significance of recent research on specifications and standards for soil-lime pavement layers --the need to reationalise the design procedure, The

Road & Transport Research, Jun 2000 by Holt, C C, Freer-Hewish, R J

After mellowing, the treated layer must be remixed before final compaction. The pulverisation requirements must be achieved before compaction can commence in some specifications. These are usually the same as those specified during initial mixing of the soil and lime. After re-mixing, the layer is then compacted to achieve specified density targets. For example, in the UK, the moisture condition value (MCV) is used as a water content control for compaction. Details of this test are given by Freer-Hewish (1984).

The lime-treated layer should then be moist-- cured for a certain period (generally 7 days) or until the next layer is placed. During this curing period only light trafficking of the layer may occur.

Minimum construction air temperatures range from -1 to 10 deg C. In temperate regions and where large seasonal fluctuations are expected, the treatment operations may be confined to certain times of the year (see Table 1). For example, in the UK, lime treatment of soils only occurs during the months of March through to September inclusive and when the shade air temperature is greater than 70 deg C. Generally, during winter months in temperate climates, treatment of soils with lime cannot be carried out unless specified by an engineer.

Of the thirty seven specifications reviewed, thirty one suggest that a mellowing period should be adopted. The requirements of the thirty seven specifications with respect to mellowing periods and treatment restrictions are listed in Table 1. Table 2 summarises the mellowing requirements of the specifications reviewed.

THE INTRODUCTION OF MELLOWING PERIODS INTO THE TREATMENT PROCESS

Early experiences in the USA in the 1950s (Hoover (1965) and Thomas, Jones and David (1965)) found that after initial mixing it was necessary to introduce a mellowing period to allow the lime to react with the soil before proceeding with compaction. This process is used to assist the break-down of large clay clods so that pulverisation requirements are achieved before compaction. The mellowing period, also assists in drying out excessively wet soils so that soil workability and traffickability of construction equipment is improved. As a result of these early experiences, mellowing periods have been included in many specifications.

Generally, a period of between 48 and 72 hours was found tobe sufficient. Further fieldwork undertaken by McDowell (1959) also supported the use of a mellowing period. However, it was suggested that the mellowing period should not exceed 48 hours unless heavy clays were to be stabilised, in which case McDowell suggested that this period could be extended to 96 hours.

As a result of these early experiences, mellowing appeared in many specifications with limited scientific justification. Two studies at that time had, however, indicated problems with long-term mellowing periods. Taylor and Arman (1960) evaluated several aspects of lime-stabilised sub-base failures and reported that treated layers mellowed for longer than 48 hours generally produced weaker material compared to layers compacted within 48 hours of lime treatment. This lead to a laboratory investigation carried out by Mitchell and Hooper (1961). Changing the mellowing period from 0 to 24 hours reduced the density by up to 7.5%, cured strengths by 30% and swelling virtually doubled. Similar decreases in density were reported by Dumbleton (1962). The significance of this research does not appear to have been fully appreciated or highlighted. Indeed, it was not until much more recently that Sweeney et al. (1988, 1989) reinforced the finding that prolonged mellowing adversely affected the strength of the lime-treated material.