Liecheng Sun

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.

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.

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.