The entire North Indian belt is receiving heavy to very heavy rainfall in this Monsoon season. Parts of Delhi, Haryana, Uttar Pradesh, Uttarakhand, Punjab, and Himachal Pradesh have been hit by heavy downpours resulting in waterlogging, flooding, structural damages, uprooting of trees, and road cave-ins among others.
Several prominent urban areas have witnessed cases of road caves-in which poses alarming life threats to commuters. At least three different cases of road cave-ins in Delhi came to light in the past few days.
On the 9th of July, Delhi Traffic Police alerted about a road cave-in incident on the Najafgarh road.
Similarly, another road cave-in was reported at the C-Hexagon India gate near the Shershah road cut.
In the Rohini Sector 24, a wide crater was formed after a road caved in after receiving heavy rainfall.
Taking action on these incidences of road cave-ins, Delhi CM Arvind Kejriwal ordered an inquiry on the 9th of July.
In Lucknow, a road cave-in incident was reported on a road near Balrampur Hospital.
Earlier, on the 5th of July, a car accidentally fell into a crater after a road caved in at Chembur, Mumbai.
Why Road cave-ins?
Road cave-ins can also be known as sinkholes or roadway subsidence. While each road cave-in incident can have its own factor, the majority of the cases can be categorised in two broad reasons.
First, if the sub-structure, that is, the underlying supporting structure on which the road was resting collapses, weakens, deteriorates substantially, or crumbles, then the constituent components (such as soil or concrete) of the road will start falling resulting in road cave-ins.
This can be possible in many ways but the moot point is that the underlying structure can’t hold the weight of the road.
- If the soil is poorly compacted, it gradually settles.
- Underground mining or excavation can weaken the supporting structures.
- Weak or Aging infrastructure can not hold the weight of the road above or of vehicular movement.
Improper Construction, excavation activities, or excessive vibrations from heavy machinery destabilise the ground. This problem arises if the soil was not fully compacted during construction.
Additionally, natural geological processes can also lead to road cave-ins. For example, the presence of soluble rock formations such as limestone or karst landscapes, can result in the dissolution or erosion of the underlying rock. This gradually creates underground voids or cavities.
Secondly, if water saturates the soil beneath a road, this can weaken the soil, making it soft. Subsequently, the soil is rendered unable to support the weight of the road and traffic.
Again this can happen in many ways. If there is a presence of underground water sources, such as leaking water pipes, sewer lines, water mains, or natural water flows, this can erode the soil beneath the road over time.
- Intense or prolonged periods of rainfall saturate the soil, causing it to become unstable and prone to road cave-ins.
- Improper drainage and wrong gradient (slope) can result in the accumulation of water on the road, resulting in seepage and saturation of soil lying below.
So basically there are two broad reasons for sinkholes or road cave-ins. That is, the lower structure becomes weak, or the soil losses strength due to erosion or saturation.
Well, the first one is self-explanatory as we all are familiar with gravity and as the lower structure collapses, so will the upper structure, causing road cave-ins.
Seepage of water has a negative impact on road stability for the following technical reasons.
When the soil gets saturated, all its pores (voids or empty spaces) get filled with water. Now, water exerts an upward pressure ( imagine it as some sort of buoyant force) which is known as pore water pressure. This increased upward pressure, in saturated soil, counters the downward force on the soil particles, thus, reducing its effective stress and shear strength of the soil.
Effective stress is that force that keeps the soil particles in contact and provides shear resistance. Shear strength is defined as the maximum shear stress that the soil may sustain without experiencing failure.
Secondly, depending on the soil type, seepage of water leads to loss of cohesion (bonding force between soil particles) which results in loss of shear strength of the soil. In saturated soil, water disrupts cohesive forces leading to a loss of cohesion.
Thirdly, in cohesive soils like clay, saturation can change the structure of the soil. The modification of the arrangement of soil particles affects compaction resulting in a decrease in shear strength.
Fourthly, saturation softens certain types of soils, such as clay. The presence of water molecules can cause the clay minerals to swell, resulting in a reduction in the soil’s shear strength and overall stability.
How it can be prevented?
The problem of road cave-ins can be mitigated by taking the following measures.
- Before the construction begins, the soundness of the sub-structure should be ensured and adequately supportive measures should be taken if alternative options are not available.
- Proper monitoring and maintenance to timely avoid or correct leakage of drainage pipe, sewer, etc.
- Using best building practices while construction, that is, giving proper drainage channel, and right gradient (slope) to discharge rain or excess water. Properly compacting the soil or filler material to avoid the presence of cavities below the road.
- Removing soluble rocks such as limestone which can later result in cavity and road sinking.