Thomas L. Brandon

Construction and Instrumentation of Geosynthetically Stabilized Secondary Road Test Sections

Nine instrumented flexible pavement test sections were constructed in a rural secondary road in southwest Virginia. The nine test sections, each 15 m (50 ft) long, were built to examine the effects of geogrid and geo-textile stabilization. Three test sections were constructed with a geogrid, three were built with a geotextile, and three were nonstabilized. The test section base course thicknesses ranged from 10.2 cm (4.0 in.) to 20.3 cm (8.0 in.), and the hot-mix asphalt (HMA) thickness averaged 8.9 cm (3.5 in.). Geosynthetic stabilization was placed on top of the subgrade layer. The pavement test sections were heavily instrumented with two types of pressure cells, soil and HMA strain gauges, thermocouples, and soil moisture cells. In addition, strain gauges were installed directly on the geogrid and geotextile. An extensive instrumentation infrastructure was constructed to locate all instrumentation, cabling, and data acquisition facilities underground. Instrument survivability has ranged from 6 percent ...

Rapid Chemical Stabilization of Soft Clay Soils

Since World War II, the military has sought methods for rapid stabilization of weak soils for support of its missions worldwide. Over the past 60 years, cement and lime have been the most effective stabilizers for road and airfield applications, although many nontraditional stabilizers also have been developed and used. The most effective stabilizer to increase the strength of two soft clay soils within 72 h for contingency airfields to support C-17 and C-130 aircraft traffic needed to be determined. The treatment of one clay with cement resulted in relatively high unconfined compressive strengths (UCS), whereas treating the same clay with quicklime and calcium carbide resulted in lower UCS. The treatment of another clay with higher plasticity resulted in similar UCS for cement, quicklime, and calcium carbide. Secondary stabilizers, including sodium silicate, superabsorbent polymers, a superplasticizer, and an accelerator, were ineffective in increasing the UCS of a soil treated with cement, quicklime, or calcium carbide.

Fiber Reinforcement for Rapid Stabilization of Soft Clay Soils

Since World War II, the military has sought methods for rapid stabilization of weak soils for support of its missions worldwide. Over the past 60 years, cement and lime have been the most effective stabilizers for road and airfield applications, although recent developments show promise from nontraditional stabilizers, such as reinforcing fibers. The benefits derived from fibers may depend on whether they are used alone or in combination with chemical stabilizers. The ability of stabilizers to increase the strength of two soft clay soils within 72 h to support C-17 and C-130 aircraft traffic on contingency airfields was investigated. Laboratory test results showed that longer fibers increased the strength and toughness the most for a clay treated only with fibers. For a clay treated with fibers in addition to a chemical stabilizer, shorter fibers increased toughness the most, but the fibers had little effect on strength. Higher dosage rates of fibers had increasing effectiveness, but mixing became difficult for fiber contents above 1%. Poly(vinyl) alcohol fibers were anticipated to perform better than other inert fibers because of hydrogen bonding between the fibers and clay minerals, but these fibers performed similarly to other fibers.

Measuring Drained Residual Strengths in the Bromhead Ring Shear

A series of Bromhead ring shear tests were conducted to measure the drained residual strength of Rancho Solano Fat Clay. Tests conducted using dierent test procedures demonstrate the signicant eect that wall friction can have in the Bromhead ring shear device. This problem was addressed by beveling the top loading platen, which led to measurements of drained residual strength that were 21-23% lower than those measured with the unmodied platen. Using the modied platen, similar test results can be achieved independent of the details of the test procedure that is followed, giving greater condence in the measured residual shear strength.

New Apparatus for Evaluating Filter Performance for Dams Containing Cracks

A new test apparatus has been developed to evaluate filter performance when both the filter and the core _base_ material have been cracked. This apparatus allows the orientation of the crack to be horizontal or vertical, and accommodates flow in the horizontal and vertical directions as well. The applied hydraulic gradient and the thickness of the crack can also be varied.Two apparatuses have been constructed and tested. The smaller apparatus allows testing of truncated cylindrical test specimens that are 10 cm in diameter by 20 cm long. The larger apparatus allows testing of specimens that are approximately 30 by 30 by 30 cm. An automated data acquisition system was developed to measure the pressures and flow rates through the test specimens as well as obtaining a video record of the filter performance. In successful tests, the filter slumped to fill the crack, the flow rate decreased, and the head water pressure increased. In failed tests, the filter never slumped to fill the crack, the flow rate remained high, and the head water pressure stayed very low. The relative success of failure of a filter system could be assessed by the video record as well.

Soil compressibility in transient unsaturated seepage analyses

Most levee underseepage and uplift analyses are based on steady-state seepage and can yield conservative results. Although computations are simpler and steady-state seepage parameters are easier to determine and readily available, transient unsaturated seepage analyses are more representative of levee seepage conditions because boundary conditions acting on the levee or floodwall and saturation change with time, which induce pore-water pressure and seepage changes with time in the embankment and foundation strata. In addition, these boundary conditions, e.g., flood surge or storm event, are rapid such that steady-state conditions may not have time to develop in the embankment and some foundation materials. Transient seepage analyses using a floodwall case study indicate that as soil compressibility of the underseepage layer decreases, rapid landside pore-water pressures increase and can approach steady-state values. The transient results also indicate that uplift factors of safety during the flood event a...

Correlations for Fully Softened Shear Strength Parameters

Fully softened shear strength is an important engineering concept for slope stability analyses of cuts in stiff clays and compacted embankments constructed of high plasticity clays. The “softening” concept has been used to explain many first-time failures for which the back-calculated shear strength is below the peak strength measured in the laboratory using undisturbed or freshly compacted samples. A comprehensive laboratory testing program was undertaken to measure the fully softened shear strength of 46 soils using over 300 direct shear tests. These results were used to develop correlations that allow estimation of a continuous and nonlinear fully softened failure envelope using soil index properties. These new correlations were developed to address some of the problems observed with existing correlations for fully softened shear strength parameters. The new correlations are presented along with the statistical parameters needed to assess reliability of the shear strength parameters and to allow use within a probabilistic framework.

Triaxial Tests on Compacted Clays for Consolidated-Undrained Conditions

Consolidated-undrained tests must be performed on compacted soils to provide undrained soil properties for embankment design scenarios, such as rapid drawdown. These soils are subject to anisotropic consolidation under field conditions. Laboratory testing of compacted clays must consider the effects of the consolidation stress system as well as the subtleties involved in testing initially unsaturated clay specimens. This study considers the effects of anisotropic consolidation on the undrained strength of a compacted lean clay. Strengths were measured using isotropically and anisotropically consolidated undrained triaxial compression tests. It was found that anisotropic consolidation has little effect on the undrained strength of compacted clay at vertical effective consolidation stresses below 50 kPa. As the consolidation stress increased, anisotropic consolidation was shown to decrease the undrained strength by 30 % at vertical effective consolidation stress of 300 kPa. It was also found that undrained strengths can increase by 50 % of more due to increases in relative compaction of a few %. Recommendations are presented for addressing the challenges of performing consolidated-undrained tests on compacted clays.

Measurement of Fully Softened Shear Strength

The fully softened shear strength, equivalent to the normally consolidated peak shear strength, can be the controlling shear strength in cut slopes in stiff-fissured clays, slopes in mudstones, and compacted embankments made of highly plastic clays subjected to weathering, that have not experienced previous slides. There has been a renewed interest in the proper techniques to measure the fully softened shear strength. A laboratory testing program was undertaken to examine the difference in the measured shear strength of ten different remolded normally consolidated natural clays using the direct shear apparatus and ring shear apparatus. The results showed that the fully softened shear strength measured with the ring shear device is always lower than the fully softened shear strength measured with the direct shear device. Based on these results, it was concluded that the ring shear device provides a very conservative measurement of the fully softened shear strength. An investigation to evaluate the influence of the sample preparation technique on the fully softened shear strength measured was also conducted. Tests were carried out on a subset of three of the soils using the direct shear apparatus. Some samples were mixed in a blender (blenderized), while others were only processed through a No. 40 (425 μm) sieve. The results showed that, in some cases, blenderizing the soil can lower the fully softened shear strength measured.

Relationship Between Backcalculated and Laboratory-Measured Resilient Moduli of Unbound Materials

The resilient moduli of an unbound granular subbase (used at the Virginia Smart Road) obtained from laboratory testing were compared with those backcalculated from in situ falling weight deflectometer deflection measurements. Testing was performed on the surface of the finished subgrade and granular subbase layer shortly after construction. The structural capacity of the constructed subgrade and the depth to a stiff layer were computed for 12 experimental sections. The in situ resilient modulus of the granular subbase layer (21-B) was then back-calculated from the deflections measured on top of that layer. The back-calculated layer moduli were clearly stress-dependent, showing an exponential behavior with the bulk stress in the center of the layer. Resilient modulus test results of laboratory-compacted specimens confirmed the stress dependence of the subbase material modulus. Three resilient modulus models were fitted to the data. Although all three models showed good coefficients of determination (R2 > 90%), the K-θ model was selected because of its simplicity. The correlation between field-backcalculated and laboratory-measured resilient moduli was found to be strong. However, when the stress in the middle of the layer was used in the K-θ model, a shift in the resilient modulus, θ, was observed. This finding suggests that a simple shift factor could be used for the range of stress values considered.