Alaska DOT – The Use of Nontraditional Stabilizers for Construction of Airports in Alaska (TPD1407103)

1        INTRODUCTION

Soil Stabilization has long been used to improve the engineering properties of soils such as soil strength and bearing capacity. It is used for temporary wearing surfaces, for enhancing marginal soils and dust management. Soil stabilization techniques include:

–          Mechanical stabilization including modifying the gradation,

–          Soil additives including asphalt, Portland cement, lime/fly ash, salt and chemicals,

–          Soil reinforcement through the use of geosynthetics, addition of geofibers and soil confinement systems.

No one soil stabilization technique can be used for every soil and every application. Consequently, engineers must understand the application of techniques and the circumstances under which they work best.

Recent advances in chemical stabilizers and soil reinforcement fibers offer new opportunities to reduce the cost of road and airport construction where quality construction materials are unavailable. Western Alaska, especially in the Kuskokwim and Yukon River deltas, lack gravel suitable for construction. Consequently, gravel is typically imported at a cost between $300 and $600/yd³ ($392 to $785/m³). As a result, the construction of a 1,220 meter runway can often reach $40 million. Soil stabilization offers the opportunity to reduce these costs by 25% (Collins and Connor 2011).

The Alaska University Transportation Center (AUTC) in partnership with the Alaska Department of Transportation and Public Facilities (ADOT&PF), and the US Department of Transportation’s Federal Highway Administration (USDOT-FHWA), evaluated the use of geofibers and chemical stabilizers to stabilize sands and silts commonly found in Alaska.

2        BACKGROUND

The use of geofibers and chemical stabilizers were introduced to Alaska at Cape Simpson in 2005 (Hazirbaba et al. 2007, 2009). The application of 2-inch (51 mm) tape geofibers and a synthetic fluid (EnviroKleen) to a sandy soil was successful in constructing a storage pad for oil exploration.

A follow on project, showed that the California Bearing Ratio (CBR) for Alaska’s Bethel silty-sand could be improved by at least 100% with the addition of 0.5% geofiber. The addition of the synthetic fluid along with the geofibers could improve the CBR by 340%. As expected, the laboratory research showed that the addition of geofibers caused the sandy-silt to exhibit strain hardening behavior, i.e. as the soil deformed, fibers were tensioned and the strength increased. As a result, it was recommended that, in the field, once the soil-geofiber mixture was compacted, no further grading operations be applied. In addition, undrained unconfined compression (UUC) tests showed that the treated silty-sand behaved more like a gravel than a sand. The material simply bulged as loading increased. UUC tests also showed that the addition of geofiber increased the apparent cohesion of the Bethel silty-sand from 2.9 psi to 23.5 psi (20 kPa to 163 kPa). The angle of friction increased by about 2 degrees. However, the addition of 3% by weight synthetic fluid showed a less positive increase in cohesion to 13.9 psi (96 kPa).

This preliminary research and field application described above showed that soil properties and bearing capacity could be improved with the addition geofibers and synthetic fluids. As a result, the ADOT&PF funded a study in partnership with AUTC to further investigate the effects of incorporating geofibers and synthetic fluids in common Alaskan soils to enhance their bearing capacity.

3        STABILIZATION WITH GEOFIBERS

The first phase of the laboratory investigation studied the effect of geofiber addition to the following soil types:

–          Ottawa sand, and modified Ottawa sand,

–          Monterey sand, and modified Monterey sand,

–          Horseshoe Lake sand, and

–          Fairbanks silt.

The Ottawa and Monterey sands were included in the study for benchmarking purposes. To evaluate the load-bearing capacity of the soil-geofiber mixes and to assess the effectiveness of the addition of geofiber, CBR tests (ASTM D1883) were performed on soil specimens prepared in the laboratory according to ASTM D1557 procedures. All tests were done in triplicates. Reported test results are the average of three specimens. Material properties, sample preparation and testing details are presented elsewhere (Collins 2011).

Two types of fibers were used in this study, tape and fibrillated. The length of the fibers were varied between 0.75 and 1.5 inches. The optimum length of fiber was found to be 1.25 inches for most soil. Collins also found that while fibrillated fibers performed slightly better for the sandy materials tested, the benefits shown were not great enough to specify one over the other.

3.1        Ottawa Sand Stabilization

The first series of tests used Ottawa sand (CBR=19) with Fairbanks silt being added as fines. To evaluate the influence of the addition of geofiber (0.5% and 0.8% by weight) and fines (10%, 20% and 30% by weight) on the Ottawa sand, an analysis of variance (ANOVA) was performed on the test results. Equation 1 shows the resulting multiple linear regression. The analysis revealed that there is a significant effect of % geofiber, % fines, and their interaction (at a 95% confidence level) on the CBR value of the stabilized Ottawa sand.

CBR = 2.245 * X1 + 81.326 * X2 – 2.052 * X1 * X2           (R² = 0.845)   Equation 1

Where X1 = % geofiber; X2 = % fines.

As shown in Figure 1, when no geofiber is added the addition of 30 % silt increased the soil CBR significantly. At 20% fines content and below, the impact was minimal on CBR. As the silt content increased, so did the effectiveness of the geofiber until the silt content reached 30%. At this point, the addition of geofiber decreased the bearing capacity of the soil. At the 20% level, the influence of the fines on the geofiber performance limits the amount of geofiber to 0.5 %. This is likely due to the silt content beginning to fill the voids in the sand likely reducing the friction between the fibers and the sand particles.

3.2        Monterey Sand Stabilization

Soil-geofiber mixtures of Monterey sand (CBR=18) and Mabel Creek silt (fines) were tested next. Using CBR test results, the ANOVA assessed the influence of the addition of geofiber on the modified Monterey sand. The resulting multiple linear regression is shown in Equation

2. The analysis showed that there is a significant effect of % geofiber, % fines, and their interaction (at a 95% confidence level) on the CBR value of the stabilized Monterey sand.

CBR = 1.427 * X1 + 98.606 * X2 – 2.379 * X1 * X2           (R² = 0.854) Equation 2

Where X1 = % geofiber; X2 = % fines.

As depicted in Figure 2, the addition of silt/fines between 20% and 50% increased the CBR of the Monterey sand. At 10%, the presence of silt had little impact. At 70% fines content, the CBR of the sand began to decrease. It appears that a 50/50 blend of sand and silt is about optimum to maximize the sand’s bearing capacity.

The greatest improvement due to the geofiber was found at 20% fines. At this point the optimum fiber content was observed to be 0.5%. At 30% fines, the optimum fiber content was found to be 0.2% after which the strength fell off rapidly. As with the Ottowa sand, it appears that above 30% fines, the number of sand particles in contact with the fibers is reduced decreasing the friction between the soil and the fibers.Above 30% fines, even small amounts of geofiber reduced the sand’s CBR.

The results described above for the sand-geofiber mixtures confirm results from other research (Tutumluer et al. 2004) where it was found that sand-geofiber mixtures should contain an optimal amount of fines to mobilize the mix shear strength effectively. Like Tutumluer, this work found that fibrillated fibers often worked better with sand than tape. However, for unexplained reasons, this was not always the case nor were the differences large enough for the sands used in this study to require one over the other. Find the data

Figure 1: Influence of geofiber and fines on Ottawa sand bearing capacity.

3.3        Horseshoe Lake Sand Stabilization

Horseshoe Lake sand was used in an experimental field application near Wasilla, Alaska, about 40 miles north of Anchorage. Horseshoe Lake sand is an aeolian sand (CBR=25) which originated in the glaciers of the area. Its gradation is shown in Figure 3 for three replicates. Figure 4 illustrates the influence of geofiber incorporation into the Horseshoe Lake sand. The addition of geofiber to the sand increases its CBR value with an optimum fiber content of 0.5%. There seems to be an optimum amount of geofiber (0.5%) that maximizes the sand CBR compared to its untreated bearing capacity.

3.4        Fairbanks Silt Stabilization

To understand the relationship between soil bearing capacity and inclusion of geofibers in silty soils, CBR tests were performed on Fairbanks silt with geofiber contents varying between 0.2 and 1%. The properties of the untreated Fairbanks silt are summarized in Table

1. Fairbanks silt is and Aeolian silt which has a uniform gradation. Consequently, this silt performs much like a very fine sand.

Figure 5 shows the effect of geofiber inclusion on the CBR of Fairbanks silt. It is seen that a small inclusion of 0.2 % geofiber almost doubled the CBR of the silt decreasing after that.

An optimum geofiber percentage exits beyond which any strength improvements are minimal or reduced. Consequently, the use of geofiber in silty soils must be carefully controlled.

In summary, from the CBR testing carried out on the four soil-geofiber mixes described above, it was clear that geofibers are beneficial in stabilizing sands and silty sands. However, geofibers’ impact is questionable when the sand contains more than about 30% silt. In this case, test results suggest that the inclusion of geofibers will cause a reduction in soil CBR. However, when 100% silt is used, there is a benefit to adding a small percentage of geofiber of about 0.2%.

Figure 2: Influence of geofiber and fines on Monterey sand bearing capacity.

Figure 3: Horseshoe Lake sand gradation.

Figure 4: Influence of geofiber on Horseshoe Lake sand bearing capacity.

4     STABILIZATION WITH CHEMICAL STABILIZERS AND GEOFIBERS

In this phase of the laboratory work, the Fairbanks silt was stabilized by adding separately two different chemical stabilizers along with the addition of 0.5 % geofibers. The chemical stabilizers used were:

–          Soiltac®: A vinyl copolymer emulsion produced by Soilworks. Also available in powdered form. It was added to the silt at dosages of 0.66%, 1.1 %, and 4% by weight.

–          DirtGlue: An aqueous acrylate polymer emulsion manufactured by DirtGlue Enterprises. It was added at a rate of 3.3%. In addition, PolyCure, a curing agent was added at two dosages of 5% and 10% by weight.

Table 1: Characteristics of untreated Fairbanks silt compacted specimens.

Figure 5: Influence of geofiber on Fairbanks silt bearing capacity.

4.1        Stabilization with Soiltac®

case, it was found that an optimum amount of Soiltac® stabilizer (1.1%) maximizes the bearing capacity of the stabilized soil (with 0.5% geofiber). A fourfold increase in CBR was measured (CBR~54) compared to the untreated silt.

CBR = 7.14 * X1 + 101.1 * X2 – 21.63 * X1 * X2      (R² = 0.735)    Equation 3

Where X1 = % stabilizer; X2 = % geofiber

4.2        Stabilization with DirtGlue

For the DirtGlue stabilization case, multiple linear regression (Equation 4) and ANOVA performed on the CBR test results revealed that the CBR of the stabilized silt is significantly affected by the presence of geofiber and the curing agent (at the 95% confidence level). In

this case, a sevenfold improvement in bearing capacity (CBR~95) was achieved by the addition of 0.5% geofiber, 3.3% DirtGlue stabilizer, and 10% PolyCure curing agent.

CBR = 7.052 * X1 + 57.613 * X2                         (R² = 0.816)                Equation 4

Where X1 = % curing agent; X2 = % geofiber

From the results of this phase of the work, it was clearly evident that the presence of a curing agent had a significant effect on the bearing capacity improvement of the stabilized silt. Therefore it was decided to use such curing agent in the next phase of the study.

5        STABILIZATION USING CHEMICAL STABILIZERS WITH CURING AGENTS

Since the test results from the addition of the curing agent PolyCure (PC) were encouraging, it was decided to test sand and silt specimens where the curing agent PolyCure is incorporated in soil samples in addition to three stabilizers separately:

–          Soiltac® (ST),

–          DirtGlue (DGI), and

–          SoilSement (SS).

SoilSement is a polymer emulsion stabilizer (aqueous acrylic vinyl acetate) manufactured by Midwest Industries.

While initial testing showed that the addition of fiber may improve strength, it was decided to delete the fiber from this phase of testing in order to isolate the effects of the combination of stabilizers and curing agents. Funding and time did not allow that phase to be explored.

As the testing proceeded, a second curing agent, Extended Use (EU), manufactured by Midwest Industries, became available. Both curing additives, PC and EU, were tested, separately, in combination with each of the three stabilizers, ST, DGI, and SS.

After preliminary testing, the CBR test proved to be inadequate to characterize the strengths achieved by the combination of stabilizer and curing additive. Consequently, unconfined compression testing was adopted to characterize the strength improvements of the stabilized soils.

In each case, the optimum stabilizer content derived from the CBR testing phase was used in testing with the curing agents PC and EU. The stabilizer contents chosen were 1.1%, 3.3%, and 4%, for the Soiltac®, DirtGlue, and SoilSement, respectively. The dosage of the PC and EU curing agents was varied between 4 and 12 % in an effort to determine the influence of curing agent content on soil strength.

5.1        Fairbanks Silt Stabilization

As shown in Figure 6, unconfined compressive strength (UCS) of Fairbanks silt (Table 1) was significantly improved. All of the combinations provided reasonable increases in strength (rather linear) although those that contained either SoilSement or EU tended to be higher. At the highest dosage of curing agents (12 %), UCS values ranged between 400 psi and 640 psi (between 2.7 MPa and 4.4 MPa). This represents an increase between 18- and 29-fold in the untreated UCS (22 psi (152 kPa)) of the Fairbanks silt.

Figure 6: Unconfined compressive strength (UCS) variation with soil stabilizer type and dosage for Fairbanks silt. (1 psi = 6.895 kPa).

5.2        Horseshoe Lake Sand Stabilization

Figure 7 shows UCS values for the Horseshoe Lake sand. The PolyCure curing agent had a pronounced peak indicating that the PC optimum content for this sand was 10% by weight for SS and for DGI. A maximum UCS of 593 psi (4.1 MPa) was reached with a combination of PC and SS. However, curing agent EU behaved differently than PC for each stabilizer. With SS, the optimum strength of about 1100 psi (7.6 MPa) was obtained at 10% EU. With DGI, the maximum UCS of 729 psi (5 MPa) was reached at 8%. When used in combination with ST, the sand was still gaining strength at a EU dosage of 12%, where a UCS of 925 psi (6.4 MPa) was recorded.

Figure 7: Unconfined compressive strength (UCS) variation with soil stabilizer type and dosage for Horseshoe Lake sand. (1 psi = 6.895 kPa).

6           SUMMARY AND CONCLUSIONS

Based on the laboratory work detailed in this paper, it was shown that bearing capacity enhancements of Alaskan marginal soils can be achieved through the use of non-traditional stabilizers, such as geofibers, chemical stabilizers and curing agents.

It was found that all three stabilization alternatives provided increases in soil strength and bearing capacity. It can be concluded that, while the two part systems worked for all soils tested the use of geofibers should generally be limited to sand and silty sands. The best improvement in bearing capacity was achieved around 0.5% geofiber by weight. Test results revealed that when the amount of silt exceeds 30% by weight, the effectiveness of geofibers is diminished.