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Explainer: Why some Karachi roads caved in after the monsoon

Contractors can skimp on adequate soil compaction

SAMAA | - Posted: Sep 2, 2020 | Last Updated: 5 months ago
Posted: Sep 2, 2020 | Last Updated: 5 months ago
Explainer: Why some Karachi roads caved in after the monsoon

Photo: Online

If you drove by Jinnah Courts and Shaheen Complex in Karachi on Monday you would have seen two shipping containers sunk into the road which had caved in. This was not the only patch where this happened. Similar collapses were reported across the city.

One of the main reasons why this happened this monsoon is inadequate compaction of the soil when these roads were built. Compaction (or pressing down the soil) is an expensive and time-consuming process. You have to test for it every 2 to3 meters when making the road. So contractors skimp on proper compaction. Only a few strips of the road are compacted properly and contractors make sure only these areas are tested in inspection. Also, more often than not, the inspection authorities work with the contractor to make selected testing possible.

The main purpose of constructing a new road is to ease traffic congestion on existing arteries. Engineers study the volume of traffic in the area and use that data to design the road. The design decides the shape, alignment, dimensions and the number of lanes. The soil is tested to analyze its properties (porosity, absorption capacity, how much load it can take).

The first step in construction is to provide the road a firm, stable foundation so that it can take the weight of traffic and its own weight. This foundation is called the ‘sub grade’. The ground is uniformly leveled. Extra soil is cut away along with any vegetation and dips or depressions are filled where needed.

This sub grade is then compacted or compressed. You will often see big yellow steel rollers doing this when a road is built. Compacting the soil at this stage is the most essential step. If the compaction is not done correctly, the road will most likely experience premature failure eventually.

The drainage and utility lines are laid upon this sub grade level of compacted soil.

The next two layers above the sub grade are called the ‘sub base’. The first layer is strong rock and stone fragments then a mixture of sand, gravel and stones is applied.

These layers prevent subsoil water from rising to the top of the road and capillary action through the layers. Water is a road’s biggest enemy. If it finds its way through its layers, it can weaken the entire infrastructure.

The last layer is a ‘wearing course’, which you see as the top of the road. This is the black layer which comes in direct contact with the tires of a vehicle. It is a mixture of asphalt or tar and concrete.

When it rained this time in Karachi, the water stayed on the roads because it could not drain away due to faulty drainage, a lack of clean sewers and improper design. The standing rainwater then seeped into the surface of the roads and down through its layers to weaken them. Roads without proper soil compaction sunk under the load of the water and vehicles. That is why big gaping holes developed in many cases.

There are, of course, other reasons why this happened. In some roads poor or badly designed drainage pipelines sunk or shoddy patchwork caved in. 

The writer is a student of journalism at CEJ-IBA, Karachi and has a Masters of Engineering in Engineering Management from the University of Alberta, Canada

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monsoon, Karachi roads, pot holes,
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