The pavement's weakest link? I think so, a survivor from Macadam?, seems to have been around so long that we have become complacent. Before stiffness measurement, performance was a particle size distribution (PSD), frost heave, and the quite strict laying and compaction specification. I should say here to foreign visitors, wish I had some!!, that I refer to the specification for highway works. A monumental work from the UK's government engineers, a species not often seen walking on subbase, it is written to protect the client (well they write it?) and provide the tax payer with a good for value trunk road. Pity for years and years they have never changed the basis for classification, the PSD, still doesn't need to have UKAS accreditation!! Cut and paste, cut and paste, if this one blog puts a N after grading (PSD) in NG table 1/1 then the tanker has turned. Where was I? finishing off the last of my Talisker, well that and espousing on the pavement's weakest link, type wan. In comparative terms when capping is necessary it has a moisture content discipline, why doesn't type 1? why is density not measured as in some of the special 6's fills. Why can sources of type 1 be mixed? Does type 1 protect capping or subgrade from rain? This material breaks rules as you build a pavement, it is a teenager getting away with it. It is familiarity, and I think it is coming to an end. English readers may wonder where my experience comes from, well igneous rocks, limestone makes good type 1, if you make it properly. It may also have pozzolanic properties providing good binding. Igneous rocks are different animals (observant readers could note that the specification for highway works is .... well from England) why not? most trunk roads are there. So we here in Northern England (Scotland) have the same trunk road specification, well yes, but we have the the ability to have national alterations to the specification. Type 1 has no national alteration. There was a time that all Scottish frost heave testing was conducted at a central Scottish government lab and it had lower heave limits for compliance than England. Anyway that's history of the frost heave test and frost rooms rather than frost cabinets. Scottish Polished Stone Value testing was centrally tested too. I know type I is variable and even from a single source can be good when laid and later heave when saturated. I consider that stiffness testing will put some numbers to this variability, and I would like to think producers could work on different gradings to produce best stiffness at various moisture contents. But why worry about about a variable product, buy scalpings, add cement, add pavement value.

4 Responses to “Type 1 Subbase”

  1. January 7, 2009 at 1:40 pm

    When working on the A55 in North Wales, the SOS’s Agent, Clwyd County Council added the stpulation that densities be taken on all type 1 before surfacing.

  2. January 7, 2009 at 1:47 pm

    The real issue with respect to density testing, lack of compaction and bumps in completed roads at structures is perhaps more complex. Is the issue lack of compaction in the 6N backfill, is it poor compaction of the adjacent embankment fill where the roller has not perhaps been hauled over the edge, or perhaps the roller used has been the now seemingly essential big FO single drum roller that the driver is too scared to take any where near the edge? Or is it the transition between something particularly hard and immovable (the structure) and something almost the complete opposite? The use of run on slabs seems to help??? Also, might it help if we were allowed to invert the traditional wedge of structural fill, such that the 6N could be completed first, followed then by the embankment fill, for which the roller driver is no longer staring into the abyss as he gets to the end of each layer??

  3. Ian January 7, 2009 at 7:37 pm

    Aye, but was it horrible flaky slaty type 1?

  4. Ian January 7, 2009 at 7:48 pm

    You make a very good point RE the transition between fill and special fill. This would be particularly the case with a boulder clay embankment even rolled with a towed roller that can safely “round” off the layer by reversing it over the slope, it would still soften up a tad. Perhaps cohesive fills should be left at a safe temporary slope and the 6N benched in against it. Good comment, lets have some more from you visitors.

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