Todds road quarry

A Visit To The Todd’s Road Area To Investigate Reports Of A Sinkhole.

Curtis Archie

As a result of recent heavy rains, extensive flooding took place along the Caparo River in the Todd’s Road and Palmiste area. When the water receded it was discovered that an old railway right of way used as a roadway had disappeared in one place. The media and residents started using a numbers of terms to describe what they believed had happened, however their usage of those words was incorrect. According to the Glossary of Geology by the American Geological Institute (1987) a Sinkhole is a circular depression in a karst area. Its drainage is subterranean and is commonly funnel shaped. While a Canyon is a long deep, relatively narrow steep sided valley confined between lofty and precipitous walls in a plateau or mountainous area.

A visit to the site (no Police as the media reported) revealed an amazing landscape. As one walked along the road, the vegetation suddenly open up and an extensively worked quarry was seen. Drains ran parallel to either side of the road, suddenly the road ended in what appeared to be space. Large tension cracks were noticed parallel to the edges and extending towards the center of the road. Water from the drains was seen to be falling over the edge like a waterfall into a deep pit, at the same time loud noises were occurring regularly. This turned out to be large blocks of sand and clay falling off the edge of the road into the pit, of course we immediately stepped back to fracture free road. The low point of the quarry was flooded, reportedly to a depth of 80’ in places.

The features seen led to some idea how it all occurred. Excessive and uncontrolled quarrying of sand took place up to the edges of the road (out of sight out of mind!), flooding of the Caparo river allowed significant amounts of water to flow down the drains, the eastern one flowed over into the pit eroding and undermining the unconsolidated sand beds, water from the western drain flowing through a underground drain did the same thing. Ultimately the road started collapsing, accelerating the erosion on either side. All the water flowing into the pit filled up the low point. It is anticipated that with further rainfall and flooding expected this year, this erosion will continue with the size of the collapse expanding.

view of collapsed road looking north

view of the new lake

In the Todd’s Road area the rocks belong to the Talparo Formation, the sand belongs to the SumSum Member. The Talparo may be as much as 4000’ in thickness with a number of thick and laterally continuous sand bodies. It has been encountered from the Venezuelan side of the Gulf of Paria to the east coast of Trinidad. An almost continuous (44 km.) outcrop (surface exposure) stretches from Claxton Bay in the west to Cumoto in the east. The SumSum sand averages 100 – 150’ thick with a maximum of 200’, in parts of the Gulf of Paria two sands are encountered.

This sand is of great economic importance to Trinidad, firstly it has served as a source of building material and secondly as a source of potable water. The recent house building boom of the last 9 year has had a number of unintended and unplanned consequences. Significant areas through central Trinidad have been affected by quarrying. In places like Claxton Bay- Soledad Road, almost all of the sand has been removed right down to the basal clay. Old Caroni quarries at Diamond Road were reopened, significantly expanded and once again in places quarried down to the basal clay. Using Google Earth one can also see the scale of the quarrying in the Ravine Sable / Caparo area. In this area the sand has also been removed to the basal clay in places, which would allow water to collect above it.

The most immediate impact of the quarrying is on the availability of water. The exposed sand served as the area (catchment area) where rain was able to soak into the ground, percolate downwards and be stored. A number of water wells exploit this aquifer in Claxton Bay (SumSum Hill), the water is used by both for residential purposes and by Petrotrin. Where the sand has been completely removed very limited recharge of the aquifer is possible in that area, leading to the water wells being unable to deliver its usual volumes of water.

What can be done with this area if quarrying is not restarted? The area is a topographic low surrounded by hills that originally was swampy. Quarrying emphasized and expanded this low area. This incident has provided an opportunity to mitigate a number of problems.

The perennial problem of flooding can be reduced if the quarry is converted to a reservoir where flood waters can be diverted and stored. This can be accomplished by the construction of a dam across the mouth of the valley and an arbitrary fill line of the 50m contour used. The outline of the proposed reservoir is dependent on the new topography resulting from quarrying. The stored water can either be discharged when the floods abate, used during the dry season for irrigation and recharge of fresh water aquifers.

Some kind of geotechnical work needs to be done to estimate the volume of material removed over time, the amount that remains and its location. If it is decided to restart mining, a better exploitation and rehabilitation plan can be designed using this data. In the short term the amount of water falling into the pit needs to be reduced.

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