A coastal hospital site at Ratnagiri: laterite foundations, a saline well and a soak pit
A four-storey hospital is proposed on a coastal plateau near Ratnagiri, about 700 m from a tidal creek. The site has 3–8 m of indurated, vesicular laterite over 12–18 m of weathered amygdaloidal Deccan basalt, with fresh basalt below 30 m. A 12 m dug well on the plot shows water at 6 m in June and at 11.5 m in May. Chloride in that well is 120 mg/L in June and 620 mg/L in May. Two dug wells 2 km inland are 8–10 m deep, with water at 4–5 m and chloride below 60 mg/L throughout the year. The contractor's plan is a strip footing founded on the laterite, with a soak pit and a septic tank at the rear of the plot, 12 m from the well.
1. Assess the well and the aquifer. What is happening to the chloride, and what should be done?
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Chloride rises five-fold as the water level falls, and the inland wells at the same time of year stay below 60 mg/L. That inverse relationship between head and salinity is the signature of saline water entering the well as the freshwater head declines - intrusion from the creek 700 m away, or upconing of saline water at the base of the aquifer. Sea spray or rock weathering would not produce a seasonal swing of this size in one well while leaving inland wells unaffected.
The well is also nearly dry at the end of May: water at 11.5 m in a 12 m well means less than half a metre of standing water, and the freshwater lens is correspondingly thin. Applying the Ghyben–Herzberg idea, the lens thickness is roughly forty times the head above sea level, so a small drop in head removes a large slice of fresh water.
Actions: stop using this well as a supply source in the pre-monsoon, log electrical conductivity monthly to track the interface, and drill a test bore to about 40 m to establish whether fresher water exists in fractures in the fresh basalt before any production well is designed. Do not simply deepen the existing dug well - that would draw the saline interface upward.
2. Is a strip footing on the laterite acceptable? Set out the reasoning and the investigation you would require.
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Laterite can be an excellent founding stratum because it hardens on exposure and drying, but three features of this site make a plain strip footing risky. First, the laterite thickness varies from 3 m to 8 m across the plot; a strip footing bearing on a layer whose base slopes will suffer differential settlement as load reaches the softer weathered basalt on the thin side. Second, vesicular laterite is not uniform - cavities and soft, clay-rich pipes are common and are invisible from the surface. Third, the weathered amygdaloidal basalt below can contain clay-filled vesicles and decomposed zones with very low penetration resistance.
Investigation required: at least three or four boreholes taken to 25–30 m, with standard penetration tests through the laterite and weathered zone, core recovery and RQD in the basalt, and groundwater level records; a plate load test on the laterite; and a slope stability check for the plateau edge near the creek.
Decision rule: if SPT values in the weathered zone are consistently adequate and the laterite base is near-horizontal beneath the footprint, a raft is preferable to strip footings because it bridges local variability. If the weathered zone is thick or irregular, use under-reamed piles socketed into the fresh basalt below 30 m.
3. Advise on the soak pit and septic tank. What is the hydrogeological reasoning behind your advice?
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The aquifer here is a thin, seasonally fluctuating body of water in laterite and weathered basalt, close to the coast, with poor dilution capacity. Any continuous source of nitrate, chloride and bacteria placed 12 m from a well screened in the same shallow horizon will reach that well, and the fine-grained laterite does not filter pathogens reliably over such a short distance.
The design principle is separation, not treatment. Move the soak pit and septic tank to the inland, down-gradient side of the plot, away from both the well and the creek, with a minimum horizontal separation of 15–30 m from any well and enough vertical separation above the water table; line the tank and provide a leach field rather than a single pit. Confirm the groundwater flow direction with levelling of static water levels in the existing wells in June, not in May, because the gradient reverses seasonally near a tidal creek.
Finally, plan for monitoring: an observation piezometer between the sanitation area and the well, sampled for nitrate and faecal indicators, is a cheap condition on the consent and gives early warning if the siting is still inadequate.