Chulmin Jung

Long-Term Performance of Chemically Modified Subgrade Soils in Indiana

Chemical modification of soils with lime in Indiana is widely used to improve the workability and compactability of weak subgrade soils. Although the modification process is primarily aimed at construction expediency, additional effects such as long-term improvement of stiffness or strength by pozzolanic and carbonation cementation processes of the treated soils have been expected but have not been quantified. Because of a lack of confidence in the long-term performance of the chemically modified soils, their enhanced stiffness has not been taken into account in pavement design, leading to a conservative design of the asphalt or concrete pavement layers. Six roads that have been in service for 5 to 11 years were selected to conduct field tests. The selection was performed considering (a) location of road project in Indiana, (b) class of road, (c) year when chemical treatment was done, (d) type of chemical modifier used, (e) type of pavement, (f) availability of geotechnical information for postconstruction evaluation, and (g) traffic and safety control for field testing. The results from the field tests show that the long-term stiffness or strength of subgrades that were chemically treated 5 to 11 years ago is 4 to 11 times higher than that of natural sub-grades. This finding suggests that the enhanced stiffness of the chemically modified subgrade could be accounted for in pavement design, which would lead to a reduction of the thickness of asphalt or concrete pavement layers.

Post-Construction Evaluation of Lime-Treated Soils

Lime is used to treat weak subgrade soils during construction of highways. A small amount of lime (4 to 7%) is used to rapidly dehydrate and modify fine grained soils. The modification process improves workability and compactability of the soils. Although the lime modification process is primarily aimed at construction expediency, additional effects such as long-term improvement of stiffness and/or strength by pozzolanic and carbonation cementation reactions are expected. Lime treatment has been employed in Indiana over several decades, but the long-term performance of lime-treated soils has not been well quantified and no field tests have been done on roads in service. A comprehensive field investigation was carried out to determine the properties of subgrade soils treated with lime in pavements that had been in service for at least five years. Six sites were selected for the field tests. At each site, SPT, DCPT, and FWD tests were performed to evaluate the in-situ stiffness and/or strength properties of the lime-treated subgrade. Laboratory tests from soil samples taken from the SPT spoon were done to obtain index properties of the lime-treated subgrade and the lime content that remains in the soil. The long-term performance of the lime-treated subgrade at each site has been evaluated by comparing the soil indices and stiffness and/or strength properties of the lime-treated subgrade soil with those of the natural soil. In addition, the lime content of the subgrade and the natural soil were measured to establish the remaining lime in the treated subgrade and detect any leaching in the underlying soil. The research has shown the following: (1) the lime remains in the soil even after 11 years of service of the road after construction; (2) the addition of lime decreases the plasticity of the soil and increases its CBR; and (3) the construction quality observed from the field tests is highly variable.

Seismic Earth Pressures behind Retaining Walls: Effects of Rigid-Body Motions

The seismic earth pressures behind a retaining wall that bends, rotates about its base, and translates horizontally and vertically are evaluated numerically using an ideally elastic model. The following factors are analyzed: (1) flexural movements of the wall; (2) rotation of the wall at its base; and (3) horizontal and vertical movements of the wall. The following results are obtained: (1) the flexural flexibility of the wall, the horizontal translation, and the rotation at the base have significant effects on the earth pressures behind the wall; and (2) within the range of relative flexibilities of typical retaining walls (relative between wall and backfill), the effects of the horizontal translation are more significant than those of the vertical translation and rotation. Four extreme cases are also analyzed: rigid wall with stiff foundation, rigid wall with soft foundation, flexible wall with stiff foundation, and flexible wall with soft foundation. The seismic pressures behind a rigid wall with a stiff foundation are the largest, and they are the smallest for a flexible wall with a soft foundation. The pressures behind a rigid wall with a soft foundation are affected the most by the horizontal translation of the wall, while the pressures behind a flexible wall with a soft foundation are affected both by the flexural flexibility of the wall and by the horizontal movements.

Field investigation of engineering properties and uniformity of subgrades chemically treated with LKD

A field investigation was performed immediately after the construction of a subgrade treated with 5% lime kiln dust (LKD) to determine the degree of uniformity and quality achieved using current construction techniques. A 280-m long section of a road was chosen for the field tests. The first 140 m was treated with LKD over a target thickness of 41 cm (16 inches), which is the current standard practice in Indiana. The second 140 m was treated with a reduced target thickness of 36-cm (14 inches). For the thicker, 41-cm treated subgrade, the increase in california bearing ratio (CBR) with respect to that of the untreated soil at the site was around 100%, as average, while for the thinner, 36-cm treated subgrade it was about 350%. All field and laboratory test results showed consistently better and more uniform results for the 36-cm thick treated subgrade than for the 41-cm thick treated subgrade.

Simple Method to Identify Marl Soils

An experimental investigation proposed a simple, practical method to identify marl soils in the laboratory and to classify the soils. The percentage of calcium carbonate (CaCO3) of the soil was determined with three methods: (a) thermogravimetric analysis (TGA), (b) sequential loss on ignition (LOI), and (c) chemical reaction following ASTM C25. The sequential LOI test has the advantage that both organic and CaCO3 content of the soil can be determined with a conventional furnace. X-ray diffraction, pH, and Atterberg limits tests were also conducted. The percentage of CaCO3 determined from the sequential LOI tests agreed well with those from the TGA tests and from the chemical tests. No correlation was found between the percentage of CaCO3 and organic content in the soil. As the organic content of the soil increases, the liquid limit increases and the plasticity of the soil increases. As the CaCO3 content of the soil increases, the liquid limit of the soil decreases and the soil becomes less plastic. The geotechnical engineering properties of marl soils depend on organic content and CaCO3 content, and so the soils should be classified according to both organic content and CaCO3 content.

Field Investigation of Subgrade Lime Modification

This is an implementation project for the completed research (INDOT SPR- 3007) Post-Construction Evaluation of Lime-Treated Soils. The objectives of the project are to investigate the degree of uniformity and quality that is obtained with soil modification with lime kiln dust (LKD) using current construction techniques, and also to explore changes in construction methods that may result in a better product. The objectives are accomplished by: (1) selecting an appropriate construction site where LKD is used for soil modification; (2) performing field and laboratory tests to ascertain the magnitude of the engineering properties of the chemically treated soil and the degree of uniformity accomplished with the treatment; (3) analyzing the field and laboratory data to provide recommendations for changes in construction methods; and (4) providing recommendations for considering the modified subgrade as a structural layer for pavement design. An INDOT road construction project, located along SR 641 south of Terre Haute, was chosen for this research. A 280-m long portion of the road was divided into two construction and test sections. The first 140-m long subgrade section was chemically treated with LKD with a target thickness of 16 inches, which is a typical standard practice based on the current design and construction protocol for chemical subgrade treatment. The remaining 140-m long section was treated with a target thickness of 14 inches. Field tests were conducted on the subgrade after seven days curing of the chemical treatment in order to evaluate in-situ engineering properties of the chemically treated subgrade. Laboratory tests were performed to estimate the lime content in the soil. The laboratory tests show an adequate presence of lime in the subgrade, with somewhat better uniformity for the test site with 14 inches target thickness for the subgrade. Field test results show consistently better and more uniform results for the 14 inches target thickness site than for the 16 inches target site. Based on the findings from this study, a decision was made to amend INDOT specifications for the following: 1) increase for design the California bearing ratio (CBR) of the subgrade treated with LKD by 25% over that of the natural soil; 2) implement a recommendation for a target thickness of the treated subgrade of 14 inches; 3) introduce a special, one-type project where quality control/quality assurance is done by the contractor for design and construction and where full advantage of the subgrade improvement may be taken into consideration to minimize pavement thickness. It was also agreed to monitor performance of new pavements where the subgrade is treated with LKD in order to build a database of the quality achieved during construction across the State. This recommendation is based on the potential for increasing the CBR of the subgrade beyond the 25% recommended, thus lowering the cost of the pavement.

Classification of Marl Soils

Field and laboratory tests were conducted to investigate the degree of uniformity and quality that is obtained with chemical treatment of the subgrade with lime kiln dust (LKD) using current construction techniques. An Indiana Department of Transportation road project under construction was selected for the research. A 140-m long subgrade section was chemically treated with LKD with a target thickness of 16 inches, which is the current standard practice, while another 140-m long section was treated with a target thickness of 14 inches. Dynamic cone penetration (DCP) tests were done at each section to obtain the stiffness (or strength) of the chemically-treated and natural (untreated) subgrade soil layers. Lightweight deflectometer (LWD) tests were performed at the same locations where the DCP tests were done to estimate the stiffness of the treated subgrade layer. Nuclear gauge and sand cone tests were carried out to obtain the water content and dry density of the chemically treated subgrade. X-ray diffraction (XRD) and thermogravimetric (TGA) tests were performed on soil samples collected in the field to identify and quantify the minerals contained in the soil. XRD and TGA laboratory tests show an adequate presence of lime in the subgrade, with somewhat better uniformity for the test site with 14 inches target thickness for the subgrade. Field tests, namely density, DCP and LWD, show consistently better and more uniform results for the 14 inches target thickness site than for the 16 inches target site. As a result of the research, it is recommended: (1) to increase for design the CBR of the subgrade treated with LKD by 25% over that of the natural soil; (2) to implement recommendation for a target thickness of the treated subgrade of 14 inches; (3) to introduce special, one type project where quality control/quality assurance is done by the contractor for design and construction, where full advantage of the subgrade improvement may be taken into consideration to minimize pavement thickness.