Dr. Lennox Honychurch/photo taken from: www.facebook.com

The ongoing slippage of the Imperial Road at Antrim Valley is a serious challenge to communications across the island. This road is the main link between the capital and Douglas-Charles Airport as well as communities along the north and east coasts. With the alternative Layou Valley road under repair and the Warner-Soltoun road seriously compromised, the situation at Antrim is even more crucial. A side road from Canefield through Cochrane village to Springfield provides an emergency by-pass but it is too narrow for regular use.

Long before Tropical Storm Erika in 2015 and Hurricane Maria last year, engineers, both local and foreign, have been debating the cause and possible solutions. The reason they do not appear to be finding the answers may be that they are not fully aware of the history of this section of the road. They may not have grasped the geology of this valley’s formation which is causing the constant slippage. It is important that all civil engineers working on our roads know the background to their construction. This includes former routes and building methods and even the administrative politics at the time.

First, the geology: The surrounding mountains are part of the rim of a huge ancient crater forming a semi-circle around Morne Trois Pitons. If you stand at a viewpoint just past Sylvania you will see this rim forming an arc of peaks to the northwest all the way around to Morne Neg Mawon. The Antrim peak itself is called Morne Cabwit Mawon and the other peaks and ridges in the line have names such as Bona Vista, Morne Cola Anglais, Morne Boyer, Despor, Deux Saisons and Morne Couronne.

This rim of mountains was formed thousands of years before Morne Trois Pitons emerged. The rim is composed of hard igneous rock that turned solid as lava came out of the volcano. The later, Trois Pitons eruption, covered these earlier igneous rock mountains with volcanic debris. This includes ignimbrite, ash, and red ferrous clays. These clays can be seen at Red Gully and all along the sides of the valley. They are constantly slipping over the smooth, older rock layers beneath. But Antrim is a special case.

What is happening at Antrim is that volcanic material from these two seperate geological periods are converging on one critical spot. A large lake once existed in this part of the valley. Eventually, its western wall collapsed and burst down the gorge to Check Hall. The old lake bed is still there, covered with thick grey silt clay that holds water and is sliding into the river.

If you cross the Antrim River to the other side of the valley, the lake bed continues. Pools of swampy water dot the landscape between huge boulders. The villagers of Massacre used to grow dasheen here and interestingly their name for the area was ‘L’etang’, Kweole for ‘lake’. The former borders of the lake can be clearly seen as a bowl spanning the entire valley.

If you go into the bush opposite where DOWASCO is now building a new tank you will see huge smooth boulders the size of small houses that once formed the sides of the old lake. Go further and you will see the cracked concrete floor of the Bunting’s house abandoned because it was splitting in half as the earth moved.

The hillside along the road is made up of earth and fallen rocks. This debris is from the top of the plateau of Morne Cabwit Mawon and it is resting against the solid mountain. Water from a ravine on the mountaintop is seeping between the hard rock and the loose debris causing it to slide into the old lake bed. The main road lies along this zone of seepage and therefore the road slides also. A bad engineering decision a few years ago to dump thousands of tons of excavated Red Gully clay on this delicate site has made it even more volatile. The more ‘tarrish’ that is dumped to shore up the road, the more the added weight causes it to slide. It is a vicious circle.

In 1900, British Administrator, Hesketh Bell, directed the construction of his ill-fated ‘Imperial Road’ through this area. Road workers were killed by a slide here. Again, in the 1916 hurricane the whole hillside collapsed. Although appearing safe for some years it has never been entirely stable. Now it has reached crisis point. Next week we look at the early engineering politics of the road and possible solutions, including the construction of a completely new highway elsewhere.

 

A diagram of the situation at Antrim Valley causing the slippage of the main road.

Article Link: Why Antrim road is slipping part 2