Maldives ground in brief

Islands built by the reef

Maldivian islands are low piles of sand and gravel sitting on coral reef platforms. Almost all of the ground is carbonate: broken coral, shell and the remains of other reef organisms. There is no clay-rich weathered rock, and no bedrock in the usual sense, within the depths most projects reach.

A typical ground profile

From the surface down, a borehole on a natural island usually finds:

1. Fine to medium coral sand, loose to medium dense, in the top few metres.
2. Coral sand with gravel and cobble-sized coral fragments, often extending to considerable depth.
3. Porous coral rock, sometimes with voids and weaker bands.

Depths and densities change from island to island, and often across a single site. A profile from a neighbouring plot is a guide, not a substitute.

Carbonate sand also behaves differently from the quartz sand most textbook correlations were built on. Its grains are angular, can crush under load, and may be lightly cemented. We check correlations against local test data before they go into a design.

Cemented layers

In places the sand is naturally cemented into hard-pan layers. They give high SPT blow counts and can stop a probe, but they are often thin and may sit over looser sand. A borehole that ends on a hard-pan layer can make the ground look stronger than it is. That is why our boreholes continue several metres into high-blow-count ground before they stop.

Reclaimed land

Much new development in the Maldives is on land reclaimed by pumping dredged coral sand onto the reef flat. Fill placed this way is variable, can be loose near the surface, and sits over the natural reef deposits. Plate load tests, dynamic probing and SPT show how well it was placed and what it can carry.

Groundwater

Groundwater is close to the surface, and on many islands a thin lens of fresh water floats on seawater, rising and falling with the tide. Groundwater can carry sulphates and chlorides that attack concrete and steel, so we sample it for chemical testing. We read water levels at the end of drilling and again after 24 hours.

Earthquakes and other hazards

The Maldives has low seismicity. The Asian Development Bank's Multi-Hazard Risk Atlas of Maldives (2020) maps earthquake ground motion across the country, with the higher values in the southern atolls. We use it in the desk study of each report, and screen loose, saturated sands for liquefaction where the design needs it.

For low-lying islands, flooding, storm surge and tsunami usually matter more than earthquakes. They are covered in the hazard sections of our environmental assessments.

What this means for an investigation

• Drill past cemented layers, not just to the first hard reading.
• Use an automatic SPT hammer, so blow counts are consistent from one hole to the next.
• Correct N values for hammer energy and overburden before using them.
• Test the groundwater, not only the soil.
• Plan for over-water drilling when structures extend into the lagoon.

See geotechnical investigation and what a SIDCO report contains.

This page is a general description for orientation. It is not a substitute for a site-specific investigation.