Report

On

EFFECT OF EcSS 3000TM SOIL STABILIZER ON THE STRENGTH OF MONTMORILLONITE

By

R. Malek, Ph.D.

Materials Research Institute, Pennsylvania State University,

University Park, PA 16802.

RQM@PSU.EDU(814) 865-7341

For

Environmental Soil Stabilization, LLC Burleson, TX

December, 2008

Final Report On

EFFECT OF EcSS 3000TM SOIL STABILIZER ON THE STRENGTH OF MONTMORILLONITE

Abstract The strength of 22% betonite/78% sand mixture was monitored at 7 and 28 days. The effect of mixing conditions on the engineering properties of the bentonite/sand mixtures was monitored. It was found that the general effect of EcSS 3000TM-stabilization process on the bentonite/sand mixtures is to increase the CBR values, produce a “dense” granular soil, increase the effective shear strength, and increase the strength value and increase the angle of internal friction at all ages. This early increase in strength of the EcSS 3000TM treated mixture has significant implications in reducing the construction delay that typically occurs in the field before sub-grade compaction due to construction operations occurs. A representative volume of bentonite consists of particles and voids which, arranged in size order, comprise: Interlayer pores < Interparticle pores (or micropores) < Intercluster pores (macropores) Compaction at optimum moisture content resulted in a dense packing of the sand within the mixture. The difference between water treated and EcSS 3000TM treated mixture is how bentonite particles effectively fill in the voids between sand particles. The disintegration of bentonite under the effect of EcSS 3000TM leads to less void ratio and in turn higher strength. One way to prove this difference in void ratio and its effect on strength is to measure the hydraulic conductivity which is planned in a forthcoming research.

Introduction

Geotechnical properties of problematic soils such as expansive soils are improved by chemical stabilization. Different methods can be used to improve and treat the geotechnical properties of the problematic soils (such as strength and the stiffness) by treating it in situ. The chemical stabilization of the problematic soils is very important for many of the geotechnical engineering applications such as pavement structures, roadways, building foundations, sports facilities, channel and reservoir linings, irrigation systems, water lines and sewer lines to avoid the damage due to the swelling action (heave) of the expansive soils. Generally, the concept of stabilization can be dated to 5000 years ago. It has been reported that stabilized earth roads were used in ancient Mesopotamia and Egypt, and that the Greeks and the Romans used soil-pozolana mixtures. The first experiments on soil stabilization were achieved in the USA with sand/clay mixtures around 1906. In the 20th century, especially in the thirties, the soil stabilization relevant to road construction was applied in Europe. These practical procedures have been improved and covered periodically by the technical standards for road and traffic. The Engineers are often faced with the problem of constructing roadbeds on or with expansive soils. These problematic soils do not possess enough strength to support the wheel loads upon them either in construction or during the service life of the pavement. These soils must be, therefore, treated to provide a stable sub-grade or a working platform for the construction of the pavement. There are two types of chemical stabilization techniniques depending to the depth of the problematic soil and the type of geotechnical application: surface or deep stabilization. The traditional surface stabilization begins by excavating and breaking up the clods of the soil followed by the addition of stabilizing agent (additive). Soil and additives are mixed together with known amounts of water and compacted. Depths of the order of 150 to 250 mm can be strengthened by this surface method. The depth of the stabilized and strengthened zone may be increased up to one meter by using heavy equipment with appropriate modification. The soil can be stabilized in multiple lifts as well, by over-excavating the subgrade and then placing the soil back into the excavation for treatment in lifts. Deeper stabilization can also be performed using the injection technique by injecting the stabilizing agent into the ground to the prescribed depth of treatment with either track-loader mounted equipment or hand-held injection rods. The sub-grade should possess desirable properties to extend the service life of the roadway section and to reduce the required thickness of the flexible pavement structure. These desirable properties include strength, drainage, ease and permanency of compaction, and permanency of strength. CBR and resilient modulus data was used for a flexible pavement design.

Rationale

When designing pavement structures, roadways, building foundations, sports facilities, channel and reservoir linings, irrigation systems, water lines and sewer lines, stabilizing the soil is an important factor. For these applications, there are three fundamental external design parameters to consider: the characteristics of the soil, the applied loads and the environment. First, the soil upon which the pavement or foundation is placed will have a large impact on structural design. Stiffness and drainage characteristics help determine treatment depth, layer thickness and the number of layers, seasonal load restrictions and any possible improvements to stiffness and drainage itself. Second, the expected loading is a primary design input (both in mix design and structural design). Third, the environment has a large impact on material performance. Environmental factors such as temperature, moisture and ice formation can affect structure life and failure.

The performance generally depends on three of its basic characteristics (all of which are interrelated):

1. Load bearing capacity. The subgrade/foundation must be able to support loads transmitted from the structure. This load bearing capacity is often affected by degree of compaction, moisture content, and soil type. A soil that can support a high amount of loading without excessive deformation is considered good.

2. Moisture content. Moisture tends to affect a number of subgrade/foundation properties including load bearing capacity, shrinkage and swelling. Moisture content can be influenced by a number of things such as drainage, groundwater table elevation, infiltration, or porosity (which can be assisted by cracks in the structure). Generally, excessively wet soils will deform excessively under load.

3. Shrinkage and/or swelling. Some soils shrink or swell depending upon their moisture content. Additionally, soils with excessive fines content may be susceptible to frost heave in northern climates. Shrinkage, swelling and frost heave will tend to deform and crack any structural type constructed over them.

Poor subgrade/foundation should be avoided if possible, but when it is necessary to build over weak or expansive soils there are several methods available to improve soil performance:

• Removal and replacement (over-excavation). Poor subgrade/foundation soil can simply be removed and replaced with high quality fill. Although this is simple in concept, it can be expensive.

• Stabilization. To increase stiffness and reduce swelling. • Additional base layers or pavement thickness. Marginally poor soils may be

compensated for by using additional base layers. These layers (usually of crushed stone – either stabilized or unstabilized) serve to spread structure’s loads over a larger area. Often, for Portland cement concrete (PCC) pavements, the designer adds an additional thickness of concrete, usually one inch. This option is rather risky; when designing structures for poor or expansive soils the temptation may be to just design a thicker section with more base or pavement material because

the thicker section will satisfy most design equations. However, these equations are at least in part empirical and were usually not intended to be used in extreme cases. In short, a thick base or pavement structure over a poor or expansive soil subgrade will not necessarily make a good construction.

The two fundamental tests to characterize a soil are:

1) The California Bearing Ratio (CBR) test is a simple strength test that compares the bearing capacity of a material with that of a well-graded crushed stone (thus, a high quality crushed stone material should have a CBR ≅ 100%). It is primarily intended for, but not limited to, evaluating the strength of cohesive materials having maximum particle sizes less than 19 mm (0.75 in.) (AASHTO, 2000). It was developed by the California Division of Highways around 1930 and was subsequently adopted by numerous states, counties, U.S. federal agencies and internationally. As a result, most agency and commercial geotechnical laboratories in the U.S. are equipped to perform CBR tests.

The basic CBR test involves applying load to a small penetration piston at a rate of 1.3 mm (0.05") per minute and recording the total load at penetrations ranging from 0.64 mm (0.025 in.) up to 7.62 mm (0.300 in.).

2) The triaxial compression test, in which the shear characteristics are measured under undrained conditions and is applicable to field conditions where soils that have been consolidated under one set of stresses are subjected to a change in stress without time for further consolidation to take place (undrained condition). Using the pore-water pressure measured during the test, the shear strength determined from this test method can be expressed in terms of effective stress, for cases where full drainage can occur.

Plan of Work The best way to test and evaluate engineering properties of EcSS 3000 TM-stabilized soil is to perform actual tests in the field and/or laboratory tests on undisturbed samples of field-treated soil. Laboratory preparation of EcSS 3000 TM-stabilized soil samples cannot accurately recreate the conditions and treatment in the field, especially for soils that are pressure-injected. Therefore, in order to compare results of EcSS 3000 TM-stabilized soil to water-treated only soil, laboratory-prepared samples of a bentonite and sand mixture was selected to significantly reduce the effects of material variability when testing a limited number of samples. Laboratory preparation procedures have been developed to somewhat simulate possible treatment conditions. Materials and methods Montmorillonite Grade (F-100) bentonite from BASF chemical company, containing 97% monmorillonite and 1-3% crystalline silica (quartz) was used. It is a light grey to off-white powder with an average particle size of 10 µm.

Silica sand Silica sand is ASTM C-778 graded sand from US Silica Company, Ottawa, IL. The gradation is as follows:

Table I: Gradation of ASTM C-778 sand

Sieve Ind. % Retained Cum. % Retained Specification Minimum Maximum

+30 0.6 0.6 0.0 4.0 +40 28.7 29.3 25.0 35.0 +50 46.2 75.5 70.0 80.0 +100 23.8 99.3 96.0 100.0 Pan 0.7 100.0

Mineralogical analysis X-ray powder diffraction technique with Cu Kα radiation was used to determine the mineralogical composition of the bentonite (Fig 1). Bentonite is composed of 14A montmorillonite and small amount of silica sand.

Figure 1. XRD of untreated (black) and EcSS-3000TM Treated (red) bentonite Effect of EcSS 3000TM Soil Stabilizer. Figure 1 includes the XRD diffractogtram of the bentonite after treatment with EcSS 3000TM soil stabilizer, which shows the normal trend of collapse of the tactoid structure and shift of the interlayer gap to a smaller value.

Procedures for Preparation and Testing of water-treated and EcSS 3000TM-stabilized soil Two mixing procedures were adopted: The first preparation procedure is termed “Daily Mixing.” This procedure consists of preparing bentonite/sand mixtures and adding either diluted EcSS 3000 TM or water only at the prescribed amount and mixing the solution into the soil. The mixtures are placed in a sealed container in a moist room and mixed once each day until the samples are ready for compaction and testing (7 days or 28 days). This procedure is meant to allow for additional treatment following the initial mixing, to somewhat simulate the continued migration of ions and treatment by EcSS 3000 TM in the field after pressure-injection. The second preparation procedure is termed “One-Time Mixing.” This procedure consists of preparing bentonite/sand mixtures in the same manner as described above for “Daily Mixing.” In this case, however, shortly after mixing with diluted EcSS 3000 TM or water only, the mixtures are compacted in test molds, ready for the applicable testing. The compacted test samples are wrapped and sealed and placed in a moist room until ready for testing (7 days or 28 days). This procedure is meant to allow for comparison of 7 day and 28 days strength for any indication of strength gain over time after compaction, to somewhat simulate the surficial mix-in-place application. A. Daily Mixing:

1- Preparation of soil sample: bentonite sample was dried in the air then it was put into oven at 50°C for 24 hours. 2- The EcSS 3000TM soil stabilizer was diluted in ratio 300:1 water:raw EcSS 3000TM. 3- Mixtures were prepared on a dry basis with a) 301 gallons of diluted EcSS 3000TM or b) 301 gallons of water only, per 600 cubic feet of soil. 4- Add silica sand in a ratio 22% bentonite + 78% sand on the dry weight basis. 5- Place the two bentonite mixtures in separate sealed containers and allowed to cure for 7 days, and 28 days (EcSS-3000TM sample only) in a humidity-temperature chamber (at ≥ 98% humidity and at temperature 25 °C ± 2), to determine the influence of the curing time factor on the geotechnical properties and on the process of EcSS-3000TM-stabilization. Mix the soils once each day during the curing period. 6- At 7 and 28 days (EcSS-3000TM sample only), carry out the standard proctor test (ASTM D 698) to determine both the maximum dry density and the optimum water content for each mixture. Adjust the water content to between optimum and 2% above optimum. 7- Run specific gravity test (AASHTO T100). 8- Run the Atterberg Limits (ASTM 4318). 9- At 7 days and 28 days (EcSS-3000TM sample only) compact samples and run the triaxial (CIU) test (ASTM D 4767), and California Bearing Ratio (CBR) test (ASTM D 1883) on both soaked and unsoaked samples.

B. One-Time Mixing:

1- Preparation of soil sample: bentonite sample was dried in the air then it was put into oven at 50°C for 24 hours. 2- The EcSS 3000TM soil stabilizer was diluted in ratio 300:1 water:EcSS 3000TM. 3- Mixtures were prepared on a dry basis with a) 301 gallons of diluted EcSS 3000TM or b) 301 gallons of water only, per 600 cubic feet of soil. 4- Add silica sand in a ratio 22% bentonite + 78% sand on the dry weight basis. 5- Carrying out of the standard proctor test (ASTM D 698) to determine both the maximum dry density and the optimum water content for each mixture. Compaction of the samples are carried out after mixing with water (2-hours delay) to simulate the typical duration between mixing and compaction that occurs in the field. 6- Run specific gravity test (AASHTO T100). 7- Run the Atterberg Limits (ASTM 4318). 8- After that the triaxial samples are prepared and after compaction, each specimen is wrapped with polyethylene paper, laid in plastic bags, and allowed to cure for 7 days, and 28 days in a humidity-temperature chamber (at ≥ 98% humidity and at temperature 25 °C ± 2), to determine the influence of the curing time factor on the geotechnical properties and on the process of EcSS-3000TM-stabilization. 9- The CBR molds are filled, compacted, surcharged with 10 pounds, and wrapped with polyethylene paper, laid in plastic bags, and allowed to cure for 7 days, and 28 days in a humidity-temperature chamber (at ≥ 98% humidity and at temperature 25 °C ± 2), to determine the influence of the curing time factor on the geotechnical properties and on the process of EcSS 3000TM-stabilization. 10- At 7 days and 28 days run the triaxial(CIU) test (ASTM D 4767), and California Bearing Ratio (CBR) test (ASTM D 1883) on both soaked and unsoaked samples.

Findings Zeta Potential Clays consist mainly of plate-like particles, which when in contact with water, usually have negatively charged faces. As a result, water molecules arrange themselves to form a layer around the clay particle. The surface charge decreases gradually as we go towards the bulk liquid. Water molecules arrange themselves in a rather less bound fashion than the first layer forming a diffuse layer. This whole arrangement is termed the electric double layer. At a certain distance from the particle surface there exists a plane of the closest approach between particles. The charge at this plane is known as the zeta potential. The physical properties of clay-water systems such as swelling are extremely sensitive to the nature of the electric double layer around the particles. Zeta potential was measured using the microelectrophoretic mobility technique where the speed and direction of moving particles, under the influence of an electric field, were measured and used to calculate the surface charge. Zeta potential was found to be – 17.92 mV before

treatment and it dropped to -13.06 mV after treatment with EcSS 3000TM. So, the interlayer spacing is reduced and the interlayer cations are exchanged as a result of the treatment with EcSS 3000TM. Engineering Properties Appendix A contains the results of the standard proctor tests (ASTM D 698), and Triaxial compression tests (ASTM D 4767). Tables II, III, IV, V, VI contain the results of standard proctor, Atterberg Limits, Effective cohesion/angle of friction/ tangent of the angle of friction, California Bearing Ratio, and specific gravity, respectively, for the daily mixed samples. Appendix B contains the results of the standard proctor tests (ASTM D 698), and Triaxial compression tests (ASTM D 4767). Tables VII, VIII, IX, X, XI contain the results of standard proctor, Atterberg Limits, Effective cohesion/angle of friction/ tangent of the angle of friction, California Bearing Ratio, and specific gravity, respectively, for the one-time mixed samples.

A) California Bearing Ratio (CBR) CBR-value is used as an index of soil strength and bearing capacity. This value is broadly used and applied in design of the base and the sub-base material for pavement. CBR-value is a familiar indicator test used to evaluate the strength of soils for these applications. The CBR test was conducted to characterize the strength and the bearing capacity of the studied bentonite mixtures with sand. The test procedures and the preparation of the specimens were achieved according to the procedures in ASTM 1883 in both soaked and unsoaked conditions. CBR-values of the tested mixtures, compacted at optimum water content, are given in Tables V and X, for the daily mixed and one-time mixing, respectively. From these Tables, one can conclude that:

Daily Mixing: There is an increase in the CBR value (soaked and unsoaked) in the EcSS 3000TM treated sample relative to the water treated sample, at 7 days. This value decreased at 28 days for the EcSS 3000TM treated sample. This is due to some irregularities in the samples surfaces, probably produced by different agglomeration rates.

One-Time Mixing: 1- CBR-values of the unsoaked are higher than the corresponding soaked values. EcSS

3000TM –treated soils tend to repel water, thus resulting in the unsoaked CBR value as being most representative.

2- There is a significant increase in the soaked CBR values of the EcSS 3000TM treated mixture over the corresponding water treated mixture. This order is reversed in the unsoaked CBR results. This is related to the efficient stabilizing effect of EcSS 3000TM soil stabilizer. Therefore, while the soaked, water treated mixture, absorbs water and become softer, the EcSS 3000TM treated mixture rejects water and remains stronger. On the other hand, in the unsoaked samples, the presence of excess free water in the interparticle spaces of the EcSS 3000TM treated samples (due to water rejection) decreases the CBR values. This is a favorable effect since

Daily Mixing

Table II: Results of Proctor Analysis (ASTM D 698)

Maximaum Dry Density (lb/ft3)

Optimum Moisture (%)

W 7d 115.2 13.7 Ec 7d 116.3 13.4 Ec 28d 115.5 15.0

Table III: Atterberg Limits (ASTM D 4318)

LL PL PI

W 7d 30 15 15 Ec 7d 30 17 13 Ec 28d 32 17 15

Table IV: Effective Cohesion, Angle of Friction and Tangent of the angle of friction.

C’ (tsf)

φ’ (Degree)

tanφ’

W 7d 0.23 25.0 0.47 Ec 7d 0.32 30.4 0.59 Ec 28d 0.42 30.1 0.58

Table V: California Bearing Ratio [CBR] (ASTM D 1883)

Soaked Unsoaked W 7d 7.7 11.5 Ec 7d 21.4 13.2 Ec 28d 5.9 4.6

Table VI: Specific Gravity [g/cc] (AASHTO T 100)

W 7d 2.662 Ec 7d 2.640

Ec 28 d 2.697

One-Time Mixing

Table VII: Results of Proctor Analysis (ASTM D 698)

Maximum Dry Density (lb/ft3)

Optimum Moisture (%)

W 7d 114.0 15.2 Ec 7d 114.2 15.1 W 28d 114.0 15.2 Ec 28d 114.2 15.1

Table VIII: Atterberg Limits (ASTM D4318)

LL PL PI

W 7d 35 17 18 Ec 7d 35 17 18 W 28d 35 17 18 Ec 28d 35 17 18

Table IX: Effective Cohesion, Angle of Friction and Tangent of the angle of friction

C’

(tsf) φ’

(Degree) tanφ’

W 7d 0.59 25.7 0.48 Ec 7d 0.61 26.0 0.49 W 28d 0.65 25.7 0.48 Ec 28d 0.65 28.0 0.53

Table X: California Bearing Ratio [CBR] (ASTM D 1883)

Soaked Unsoaked W 7d 13.3 19.9 Ec 7d 15.4 15.5 W 28d 12.8 24 Ec 28d 16.3 22.3

Table XI: Specific Gravity [g/cc] (AASHTO T 100)

W 7d 2.645 Ec 7d 2.647 W 28d 2.645 Ec 28 d 2.647

the excess water will eventually evaporate leading to a net increase in the CBR values, as is noticed in the 28-day cured samples.

3- The CBR value of the soaked water treated samples did not change with curing time (13.3 @ 7 days and 12.8 @ 28 days). On the other hand, there is about 6% increase in the soaked EcSS 3000TM treated samples (15.4 @ 7 days and 16.3 @ 28 days), due to the continuation of the stabilizing mechanism operating in the EcSS 3000TM treated samples.

It is thus concluded that the general effect of EcSS 3000TM-stabilization process on the bentonite/sand mixtures is to increase the CBR values. The ratio of the CBR-value of the EcSS 3000TM -stabilized soil to that of the water treated compacted soil is known as the CBR-gain factor. The CBR-gain factor (due to addition of EcSS 3000TM) is 1.2 in the one-time mixed samples. It appears that the daily mixing procedure has increased the CBR of the EcSS 3000TM –stabilized soil at 7 days, due to exposing more soil to the effect of the liquid. Further studies are needed to explain the drop at 28 days.

B) Consolidated Undrained Triaxial compressive strength Compressive strength of a soil is a significant factor to estimate the design criteria for the use as a pavement and construction material. The EcSS 3000TM-stabilization of soil, generally, leads to increase in the strength of the soil. Therefore, EcSS 3000TM soil stabilizer become cost-effective and efficient material for use in road construction, building subgrades, embankments, and earth fills. The strength gain of EcSS 3000TM -stabilized soil is primarily caused by the formation of flocs that result in a general densification of soil. The test procedures and the preparation of the specimens were performed according to the procedures in ASTM D 4767. Results of effective strength, angle of friction, and tangent of the angle of friction of the water treated and EcSS 3000TM treated bentonite/sand mixtures, compacted at optimum water content are given in Table IV. All the specimens were prepared using a standard proctor test. All the mixtures were compacted at the optimum water content. The mixtures were cured for 7 and 28 days to estimate the influence of curing time on the strength and angle of internal friction.

Effective shear strength at various depths below grade

Tables XIV and XV show the effective shear strength at various depths below grade (psf), for the daily mixed and one-time mixed samples, respectively.

Table XIV: Effective shear strength at various depths below grade (psf), for the daily mixed samples.

Depth Below Grade (ft)

Water (7d)

EcSS (7d)

EcSS (28d)

1 516 711 910 2 573 782 979 3 629 852 1048 4 686 923 1118 5 742 994 1188 6 798 1065 1257 7 855 1136 1327 8 911 1206 1397 9 968 1277 1466 10 1024 1348 1536

Table XV: Effective shear strength at various depths below grade (psf), for the one-

time mixed samples

Depth Below Grade (ft)

Water (7d)

Water (28d)

EcSS (7d)

EcSS (28d)

1 1238 1358 1279 1364 2 1295 1415 1338 1427 3 1353 1473 1396 1491 4 1410 1530 1455 1554 5 1468 1588 1514 1618 6 1525 1646 1572 1682 7 1583 1703 1632 1745 8 1641 1760 1690 1809 9 1698 1818 1749 1872 10 1756 1876 1808 1936

The results are presented graphically in Figure 2 for the daily mixed mixtures; and 3 and 4 for 7 day and 28 day one-time mixed mixtures, respectively.

Daily Mixing: It appears that there is an average increase in the effective shear strength, at 7 days, of about 34% in the EcSS 3000TM treated mixture over the water treated one. This value increased to an average of 60% at 28 days of age. This is an indication of the consolidation and densifying effect of the EcSS 3000TM soil stabilizer.

One-Time Mixing:

It appears that there is an average increase in the effective shear strength, at 7 days, of about 3.5% in the EcSS 3000TM treated mixture over the water treated one. This value

Figure 2. Effective shear strength vs. overburden pressure of water treated and EcSS-3000TM Treated for the daily mixed bentonite/sand mixtures.

7 Days

1200

1300

1400

1500

1600

1700

1800

1900

0 200 400 600 800 1000 1200 1400

Overburden Pressure (Psf)

Effe

ctive

Shea

r St

reng

h (P

sf)

Figure 3. Effective shear strength vs. overburden pressure of water treated (blue)

and EcSS-3000TM Treated (red) bentonite/sand mixtures at 7 days. One-time mixed.

28 Days

1300

1400

1500

1600

1700

1800

1900

2000

0 200 400 600 800 1000 1200 1400

Overburden Pressure (Psf)

Effe

ctive

Shea

r St

reng

h (P

sf)

Figure 4. Effective shear strength vs. overburden pressure of water treated (blue) and EcSS-3000TM Treated (red) bentonite/sand mixtures at 28 days. One-time mixed.

increased to an average of 4.5% at 28 days of age. This is an indication of the consolidation and densifying effect of the EcSS 3000TM soil stabilizer.

Daily Mixing: From Table IV and Figure 2, it is concluded that:

1- There is an increase of an average of about 34% and 60% (7 day and 28 days,

respectively) strength of EcSS 3000TM-treated mixture over the water-treated mixture.

. 2- The angle of internal friction in the EcSS 3000TM treated mixture stayed the

same high value as it was aged from 7 days to 28 days. On the other hand, the angle of internal friction in the water treated mixture was lower than the EcSS 3000TM treated mixture at 7 days.

It is thus concluded that the general effect of EcSS 3000TM-stabilization process on the bentonite mixtures is to increase the strength value at all ages and increase the angle of internal friction at 7 days. This early increase in strength of the EcSS 3000TM treated mixture has significant implications in reducing the construction delay that typically occurs in the field before sub-grade compaction due to construction operations occurs.

One-Time Mixing:

From Table IX and Figures 3 and 4, it is concluded that:

3- There is an increase of about 3.5% in the 7 day strength of EcSS 3000TM treated mixture over the water treated mixture.

4- Both mixtures reach equal strength at 28 days. 5- The angle of internal friction in the water treated mixture stayed the same low

value as it was aged from 7 days to 28 days. On the other hand, the angle of internal friction in the EcSS 3000TM treated mixture was higher than the water treated mixture at 7 days, and increased even more at 28 days.

It is thus concluded that the general effect of EcSS 3000TM-stabilization process on the bentonite mixtures is to increase the strength value at 7 days and increase the angle of internal friction at all ages. This early increase in strength of the EcSS 3000TM treated mixture has significant implications in reducing the construction delay that typically occurs in the field before sub-grade compaction due to construction operations occurs. C) Earth Pressure Coefficient at Rest One can calculate the earth pressure coefficient at rest, k0, from the relationship: k0 = 1 – sin φ’ Where φ’ is the effective angle of internal friction obtained from the CIU tests.

• For normally consolidated soil, the critical value of φ’ which separates “loose” and “dense” range of soils is 30o and Ko is 0.5.

• For over-consolidated and compacted soils the range of Ko may be in the order of 1.0.

The earth pressure coefficients ae presented in Tables XII and XIII, for the daily mixed and one-time mixed samples, respectively.

Daily Mixing: At 7 and 28 daysdays, it appears, from Table XII, that the EcSS 3000TM treated sample falls within the “dense” range of granular soil, whereas the water treated sample falls within the “loose” range of granular soils.

One-Time Mixing:

For the current mixture, with considerable amount of consolidable clay (>15%), it is reasonable to assume a value of Ko >0.50 - <1.00. Looking at the earth pressure coefficients (Table XIII), all samples fall within the “loose” range for granular soils. However, the EcSS 3000TM-treated samples had lower Ko values than the water-treated samples. This is an indication of the densifying effect of EcSS 3000TM.

Table XII: Earth Pressure Coefficient at Rest, k0 Daily-Mixed

Sample Effective Angle of Friction φ’ k0

W 7d 25.0 0.58 Ec 7d 30.4 0.48

Ec 28 d 30.1 0.50

Table XIII: Earth Pressure Coefficient at Rest, k0 One-Time-Mixed

Sample Effective Angle of Friction φ’ k0

W 7d 25.7 0.57 Ec 7d 26.0 0.56 W 28d 25.7 0.57 Ec 28 d 28.0 0.53

Summary and Conclusions The strength of betonite/sand mixture was monitored at 7 and 28 days. This is achieved by using a mixture that contains sufficient sand (78%) to ensure the stability of the compacted mixture and enough bentonite (22%) to seal the voids between the sand particles. The results can be summarized as follows:

1- This investigation has shown the importance of mixing conditions and the need to effectively distribute the chemical stabilizer to give its maximum effect. However, further studies are needed to optimize the mixing conditions for maximum stabilization effectiveness.

2- The general effect of EcSS 3000TM-stabilization process on the bentonite/sand mixtures is to increase the CBR values.

3- Generally, it appears that there is an average increase in the effective shear strength in the EcSS 3000TM treated mixture over the water treated one. This is an indication of the consolidation and dansifying effect of the EcSS 3000TM soil stabilizer.

4- The general effect of EcSS 3000TM-stabilization process on the bentonite/sand mixtures is to increase the strength value at 7 days and increase the angle of internal friction at all ages. This early increase in strength of the EcSS 3000TM treated mixture has significant implications in reducing the construction delay that typically occurs in the field before sub-grade compaction due to construction operations occurs.

5- Generally, the EcSS 3000TM treated sample falls within or close to the “dense” range of granular soil, whereas the water treated sample falls within the “loose” range of granular soils. This is an indication of the densifying effect of EcSS 3000TM.

Montmorillonite is an alumino-silicate mineral with a 2:1 unit layer structure. Individual layers (or lamellae) are about 10Å (1nm) thick, but up to several orders of magnitude larger in the other directions (Mitchell, 1993). These unit particles can aggregate together to form clusters. Thus, structural elements exist at several scales and can be arranged in the size order: Unit layers (Lamellae) < Unit particles < Particle clusters A representative volume of bentonite consists of particles and voids which, arranged in size order, comprise: Interlayer pores < Interparticle pores (or micropores) < Intercluster pores (macropores) [Effect of EcSS 3000TM on Internal Pore structure and Water Distribution in Montmorillonite, Report to ESSL, 2008] Exposure of bentonite powder to water results in surface hydration beyond that needed to give an interlayer separation, which results from diffuse double layer forces between the hydrated montmorillonite unit layers. However, complete dispersion of lamellae is prevented by the formation of tactoids, which are stable regions where the lamellae are orientated essentially parallel to one another. It is proposed that compaction at optimum moisture content resulted in a dense packing of the sand within the mixture. The difference between water treated and EcSS 3000TM treated mixture is how bentonite particles effectively fill in the voids between sand particles. The disintegration of bentonite under the effect of EcSS 3000TM leads to less void ratio and in turn higher strength. One way to prove this difference in void ratio and its effect on strength is to measure the hydraulic conductivity which is planned in a forthcoming research.

Acknowledgement The author appreciates the invaluable input of Russell Scharlin, Vice President of Environmental Soil Stabilization, LLC, in suggesting the scope of this work, providing the mixture composition, and discussions on several parts of this report.

Appendix A

Proctor and Triaxial Test Results of the daily mixed samples

Appendix B

Proctor and Triaxial Test Results of the one-time mixed samples