Arab J Sci Eng (2014) 39:6825–6834DOI 10.1007/s13369-014-1247-8

RESEARCH ARTICLE - CIVIL ENGINEERING

Sand Samples’ Preparation Using Mobile Pluviator

Mahdy Khari · Khairul Anuar Kassim · Azlan Adnan

Received: 9 October 2012 / Accepted: 31 October 2013 / Published online: 26 June 2014© King Fahd University of Petroleum and Minerals 2014

Abstract It is important to have a good estimation of soilproperties in the laboratory when simulation of natural soilcondition is required, especially if the soil profile is subjectedto static and dynamic loadings. Soil configuration is one ofthe most important parameters to maintain when preparingsoil samples in large area. In this study, the newly designedMobile Pluviator adopted the air pluviation method for depo-sition of sand samples. The apparatus is operated based onthe uniformity distribution of sand particles with the termi-nal velocity concept where a series of shutter porosities arechosen for different sizes of sand particles. The densities ofthe soil samples were determined based on the correlationbetween the falling height and the rate of soil discharge ofthe shutter. Using this apparatus, a wide range of Dr (rela-tive density) ranging from 10 to 98 % can be achieved. Theadvantage of the Mobile Pluviator is that it can be moved tocover a wider area during the raining process and does notinterfere with monitoring instrumentation.

Keywords Mobile Pluviator · Sand sample · Relativedensity · Falling height · Air pluviation

M. Khari (B) · K. A. Kassim · A. AdnanDepartment of Geotechnics and Transportation,Faculty of Civil Engineering, Universiti Teknologi Malaysia,Skudai 81300, Johor Bahru, Malaysiae-mail: mehdikhari@gmail.com

K. A. Kassime-mail: kanuar@utm.my

A. Adnane-mail: azlanadnan@utm.my

List of symbols

Dr Relative densityV Terminal velocityP Mass densityD50 Mean size of soil particlesH Distance between bottom sieve and sand surfaceF Distance between top sieve and shutter plateGs Specific gravityCu Uniformity coefficientγd max Maximum dry unit weightγd min Minimum dry unit weightγd Dry unit weightemax Maximum void ratioemin Minimum void ratioHcrit Critical falling heightP1 1th Pattern of shutter platP2 2th Pattern of shutter platP3 3th Pattern of shutter platP4 4th Pattern of shutter plat

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1 Introduction

Accurate evaluation of soil properties in the laboratory tosimulate the natural soil conditions is important, especiallywhen the soil profile is under static and dynamic loadings.For example, the dynamic properties of a soil deposit (i.e.,shear modulus and damping) are remarkably significant inthe evaluation of the site response analyses and the soil–deepfoundation interaction [1,2]. On the other hand, to reconstructsoil samples in the laboratory, similar to natural conditions ofsoil configuration, is dependent on soil deposition method.According to previous studies carried out, the mechanicalbehavior of sandy soils is influenced by the effective stressand the void ratio [3]. Obtaining uniform sand sample isessential in order to achieve high accuracy and consistencywhen investigating soil characteristics. While repeatable anduniform clay specimens can be easily attained by trimminga sample from a large consolidated block; this process isnot applicable for sands due to little or no natural cohesionin sandy soils. Consequently, obtaining a uniform sampleis extremely difficult since the sandy specimens should beindividually prepared for each test. For this reason, reconsti-tuting sandy samples in the laboratory is desirable. There-fore, preparation methods of the sandy soil samples have toinvolve several criteria such as the preparation of dense andloose samples, having a uniform void ratio, and a well-mixedsample without separation of soil particles [4]. Based on thesefactors, several methods have been developed by investiga-tors such as sedimentation, dry funnel, and wet deposition[5].

The properties of the specimens prepared by each of theseapproaches have been proposed by researchers [4,6,9]. Forexample, the resistance to liquefaction for samples preparedby the sedimentation method is more than the resistanceexhibited by the wet deposition and the dry methods [6].However, the reported resistance of the latter two methodsdiffers from one another [7]. The particle size gradation isknown to be problematic in the wet deposition method, owingto falling of sand particles through water and the consequen-tial reduction of velocity of sand particles during deposition.Among the methods mentioned, the air pluviation is the mostpopular approach in sand samples’ preparation. Using thisapproach, samples can be produced over a wide range of den-sities. Different types of devices have been deployed to recon-struct samples by the air pluviation method [8]. Althoughthese devices have been utilized to prepare the sand samples,they are not able to reconstruct soil samples for large areas,owing to the static properties of these devices. For this rea-son, the sand collector or the test chamber has to be movedin order to cover a larger area. When using the Mobile Plu-viator, the test chamber can be fixed and the sampled soilcan be spread by a raining process over the whole area ofthe test chamber. This is particularly applicable for soil mod-

eling in centrifuge and shaking table applications, as theirinstrumentations are very sensitive to movement and stress.

2 Mobile Pluviator Apparatus

The pluviation methods have been developed based on thefree fall theory of spherical particles. In this theory, we con-sider the mass m of a falling body toward the surface of theearth, and the drag force resistance against it in an upwarddirection, and therefore, the particle velocity remains con-stant; and as a result, the particle acceleration will be equalto zero. Therefore, it can be mathematically shown as:

V =√

2mg

ρ ACd(1)

where ρ, g, and A are mass density, gravitational accelera-tion, and particle acceleration, respectively. Cd is the dragcoefficient. The mean size of the sand particles (D50) canbe idealized as a free fall of spheres during pluviation. Theinterference effect due to simultaneous fall of many particlesis usually neglected.

As mentioned in the foregoing section, the static prop-erties of the previously designed apparatuses are consid-ered a deficiency. Based on this defect, the newly designedMobile Pluviator was utilized in this research to reconstructthe dry sandy soil samples using the dry pluviation method.As shown in Fig. 1a, the newly developed Mobile Pluviatorconsists mainly of a soil bin (hopper), the diffuser system(the three sieves), the sand collector, a fixing device to set upthese components so that the whole system is carried by amoveable steel frame. The fixing device has sections of threesteel pipes arranged around a circle with 20 cm in diame-ter. The sand hopper used to stock up the sand has a conicalshape with a base diameter equal to 20 cm. The intensity ofthe sand deposition plays a very important role in obtainingthe desired relative density and in terms of controlling theflow rate of the sand mass. For this reason, different arrange-ments of holes, in terms of diameter and number, were tested(Fig. 1b). These patterns were formed on a wooden perfo-rated plate with a diameter of 20 cm (known as the shutterplate). The four patterns of the shutter plates were formedof different arrangements of holes to control the rate of thesoil discharge. The interchangeable circular wooden plateswere installed at the bottom of the sand hopper. The porosi-ties of the shutters were equal to 0.8, 1.88, 5.25, and 5.5 %for the shutters P1 (21 holes with diameter = 5 mm), P2(21 holes with diameter = 7 mm), P3 (21 holes with diam-eter = 10 mm), and P4 (16 holes with diameter = 13 mm),respectively. To attain the best diffusion results and to insureuniform deposition of particles, the utilized diffuser systemmade use of three standard sieves with different sizes based

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(a)

(b)

P4 p3 p2 p1

Sand Hopper

Sand Jets

Sand Collector

Sieves

Sand Rainy

Steel Profile

F

H

Fig. 1 Mobile Pluviator system comprised of: a Mobile Pluviator, and b shutter plates

on the maximum sand particle size without any sand accu-mulation on the sieves. The distance between sieves was 5cm; and it remained constant during the tests. As mentionedearlier, the drop (distance between the bottom sieve and thesand surface within the collector—referred to as H ) is animportant parameter in obtaining terminal velocity and sub-sequently the desired relative density. For this reason, thisdistance should be equal or more than the height determinedduring sand pluviation. In addition, the height F (distancebetween shutter plate and the top sieve) and H complementeach other. In other words, when F increases, H will decreaseand visa versa.

The sand collectors, in this research, were the three cylin-drical molds with 45 and 100 mm standard radius and height,respectively, constructed of PVC. A 225-cm2 plastic baseplate was glued to the bottom of each of the molds. A vac-

uum cleaner was used to level the sand surface of a sandsample by removing extra sand particles. The Mobile Pluvi-ator system moves manually over the area of the molds in thetwo horizontal directions.

It should be mentioned that vertical and horizontal veloc-ities of the particle are independent when the moving parti-cle is uniform and slow. Under these conditions, movementdirections have insignificant influence on the desired relativedensity.

3 Sands’ Properties and Testing Procedure

Before the performance of tests using the Mobile Pluviator,an initial series of tests were performed on the sand of JohorBahru, Malaysia. The grain size distributions for the sandy

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0

10

20

30

40

50

60

70

80

90

100

1010.10.01

Pre

cen

t F

iner

(%)

Grain Size(mm)

Soil A Soil B Soil C

Fig. 2 Grain size distribution curves of the three soils used in this study

Table 1 Properties of the three types of tested soils

Soil Gs Cu D50(mm)

ϒd max(kN/m3)

ϒd min(kN/m3)

emax emin

A 2.625 5.00 0.65 17.07 13.97 1.00 0.63

B 2.625 3.82 0.52 16.38 13.73 1.03 0.70

C 2.625 1.85 0.33 16.18 13.05 1.14 0.73

soils used in this study are shown in Fig. 2. As shown in Fig.2, the grain size distributions of soil A and soil B are verysimilar. Soil A is coarser than Soil B, while soil C is finer. Allof the soils are uniform and are classified as SP, accordingto the Unified Soil Classification System (USCS). As shownin Table 1, the medium diameter (mean size = D50) of thethree types of sand are 0.65, 0.53, and 0.33 for soil A, soil B,and soil C , respectively. Physical characteristics includingspecific gravity and maximum and minimum dry densitieswere measured using methods proposed by the AmericanSociety of Testing and Materials (ASTM). Other propertiesof these soils are shown in the Table 1.

The tests were carried out by selecting one of the para-meters as a variable and keeping other parameters constantduring each of the tests. After that, each of the three soilswas poured into the sand hopper and allowed to rain downwhen the plastic plate was removed from behind the woodenperforated plate. The sand jets were diffused by the threesieves (according to ASTM-D2487, No#4 sieve having open-ing 4.75 mm size) after falling through the F height. Thevolume and the weight of sand collected were measured, andthe relative density was computed as follows:

Dr =(

γd − γd min

γd max − γd min

)γd max

γd(2)

where γd min,γd max, and γd are the minimum, maximum, anddry unit weights of sand, respectively.

4 Results of the Tests Conducted

To establish a relationship between relative densities and thedifferent parameters, 180 sets of tests were carried out. Theinitial trial results illustrated that the different relative densi-ties could be successfully arrived at using the Mobile Pluvi-ator. Based on previous studies [9], the H distance should belarge enough so that soil particles reach the terminal veloc-ity at this height known as the critical falling height (Hcrit).Consequently, the falling height for the last particle has to beequal to the Hcrit . The results obtained for the relative densityproduced under different H distances and the holes’ patterns,for the three types of soils, are shown in Fig. 3. Figure 3 showsthe different relative densities (Dr) that can be achieved overa wide range from around 10 to 98 % of relative density usingthe Mobile Pluviator. The effects of the mean size of soil par-ticles on Dr are illustrated in Fig. 3. It is also worth notingin Fig. 3 that the value of relative density increased with theincrease of D50. This is due primarily to the fact that as thesoil particle increases in size, its impact on the soil particlesbelow will be higher. The falling height effect on Dr has beenreported by other researchers as well [10,11]. However, it isnoteworthy that when the falling height was less than thecritical falling height, this increase was significant. Relativedensities do vary with different shutter plates and with var-ied hole patterns. However, the contribution of H per sayis not significant. Therefore, when the size of the soil parti-cle increases, the maximum dry density can be attained byselecting the shutter plate pattern and adjusting the fallingheight.

The influence of the shutter porosities on relative densitycan be seen in Fig. 4. As the figure shows, there is a decreaseof Dr with an increase of the porosity value (=holes’ areaon shutter plate/shutter plate’s area) of the shutter plates, andthis decline in soil C(D50 = 0.33) is more than the other twosoils. It is theoretically apparent that the upward force wasmore against the heavier sand particles; however, the fallingheight required to attain the terminal velocity was increased.Based on previous studies carried out, the impact effects ondeposition are significantly dependent on the falling heightuntil the terminal velocity is reached by the sand particles.As would be expected, the impact energies decline with thedecrease of D50 owing to the decrease of particles’ weight.

As can be seen in Fig. 3, Hcrit is 60, 50, and 45 cm for soilA, B, and C , respectively. The horizontal movement of theMobile Pluviator does not have any effect on the relative den-sity. The relative density obtained is inversely proportionalto the shutter porosity.

Based on Figs. 3 and 4, the falling height and the rate ofpouring have opposite effects on relative density.

As indicated, the slow uniform movement of the Pluvia-tor can produce the desirable relative densities. The smallerporosity had the higher relative density. The results presented,

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Fig. 3 H distance versusrelative density at differentshutter porosities; a soil A(D50 = 0.65); b soil B(D50 = 0.53); c soil C(D50 = 0.33) (P1: shutter platewith 21 holes and diameter = 5mm, P2: shutter plate with 21holes and diameter = 7 mm, P3:shutter plate with 21 holes anddiameter = 10 mm, and P4:shutter plate with 16 holes anddiameter = 13 mm)

(c)

(b)

(a)

0102030405060708090

100

0 10 20 30 40 50 60 70

Dr(

%)

H(cm)

Soil A; F=50cm

P1 P2 P3 P4

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70

Dr(

%)

H-cm

SOIL B,F=50 cm

p1 p2 p3 p4

0102030405060708090

100

0 10 20 30 40 50 60 70

Dr(

%)

H(cm)

Soil C; F=50 cm

P1 P2 P3 P4

using the Mobile Pluviator, were in good agreement with theconclusions attained by previous researchers [12,13] with Has a variable. This outcome is supported by Fig. 5 where Fwas equal to 50 cm and the different values of H were tested.The lower Dr varied with soil type in the higher rate of sanddischarge per unit time and per unit deposition area. With

regard to uniform soils included, the relative density mayhave been affected by the actual grain size, but there was nota clear trend.

It is worth noting that the influence of the horizontal move-ment of the apparatus on the vertical terminal velocity of soilparticles can be ignored. It should be noted, as shown in

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0

10

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60

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90

100

15 25 35 45 55 65 75

Dr(

%)

H(cm)

Holes Pattern 1

Soil A Soil B Soil C

0

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30

40

50

60

70

80

90

100

15 25 35 45 55 65 75

Dr(

%)

H(cm)

Holes Pattern 2

Soil A Soil B Soil C

0

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40

50

60

70

80

90

100

15 25 35 45 55 65 75

Dr(

%)

H(cm)

Holes Pattern 3

Soil A Soil B Soil C

0

10

20

30

40

50

60

70

80

90

100

15 25 35 45 55 65 75

Dr(

%)

H(cm)

Holes Pattern 4

Soil A Soil B Soil C

Fig. 4 H distance versus relative density at different holes’ patterns

Fig. 5, that the lower magnitude of the relative density wasreached with higher shutter porosities using the Mobile Plu-viator system.

To verify the effects of F on Dr, the results obtainedare presented in Fig. 6. To avoid the falling height effectson the F distance, the falling height was kept constant andequal to 70 cm. In other words, the value was more thanHcrit reached from previous tests. Figure 6 shows that if dis-tance F increases beyond the critical value (almost 40 cm),the increase in the relative density is not significant. Thisincreasing trend was exhibited in the relationship betweenthe relative density and the falling height and has also beenreported by other researchers [14,15]. In addition, the meangrain size affected the value of the Dr. Moreover, the shutterporosities have an effect on Dr at different F distances. Sothat, the higher relative density was obtained with the leastshutter porosity.

Figure 7 illustrates the dependence of sample preparationmethod, vis-a-vis the Mobile Pluviator, on F and H . [13]reported this relationship. In fact, both terms (F and H ) con-trol the terminal velocity, because the velocity of the sand

particles that jet out from the shutter, and their impacts onthe top sieve are dependent on F . Therefore, the F distanceaffects the initial velocity of the rained soil particles fromthe bottom sieve and also affects the falling height (H ). Thisresult can be derived from Fig. 7, where an increase in Fresults in a decline of the H distance. It is worth noting thatthe influence of D50 on this interrelationship between F andH was more significant than the shutter porosity effects. Acomparison of the results shown in Fig. 7 for soil B and soilC explains the different magnitude of Dr for the differentgrain size, using the Mobile Pluviator.

5 Maximum Dry Density

The compaction effects on the dry density of cohesionlesssoils have been recognized so that vibratory compaction isused to obtain the maximum density in laboratory tests. In thisstudy, a series of tests were performed to compare the peakdry densities attained by ASTM method versus the MobilePluviator. A similar mold of ASTM method was utilized toavoid the size effects of the mold on the results obtained.

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Fig. 5 Shutter porosity versusrelative density for soils A, B,and C. H = distance betweenbottom sieve and sand surface

0

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20

30

40

50

60

70

80

90

100

0 5 10 15 20

Dr(

%)

Pi(%)

Soil A

H=20 CM H=30 cm H=40 cm H=50 cm H=60 cm

0102030405060708090

100

0 5 10 15 20

Dr(

%)

Pi(%)

Soil B

H=20 cm H=30 cm H=40 cm H=50 cm H=60 cm

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20

Dr(

%)

Pi(%)

Soil C

H= 20 cm H=30 cm H=40 cm H=50 cm H=60 cm

The results are shown in Table 2. It is remarkable that thepeak density obtained is slightly less than the one by ASTMmethod.

The results of Tables 1 and 2 indicate that the peak drydensities obtained using the Mobile Pluviator can be 10 %less than the dry densities produced by ASTM method forsandy soils with smaller and higher uniformity coefficients.To the contrary, the peak dry densities obtained from the twomethods are almost same in soil B, with uniformity coeffi-cient equal to 3.50. Similar results were obtained by Miuraand Toki [16], who used materials with different uniformity

coefficients. In fact, the flow of the non-uniform particlescauses segregation during pluviation.

A series of static direct shear tests were carried out toestimate the mechanical properties such as shear stress andfriction angle of the reconstructed samples. With referenceto the forgoing results, the samples were prepared from soilB using the Mobile Pluviator apparatus. The specimens hada height and a horizontal direction of 25 and 60 mm, respec-tively. The axial loads applied were 110.7, 219.7, and 437.7kPa. The samples were prepared with Dr of 35 and 75 % asloose and dense sand, respectively. After each sample was

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0102030405060708090

100

0 20 40 60 80 100

Dr(

%)

F(cm)

Soil B; H=70 cm

P1 P2 P3 P4

0102030405060708090

100

0 20 40 60 80 100

Dr(

%)

F(cm)

Soil C; H=70 cm

P1 P2 P3 P4

Fig. 6 Relative density versus F distance at different porosities

set up in the shear direct box, the test was conducted in twostages: consolidation and shearing.

The results shown in Fig. 7 indicate that the observeddecrease in density with increase in plate porosity varies sig-nificantly with soil type.

Figure 8 illustrates that shear failure loading in the densesand was slightly more than that in the loose sand. Therefore,

Table 2 Peak dry densities achieved by Mobile Pluviator

ϒd max (kN/m3)

H = 40 cm H = 50 cm H = 60 cm

Soil A

P1 16.6 16.77 16.84

P2 16.50 16.50 16.53

P3 16.50 16.50 16.52

P4 16.22 16.25 16.52

Soil B

P1 16.10 16.24 16.35

P2 16.12 16.10 16.30

P3 16.00 15.95 16.04

P4 15.20 15.73 15.64

Soil C

P1 15.44 15.44 15.44

P2 15.42 15.35 15.42

P3 15.15 15.25 15.25

P4 14.80 14.65 14.78

the mechanical characteristics of a soil are retained whenusing the Mobile Pluviator.

As would be expected, the peak shear strength dependson the confining stress. This phenomenon is shown in Fig.9, where the friction angle of two different densities of sandB was computed. Della et al. [5] attained similar results. Ascan be seen in Fig. 9, the friction angle is equal to 29.5◦ and35.7◦ for the loose and the dense sand, respectively.

6 Conclusion

In an attempt of understanding sand samples preparation ina large area for laboratory tests, a new Mobile Pluviator was

0

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50

60

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0 5 10 15 20

Dr(

%)

Pi(%)

Soil B

F=20cm,H=80cm F=30cm, H=70cm F=40cm,H=60cm

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20

Dr(

%)

Pi(%)

Soil C

F=20cm,H=80cm F=30cm,H=70cm F=40cm,H=60cm

Fig. 7 Shutter porosity distance versus relative density (effect of varying F on H )

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0

50

100

150

200

250

300

350

0 1 2 3 4 5

Stre

ss(k

Pa)

Strain(mm)

Soil B;Dr=35%

laoding 4 kG loading 8 kG loading 16 kG

0

50

100

150

200

250

300

350

400

0 1 2 3 4

Str

ess(

kPa)

Strain(mm)

Soil B;Dr=75%

loading 4 kG Loading 8 kG Loading 16 kG

Fig. 8 Obtained results of direct shear tests conducted on samplesextracted by Mobile Pluviator for Soil B at two different densities underthree different loads

designed using the air pluviation method. The pluviationmethods have been developed based on the free fall theoryof spherical particles. The Mobile Pluviator is able to movehorizontally in two directions. The newly developed MobilePluviator consists mainly of a soil bin (hopper), the diffusersystem (the three selected sieves), the sand collector, a fixingdevice to set up these components so that the whole systemis carried by a moveable steel frame. To control the flowrate of the sand mass, four shutter plates (wooden perforatedplates) with different arrangements of holes in terms of diam-eter and number were tested. Distances H and F were alsoimportant parameters in obtaining the terminal velocity and,subsequently, the desired relative density. While the appa-ratus is movable, different factors were examined to obtaina wide range of relative densities. Three sandy soils withdifferent grain size distributions were used. Based on resultsobtained, soil particles reached the terminal velocity at a criti-cal falling height (Hcrit) equal to 60, 50, and 45 for soils A, B,and C , respectively. The results show that if the distance F

y = 0.565x R² = 0.999

0

20

40

60

80

100

120

140

0 50 100 150 200 250

Shea

r St

ress

(kPa

)

Normal Stress(kPa)

Soil B; Dr=30%

y = 0.719x R² = 0.998

0

20

40

60

80

100

120

140

160

180

0 50 100 150 200 250Sh

ear

Stre

ss(k

Pa)

Normal Stress(kPa)

Soil B; Dr=75%

Fig. 9 Friction angle of samples prepared by Mobile Pluviator for SoilB at two different densities using direct shear test

increases beyond the critical value, the increase in the rel-ative density is not significant. The shutter porosities havean effect on Dr at different F distances. The higher relativedensity was obtained with the least shutter porosity. Basedon the results obtained, the mean grain size of the soil hasaffected the relative density so that the three sizes used couldproduce the sand samples repeatedly over a wide range ofDr ranging from 10 to 98 %. A series of tests were also per-formed to compare the peak dry densities attained by ASTMmethod versus the Mobile Pluviator. The peak dry densitiesobtained using the Mobile Pluviator can be 10 % less thanthe dry densities produced by ASTM method. The frictionangle of samples prepared by the Mobile Pluviator was equalto 29.5◦ and 35.7◦ for the loose and the dense sand, respec-tively. They were in good agreement with previous studies.In addition, and as observed, the influence of the horizontalmovement of the Mobile Pluviator had insignificant effecton the results obtained. However, there is yet some work thatremains to be done on Mobile Pluviator such as investigat-ing soils that have a combination of two or more uniformlygraded fractions.

Acknowledgments The research was undertaken with support fromthe ERGS grant (No. 4L026) of the Universiti Teknologi Malaysia.The first author would like to thank the Ministry of Higher Education(MOHE) for the financial supports during this study.

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