Fig. 21 illustrates one ofthe defects discovered in some of the large diameter bored piles involved in the bridge construction. Three main lessons may be learned from these defects.(a) The value of 10 kN/mZ for ground cohesion normally adopted to determine whether permanent liners or only temporary liners are required for casing the pile bore may be too low. It would be more prudent to use a higher value, for example 15 kN/m*.

The planning of maintenance work, particularly mechanized maintenance, using ontrack machines has been based mainly on the monthly 'black box' records (cf. 66)depicting aspects of the vehicle ride. An analysis of the mechanized maintenance carried out during the first year has shown that, as an overall average, the frequency of tamping/lining has been about twice per year. There has, however, been a considerable variation over different sections of the route, ranging from 1-3 times per year to 2.8 times per year.71. Records from the new British Railways high speed track recording car and its predecessor, the Research Department's TRIM vehicle, have been used to detect changes in track geometry, both in the top and in the alignment, which are measured in terms of standard deviations from the ideal profiles. These showed that the overall quality did not vary much during the year, although there was a slight improvement in alignment. The latter tended to be better than the top, confirming the initial impression that greater emphasis is needed on the maintenance of the vertical profile. This applies particularly to such aspects as welds and washy places where the ballast condition has deteriorated or failed.72. Local washy places have had to be dealt with quickly to avoid the usual spreading into a full ballast cleaning item. The most effective way of dealing with them has been to dig out the cribs and beds using lightweight hammers with spade attachments and then to use the same basic tools with compactor head attachments to consolidate the new ballast. This has been successfully carried out without a temporary restriction of speed even on the high speed route. The fact that washy places do still appear, usually'where the full 300 mm depth of ballast had not been achieved, has emphasized the need for the large programme of ballasting described in the Paper.73. The first year has amply demonstrated that in order to retain the high standards of track geometry required for satisfactory high speed running, preventive maintenance of the track is essential in every aspect. The unplanned imposition of temporary restrictions of speed would quickly destroy the high speed service which depends, not only on the high speed running, but on a short turn-round time at the terminals.M r P. Rees, British Rail, Western Region According to the Paper, reballasting, including blanketing, was carried out on 108 miles of track prior to the introduction of the high speed trains, and that was only 43% of the total track length between Paddington and Bristol Parkway on the one hand and Box