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

The initial moisture content affects changes in strength with mellowing duration. Typically, an optimum mellowing period, of not more than 1 day, for maximum strength and durability criteria was observed for soils that were initially very wet prior to lime addition, see Table 6. For soils that were initially dry (i.e. water had to be added to the soil-lime mix), maximum strength and durability were recorded with no mellowing prior to compaction, see Table 6.

Soil-lime mixes were mellowed and cured at 5, 10 or 20 deg C. At cool temperatures (10 deg C or below), the soil-lime reaction was much slower and consequently the changes to the properties of the soil were less significant. Typically, at cool temperatures, the decreases to the pH, plasticity index and linear shrinkage were less significant than those experienced at warm temperatures, see Table 7. Changes to the compaction characteristics were also slightly less significant at cool temperatures, also shown in Table 7.

DIFFERENCES BETWEEN CURRENT SPECIFICATIONS AND TESTING STANDARD PROCEDURES

The purpose of laboratory testing methods are to determine whether lime treatment will enhance the workability, strength and durability characteristics of a soil. Ideally, the laboratory testing procedure adopted should emulate site conditions as closely as possible to quantify reactivity of the soil with lime.

There do, however, appear to be significant discrepancies between the recognised procedures for testing standards and specifications used for lime treatment, particularly with respect to the mellowing period. Examples of these differences are highlighted below.

Mellowing periods used in the USA

Specifications for highway works in the USA are primarily 'State' generated rather than derived at a national level. This appears to be reasonable in the context of soil types and climates in the different regions of the USA. Local factors can therefore be more easily addressed. However, from Table 2, it does appear that the duration of mellowing from State to State is inconsistent and not explained solely by climate, geographical and geomorphological differences.

The current American Standard Testing Method (ASTM 1990) that specifies the preparation of soil-- lime mixes for testing is D 3551-90. Clause 5.6 of this test method states that:

'If mellowing of the mixture is permitted ..... Operations should be planned so that the mixture is used immediately after the mixing operation or after a mellowing period, if such is desired.'

This statement would appear to leave the mellowing period as optional. Test methods generated by individual States, however, are more specific about the mellowing duration used during laboratory testing. A typical example of this is given below for the State of Illinois.

Clause 2.c.4 of the State of Illinois Department of Transportation laboratory design procedures (1992) for lime stabilised/lime modified soils states that:

'The mixture shall be allowed to mellow for a period of one hour before compaction.'