Selecting dimension stone for a salt-exposed jetty on the Konkan coast
A contractor must choose dimension stone for a 60 m jetty arm on the Sindhudurg coast, exposed to direct monsoon swell, tidal wetting and drying, and salt spray; the design life is 30 years. Three quarries are within haul distance. Quarry A: amygdaloidal basalt, vesicles filled with soft green zeolite, water absorption 8.5 percent, specific gravity 2.4, lowest price. Quarry B: compact fine-grained basalt, absorption 1.2 percent, specific gravity 2.9, crushing strength 180 MPa, priced 18 percent above A. Quarry C: laterite blocks, absorption 11 percent, soft and clayey when saturated, cheapest of all. A fourth supplier offers pale, uniform blocks that are scratched by a copper coin and effervesce with dilute hydrochloric acid, and markets them as granite. The client wants the lowest first cost; the engineer wants no maintenance beyond routine pointing over the design life.
1. Which quarry should be specified for the jetty arm? Justify your choice and reject the alternatives using the given properties and the deterioration mechanisms at this site.
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The controlling mechanisms in a tidal, spray-soaked jetty are salt crystallisation inside connected pores, wetting-drying fatigue and wave abrasion, so connected porosity matters more than first cost.
Quarry B is the correct choice: 1.2 percent absorption with specific gravity 2.9 indicates a dense, nearly impermeable rock, and 180 MPa crushing strength is far above any wave or berthing load. Low porosity leaves little space for salt crystals to grow and generate internal pressure.
Quarry A must be rejected: 8.5 percent absorption and specific gravity 2.4 show abundant open or clay-filled vesicles. Zeolite amygdules are secondary, softer minerals that alter and dissolve, and salt crystallisation in vesicles will spall the face within a few years, well inside 30 years.
Quarry C is worst: laterite combines 11 percent absorption with an iron-clay matrix that softens when saturated, so it cannot be used below high-tide level.
The 18 percent premium for B is smaller than a single replacement cycle of A, so the client's lowest-cost instruction is actually met over the design life. As a precaution, test B for alkali-silica reactivity: some Deccan basalts carry opal or chalcedony that reacts with cement alkalis, so the choice should be conditional on that test.
2. How would you identify the pale blocks offered as granite, and should they be accepted for the jetty?
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The two simple field tests already identify them. A copper coin has a Mohs hardness of about 3, and calcite is also 3, so a rock scratched by a coin contains a mineral no harder than calcite; granite is dominated by quartz (7) and feldspar (6) and would not be scratched. Effervescence with dilute hydrochloric acid confirms calcium carbonate. Together these point to a limestone or calcite-cemented rock, not granite.
For the jetty the answer is no. Calcium carbonate dissolves slowly in seawater and in water carrying dissolved carbon dioxide, and limestone has low abrasion resistance against wave-borne sand, so a marine splash zone is the worst possible place for it.
The blocks are not valueless. If crushing strength and absorption are acceptable, they can be used for dry plinth masonry, internal walls or flooring. Before any use, confirm on a freshly broken surface with acid, check for chert nodules (which polish differently and cause pop-outs) and check for clay seams. Report the mislabelling to the client, because a supplier who misnames one rock may misname others.
3. Set out an acceptance-test programme for the chosen stone before the supply order is placed.
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The programme should test rock, not a single hand specimen, because a Deccan lava flow is zoned: a vesicular or amygdaloidal top, a compact core and a weathered base can all be quarried from one lease. Take samples from at least three benches or faces.
Tests, in order of decision value:
- Petrographic examination of thin sections - confirms mineralogy, grain size, degree of alteration and the presence of reactive silica.
- Water absorption and specific gravity - the single best predictor of salt durability; reject anything above about 2 percent for marine work.
- Crushing strength on saturated 50 mm cubes - saturation, not oven-dry state, represents the jetty.
- Abrasion resistance - simulates wave-driven sand.
- Soundness by repeated sodium sulphate cycles - accelerates salt crystallisation and is the closest laboratory proxy for this site.
- Alkali-silica reactivity - mortar bar or accelerated test, since secondary silica is common in amygdaloidal flows.
Write pass and fail limits into the tender before quotations are opened, so the cheapest bidder cannot argue the specification afterwards, and require quarterly re-testing during supply.