Effect of Grain Crushing and Bedding Plane Inclination on Ras en-Naqab

Natural Sand Behavior

Reyad Al Dwairi Department of Natural Resources and Chemical Engineering, Tafila Technical

University, Tafila, Jordan. e-mail: rdwairi@ttue du.jo

Omar Al-Hattamleh, Faisal Al-Shalabi, and Talib Al-Rousan

Department of Civil Engineering, The Hashemite University, Zarqa, Jordan e-mail: hattam@hue du.jo

ABSTRACT This pa per p resents an e xperimental investigation f or i nherent ani sotropy an d particle breakage for natural Ras en-Naqab sand southern Jordan. The nat ural sand specimens were subjected to one dimensional compression to induce breakage. Th e grain size distributions of the specimens were reported before and aft er the application of the stresses. Thereafter, the sand s hear st rength parameters were asses sed using direct shear box t ests. E xamining t he obtained results revealed that the amount of breakage due to one dimensional compression is of order higher t han t he amount occ urring d uring di rect shear t est. Peak shear s trength parameters h ave little o r no ch ange w ith in creasing particles b reakage. H owever, d ilatancy component o f shear st rength di minishes w ith i ncreasing the am ount of part icles breakage. Moreover, exa mining t he effect of a ngle of de position shown a cons iderable am ount of dilation occurs on the higher deposition angle regardless the extent of breakage reported. KEYWORDS: natural sand, granular materials, crushing, breakage, anisotropy, fabric, shear strength, dilatancy, direct shear.

INTRODUCTION The mechanics of particle crushing, or partic le breakage, is o ne of the most intractable

problems in geosciences. The topic is of interest to many research disciplines including powder technology, minerals and mining engineering, geology , geophy sics and geo mechanics (Einav, 2007). Grain crushing is one of the micro-mechanisms that govern the stress–strain behavior of a granular material, and also its permeab ility by altering the grain size distribution (Marketos and Bolton, 2007).

The description of the mechanical behavior of granular materials, such a s sands, clays, and powders is im portant to many fields of science and engineering. Granular asse mblies are intriguing s ystems rich with unusual properties such as dilatancy , arching, instabilit y, and thixotropy. These properties conspire to create a co mplex sy stem with numerous instabilities.

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Examples include liq uefaction failures, density waves in h opper f lows, and sti ck-slip motion in shear flows.

Thus, the behavior of granular materials is very co mplicated, and presently not well understood, behaving nei ther like solids nor like vi scous fluids. Stresses inside the granular medium are composed by multiple of stress chains, which can lead to local high-stress concentrations within the system. Such high stress concentration may lead to crushing of s ingle particle even under relatively low stresses. If gra nules are broken into smaller particles due to t he application of external fo rce, phy sical and mechanical properties of the granular matter will undergo significant change. Therefore, the original engineering properties with which a structure was designed will change during its engineering life. Exam ples of these stru ctures which will suffer from such changes are pavements, highway embankments, earth dams’ embankment, rock fill dams, and etc. Changes in the original engi neering properties could put the stability of such structures in jeopardy and make it unsafe during th eir life of operation. Therefor e, understanding the crushing in granular material s a nd its e volution during the process of com pression and shearing is of highly demands.

Literature is depleted in many experimental and numerical stud ies to thoro ughly understand the particle crushing phenomenon and to be able to figure out quantitativel y its im pact on the engineering properties of granular materials. Lobo-Guerrero and Vallejo (2005, 200 6) examined the effect of particle crushing on the ca pacity of driven piles. Their r esults indicated that the particles crushing negatively affect the capacity of piles. Okada et al. (2004) reported that grain crushing was in onset for trigging a landslide. An investigation of excess por e water pressure generation of a weathered granitic sand, taken fro m the source area of a ty pical landslide caused as a re sult of liquefaction, indicated that grai n crushing withi n the failure zone is the key phenomenon of rapid long -runout motion of landslides. Moreover, Fragaszy and Voss (1986) reported that particle breakage will cause settlements and reduction in hydraulic conductivity.

Experimental works on t he crushing of the gra nular materials point out m any ke y factors associated with the occurrence of particle crushing. It has been established that grain crushing is influenced b y soil particle strength, topolo gy such as angularity, granular materials phy sical properties such as gradati on, porosity, moisture content, and induced stres s level and anisotropy (Lee and Farhoomand 1967; Hardin 1985; Hagerty et al. 1993; Lade et al. 1996; McDowell and Bolton 1998; Takei et al. 2001; Coop et al. 2004; Tarantino and Hyde, 2005; and Lobo-Guerrero and Vallejo, 2005). Mo reover, previous research has indicated that the angle of shearing resistance of granular materials undergoing cr ushing decreases as a conseq uence of particle crushing (Bolton, 19 86; Feda, 2002). Co op et al . (2004) conducted ring shear tests on carbonate sand and rep orted that cru shing of particles occurred without a lo ss of residual angle of inte rnal friction. In this regards, the reported results gi ve the impression that crushable granular materials experience a reduction in the internal friction angle as a consequence of particle breakage prior to achieving a constant value of residual strength.

According to Lee and Farhoom and (1967), one of the most important factors influencing th e crushing of a mass of granular materials is the crushing resistance of the grains. Coarse granitic sand particles with an aver age diameter of 2.8 mm ex perienced breakage at pre ssures equal t o 2 MPa, while calcareous shells began crushing at 0.05 to 0.2 MPa (Lee and Farhoom and, 1967). Angular particles of freshl y quarried materials undergo fragmentation under or dinary pressures (about 0.98 MPa) due to breakdown of sharp angularities (Ramam urthy, 1974). When a granular

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mass is subjected to a compressive load, the partic les resist the load through a series of contacts between the grains in the shearing zone as pointed out by Oda et al. (2005), (Fig. 1).

Figure 1: Results of numerical simulation tests by distinct element method (DEM), taking into account rotational resistance at contacts (all results taken from same particle assembly sheared

above peak stress): (a) distribution of shear strain accumulated in a major shear band; (b) transmission of contact forces through columnar structure; (c) distribution of volumetric strain

As shown in Fig. l, particles do not share equa lly in the bearing of the applied load. Some particles carry m ore load than others. In fact , so me particles can actually be rem oved without affecting the mechanical equilibrium of the packing. The particles with highly loaded contacts are usually aligned in chains (Cundall and Strack, 19 79)e Crushing s tarts when these highl y loaded particles fail and break into smaller pieces that move into the voids of the original material. These load chains change in inte nsity and direction as the crushing develops in the particle assemblage. On crushing, fine grains a re produced and the grain size distribution curve be comes less st eep. Consequently, with contin uing crushing, the soil becomes less permeable and more resistant to crushing. Therefore, the grain size distribution is a suitable measure of extent of crushing material (Hardin, 1985; Lade et al. 1996; Tarantino and Hyde, 2005; and Einav, 2007).

Lade et al. (1996) found that if uniform sand is cr ushed, the resulting grain size distribution approaches that of a well graded soil for a large com pressive loads. However, before reaching a well graded particle distribution, the granular assem bly will experience gradual changes in particle size depending in the level of progressive load being applied.

Tarantino an d H yde (2 005) perform ed sim ple direct shear tests on carbon ate sand to demonstrate the affect of c rushing on shear strength properties. The shear te sts have been carried out on m ono-granular and fractal grain size distributions of crushable carbonate sand at vertical stresses ranging from 0.2 MPa to 1.4 MPa and horizontal displace ments from 0.5 mm to 8 mme In order to c ompare particle breakage of specimen s of different particle sizes, speci mens were

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prepared with a sample h eight of abou t 20D50e Grain size distributions were measured before and after shearing. They e stablished a link betw een grain crushing, shear strength and general mechanical behavior of sands. The y found that, the apparent critical state fricti on angle has been shown to contain both frictional and elastic components. They concluded that the apparent critical state angle o f friction increase s a s the rate of parti cle crushing norm alized with respect to the normal force, increases.

Figure 2: Location map of Ras en-Naqab Natural Sand deposits.

Moreover, with progression of particle breakage due to applied load, the void s pace between large particles filled with crushed material. Due to this process, the hydraulic conductivity for the material is r educed. For instance, for a sphalt pavement crushing of granular material can occur during installation and placement, compaction, and through daily vehicular traffic. Most serious problems are caused to as phalt paveme nts when th eir granular bases are una ble to rem ove the water that enters the pavement (Al-Qadi et al., 2004). Hence crushed material is full of voids in which water flows; as a result, an excess pore wa ter pressure will develop and produce failure in the granular base as well as in the pavement.

The motivation of this work is the fact that the strength and stress characteristics of sands as determined from conventional testing su ch as tria xial compression are a function of their i nitial fabric. The initial fabric of sand specimen is a function of t he method of sam ple preparation as pointed by Zlatovic and Ishihara (1997). Moreover, with altering initial (inherent) fabric, which is a function of bedding plane inclination (i.e., depos itional angle of sand particles) at a fixed void ratio and confining stress, Oda (1972) showed that the initial stiffness, peak strength, and volume dilation are varied.

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The main focus of this study is to separate the effect of particle breakage and depositional angle in the s hear strength parameters for granular material subjected to direct shear testse Up to the knowledge of the authors, this is the first ti me such an effort is being reportede The results from direct shear tests conducted on Ras en-Naqab sand will be presented and analyzed.

REGONAL GEOLOGY

The material used for the tests was nat ural sand fro m Ras en-Naqab area sout hern Jordan. Huge amount of silica sand resources of early Ordovician are found in this are a south of the Ras en-Naqab escarpment (70 km north of Ras en-Naqab) (Fig. 2). T he exposures are extended over an area of more than 150km2 with a thickness of about 265-350 m.

MATERIALS

In this study, Twenty fresh representative ra w silica sand channel sam ples w ere collected from Ras en-Naqab area. The Che mical co mposition of the used sa mples are listed in Table 1 (NRA, 2006) . Ras en-Naqab silica sand com posed mainly of SiO 2 >98.72%, with ver y l ittle contaminant oxide contents (i.e., Al2O3, TiO2, CaO, and Fe2O3) and heavy minerals of < 0.1% .

Table 1: Chemical analysis of sand samples from Ras en-Naqab area (NRA 2006).

Major Oxides % SiO2 98.7 2 Al2O3 0.52 Fe2O3 0.04 TiO2 0.09

CaO+ MgO 0.08 Na2O+K2O 0.11

Mineralogical investigation of Ras en-Naqab raw s and size fract ion indicated that they are consist mainly of q uartz as a major m ineral with m inor a m ounts of kaolin, where as mineralogical investigation of fine fraction (<63 micron) reveals that the fine fractions of the glass sand consist mainly of Kaolin, Feldspar and quartz as a major minerals with traces of heavy minerals such as rutile, illmenite (NRA, 2006) . Physical and engineering properties for Ras en- Naqab silica sand were determ ined and evaluated by Al Dwairi (1995). The specific gravity of Ras en-Naqab sand is 2. 63. Deter mination of particles shape was carri ed out by calculating sphericity. Sphericity was cal culated by Hydraulic sphericity and automatic i mage analyzer (Al Dwairi, 1995), the calculated sphericity b y using the two methods ranges between 0.87 - 0.90e Thus, the par ticles shapes are subrounded in shape. The grain size distribution of Ras en-Naqab sand indicates that it contains more than 98% of sandy fraction. The grain siz e diameters D10, D30 and D60 are 0.922mm, 1.135mm and 1.579mm respectively. The coefficient of cur vature is 0.88 while the coefficient of uniform ity is 1.7. Therefore, Ras en-Naqab sand is classified as poorly graded sand, SP according to th e Unified Soil Classification S ystem (USCS)e Moreover, the maximum and the minimum void ratios were found to be 0.78 and 0.54.

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EQUIPMENT AND PROCEDURES

The natural sand samples were prepared as follows. The sand was rained from specific height to a standard com paction test mold of the size 1/30ft 3. Thereafter the sand was subjected to one dimensional co mpression using a hy draulic jack to the desired pressure. The sand speci men is then sieved. These specimens were then subjected to direct shear tests using a standard laboratory shear box apparatus with an initial sample cross-section of 60 mm x 50 mm. Followed Tarantino and Hyde (2005) the sam ple height in all testing w as kept above 28 D50. T his is due to t he fact that experimental results reported b y Finno et al. (1997) revealed t hat the thickness of the shear zone lies between 10 and 25 times D50.

The sand samples were prepared by sieving a predetermined mass of sand over a n open box-shaped metal grid into the shear box, and then slowly raising the grid. This technique assures that there is no segregation of the particle si zes during the deposition process. Moreover, to achieve different bedding angles, the Shear box was placed in adjustable plate connected with pin rotating around the horizontal plane through 90 degrees (Fig. 3).

Figure 3: Sample preparation techniques to achieve well-graded samples in given depositional angle, θ

A se ries of s hear test s was ca rried out, each at a co nstant vertica l effective str ess σv, with values of 55, 110, and 16 5 kPa and total horizont al displacement of 12 mm. Other sets of tests were also conducted after appl ying one dimensional compression in order to quantify the amount of grain crushing due to vertical co mpression and horizontal shearing. This made it possible t o differentiate crushing occurring during the horizontal shearing stage. Table 2 summarizes all tests performed, including void ratios after o ne dimensional compression, and af ter pluviation before direct shear tests.

Adjustable Height Sieve

Rained Sand Grains

Direct Shear Cell

θ

Adjustable Rack Angles

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Table 2: Test Summary

One dimensional compression

(MPa)

Initial Voids

Ratio, e

Voids Ratio, e (after

compression)

Deposition Angle, θ (o)

Vertical Stress, σv: kPa

Horizontal Displacement,

x: mm

Voids Ratio, e (after

pluviation)

0 - -

0 15 30 45 60

55, 110, 165 55, 110, 165 55, 110, 165 55, 110, 165 55, 110, 165

12 12 12 12 12

0.644 0.632 0.644 0.640 0.631

4.87 0. 775 0.656

0 15 30 45 60

55, 110, 165 55, 110, 165 55, 110, 165 55, 110, 165 55, 110, 165

12 12 12 12 12

0.644 0.638 0.641 0.644 0.641

14.62 0 .775 0.571

0 15 30 45 60

55, 110, 165 55, 110, 165 55, 110, 165 55, 110, 165 55, 110, 165

12 12 12 12 12

0.644 0.639 0.644 0.642 0.644

19.49 0 .775 0.552

0 15 30 45 60

55, 110, 165 55, 110, 165 55, 110, 165 55, 110, 165 55, 110, 165

12 12 12 12 12

0.638 0.641 0.644 0.639 0.644

Figure 4: (a)

0

50

100

150

200

250

0 2 4 6 8 10 12 14

Shea

r Stre

ss, τ

(kPa

)

Horizontal Displacement (mm)

55110165

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(b)

Figure 4: Direct shear test on natural Ras en-Naqab sand (a) shear stresses (b) vertical displacement, legend values are σn in kPa

EXPERIMENTAL RESULTS Fig. 4 shows the results of direct shear t ests on the sand sample without prior to one-

dimensional compression. The figure clearly shows that the she ar stress on the sand spe cimens increases gradually to the peak value and then gradual softening to the residual value, Fig. 4ae On the other hand, the volume change increases until a displacement of 7 mm has been reached, after which it becomes asymptotic to horizontal.

-0.5

0

0.5

1

1.5

0 2 4 6 8 10 12 14

Ver

tical

Dis

plac

emen

t (m

m)

Horizontal Displacement (mm)

55

110

165

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(b)

Figure 5: Influence of bedding plane orientation on sand behavior under direct shear test on

natural Ras en-Naqab sand, (a) shear stresses (b) vertical displacement, σn=55kPa

(a)

-0.5

0

0.5

1

1.5

0 2 4 6 8 10 12 14

Ver

tical

Dis

plac

emen

t (m

m)

Horizontal Displacement (mm)

θ=0θ=15θ=30θ=45θ=60

0102030405060708090

100

0 2 4 6 8 10 12 14

Horizontal Displacement (mm)

Shea

r Stre

ss (k

Pa)

θ=0θ=15θ=30θ=45θ=60

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(b)

Figure 6: Influence of bedding plane orientation on sand behavior under direct shear test on natural Ras en-Naqab sand, (a) shear stresses (b) vertical displacement, σn=110kPa

The corresponding peak and residual shear stres ses as a function of the app lied vertical stresses is shown in Fig.8a and 8be It is clearly shown that the bedding plane affects the r esponse of the sand speci men to shearing in all reported range of vertical stresse s. The speci mens of a deposition angle of 30o w ith respect to the horizont al plane (ie . parallel to the shearing plane) show the lowest values of shear stresses. Moreover, the same specimens show the higher amount

(a)

0

20

40

60

80

100

120

140

160

180

0 2 4 6 8 10 12 14

Horizontal Displacement (mm)

She

ar S

tress

(kP

a)θ=0θ=15θ=30θ=45θ=60

-0.5

0

0.5

1

1.5

0 2 4 6 8 10 12 14

Ver

tical

Dis

plac

emen

t (m

m)

Horizontal Displacement (mm)

θ=0θ=15θ=30θ=45θ=60

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of initial com pression an d the lowest amount of dilation. However, speci mens of a deposition angle of 60o and 45o sh ow the higher values of shear stresses response. Similar Results were reported b y Oda (1972) and Tatsuok a (1990) for sand tested on conventi onal triaxial teste Furthermore, it is noticed that after the constant volume has been reached, the s tress strain curve start to depart away from this condition after a displacement of 6 mm (see Fig. 4 and Fig. 5)e This mainly due to the fact that in direct shear tester, n on-uniform shearing caused non-horiz ontal sliding, therefore, it prevents constant-volume conditions to be stabilized as clarified by Tarantino and Hyde (2005).

(b)

Figure 7: Influence of bedding plane orientation on sand behavior under direct shear test on natural Ras en-Naqab sand, (a) shear stresses (b) vertical displacement, σn=165kPa

-0.5

0

0.5

1

1.5

0 2 4 6 8 10 12 14

Ver

tical

Dis

plac

emen

t (m

m)

Horizontal Displacement (mm)

θ=0

θ=15θ=30

0

20

40

60

80

100

120

140

160

180

200

0 2 4 6 8 10 12 14

She

ar S

tress

(kP

a)

Horizontal Displacement (mm)

θ=0

θ=15θ=30

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(b)

Figure 8: (a) Peak (b) Residual horizontal (shear) stresses versus vertical stresses for different bedding plane angles, θ.

(a)

0

50

100

150

200

250

0 50 100 150 200

Hor

izon

tal S

tress

, τ(k

Pa)

Vertical Stress, σn (kPa)

θ=0θ=15θ=30θ=45θ=60

0

50

100

150

200

250

0 50 100 150 200

Vertical Stress, σn (kPa)

Hor

izon

tal S

tress

, τ (k

Pa)

θ=0θ=15θ=30θ=45θ=60

Vol. 14 [2009], Bund. L 13

The effect of one dimensional co mpression on gr ain size distribution is shown in Figure 9e The initial and final grain size distrib ution afte r com pression is shown. It is obvious as the vertical stress increases the amount of particle breakage increases.

The amounts of particles crushing (bre akage) were evaluated based on the m odified Hardin (1985) formula by Einav (2007)e Einav (2007) postulates that the grain size distribution will start from an initial grading and ultim ately reaches a final grading due to shearing and com pression. The relative breakage index, Br, is defined as an area ratio

BpBtBr = (1)

where Bt and Bp are shown in Figure 10.

In this fi gure Bp, the ‘bre akage potential’ , is defined by integrating the entire area confined between the initial and final grain size distribution whilst Bt is the area betw een the current (at given com pression stres s), and the initial one, wh en there is no applied shear or com pression stresses. The relative breakage index ev olution as fu nction of com pression stresses i s shown in Fig. 11. It is obvious from this figure that ther e is an asymptotic value for frag mentation of sand particles upon increasing the co mpression pressure app lied to the sand particl es. Similar results were also deduced by Einav (2007).

Figure 9: Final grain size curve of samples after vertical one-dimensional compression.

0102030405060708090

100

0.01 0.1 1 10

Grain Size Diameter (mm)

% F

iner

0.00 MPa4.87 MPa9.74 MPa14.62 MPa19.49 MPa

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Figure 10: Einav (2007) modification definition of Hardin’s breakage index Br.

Figure 11: Evolution of The relative breakage index in one dimensional compression

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20 25

Compression Stresses (MPa)

Rel

ativ

e B

reak

age

Inde

x, B

r

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The response of stress strain curve at relativ e breakage index, Br=0.74 for diff erent bedding plane under normal stresses of σn=55kPa, 110kPa and 165kPa are shown in Fi gs. 12, 13 and 14e It is clearly shown from these figures that in case the bedding plane is horizontal ( θ=0o) and parallel to the direction of shearing, the sa nd has the larger peak strength and the h igher volumetric dilation. Hence, the bedd ing plane be ing parallel to t he horizontal im plies simply more contacts normals are oriented hori zontally so that the specimen appears to be strong in this direction. Therefore, there is le ss poten tial for volume changes to occur. On other hand, if the bedding plan e ( θ=60o) most contact normals ar e vertical and then the materi als appears t o be overly weak.

The grain size distribution for the sand speci men before and after shearing at Br=0.74 is shown in Fig. 15e It is obvious that the am ount of particle breakage for this sand under direct shear condition is insignificant; the change in Br is less than 1.5%e This is because the quartz and feldspars are generally hard particles which resi st disintegration during shear process. There fore, crushing of this sand under conven tional applied s hear stress is in consequential. Similar t rends were also observed for the other reported values of Br.

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(b)

Figure 12: (a) Stress ratio τ/σn (b) vertical displacement at Br=0.74 for different bedding plane, Normal stress=55 kPa

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 2 4 6 8 10 12 14

Horizontal Deformation (mm)

τ/σ

n θ=0

θ=15

θ=30

θ=45

θ=60

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14

Horizontal Deformation (mm)

Ver

tical

Dis

plac

emen

t (m

m)

θ=0

θ=15

θ=30

θ=45

θ=60

(a)

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(b)

Figure 13: (a) Stress rati o τ/σn (b) vertical displacem ent at Br=0.74 for diff erent bedding plane, Normal stress=110 kPa

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 2 4 6 8 10 12 14

Horizontal Deformation (mm)

τ/σ

n

θ=0θ=15θ=30θ=45θ=60

-0.2

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10 12 14

Horizontal Deformation (mm)

Ver

tical

Dis

plac

emen

t (m

m)

θ=0θ=15θ=30θ=45θ=60

(a)

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Figure 14: (a) Stress ratio τ/σn (b) vertical displacement at Br=0.74 for different bedding plane,

Normal stress=165 kPa

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 2 4 6 8 10 12 14

Horizontal Deformation (mm)

τ/σ

n θ=0θ=15θ=30θ=45θ=60

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10 12 14

Horizontal Deformation (mm)

Verti

cal D

ispl

acem

ent (

mm

)

θ=0θ=15θ=30θ=45θ=60

(a)

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Figure 15: Contributions to shear resistance of granular materials (Guo and So (2007)).

Effect of Sand Breakage on the Shear Strength Parameters

For soil sheared in a direc t shear box, Taylor (1948) derived a si mple soil m odel based on energy considerations. In this model, the peak internal friction angle is given as

ψφφ tantantan residualmax += (2)

where

n

imummaxpeakmax tantan

στφφ == (2a)

in which τ and σn are the applied maximum shear and normal stresses;

n

residualcvresidual tantan

στφφ == (2b)

φresidual, residual internal friction angle and φcv, internal friction angle at constant volum e, τresidual, the residual shear stresses or the shear stresses at constant volume;

γεψ

ddtan −= (2c)

in which dε is the increment of normal stress and dγ is the increment of shear strain.

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In order to evaluate the effect of particles breakage on the shear strength parameters, the peak and residual internal friction angles were evaluated for different dire ct shear stress tests preceded with one dimensional co mpression te stse Fig. 16 shows the difference bet ween the maximum internal friction angle and the residual friction angle measured in the direct shear box. This figure clearly shows that as the one dimensional compression stress increases the difference between the peak and residual angles decreases. On other wo rds, the dilatancy com ponent of the shear strength suppressed (i.e., the dilatancy angl e diminishes wit h increasing one dim ensional compression)e Moreover, at Br=0.49 the mea sured values of dilatancy angle s were de creased then started to increase again at Br=0.74e This behavior is due to the fa ct that with the increases in crushing stresses, the asperities of the sand grai ns were flattened, howev er, with increases the compressive crushing stresses n ew asperities w ere developed due to the fract ure of sand grain, which in turn increas es t he interlocking energy due to the rearrangem ent of the soil parti clese Furthermore, at depositional angle of θ=30o, the am ount of difference between the peak and residual frictional angles is the lowest regardless the amount of grains breakage.

Figure 16: Evolution of dilatancy angle with relative breakage index

CONCLUSIONS Inherent anisotropy and p article breakage for natural Ras en-Naqab sand were investigated

experimentally. The natural sand speci mens were subjected to one dimensional co mpression to induce breakage. The grain size distributions of the specimens were reported before and after the application of the stres ses. Thereafter, the sa nd she ar strength para meters we re as sessed using direct si mple shear. Examining the obtained resu lts revealed that th e peak shear strength parameters h ave little or no change. However, i nspection of the residual shear strength parameters, ie . the apparent critical stat e angle of friction, showed an increase as the am ount of particle crus hing increases, regardle ss of the level of the ap plied norm al stres s. Mor eover,

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0 0.2 0.4 0.6 0.8 1

Relative Breakage Index, Br

ω=t

an−1

(tan

φ'm

ax−t

anφ'

resi

dual

) θ=0θ=15θ=30θ=45θ=60

Vol. 14 [2009], Bund. L 21

examining the effect of angle of dep osition, it shows a considerable amount of the dilation on the higher deposition angle regardless the extent of breakage reported

ACKNOWLEDGEMENTS The authors wish to acknowledge the contribution of Eng. Hussein Al-Deeky, who carri ed

out the experimental tests.

REFERENCES 1. Al Dwairi, R. A. (1995) “Evaluation of Jordanian Ordovician Glass Sand as Filter Media

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