Over the next five years road construction in Ireland will be carried out at an unprecedented scale. The main driver for this work is the National Development Plan which allows for a total investment of up to 6 Billion Euro. Due to the scale of these major road projects, Irish companies are not in a position to carry out the work on their own and are forming partnerships with some of Europe´s largest road building contractors. A number of these projects are being tendered on a design and build contract basis and therefore European road building techniques are likely to be used in this country. Soil modification and stabilisation using lime or lime/cement is a commonly used method of providing a construction platform in many countries. The advantages of in-situ stabilisation are many. They include reduced disposal of sub-standard clays, reduced importing of aggregates, far less truck movements and reduced costs. In the UK last year, approx. 1.3 million m3 of soil were stabilised in either road construction or in private site work. So far this year in Ireland, almost 250,000 m3 of soil have been modified / stabilised in the provision of factories, shopping areas and housing estates etc. Soil treatment´s greatest popularity however is in the U.S.A. where they have amassed considerable information and expertise over the last 50 years. Last year, they stabilised up to 40 million m3 of soil. Seminars on the subject were held in Ireland pver the last few years which gave detailed information on the use of soil stabilisation. The keynote speaker was Dr. Dallas N. Little who is the leading soil stabilisation person in the U.S.A. and who holds the E. B. Snead Chair Professor at the Texas A&M University and is a Senior Research Fellow at the Texas Transportation Institute. In his address he discussed the technology and impact of lime and cement stabilisation in the U.S. Some of his views are given here. Lime (calcium oxide or calcium hydroxide) and Portland cement are used widely in the United States in roadway construction not only to provide a construction platform but also to provide a durable structural layer. It is very important to select the appropriate stabiliser based on the type of soil to be stabilised. It is equally important to understand the mechanisms of stabilisation, the expected engineering improvements due to stabilisation and the design and construction practices necessary to achieve satisfactory results. A wide range of case histories now exist that document the improvements in engineering properties as well as the durability of stabilisation. Many of these case histories deal with in situ strength and modulus measurements. A 1995 field study on over forty pavements in the state of Texas compared properties of unstabilised clay soils with the same soils stabilised with lime. When lime was used to achieve stabilisation and not merely modification, compressive strengths and moduli of the stabilised soils were typically five to twenty-five times greater than those of the unstabilised soils. These stabilised layers were typically between ten and twenty-five years old. One must have at least a rudimentary understanding of the means by which lime and cement react with soil in the stabilisation process in order to appreciate the efficacy of stabilisation. This understanding sets the stage for discussions on mixture design, layer thickness design, construction techniques, and quality control/quality assurance requirements. In the upcoming seminar mixture design and predicted engineering properties (plasticity reduction, strength and modulus) will be linked with in-situ properties from two major studies in the states of Texas and Mississippi. Research studies in these states used dynamic cone penetrometer tests and Falling Weight Deflectometer tests to assess in-situ strength and resilient moduli of the stabilised layers. Most of the fifty pavements tested were over 20 years old, and the results document the durability of the properties derived from stabilisation. The presence of high levels of sulphate salts (such as calcium sulphate) in clay soils can cause disruptive heaving due to the formation of expansive hydrous minerals. However, recent research has demonstrated how construction techniques can be modified to minimise the risk of stabilising such high sulphate soils. The successful stabilisation at Denver International Airport in Colorado supports the validity of the approach. Soils rich in organics can also be difficult to stabilise with lime or cement due to the tendency of organic molecules to adsorb calcium. However this type of soil was successfully addressed during the construction of Interstate Highway 10 across the state of Louisiana. The success of lime and cement stabilisation has been so striking in Texas, that the Texas Department of Transportation is requiring stabilisation with either lime or cement on the proposed Toll Road project, State Highway (SH) 130. The soils along the corridor of SH 130 are notorious for their expansive properties with plasticity indices often exceeding 60 percent. This $2.4 billion project will be the first major design-build, warranty project in central Texas. This project is an excellent example of how expansive clay soils and the non-plastic soils can be stabilised with lime and cement. During the construction of the $4.5 billion Denver International Airport, which was completed in 1995, over 110,000 tons of lime was used to stabilise the expansive claystone subgrade. The process was so successful that the same stabilisation specifications and pavement designs are being used to construct a sixth runway to be used exclusively for international flights. The combination of highly plastic, sulfate-bearing claystones made the Denver soils a particular challenge to stabilise. The stabilisation process was cited as one of the major reasons that the Denver International Airport was selected by the American Society of Civil Engineers as the outstanding engineering project of 1996. Lime slurry being added to claystone soil at Denver International Airport. Lime and cement have been used in novel ways by the City of Houston Texas at George Bush Intercontinental Airport (BIAH). Houston has poor quality aggregates. Yet the native siliceous river gravel was stabilised with lime, cement, and a locally produced fly ash to produce a pozzolanically stabilised base for a runway and a taxiway at BIAH. The runway consists of 75 mm of asphalt concrete over a 750 mm layer of lime-cement-fly ash stabilised base. The taxiway consists of 350 mm of Portland cement concrete (PCC) over a 500 mm layer of lime-cement-fly ash stabilised base. Each pavement saved the City of Houston an estimated $10,000,000 in initial construction costs. The runway has been in service for over 15 years and is performing extremely well. Soil Stabilisation is not a new technology, in fact parts of the Appian Way in Rome were stabilised and are still in use two thousand years later. Contrary to worldwide practices Ireland has been slow to adopt this technology due mainly to the abundance of aggregates. However with the extensive development program planned for the next 5 years, stabilisation technology can have a vital role to play in protecting our environment and providing alternative practical and cost saving options for our road-builders. Pavement cross-section containing lime, cement, fly ash (LCF) base at Bush Intercontinental Airport, Houston, Texas.