B horizondecreased suddenly at a soil water content of 30%d.b. and the airflow became poor. This is acharacteristic of the clayey soil.

(7) The flow through the Ap horizon of the meadowsoil hardly changed with the greater soil watercontent similar to the Ap horizon of the planosol.The A and Cg1 horizons of the meadow soil wereall clay and the trend in the value of k was similarto the B horizon of planosol and the airflowsuddenly decreased at a soil water content of 30%d.b.

(8) The value of k of 10m2 s�1MPa�1 is a boundaryvalue between fluidization and the V-shaped soilfailure, mechanism.

(9) When the value of k ranged from 10 to0�1m2 s�1MPa�1, the V-shaped soil failure tookplace and when it was less than 0�1m2 s�1MPa�1,the soil cavity production took place.

(10) In order to produce the soil cavity in the B horizonof the planosol or Cg1 horizon of the meadow soilby air injection, the value of k for these soils shouldbe less than 0�1m2 s�1MPa�1 and hence, the soilwater content of these soils should be more than30% d.b.

(11) An artificial perched watertable could be formed byair injection into the soil subject to the successfuldevelopment of a suitable experimental machine toconstruct the underground soil cavity.

References

Akai K (1966). Soil Dynamics, p 30. Asakura Ltd, TokyoAraya K (1982). Soil failure by introducing fluid underpressure. PhD Thesis, Journal of Senshu University-Hokkaido, 15, 1–85

Araya K (1991). Influence of particle size distribution in soilcompaction of planosol. Journal of Environmental ScienceLaboratory, Senshu University, 2, 181–192

Araya K; Kawanishi K (1984). Soil failure introducing airunder pressure. Transactions of the ASAE, 27(5),1292–1297

Araya K; Kudoh M; Zhao D; Liu F; Jia H (1996a).Improvement of planosol solum, part 1: experimentalequipment, methods and preliminary soil bin experimentswith ploughs. Journal of Agricultural Engineering Research,63, 251–260

Araya K; Kudoh M; Zhao D; Liu F; Jia H (1996b).Improvement of planosol solum, part 5: soil bin experimentswith a three-stage subsoil mixing plough. Journal ofAgricultural Engineering Research, 65, 143–149

Araya K; Kudoh M; Zhao D; Liu F; Jia H (1996c).Improvement of planosol solum, part 6: field experimentswith a three-stage subsoil mixing plough. Journal ofAgricultural Engineering Research, 65, 151–158

Fujimoto T (1963). Hydrodynamics, p 36. Yokendo Ltd,Tokyo

Jia H; Liu F; Zhang H; Zhang C; Araya K; Kudoh M; Kawabe

H (1998a). Improvement of planosol solum, part 7:mechanical properties of soils. Journal of AgriculturalEngineering Research, 70, 177–183

Jia H; Liu F; Zhang H; Zhang C; Araya K; Kudoh M;

Kawabe H (1998b). Improvement of planosol solum, part 8:analysis of draught of a three-stage subsoil mixingplough. Journal of Agricultural Engineering Research, 70,185–193

Jia H; Liu F; Zhang H; Zhang C; Araya K; Kudoh M; Kawabe

H (1998c). Improvement of planosol solum, part 10: mixingof wheat straw and corn stalk into subsoil. Journal ofAgricultural Engineering Research, 71, 221–226

Liu F; Jia H; Zhang H; Zhang C; Araya K; Kudoh M; Kawabe

H (1998). Improvement of planosol solum, part 9: fertilizerdistributor for subsoil. Journal of Agricultural EngineeringResearch, 71, 213–219

Leva Max (1948). Fluidisation of an anthracite coal. Industrialand Engineering Chemistry, 41(6), 1206–1212

Shirai T (1975). Fluidisation, p 25. Science and TechnologyLtd, Tokyo

Society of Soil Physics (1987). Soil Physical Properties, pp 29–46. Yokendo, Tokyo

Society of Soil Dynamics (1990). Soil Test, Society of SoilDynamics, pp 49–53. Tokyo

Page 13: SW—Soil and Water: Construction of an Artificial Perched Watertable, Part 1: Air Permeability of Soils and Aspects of Soil Failure

ARTIFICIAL PERCHED WATERTABLE, PART 1: AIR PERMEABILITY OF SOILS AND ASPECTS OF SOIL 345

Zhang C; Araya K (2001a). A three-stage soil layer mixingplough for improvement of meadow soil, part 1: mechanicalproperties of soils. Journal of Agricultural EngineeringResearch, 78(3), 253–260

Zhang C; Araya K (2001b). A three-stage soil layer mixingplough for improvement of meadow soil, part 2: soil binexperiments. Journal of Agricultural Engineering Research,78(4), 359–367

Zhang C; Araya K; Kudoh M; Liu F; Jia H; Zhang H;Yang S (2001). A three-stage soil layer mixing plough forimprovement of meadow soil, part 3: field evaluation.Journal of Agricultural Engineering Research, 79(1),47–53

Zhao D; Liu F; Jia H (1992). Improvement of Low Yield Soilsin Three-river Plain, China, pp 30–59. Black Dragon SciencePress, Harbin