Tony L. Beckham

Resilient Modulus of Compacted Crushed Stone Aggregate Bases

In recent years, the American Association of State Highway Transportation Officials (AASHTO) has recommended the use of resilient modulus for characterizing highway materials for pavement design. This recommendation evolved as result of a trend in pavement design of using mechanistic models. Although much progress has been made in recent years in developing mathematical, mechanistic pavement design models, results obtained from those models are only as good as the material parameters used in the models. Resilient modulus of aggregate bases is an important parameter in the mechanistic models. The main goal of this study was to establish a simple and efficient means of predicting the resilient modulus of different types of Kentucky crushed stone aggregate bases. To accomplish this purpose, resilient modulus tests were performed on several different types of aggregate bases commonly used in pavements in Kentucky. Specimens were remolded to simulate compaction conditions typically encountered in the field. Tests were performed on wet and dry specimens. The compacted specimens were 6 in. in diameter and 12 in. in height Crushed limestone base materials included Dense Graded Aggregate (DGA), and Crushed Stone Base (CSB). Number 57s, crushed river gravel, recycled concrete, and asphalt drainage blanket samples were submitted for testing by engineers of the Kentucky Transportation Cabinet. A new mathematical resilient modulus model, developed in a previous study by researchers at the University of Kentucky Transportation Center (UKTC), was used to relate resilient modulus to any selected, or calculated, principal stresses in the aggregate base. This model improves the means of obtaining best data “fits” between resilient modulus and stresses. Furthermore, the resilient modulus can be predicted, using the UKTC resilient modulus model, when the stress condition and type of Kentucky base aggregate are known. Multiple regression analysis is used to obtain model coefficients, k1, k2, and k3, of the relationships between resilient modulus and confining and deviator stresses used in the testing procedure. Also, multiple regression analysis was performed using other models developed by the National Cooperative Highway Research Program (NCHRP Project 1-37A, 2001) and Uzan (1985) to obtain the model coefficient, k1, k2, and k3. The resilient modulus data and the UKTC model, as well as models developed by NCHRP and Uzan, are readily available to design personnel of the Kentucky Transportation Cabinet. Computer software was developed in a client/server and Windows environment. This program is embedded in the Kentucky Geotechnical Database, which resides on a Cabinet server in Frankfort, Kentucky.

Modification of Highway Soil Subgrades

Major study objectives were to develop highway pavement subgrade stabilization guidelines, to examine long-term benefits of chemical stabilizers, such as cement, hydrated lime, and two byproducts from industrial processes, and to establish a subgrade stabilization program in Kentucky. In developing a program, a number of design and construction issues had to be resolved. Factors affecting subgrade behavior are examined. Changes in moisture content and California bearing ratio (CBR) strengths of untreated and chemically treated subgrades at three experimental highway routes were monitored over a 7-year period. CBR strengths of the untreated subgrades decreased dramatically while moisture contents increased. CBR strengths of subgrade sections treated with hydrated lime, cement, and multicone kiln dust generally exceeded 12 and increased over the study period. At four other highway routes ranging in ages from 10 to 30 years, CBR strengths of soil-cement subgrades exceeded 90. Knowing when subgrade stabilization is needed is critical to the development of an economical design and to insure the efficient construction of pavements. Bearing capacity analyses using a newly developed, stability model based on limit equilibrium and assuming a tire contact stress of 552 kPa show that stabilization should be considered when the CBR strength is less than 6.5. For other tire contact stresses, relationships corresponding to factors of safety of 1 and 1.5 are presented. Stability analyses of the first lifts of the paving materials show that CBR strengths of the untreated subgrade should be about 9 or greater. Guidelines for using geogrids as subgrade reinforcement are presented. Factors of safety of geogrid reinforced granular bases are approximately 10 to 25% larger than granular bases without reinforcement. As shown by strength tests and stability analysis, when the percent finer than the 0.002 mm-particle size of a soil increases to a value greater than about 15%, the factor of safety decreases significantly. Guidelines are also presented for the selection of the design strengths of untreated and treated subgrades with hydrated lime and cement. Based on a number of stabilization projects, recommended design undrained shear strengths of hydrated lime- and cement-treated subgrades are about 300 and 690 kPa, respectively. A laboratory testing procedure for determining the optimum percentage of chemical admixture is described. Correlations of Dynamic Cone Penetrometer and the Clegg Impact Hammer values and in situ CBR strengths and unconfined compressive strengths are presented.

Long-Term Monitoring of Culvert Load Reduction Using an Imperfect Ditch Backfilled with Geofoam

Rigid culverts resting on unyielding foundations are frequently required in Kentucky for routing streams beneath highway embankments because of shallow depths to bedrock, rolling terrain, numerous streams, and the need to use high fills, which create large vertical stresses. Expensive structures are usually required. To find a way to reduce large vertical pressures, ultralightweight geofoam was placed in a 2-ft-thick trench above a reinforced, rigid concrete box culvert resting on an unyielding foundation at a roadway site in Kentucky. In situ measurements of stresses, strains, and geofoam settlements obtained over 5 years showed that geofoam was an ideal compressible material to use in the imperfect trench. At two culvert sections where geofoam was used, vertical pressures were reduced to about 10% of the normal pressures measured at the top of a culvert section where geofoam was not used. Stresses on top of two culvert sections where geofoam was used remained relatively stable after final grade elevation was reached. Earth pressures on the sidewalls of all three sections of the culvert did not change significantly after completion of the fill and were much lower than the vertical pressures measured in the culvert section without geofoam. Geofoam settlement of 60% has been recorded. Settlement behavior of the geofoam showed that movement of the soil prism above the imperfect trench was rapidly decreasing with increasing time and suggested that the reduced vertical stresses observed under the geofoam culvert sections could remain throughout the culvert design life.

Selection of Design Strengths of Untreated Soil Subgrades and Subgrades Treated with Cement and Hydrated Lime

Selection of design strengths of soil subgrades and subgrades treated with cement or hydrated lime is a problem in pavement design analysis and construction. Different types of soils may exist in a highway corridor and different strengths may exist after the soils are compacted to form the pavement subgrade. The selected subgrade strength will largely affect the pavement thickness obtained from the design analysis, future pavement performance, and the overall bearing capacities of the subgrade during construction and the pavement structure after construction. In developing the proposed selection scheme, a newly developed mathematical model, based on limit equilibrium, is used. Relationships among undrained shear strength [or California bearing ratio (CBR)] and tire contact stresses are developed for factors of safety 1.0 and 1.5. The minimum subgrade strength required to sustain anticipated construction tire contact stresses during construction is determined. A criterion is proposed for determining when subgrade stabilization is needed and methods of selecting the design subgrade strength are examined. A least-cost analysis appears to be an appropriate approach as shown by analysis of a case study involving pavement failures. Two case studies show that soaked laboratory strengths appear to be fairly representative of long-term field subgrade strengths. Hence, using soaked laboratory strengths and least-cost analysis appears to be a reasonable means for selecting the design strength of subgrades for pavement analysis. To avoid failures of chemically stabilized layers, relationships among thicknesses of chemically treated layers and the CBR values of the untreated subgrade for a factor of safety of 1.5 are presented.

Stress Reduction by Ultra-Lightweight Geofoam for High Fill Culvert: Numerical Analysis

The study of earth pressure distribution on buried structures has a great practical importance in constructing highway embankments above pipes and culverts. Based on Spanglers research, the supporting strength of a conduit depends primarily on three factors: 1. the inherent strength of the conduit; 2. the distribution of the vertical load and bottom reaction; and, 3. the magnitude and distribution of lateral earth pressures which act against the sides of the structure. Considering high fills above them and high earth pressures they may experience, rigid culverts are usually used underneath highway embankments. To reduce high vertical earth pressures acting on buried structure, ultra-lightweight Geofoam was placed above a culvert in the field, in Russell County, KY. Before construction began, numerical analysis using FLAC 4.00 (Fast Lagrangian Analysis of Continua) had been performed to predict stresses on the culvert. Results of the analysis show that Geofoam has a great effect in reducing vertical stresses above and below the culvert. There were areas of high stress concentrations at the top and bottom of the concrete culvert if no Geofoam was placed above the culvert. After placing Geofoam above the culvert, the concentrated stress at the top can be reduced to 28 percent of the stress without Geofoam. The high stress at the bottom of culvert can be reduced to 42 percent of the stress without Geofoam. Stresses on the two sidewalls of the culvert were observed to have no significant change in values with and without Geofoam.

REDUCTION OF STRESSES ON BURIED RIGID HIGHWAY STRUCTURES USING THE IMPERFECT DITCH METHOD AND EXPANDED POLYSTYRENE (GEOFOAM)

The study of earth pressure distribution on buried structures has a great practical importance in constructing highway embankments above pipes and culverts. Based on Spangler’s research, the supporting strength of a conduit depends primarily on three factors: 1) the inherent strength of the conduit; 2) the distribution of the vertical load and bottom reaction; and 3) the magnitude and distribution of lateral earth pressures which act against the sides of the structure. Rigid culverts are frequently used in Kentucky for routing streams beneath highway embankments because of rolling and mountainous terrain, numerous streams, shallow depths to bedrock, which creates unyielding foundations, and the necessity of using high fills which create large vertical stresses acting on culverts. As a means of exploring ways of reducing large vertical earth pressures acting on a buried structure, ultra-lightweight geofoam was placed in a trench above a reinforced rigid box culvert in Russell County, KY. This study provides strong evidences from both numerical model analysis and in-situ test data to indicate that geofoam is an ideal elasto-plastic material to reduce vertical load on top of a rigid culvert resting on a rigid foundation. The load on the top of a culvert can be reduced to 20% of traditional design load after 2-ft-thick geofoam is placed on top of it. Results from numerical model are more conservative when compared to actual test data. As much as 57% of settlement from geofoam has been recorded. Stresses on the top of a culvert where geofoam was placed have reached a relatively stable level which is expected at the yield point of the geofoam. This technology can be used in applications which require controlled pressure on rigid underground structure. Whether geogoam is used or not used, the model analysis and test data show that the earth pressure acting on the sidewall does not change significantly. Although the pressure acting on the sidewall is slightly higher when geofoam is used on top of the culvert only, the value is still below the design value used by the Kentucky Transportation Cabinet. Use of geofoam placed in an imperfect trench significantly reduces the vertical stresses acting on the top of the culvert.

Characteristics and Engineering Properties of the Soft Soil Layer in Highway Soil Subgrades

The objective of this research was to examine the conditions and characteristics of soil subgrades that had been stabilized using mechanical compaction. Goals of the study are to identify and examine the engineering properties and behavior of the “soft layer’ of material observed at the top of untreated, highway pavement soil subgrades. Alternative methods of preventing, or mitigating, the development of the soft layer are discussed. Evidence is presented that shows that a soft layer of soil frequently develops at the top of untreated, highway soil subgrades. Data are presented that show strengths obtained from mechanical compaction are largely destroyed when untreated compacted soils are exposed to moisture. California Bearing Ratio (CBR) values of compacted clayey soils initially are high but become small when exposed to saturation. In situ CBR values measured at the tops of untreated subgrades, where mechanical compaction was the only means used to stabilize the soil subgrade, were smaller than unsoaked and soaked laboratory Kentucky CBR values. At the 85th percentile test value, the laboratory KYCBR value of compacted, unsoaked clayey specimens was 11.5 while the CBR value of soaked specimens was 3.0. For comparison, the in situ CBR value of untreated subgrades at the 85th percentile test value, as shown in this study, was only 2. Using a bearing capacity model, based on limit equilibrium of layered media, bearing capacity analyses of flexible pavement sections were performed. The analyses show that when the in situ CBR is equal to or below 3, the pavement was unstable, i. e., the factor of safety against failure was 1.0 or below. However, when the in situ CBR value was 6, or greater, the pavement was generally stable and the factor of safety was 1.5, or greater. Chemical admixture stabilization of soil subgrades is the most effective means of maintaining large CBR values during construction and throughout the life of the pavement. In situ CBR values at the 85th percentile of tests performed on the tops of soil subgrades treated chemically with lime kiln dust, hydrated lime, and portland cement and that had been in place for 8 to 15 years were 24, 27, and 59, respectively. At the 85th percentile test value, in situ CBR values of chemically treated subgrades were about 12 to 30 times larger than the in situ CBR value of 2 of untreated subgrades.

Load Reduction by Geofoam for Culvert Extension: Numerical Analysis

Widening of a highway embankment may increase loads acting on the weaker portions of the culvert. To accommodate increased loading, geofoam will be placed above the weaker portions of the culvert. Numerical analysis using FLAC was performed to predict loads on the culvert. Results of the analysis show that geofoam significantly reduces maximum moments on top and bottom of the culvert. The moment reduction is a function of the size of geofoam and the distance between top of culvert and the geofoam. By considering these factors, effectual curves are obtained from the numerical analysis.

Corrosion evaluation of mechanically stabilized earth walls.

Numerous reinforced walls and slopes have been built over the past four decades in Kentucky, the United States, as well as worldwide. Tensile elements used in constructing low-cost reinforcing walls and slopes consist of metal polymer strips or grids. Although reinforced structures have been used extensively, the effects of corrosion on the metal tensile elements are unknown. Mechanically stabilized earth walls are expected to remain stable for many decades. An examination of the effects of corrosion of metal tensile elements used to construct these walls can provide invaluable data regarding the longevity of reinforced walls and slopes. Four mechanically stabilized earth walls constructed with galvanized steel reinforcing elements were instrumented and corrosion rates obtained. Corrosion data obtained indicate the designed sacrificial thickness will not be used during the design life of the structures. No visible corrosion was observed in reinforcing elements removed from a mechanically stabilized earth wall that had been in service for more than 20 years. A database was constructed to manage inventory of mechanically stabilized earth walls constructed and maintained by the Kentucky Transportation Cabinet.

Use of Ultra-Lightweight Geofoam to Reduce Stresses in Highway Culvert Extensions

Culvert extension under highway embankment construction is a regular and important practice when roadway widening occurs. At some existing sites, concrete thickness and reinforcing steel of culvert tops and walls were stepped-down in sections of the culvert under the embankment slopes. The part of the culvert positioned under the embankment slopes was constructed weaker because the stresses under the portions of the slopes are much less than the stresses acting on the culvert section located under the main portion of the embankment. When additional fill is placed over the culvert due to roadway widening, much greater stresses are imposed on the weaker portions of the culvert. To accommodate the increased stresses on the weaker portions of the culvert, lightweight material will be placed above the weaker portions of the culvert in the field. Before construction begins, numerical analysis using Fast Lagrangian Analysis of Continua (FLAC) 4.0 was performed to predict stresses on the culvert. Results of the analysis show that geofoam has a great effect in reducing vertical stresses above and below the culvert. There are areas of high stress concentrations at the top and bottom of the concrete culvert if no geofoam is placed above the culvert. Placing geofoam above the culvert reduces the concentrated stresses at the top and bottom significantly. The stress reduction is a function of the size of geofoam and the distance between top of culvert and geofoam. To obtain an optimal practical situation, a numerical model was created to thoroughly analyze these factors. By considering these factors, effectual curves are obtained from the numerical analysis. When geofoam is placed directly on top of the culvert, the results indicate that the concentrated stresses at the top and bottom will be minimized, but it will require excavating the fill and replacing it with geofoam. The optimal situation for each culvert should be analyzed case by case. On the other hand, foam concrete can reduce load on the culvert if it is placed correctly. Valuable results using geofoam and foam concrete to reduce loads on a culvert are discussed in this report.