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

3. Mellow the soil-lime mix that has been placed in the container at the expected average daily ground temperature for 24 hours. Note that the expected average ground temperature used for curing and mellowing in 2 and 3 will be dependent upon the time of year that the work is undertaken. For example, if treatment work is to be planned for autumn in temperate areas then a realistic mellow and cure temperature might be 10 deg C.

4. Determine the modified compaction characteristics of the 24-hour-mellowed specimens.

5. Once the modified OMC has been determined for the 24-hour-mellowing period, a similar soil-- lime batch can be prepared and mellowed under the same conditions reported above.

6. After mellowing, prepare strength specimens compacted at the modified OMC determined in 4 (this will be greater than the immediate OMC with no mellow period) and cure for a specified period at the mellowing temperature for strength assessment.

7. If the specimens that receive no mellowing before compaction generate the greatest strength gain, then it must be recommended that the soil-lime mix is compacted immediately, or as soon after lime addition as possible. However, if a 24hour-mellowing period, before curing, produces a greater strength, then more mellowing periods should be investigated to determine the optimum mellowing period for maximum strength gain. The optimum period should then be recommended as the mellowing period used on site.

The procedure described above is valid where good pulverisation plant on site is to be used. However, mellowing periods should be considered where crude mixing techniques are utilised to avoid the possibility of unchanged calcium oxide being present in the compacted mix as it will subsequently expand on hydration and destroy the integrity of the limetreated material.

When projects stipulate the use of a specific mellowing period, then the above testing procedure may be followed, using the specified mellowing period given, or chosen to determine the modified compaction characteristics and to assess the expected strength gain. A summary of both procedures is presented in Figure 4.

(Note: It is important to determine the compaction characteristics after various mellowing periods because the compaction characteristics alter with time (MDD decreases and the OMC increases with mellowing duration). Different mixing moisture contents also affect the compaction characteristics. Therefore, it is critical that the soil to be treated is tested at the current in situ, or anticipated operational, moisture content. Failing to do so may lead to the soil-lime mix being compacted at the incorrect moisture content. If the layer is too wet at mixing, then the strength reduces due to decreased density. If the layer is too dry, more air voids are incorporated which affects the durability of the layer as moisture can penetrate the layer more easily at a later date. This may ultimately lead to failure.)

Procedure for material that is to be modified only

If only modification of the soil is required, often to dry out wet material, then the treated soil must be prepared and mellowed as described in section 6.1 at a temperature corresponding to the expected temperature on site. The mellowing period may exceed the maximum period recommended by the specification depending upon the criteria required and the condition of the material to be treated. The testing programme should determine the most effective mellow period required to achieve the required modification. It has been reported by Holt and Freer-Hewish (1998) that property changes to the treated soil during the modification process are enhanced with mellowing duration. By adding less lime than originally thought necessary and increasing the mellowing period to achieve the desired modification of the soil savings in cost could possibly be achieved. However, it must be borne in mind that any delays in compaction will result in plant being required for longer periods. Therefore, any cost savings made from using less lime may be negated by increased plant costs. A summary of this procedure is presented in Figure 5.