Transparent soil in Geotechnical engineering

INTRODUCTION TO

TRANSPARENT SOILS IN

GEOTECHNICAL ENGINEERING(PARTICLE PROPERTIES)

Emad Maleksaeedi

Université de Sherbrooke

Decembre 2015

What Is a Transparent Soil?

• Transparent soil is a reflective index-matched system which can be monitored by image processat high spatial and temporal resolutions at small laboratory scale (Optical Methods).

• Most transparent materials are dielectric which means that they are poor conductors of electricity

What Is The Philosophy of Transparent System ?

• When a light beam strikes a transparent medium itgoes through with a different speed than its speed invacuum and it passes through the dielectric material atspeed “v” less than the speed of light in vacuum.

• The ratio between the speed of light in a matter and in vacuum is expressed by the refractive index (𝑹𝑰 𝒐𝒓 𝒏),which is a characteristic of a material.

• The velocity change of light beams within transparent materials results in a changein orientation of light beams.

• In granular material, the voids between the particles can be filled with a material orliquid that has the same refractive index. Thus, the whole medium appearstransparent because no reflection occurs at the particle boundaries.

The basic concept of transparent synthetic soils is to use a matched refractiveindex liquid to fill up the voids in between transparent solid particles

What Are The Transparency Degradations?

• Air diffusion and trapped air in the solid-liquid mix.• Failing to exact matching the refractive indices of solids and pore fluid.• Temperature variation may affect the refractive index match especially for liquid phase.• Changes in the properties of the pore fluid such as transparency with time.

• Using transparent soil to assess soil-structure interaction during geogrid pull-out tests

• Using Transparent soil to study the ground displacements during pile placement and tunneling.

• Using transparent soil for evaluation of colloid transport

• Using transparent soil to determine fluid distribution pattern in multi-phase media

• Introducing air to a transparent soil to examine multi-phase flow

Some Examples of Studies By Transparent Soils !

What Types of Soil Particles Are Used In Transparent Soil?

• Glass beads• Quartz Powders• Transparent amorphous silica powders• Colloidal form of silica known as Silica gel• A water-based transparent synthetic polymer, named Aquabeads,

What Types of Materials Are Used To Model Liquid Phase ?

• Water• Mineral oils• Petroleum-based oils• Sucrose solutions

General Comments On Solid Particles!

• Glass beans and quartz powders are less used due to their inability to represent the geotechnicalproperties of a wide range of natural soils and also by their poor transparency

• Quartz powders only exhibits single porosity behavior.• Amorphous silica powder are suitable to model the geotechnical properties of natural clays• Silica gels are proper to model sand• Both silica gels and amorphous silica powder have the same reflective index and can be used with the

same pore fluids.• Silica gels can have both intra- and inter- aggregated porosity.• Aquabeads are weak particles suitable for some natural soils or sediments with weak strength.

General Comments On Liquid Phase!

• Common mineral oils in studying transparent soils are the mixture of Petro Canada Krystol40 and PetroCanada Puretol7, Petro Canada Krystol40 and Life Brand™ Unscented Baby Oil, the blend of Drakeol 35mineral oil and Norpar® 12 paraffinic solvent or Calcium Bromide and water

• The mixtures are quite colorless, odorless and stable with a neutral pH. Also the RI and transparency ofthe oil mixture is stable over time.

• Both mineral oil and sucrose solution are highly viscose.• Mineral oils might degrade the latex membrane used in shear strength tests.

AquabeadsWhat is aquabeads?

• Aquabeads is a strong water adsorption polymer sold.• It has the same reflective index like water and is compatible to water, oil, alcohols.• Chemically it is composed of isobutylene and maleic anhydride copolymer• In dry condition it is light yellowish spherical particles and in saturated state, they become very

clear if colorless fluid is chosen (transparent).• Its dry density is about 980 g/lit, a pH of 9-10 and is have good durability and stable under

different temperature conditions• Aquabeads can absorb water up to 200 times their own weight.

What are the practical application of aquabeads?

• Designed for ground improvement purposes such as stabilization of soft soils or mud by mixingwith in-situ soils that contain high water contents during excavation.

• It is utilized to manufacture chemical pocket heaters, and perfume carriers

AquabeadsWhat types of aquabeads are available?

• Aquabeads M:o dry diameter of 0.6 to 2 mm while by absorbing water it reaches to 5-7mmo It can be crushed to produce lower hydraulic permeabilityo The crushed materials are named C50 and C75

• Aquabeads 200:o it is powder passing #200 sieve and it becomes transparent gel in mix with water.

AquabeadsGrain Size Distribution:

• The grain size distributions of both dry and hydrated Aquabeads exhibit poorly graded or uniform behavior.

• It is possible to a wider range of particles by mixing the aquabeads.

AquabeadsHydraulic Conductivity:

• Fixed wall permeability tests indicate that the hydraulic conductivity of different types of Aquabeads ranges from 7×10-2 to 2x10-5 cm/sec

• It undergoes volume change due to small variations in confining pressure. Thus, compressibility affects permeability.

• It is suitable for representing the hydraulic conductivities of sands and silts.

• Consolidated Aquabeads are suitable for modeling clays.

AquabeadsCompressibility of aquabeads:

• Aquabeads changes its volume up to 200 times when it absorbs water.

• Aquabeads exhibits double-porosity behavior due to construction process.

• It has high apparent void ratio due to inter-aggregate pores.

• Volume change is time- dependent and the end of primarily consolidation is not always identifiable.

• Since aquabeads have intra- and inter-aggregate pore, their behavior is similar to peat and organic clays• The compression and recompression indices (Cc and Cr) are 0.1-0.15 and 0.02-0.03. their behavior is

similar to Ft. Gordon Clay, Georgia: Cc=0.12; New Orleans clay: Cr=0.05; Montana clay: Cr=0.05 andMontmorillonite minerals. The ratio Cr/Cc is 0.2 for aquabeads, which is within the normal range of 0.02to 0.2 for natural soils

AquabeadsStrength of aquabeads:

• Aquabeads are very weak . They exhibit solid-like behavior at low stresses and liquid-like behavior athigh stresses.

• It is difficult to form a Triaxial specimens from aquabeads.• In order to assess their strength, it is proposed to use Laboratory vane test.• Aquabeads are proper to evaluate the yield behavior of super soft clays.

Silica PowdersWhat are Silica Powders?

• Silica powders or precipitated silica are a type of compound among the amorphous solids.• Amorphous solids are any non-crystalline materials such as glass, plastics, and gels• Amorphous solids do not have a definite melting pressure.• Amorphous solids are prepared by cooling from their liquid or gas state.• Amorphous silica can be in prepared in forms of gels or powders• Amorphous silica can have different variety range from dense to loose, impervious to

porous, or even synthetic products.• Amorphous silica can be either hygroscopic, hydrophilic, or hydrophobicWhat are the Experimental Application of Amorphous Silica?

• Silica powders have the shear strength, consolidation and permeability, macroscopicproperties of many natural clays but not all of them.

• Silica powders can be used to model both over-consolidated and normally consolidatedclays

Silica PowdersPhysical properties:

• They consist of elementary particle arrangements with individual size of almost 0.02𝜇𝑚• They can combine to have a porous aggregate medium with large surface area like clays• They have double porosity behavior thus they have very low bulk density compared with clays• They are available in colorless or white packs• They have the reflective index range of 1.41-1.46• Different particle size range available. The most common is 1.4𝜇𝑚 to 1.46𝜇𝑚

Silica Powders

What Type of Pore Fluids Can Be Used?

• Two mixtures have been proposed :1. 1: 1 blend of Drakeol 35 mineral oil and Norpar 12 paraffinic solvent by weight

- The reflective index of 1.447, viscosity of 5.0 cP, and density of 800 𝑘𝑔 𝑚3 at room temperature

2. The mixture of calcium bromide and water- The reflective index of 1.448, viscosity of 3.6 cP, and density of 1572 𝑘𝑔 𝑚3 at room temperature

• Due to high boiling temperature, fluid content can not be measured in conventional way.

Physical properties:

• Due to double porosity behavior of silica powders, they have higher apparent void ratio.• It is proposed to use the inter-aggregate void ratio rather than geotechnical void ratio.• The absorption factor of silica powders are about 2.1 𝑐𝑚3 per gram of silica

Silica PowdersUndrained Triaxial Test:

• Researchers perfomed conventional triaxial tests on two sizes of particles, FGSP (25𝜇𝑚) and HST600 (1.4𝜇𝑚)• Due to applying oil pressure, special set-up must be used for maintaining the back pressure.• Normally consolidated specimens were tested under

various pressure (≈ 70𝑘𝑃𝑎 − 400𝑘𝑃𝑎)• Over-consolidated specimens with ranging OCR of 2-8

were tested under (≈ 30𝑘𝑃𝑎 − 140𝑘𝑃𝑎) pressure.

Silica Powders• Normally Consolidated behavior:

• The behavior of silica powders are consistent with many natural clays

• They have higher normalized strength but lower modulus than natural clays

• The peak strength of silica powders is larger than natural clays

• The initial Young’s modulus, calculated from undrained triaxial tests was 3–19 MPa for both FGSP and HST600.

• For normally consolidated, 𝐸𝑠 𝜎3𝑐is about 30–60, which is smaller than natural clays.

• For undrained tests Poisson’s ratio was 0.45–0.5.

Silica Powders• Normally Consolidated behavior:

• For silica powders, the average value of Skempton’s parameter at failure, 𝐴𝑓 , is about 0.35–0.7 which for natural clays in normally consolidated condition is almost 0.5–1.3 and for over-consolidated clays is about 0.25–0.7.

• The modified Mohr Coulomb failure envelope for normally consolidated undrained silica powder demonstrates that the total stress parameters are c = 0–16 kPa and φ = 19–21° while the effective stress parameters are c =0 and φ =36°.

• The undrained shear parameters are higher than the strength reported for most clays but within the range of natural clays.

Silica Powders• Over-Consolidated behavior:

• The over-consolidated behavior of silica powders are consistent with the behavior of over-consolidated natural clays. The strength is within the range reported for natural clay, but near the upper boundary.

• They exhibit the same strain softening behavior as the clays • Positive pore water pressures increase with strain, and then the familiar

drop in pore pressure occurred with further strain.• Negative pore pressures decrease with increasing overconsolidation.

Silica Powders• The pore pressure coefficient decreases with over-consolidation.• 𝐴𝑓 decreases from positive to negative with increasing over-consolidation.• For Over-consolidation condition, 𝐴𝑓 within the range reported for natural

clays. However, 𝐴𝑓is lower for normally consolidated silica powder than that of natural clays.

• In over-consolidation condition, 𝐸𝑠 𝜎3𝑐 approaches the reported values for natural clays.

Silica PowdersDrained Triaxial Test:

• Researchers performed conventional triaxial tests on two sizes of particles, FGSP (25𝜇𝑚) and HST6001.4𝜇𝑚

• The volume change in drained tests is similar to the pore pressure build up in undrained tests for bothnormally and over-consolidated specimens

• Volume change shows a remarkable agreement with natural clays in both normally and over-consolidatedconditions for drained condition.

• The effective-stress strength parameters for the drained tests is slightly smaller than the effective stressparameters obtained from undrained tests.

• For drained tests, Poisson’s ratio were between 0.2 and 0.3.• The initial Young’s modulus for drained tests were between 4–15 MPa

Silica Powders

Silica PowdersConsolidation Properties:

• Standard one dimensional (Ko) consolidation tests were performed using conventional oedometer apparatuswith fixed weights.

• Isotropic consolidation tests were also carried out in a triaxial cell using computerized apparatus.

Silica Powders• The main difficulty relating to consolidation behavior is identifying the end of primary consolidation.• Normally the void ratio is read after 24hours under constant loading.• The compression and recompression indices (𝐶𝑐 , 𝐶𝑟) are about 1.6–3.0 and 0.15–0.3 for silica powders. Both

indices are within the range typically reported for clay Montmorillonite minerals, The ratio Cr/Cc is around0.1 which is within the normal range of 0.02 to 0.2 for natural soils.

Silica Powders• For Ko consolidation, the volume change was calculated based on the sample settlement. However, for

isotropic consolidation, it was determined by measuring the amount of oil flowing in and out of the pumpconnected to the sample.

• The main problem of consolidation tests on silica powders was the fact that the volume change continued toincrease with time and the end of primarily consolidation was not identifiable.

• Terzaghi’s theory of consolidation cannot account for volume change due to the occurrence of largesecondary consolidation this phenomenon is due to the micro-pore structure of a typical amorphous silicapowders

• Similar behaviors were reported in the consolidation of peat and organic clays.

• Reduction in permeability is the main reason for observing such variation from Terzaghi’s theory of consolidation.

Silica PowdersPermeability Properties:

• The permeability of silica powders were studied by performing a constant head setup and flexible wall (FW)permeability tests and rigid wall (RW) permeability tests were performed on HST600 and FGSP). Tests werecarried out on specimens 64 mm

• Hydraulic conductivities ranging between 2.3×10-9 to 2.5×10-7 m/sec were measured, within the range ofnatural soils with hydraulic conductivities to water ranging between 1.5×10-8 to 1.6×10-6 m/sec.

• The permeability decreased linearly with void ratio, similar to the behavior of clay minerals.• The permeability decreased by increasing pressure. The permeability was linear for RW tests but not for FW

tests.

Silica GelsWhat are Silica Gels?

• Silica gel is a colloidal form of amorphous silica. They are similar to coarse white sand.• Mainly used as a desiccant for packaging, drying agent, as a catalyst or catalyst carrier, and in purifying

various substances, buffing media in toothpaste and in chromatography.• First time appearance was in 17th century as a powerful adsorbing agent in gas masks during World

War I.• Can be produced in different sizes (0.5mm to 7.0mm) and shapes (rounded beads and angular particles)

Silica GelsWhat are Silica Gels?

• Silica Gel is highly porous with double or multi-porosity behavior.• Silica gel is inert and non-toxic.• silica gel manufacturing happens in three steps. The first step is production of sodium silicate from

crushed quartz or sand. The second step is the release of silicic acid from the sodium silicate by means of asuitable acid followed by the conversion of the hydrogel into xerogel (dried gel). The third stage is thegrinding and grading of the xerogel to different shape and size

• Spherical silica gel is not grinded. Usually it is prepared by spraying a neutralized silicate solution intofine droplets before gelling. Drying the droplets in a stream of hot air results in the required sphericalsolid shape of silica gel.

Silica GelsPhysical properties:• The internal structure of silica gel particles is very different from sands because it is highly porous.• The dried silica gel (xerogel) may have particle size range from 3-30 nanometer.• The specific gravity of silica gels is about 2.2 which is approximately 80% of the specific gravity of

natural silicate sands.• Depending on shape and grain size, the dry unit weight is 6–9 kN/m3 (40–60 pcf).• Depending on the pore fluid, The saturated unit weight ia about11-14 kN/m3 (70-90 pcf).• Due to multi- porosity behavior of silica gels, they have higher apparent void ratio.• It is proposed to use the inter-aggregate void ratio rather than geotechnical void ratio.• The absorption factor of silica powders are about 0.43 𝑐𝑚3 per gram of silica

Silica Gels

What Type of Pore Fluids Can Be Used?

• The same type of pore fluids that are used for silica powders can be used for silica gels.• Stability of the colorless appearance may vary with time.

Physical properties:• Commercial silica gels are usually sold in uniform size ranges, i.e. the purchased silica will have a

pre-defined maximum particle size and minimum particle size.

Silica GelsShear Strength under Static Loading:Triaxial Test and Direct Shear Test Preparation:

• The main problem in sample preparation is due to theexposure of latex membrane to pore fluid particularlywhen pore fluid is mineral oil and solvent.

• Reaching saturation is time-consuming therefore,soaking before forming the specimen should be done.

• Conventionally two types of silica gels were selected inlaboratory for geotechnical purposes; SG1 which isangular, 0.5-1.5mm in size and SG2 which is round,2.0-5.0mm in size.

• Silica gels can not be used to identify the fine sands.• Traditional procedure for test preparation can be done

for dry state.• CD triaxial tests are common to preform in literature.

Triaxial test results:

Silica Gels• For angular silica beads (SG1), in both dense and loose condition, the results are consistent with almost all types

of sands.• Dense specimens exhibit softening behavior particularly at low confining pressure.• Dense specimens have higher peak strength compared to dense sands, probably due to compressibility of beads• Saturation increases the shear strength due to the increase in unit weight of saturated samples or because pore

fluids are like anti-lubricant.

Triaxial test results :• For round silica beads (SG2), the stress-strain behavior is more consistent with typical coarse sands.• The peak strength value occurs at lower strains values compared to SG1• Saturation has negligible effect on strength.• A large fluctuation in stress-strain behaviors is observed post-peak in all tests, due to particle breakage.• Young’s Modulus for all SG1 and SG2 Triaxial tests was calculated based on• stress associated with 50% of failure strain.• For SG1 Young’s modulus was found to be 15-22 MPa (2-3 ksi) for loose samples and 26-32 MPa (3-5 ksi)• for dense samples. For SG2 it was found to be 24-52 MPa (3-8 ksi) and 36-84 MPa (5-12 ksi) for loose and

dense respectively.

Silica Gels

Silica GelsTriaxial test results:• Assuming no cohesion, the internal friction angle of silica gels are about 29𝑜 − 42𝑜 which is consistent for

medium to coarse sands.• The internal friction angle decreases with increase of confining pressure, causing the failure envelope to be

curved. This phenomenon happens in natural sands at higher level of confining pressure.

Silica Gels• According to ASTM test method D3080, direct shear test can be performed.• The behavior of SG1 silica is consistent with typical behavior of sand, for both dense and loose conditions.• The dense samples usually have a defined peak an a tendency to compress prior to expanding, like natural

sands. Loose sands exhibit strain hardening and contractive behavior, particularly at higher normalpressures.

• The peak strength coincides with peak volume change for both dense and loose samples.• For SG2 samples a higher strength and volume change than SG1 is observed.• A large fluctuation in stress-strain behaviors was observed post-peak in all tests, due to particle breakage.

All specimens• The results can be influenced by the large diameter of the SG2 particles (2–5 mm) relative to the height of

the direct shear specimen (20 mm).

Direct Shear Test Results:

Silica Gels

Compressibity:

Silica Gels• On both SG1 and SG2 specimens according to ASTM test method D2439-96, One Dimensional Consolidation

can be performed.• Both SG1 and SG2 have nonlinear stress-strain behavior like most sands (including Calcareous and Ottawa

sand).• The “virgin” compressibility of SG1 is larger than some type of sands. SG1 has large plastic deformation,

probably due to particle breakage.• The high non-linear elastic compressibility of SG1 explains the lack of a defined peak strength in the triaxial

compression tests and also explains why the peak strength of dense SG1 specimens was reached at a strain of8–15%, which is relatively high.

• Although SG2 specimens exhibit the same virgin compression like sandy soils, their elastic rebound isconsiderably larger. It is therefore believed that elastic compression of SG2 specimens is the dominantcompressive mechanism, which is probably due to the relatively small odometer sample height

Silica Gels

Silica GelsHydraulic Conductivity:

• according to ASTM test method D5084 for Measurement of HydraulicConductivity of Saturated Porous Materials was done on SG1 and SG2.

• Hydraulic conductivities of 1.5 × 10−4 cm/sec for SG1 and7 × 10−3cm/sec for SG2 were measured.

• These conductivities correspond to intrinsic permeabilities of 1 and 45darcys, respectively, similar to natural soils.

FinThe End