Environmental Engineering Reference
In-Depth Information
Schmertmann (1991) are more likely causes of the initial strength gains than solution
and recementation of the carbonate material.
One of the mechanisms described by Schmertmann was interlocking of grains due
to pressure solution and the authors consider it possible that this may be the cause of
the small remaining strength after resoaking.
Based on their experience with the gypsum-cemented marble (see
Section 3.7.7.1
) the
authors suggested that the initial strength increase might have been caused by the presence
of minute amounts of gypsum and that this could have been present in the coquina or in
the waters used. McClellan (2003) has advised that the presence of gypsum is possible,
even likely, during road construction in Florida. Traces of pyrites occur in some of the
limestones and sea sprays (containing sulphates) occur widely. However he believes that
gypsum is not a factor in their laboratory tests. SEM/EDX examination of the cemented
areas of their specimens has never shown any significant sulphur.
Some very weak, porous calcarenites are sawn into blocks, which are then used for the
walls of small buildings. Newly-sawn blocks are soft to touch and easily scratched,
gouged or broken. After exposure to the weather (wetting and drying) for several years,
the surfaces of the blocks become noticeably stronger and more durable. It appears that
this “case-hardening” happens because carbonate derived from dissolution within the
blocks becomes deposited near their surfaces. The Gambier Limestone (Tertiary Age) in
South Australia is an example. Millard (1993) notes that “soft local limestone” on Malta,
shows similar behaviour. Similar “case-hardening” effects can be seen in the faces of many
cliffs formed by
Category Y
limestones, and carbonate-cemented sands.
The following conclusions can be drawn about
Category Y
carbonates:
1. When subjected to compaction, followed by soaking or wetting and partial drying,
some of them have become strengthened by solution and recementation of carbonate
grains in a few days to a few years. In most cases the above has involved some very fine
carbonate material. In some cases most of the strength gains have been lost on re-soaking;
2. Exposed surfaces of some very weak, porous carbonate rocks become strengthened in
a few years by solution and redeposition of carbonate;
3. In each of the above cases it is possible that the strengthening has been partly or wholly
caused by gypsum.
The moisture environments in filter materials (in chimney zones, in particular) may at
times be similar to those in which cementation was postulated, in 1 above. A carbonate
filter zone would not normally contain significant amounts of fines. However the authors
consider that the possibility exists that during the lifetime of a dam, solution and rece-
mentation and/or grain interlock from pressure solution, could render it cohesive and
ineffective.
The authors consider that carbonate materials should not be adopted for filters until
the possible long-term effects of dissolution have been assessed by geotechnical specialists.
3.7.8
Suitability of carbonate rocks for embankment materials
Category O
carbonate rocks have been used widely and with apparent success as rockfill,
random fill and riprap. However, there is some potential for deleterious effects, if sulphide
minerals are present:
- in the carbonate rocks;
- in other materials in the embankment, or
- in the foundation or storage area.
