EM 1110-2-1100 (Part III)
30 Apr 02
which is equivalent to crushing strength. Tensile strength, which is significantly less than compressive
strength, is not discussed here, but tensile strength is usually proportional to compressive strength.
(2) The most commonly encountered material in coastal engineering is the quartz sand grain, which is
very strong indeed. A single crystal of quartz has a strength on the order of 2,500 MPa. However, a
sandstone, which is a composite of many sand grains, is surprisingly weak (top histogram on Figure III-1-4),
being typically less than 100 MPa, or less than 4 percent of the strength of the single crystal. For this reason,
sandstone is rarely used in coastal engineering construction.
(3) The difference in strength between quartz crystal and composite sandstone is due to weak
intergranular cement and to flaws such as grain boundaries, bedding planes, cleavage, and joints that have
a higher probability of being present in larger pieces. The data on Figure III-1-4 are excerpted from the
extensive tabulation of Handin (1966), using all samples which were tested at room temperature and zero
confining pressure.
(4) For calcium carbonate, strength varies with size in a direction opposite to that of quartz. Single
crystals of calcite are weak (~14 MPa, depending on orientation) compared to single crystals of quartz
(~2,500 MPa). But limestone rocks, made from interlocking calcium carbonate crystals, are much stronger
than single crystals of calcium carbonate, and even somewhat stronger than sandstones, as shown by
comparing the top two histograms on Figure III-1-4.
(5) The weaker rocks on the left end of the histograms of Figure III-1-4 are those with macroscopic flaws
such as bedding planes, rather than flaws in single grains. The high-strength outliers on the right end of the
sandstone and limestone histograms of Figure III-1-4 are special cases. The sandstone outlier is a quartzite,
a metamorphosed sandstone recrystallized with silica cement. The limestone outlier is Solnhofen limestone,
which is a dense, fine-grained, uniform limestone.
(6) Dolomite is a carbonate rock allied to limestone in which about half of the calcium of calcite has been
replaced by magnesium (both the mineral and the rock are called dolomite). On average, dolomite and
dolomitic limestones make better riprap than limestone and sandstone, as suggested by comparison of the
histograms on Figure III-1-4.
(7) Where available, rocks classified commercially as trap rock (dense basalt, diorite, and related rocks)
or granite (including rhyolite and dense gneiss) make even better riprap, with strength typically on the order
of 140 to 200 MPa under conditions comparable to those in Figure III-1-4.
(8) A typical specification for rock used as riprap in coastal engineering, extracted from "Low Cost Shore
Protection," Report on Section 54, U.S. Army Corps of Engineers (1981, p. 785), is as follows:
The stone shall be free of cracks, seams, and other defects that would tend to increase unduly its
deterioration from natural causes or breakage in handling or dumping. The stone shall weigh, when
dry, not less than 150 pounds per cubic foot.* The inclusion of objectionable quantities of sand, dirt,
clay, and rock fines will not be permitted. Selected granite and quartzite, rhyolite, traprock and
certain dolomitic limestones generally meet the requirements of these specifications.
*150 PCF is equivalent to 2,400 kg/m3.
Coastal Sediment Properties
III-1-17
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