Biosystems Engineering (2004) 87 (1), 47–55doi:10.1016/j.biosystemseng.2003.10.008

Available online at www.sciencedirect.com

PM}Power and Machinery

Machine for Construction of an Artificial Perched Watertable, Part 1: Hydraulicand Mechanical Properties of Sands to be deposited

G. Guo; K. Araya; Y. Shibutani; H. Zhang

Environmental Science Laboratory, Senshu University, Bibai, Hokkaido 079-0197, Japan; e-mail of corresponding author:araya@senshu-hc.ac.jp

(Received 20 May 2003; accepted in revised form 9 October 2003; published online 21 November 2003)

By constructing an artificial perched watertable at about a 1m depth from the soil surface in an area where theannual precipitation occurs only in the summer season, it was intended to retain summer runoff in thiswatertable and prevent excess moisture loss. The water in the artificially permeable layer could be used forplants as capillary water in the dry spring season. For this purpose, a machine to construct the artificialperched watertable was developed to deposit sand in the horizontal and cylindrical soil cavity underground,produced by high-pressure air. This paper deals with the hydraulic and mechanical properties of sand to bedeposited as an aid to the development of a sand gun, which is a part of the machine.

The results showed that when the soil water content increased from 0 to 35% dry basis (d.b.), the sand bulkdensity increased from 1250 to 1600 kgm�3. The specific surface area for each sand type was 3� 104–4� 104m�1 when the soil water content was nearly zero. However, when the soil water content increased andthe sand was saturated, the minimum values of the specific surface area decreased to 6� 103–9� 103m�1. Theair permeability for each sand type decreased with greater soil water content because air passed through withmore resistance in the sand. The minimum air permeability of the Chinese river sand was the highest at4 kgMPa�1 s�1m�1 because of its roughness which resisted airflow. This was followed by the Japanese coastalsand at 3 kgMPa�1 s�1m�1 and the Toyoura standard sand at 2 kgMPa�1 s�1m�1 because of its fineness andgreater resistance to airflow. The cohesion of each sand showed a maximum at a particular soil water content.These soil water contents were about 10% d.b. for each sand. The maximum cohesion for each sand wasnearly the same at about 8�0 kPa. The adhesion of each sand also showed a maximum at a particular soil watercontent. These soil water contents were about 10% d.b., which is the same as that of the cohesion.# 2003 Silsoe Research Institute. All rights reserved

Published by Elsevier Ltd

1. Introduction

The annual precipitation in the Three-river Plainof the Black Dragon province of People’s Republicof China is only 500–600mm. Moreover, themonthly rainfall is uneven; that is, about 60–70% ofthe annual precipitation occurs in July and August,and there is almost no rainfall in the winter andspring seasons. Plants often suffer due to excessivemoisture during the growing season in the summerand, alternately, to drought during the seeding seasonin the spring. If heavy rain occurs in the summerseason, the runoff flows on the soil surface and gathersat the lowest place in the field because soils in this areaare planosol (Araya et al., 1996) and meadow soil

(Zhang & Araya, 2001), both of which are quiteimpermeable. The lowest place becomes a pond duringevery rainfall, and the plants there are submerged at thistime.

The annual precipitation in the North of River andInner Mongolia provinces is much less, only 300–400mm. Here, too, the monthly rainfall is uneven;about 70% of the annual precipitation occurs once withhail in July and August. Soil in this area is whitish oasissoil (Guo & Araya, 2002), which is also quite imperme-able. Hence, almost all rain water cannot penetrate intothe soil but becomes runoff loss on the soil surface andeventually flows into the rivers. As a result, there iswater in the rivers only in the rainy season of July andAugust in this area.

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1537-5110/$30.00 47 # 2003 Silsoe Research Institute. All rights reserved

Published by Elsevier Ltd

By constructing an artificial perched watertable atabout a 1m depth from the soil surface as described inthe previous papers (Araya & Guo, 2002a, 2002b), itwas intended that the runoff caused in the summer couldpreferentially be held in this watertable, and the excessmoisture loss could therefore be prevented. The water inthe artificially permeable layer could be available forcrops and grasses as capillary water in the dry springseason in the Black Dragon, the North of River and theInner Mongolia provinces. To accomplish these results,a machine to construct the artificial perched watertablewas needed (Fig. 1). This machine first produces ahorizontal and cylindrical soil cavity underground byhigh-pressure air; then, sand is deposited in the soilcavity to serve as the watertable.

For this purpose, a sand gun which depositsthe sand into the soil cavity was developed (Guo &Araya, 2003). The injector is first charged with sand,and then, the sand is pressurised by high pressureair and is injected from the nozzle. The hydraulicand mechanical properties of the sand to be depositedhad to be determined as an aid in the developmentof sand gun. In Fig. 1, the air pressure is givenartificially by pumping into the sand, and soil moisturebetween the sand particles moves along the airflowwhich is quite different from the natural situation.These properties of the sands will be used as an aidof the development of sand gun. Design and test resultsfor the sand gun will be presented in a subsequentreport.

2. Experimental details

2.1. Description of sands in this study

The sand which will actually be used for the sand gunin China is river sand in which many large particles arefound [Fig. 2(a)]. This river sand is now used as repairmaterial for roads in China. The sand used for testing inthis study was Japanese coastal sand because it was notfeasible to transport a sufficient amount of the river sandfrom China to our test site in Japan. The particle size ofthe Japanese coastal sand is comparatively uniform, asshown in Fig. 2(b). For comparison, standard sand fromJapan [Toyoura standard sand, shown in Fig. 2(c)] wasalso tested. The particle size distribution of these threekinds of sand is shown in Fig. 3. The sand was separatedby 4�76, 3�36, 2�38, 1�68, 1�19, 0�84, 0�59, 0�3 and 0�25mmsize sieves. Soil particles�smaller than 0�02mm are silt,those between 0�02 and 0�2mm are fine sand, thosebetween 0�2 and 2mm are coarse sand and those largerthan 2mm are gravel. The largest percentage of particleswere 0�4mm in diameter in the Chinese river andJapanese coastal sands. The Japanese sand has virtuallyno gravel particles, but the Chinese sand has gravelparticles of various sizes. The particle size range of theToyoura standard sand is not scattered, with 70% of thesand particles less than 0�25mm, and the remainderbetween 0�25 and 0�30mm.

With the values in Fig. 3, the mean particle size

dm (Guo & Araya, 2003) of the three kinds of sand was

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Notation

am adhesion of sand–steel, Paap adhesion of sand–polyethylene, PaAc sectional area of soil cell, m2

c cohesion of sand, Padm mean particle size of sand (diameter), mGt air mass flow rate, kg s�1

ka air permeability, kg Pa�1 s�1m�1

l thickness of soil cell, mMs mass of solid phase in soil cell, kgMw mass of liquid phase in soil cell, kgN normal (perpendicular) load, Np0 absolute atmospheric pressure, 1�013� 105, Pap1 gauge pressure at inlet of soil cell, Papf resistance gauge pressure caused by soil in soil

cell, Par1 outer radius of ring (¼ 0�05m)r2 inner radius of ring (¼ 0�03m)R gas constant of air, 286�8Nmkg�1 K�1

Res reynolds number modified for soils degree of saturation

Sv surface area per unit volume of soil particleswith moisture (specific surface area), m�1

t air temperature, 8CT torque measured, Nmwa air velocity (no sand-charge basis), m s�1

Wa air mass flow velocity (no sand-charge basis),kg s�1 m�2

dm angle of sand–steel friction, degdp angle of sand–polyethylene friction, dege porosityy soil water content, % d.b.m coefficient of air viscosity, N sm�2

r0 atmospheric air density, kgm�3

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

ra air density, kgm�3

rsb sand bulk density, kgm�3

s normal (perpendicular) stress, Pat shear stress, Paj angle of soil–internal friction of soil, deg

G. GUO ET AL.48

determined. The dm of the Chinese river sand was0�975mm, that of the Japanese coastal sandwas 0�445mm and that of the Toyoura standard soilwas 0�221mm.

2.2. Hydraulic properties

The apparatus used to measure the air permeability ofsoils pumps air into the soil in the soil cell underpressure (Araya, 1982; Guo & Araya, 2002a). Its basicoperation is as follows. The air compressor first chargesair into the charge tank (Fig. 4) to about 1MPa.Byadjusting the air valve, the flow rate to the soil cell iscontrolled, and the resistance pressure produced in thesoil cell can be determined. The airflow rate wasmeasured by the venturi. The differential pressurebetween the port and the venturi was monitored by adifferential pressure sensor. Static pressure produced atthe port was monitored by a pressure sensor mountedon the port. Resistance pressure produced in the soil cellwas monitored by another pressure sensor mounted onthe soil cell.

The soil cell consisted of acrylic resin plates with ahole, and it had a filter at the bottom to retain the soilcharge. The soil cell was held together by eight boltsthrough rubber packing and air entered from the upperpart.

Three to four runs were made for each condition. Thesoil water content was first adjusted to the requiredvalue, and then the sand was charged in the soil cell.

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Sandtank

Air compressor

Sandvalve

Chargetank

Airvalve

Nozzle Cavity

Injector

Fig. 1. Schematic diagram of a machine which constructs anartificial perched watertable

Fig. 2. Microscopic photographs; (a) Chinese river sand, � 20;(b) Japanese coastal sand, � 20; (c) Toyoura standard

sand, � 20

MACHINE FOR CONSTRUCTION OF PERCHED WATERTABLE-1 49

2.3. Mechanical properties

Sand shear strength, sand–metal friction and sand–plastic friction were determined.

The sand shear and interface friction tests wereconducted with a ring (annular) shear tester (50mmouter radius and 30mm inner radius, The JapaneseSociety of Soil Mechanics and Foundation Engineering,1992). A ring with vanes was used for the sand sheartests. A flat steel ring plated with chromium was used forthe sand–metal friction tests, and another flat ring linedwith polyethylene was used for the sand–plastic frictiontests.

Three to four runs were made for each condition.The soil water content was first adjusted to therequired value, and then the soil was compacted in asteel mould.

3. Experimental design

3.1. Hydraulic characteristics

The hydraulic characteristics of specific surface areaSv and air permeability ka of the sands were determined.A description of these characteristics is given below.

The specific surface area Sv was defined for theunsaturated flow as the surface area divided by the unitvolume of soil particles having soil water. A greatervalue of Sv means that the surface of the soil particles isrougher, and air cannot pass through easily. The valueof Sv is a function of the soil water content, and each soilpossesses a unique relationship between Sv and soilwater content. The values of Sv were experimentallydetermined as described later.

The specific surface area Sv in m�1 is given by

Sv ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi9pf Acra

50mlGt

e3 1 � sð Þ3

1 � eþ esð Þ2

sð1Þ

where: pf is the gauge pressure in Pa caused by soil inthe soil cell; Ac is the sectional area of the soil cell in m2;ra is air density in kgm�3; e is porosity; m is thecoefficient of air viscosity in N sm�2; l is the thickness ofthe soil cell in m; Gt is air mass flow rate in kg s�1; and s

is degree of saturation.The modified Reynolds number Res for the soil is

Res ¼6Gt

1 � eþ esð ÞSvmAc

ð2Þ

Equation (1) has the air density ra and hence, thecompressibility of air should be considered. It isassumed that the average value of the air density atthe inlet of the soil cell r1and the outlet of the soil cell r0

in Fig. 4 is correct. The air density is obtained from the

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Pressuresensor

Soilcell

Recorder

Differentialpressuresensor

Venturi

Air compressorAirvalve

Chargetank

Fig. 4. Schematic diagram of an apparatus which providesairflow into a soil cell under pressure

80

70

60

50

40

30

20

10

00 1 2 3 4 5 6

Particle size, mm

Perc

enta

ge, %

Fig. 3. Particle size distribution of sands; *, Chinese river sand; *, Japanese coastal sand; 4, Toyoura standard sand

G. GUO ET AL.50

characteristic equation for a perfect gas as

ra ¼r1 þ r0

2¼

p1 þ 2p0

2R 273 þ tð Þð3Þ

where: r0 is the atmospheric air density; p0 is theabsolute atmospheric pressure in Pa; p1 is the gaugepressure in Pa at the inlet of the soil cell; R is the gasconstant of air (286�8Nmkg�1K�1); and t is airtemperature in 8C.

The value of Sv of a soil whose physical properties(porosity e and the degree of saturation s) are known,can be determined from Eqn (1) by substituting thevalue of the pressure pf obtained when air is slowlypumped into the soil under laminar flow [when the valueof Eqn (2) is less than 10].

On the other hand, the air permeability ka of thesands is given by Darcy,s law (Akai, 1966)

ka ¼Wal

pf

ð4Þ

where: ka has the dimension of kgMPa�1 s�1m�1 andWa is air mass flow velocity (no sand-charge basis) inkgm�2 s�1, and is

Wa ¼ wara ð5Þ

where wa is air velocity in m s�1.Smaller ka values for Eqn (4) mean that air cannot be

easily pumped into the sand layer and a high pressure isproduced. The values of Sv [Eqn (1)] and ka [Eqn (4)]were experimentally determined for the sands in thisstudy. These values are a function of the soil watercontent y in % (d.b.).

The soil water content y is defined as

y ¼Mw

Ms

ð6Þ

where: Ms is the mass of the solid phase in kg; and Mw isthe mass of the liquid phase in kg.

The air pressure was generated artificially by pumpinginto the sand as shown in Fig. 4, and soil moisture(gravitational and capillary water) among the sandparticles moved along the airflow (which is quitedifferent from the natural situation). Hence, even whenthe sand is nearly at saturation (the degree of saturations is nearly 1), the measured value of pf did not becomeinfinity and the measured value of Sv of Eqn (1) and ka

of Eqn (4) did not become zero.

3.2. Mechanical properties

From the measured load and torque, the normaland shear stresses were calculated. Normal stress s is

given by

s ¼N

p r21 � r22� � ð7Þ

where: r1 is the outer radius of the ring (0�05m), r2 is theinner radius of the ring (0�03m); and N is the normalload in N.

With the torque T measured by a torque wrench inNm, the shear stress or the frictional resistance t in Pa isgiven by

t ¼3T

2p r31 � r32� � ð8Þ

Graphs were drawn of t versus f ðsÞ for each soil andeach soil water content y. The cohesion c or adhesion a

was determined as the value when s ¼ 0. The angle ofinternal friction j or the angle of soils–teel or soil–polyethylene friction d was determined from the slope ofthe plot of t versus f ðsÞ.

4. Results and discussion

4.1. Soil bulk density

Figure 5 shows the measured sand bulk density rsb

which was determined in the soil cell in Fig. 4. There wasno difference in the values among the three kinds ofsand. When the soil water content y increased from 0 to35% d.b., the sand bulk density rsb increased from 1250to 1600 kgm�3.

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3

2

1

0 10 20 30 40

Soil water content θ, % d.b.

Sand

bul

k de

nsity

ρsb

, Mg

m−3

Fig. 5. Sand bulk density as a function of soil water content; *,Chinese river sand; *, Japanese coastal sand; 4, Toyoura

standard sand

MACHINE FOR CONSTRUCTION OF PERCHED WATERTABLE-1 51

4.2. Specific surface area of sands

Figure 6 shows the measured specific surface area Sv

defined by Eqn (1). The values of Sv for each sand were3� 104–4� 104 m�1 when the soil water content wasnearly zero. Here, the sands were dry, and the surface ofthe soil particles were rough, as shown in the micro-scopic photographs in Fig. 2. However, when the soilwater content increased, and the sand was saturated, thevalue of Sv decreased, and the minimum values of Sv forthe Chinese river and Japanese coastal sands became6� 103 m�1, and that of Toyoura standard sand became9� 103 m�1. The Sv value of 6� 103 m�1 is the same asthat of glass beads (Araya & Guo, 2002a). This isbecause the soil moisture covered the surface of the sandparticles and made it smoother; therefore air passed witha lower resistance. The minimum value of Sv would be aproper value for each sand.

In the case of clay, whose soil particles are finer thanthose of sand, the value of Sv decreased more atsaturation, and the minimum value of Sv was 3� 102

m�1 (Araya & Guo, 2002a).

In Fig. 6, the curve of Sv shifted to the right when thesand consisted of finer particles.

4.3. Air permeability of sands

Figure 7 shows the measured air permeability ka

defined by Eqn (4). The value of Sv in Fig. 6 showsthe resistance of airflow produced by the roughness ofthe sand particles on a microbasis. On the other hand,the value of ka in Fig. 7 shows the flow resistance of theentire soil layer, namely, the total flow resistance on amacrobasis.

In Fig. 7, the values of ka for each sand decreased withthe greater soil water content because air met with moreresistance in the sand. The minimum value of ka atsand saturation was different in the three kinds of sand.The minimum value of ka of the Chinese river sand wasthe largest of the three sands at 4 kgMPa�1 s�1 m�1

because of its roughness and least resistance to airflow.The value for Japanese coastal sand was lower at3 kgMPa�1 s�1m�1 and the Toyoura standard sand had

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105

5x104

104

103

5x103

10 20 30 40

Soil water content θ, % d.b.

Spec

ific

sur

face

are

a S �

, m−1

Fig. 6. Specific surface area Sv of sands as a function of soilwater content; *, Chinese river sand; *, Japanese coastal sand;

4, Toyoura standard sand

100

50

10

5

1

10 20 30 40

Soil water content θ, % d.b.

Air

per

mea

bilit

y k a

, kg

MPa

−1 s−1

m−1

Fig. 7. Air permeability ka of sands as a function of soil watercontent; *, Chinese river sand; *, Japanese coastal sand; 4,

Toyoura standard sand

G. GUO ET AL.52

the smallest value at 2 kgMPa�1 s�1m�1 because of itsfineness and greater resistance of airflow.

When air pressure was pumped into the nearlysaturated sand, the gravitational and capillary wateramong the sand particles moved along the airflow. Thisphenomenon cannot be found in normal situation.Hence, the value of ka decreased with greater soil watercontent and reached a particular value.

In the case of clay, the value of ka decreased more atsaturation, and the minimum value of ka was0�1 kgMPa�1 s�1m�1 (Araya & Guo, 2002a).

As shown in Fig. 7, the curve of the value of ka shiftedto the left when the sand particles were smallerbecause of the poorer airflow. Air permeability ofthe sand was from 2 to 10 kgMPa�1 s�1m�1. Thisvalue is much larger than the water permeability(0�01 kgMPa�1 s�1 m�1) for the sand when water flowsinto sands under saturation (Akai, 1966). Hence, air canflow much more easily in the sands than water.

4.4. Cohesion of sands

From the data of t versus f ðsÞ, the values when s ¼ 0(which denote the cohesion c) were determined (Fig. 8).The cohesion of each sand showed a maximum at aparticular soil water content. These soil water contentswere about 10% d.b. for each sand. The maximum valuec for each sand was nearly the same at about 8�0 kPa.

The cohesion of general soils also showed maxima atparticular soil water contents (Jia et al., 1998; Zhang &Araya, 2001).

4.5. Adhesion of sands

The adhesion to steel am for the three kinds of sand isshown in Fig. 9. The adhesion of each sand showed amaximum at a particular soil water content. These soilwater contents were also about 10% d.b., which is thesame as that of the cohesion.

The adhesion to polyethylene ap is shown in Fig. 10.The soil water contents at which adhesion showed amaximum were the same as those in Fig. 9. The values ofap on polyethylene were somewhat larger than those ofam on steel in Fig. 9. This is because the sand particlestend to scar the polyethylene. In the case of general soilswhose soil particles were finer than those of sands, thevalues of ap were always smaller than those of am (Jiaet al., 1998; Zhang & Araya, 2001).

4.6. Angle of internal friction of sands

From the data of t versus f ðsÞ, the slopes of the linesj, which denote the angles of internal friction, were

determined (Fig. 11). The angles of internal friction ofeach sand increased with greater soil water content andreached a particular value. This is quite different fromthe trend of the general soils, where the angles ofinternal friction decreased as soil water content in-creased and reached a particular value (Jia et al., 1998;Zhang & Araya, 2001).

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10

8

6

4

2

0 10 20 30

Soil water content θ, % d.b.

Coh

esio

n c,

kPa

Fig. 8. Cohesion c of sands as a function of soil water content;*, Chinese river sand; *, Japanese coastal sand; 4, Toyoura

standard sand

6

4

2

0 10 20 30Soil water content θ, % d.b.

Adh

esio

n of

san

d-st

eel a

m, k

Pa

Fig. 9. Adhesion am of sand–steel friction as a function of soilwater content; *, Chinese river sand; *, Japanese coastal sand;

4, Toyoura standard sand

MACHINE FOR CONSTRUCTION OF PERCHED WATERTABLE-1 53

4.7. Angles of sand–steel and sand–polyethylene friction

Figure 12 shows the values for the angle of sand–steelfriction dm for the sands. The values for the angle ofsand-steel friction dm showed a maximum at a particularsoil water content of 15–20% d.b., which was largerthan those in Figs 8–10.

Figure 13 shows the angle of sand–polyethylenefriction dp. The trend in Fig. 13 was the same as thatof sand-steel friction in Fig. 12, and the values of dp werenearly the same as those of dm.

5. Conclusions

(1) When the soil water content increased from 0 to35% d.b., the sand bulk density increased from 1250to 1600 kgm�3.

(2) The specific surface area for each sand were 3� 104–4� 104m�1 when the soil water content was nearlyzero. However, when the soil water content in-creased, and the sand was saturated, the minimumvalues of the specific surface area for the Chineseriver and Japanese coastal sands became6� 103m�1, and that of Toyoura standard sandbecame 9� 103m�1.

(3) The air permeability for each sand decreased withgreater soil water content because air passed withmore resistance in the sand. The minimum air permea-bility of the Chinese river sand was the largest at4kgMPa�1 s�1m�1, followed by the Japanese coastalsand at 3kgMPa�1 s�1m�1 and the Toyourastandard sand was at 2kgMPa�1 s�1m�1.

(4) The cohesion of each sand showed a maximum at aparticular soil water content. These soil watercontents were about 10% d.b. for each sand. Themaximum cohesion for each sand was nearly thesame at about 8�0 kPa.

(5) The adhesion of each sand also showed a maximumat a particular soil water content. These soil watercontents were about 10% d.b., which is the same asthat of the cohesion.

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6

4

2

10 20 30Soil water content θ, d.b.

Adh

esio

n of

san

d-po

lyet

hyle

ne a

p, k

Pa

Fig. 10. Adhesion ap of sand–polyethylene friction as a functionof soil water content; *, Chinese river sand; *, Japanese

coastal sand; 4, Toyoura standard sand

Ang

le o

f in

tern

al f

rict

ion

φ, d

eg

Soil water content θ, % d.b.3020100

20

40

60

Fig. 11. Angle of internal friction of sands f as a function of soilwater content; *, Chinese river sand; *, Japanese coastal sand;

4, Toyoura standard sand

40

20

0 10 20 30Soil water content θ, % d.b.

Ang

le o

f sa

nd-

stee

l fri

ctio

n � m

, deg

Fig. 12. Angle of sand-steel friction dm as a function of soilwater content; *, Chinese river sand; *, Japanese coastal sand;

4, Toyoura standard sand

40

20

2010 300Soil water content θ, % d.b.

Ang

le o

f sa

nd-

poly

ethl

ene

fric

tion

� p, k

Pa

Fig. 13. Angle of sand-polyethylene friction dp as a function ofsoil water content; Chinese river sand; *, Japanese coastal

sand; 4, Toyoura standard sand

G. GUO ET AL.54

(6) The angles of internal friction of each sand increasedwith greater soil water content and reached amaximum value. This is opposite to the trend ofgeneral soils.

(7) The values for the angles of sand–steel and sand–polyethylene friction showed a maximum at aparticular soil water content of 15–20% d.b.

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