VALUE ENGINEERING- ACASE STUDY

Behaviour of soil is always a surprise.?At least to me, whenever I tried to use soil for building roads. Engineers judge a soil based upon the colour, feel, the ground condition etc. and judge the soil without testing it thoroughly.

One such interesting story happened in construction of Dholera Smart City roads.

I first visited the site in first week of March 2018 before formally taking charge as the Project Manager. What a sight this site was. Barren till the horizon. I felt like Neil Armstrong.

I had some information about the site as the tests during pre-bid study were conducted in the laboratory of my project. According to that information the locally available soil could not be used in road construction. It is “black cotton” (because it was black in colour), fine, clayey. The ground condition was such that it was difficult to even walk after the rain.

What I noticed about the site during my first visit was that the ground was flat with no much vegetation and of course no agriculture. The soil was fine grained but I could not find any black-coloured soil. The plasticity, when tested in field according to palm test of IS:1498, was not observed very high. It was only later that I came to know that the so-called black cotton soil was 1m below the ground level.

When I saw the soil, I was not convinced with the assessment made during pre-bid that this can not be used for road construction. I also observed that this soil type was not limited to the project area but was widely spread. I asked the local people about the soil used in the roads in and nearby Dholera. I was told that the practice was to cut the soil from ROW itself and fill it in embankment (including Sub-grade).

So, my next obvious step was to check the condition of the roads. These were 2- lane and single lane roads connecting nearby district and tehsil headquarters. I drove quite a few kilometres in each direction. The roads were not in a very good condition. But there was a catch. I noted that the poor condition of asphalt wearing course is the reason of poor condition of road. I did not observe base or subgrade subsidence anywhere. This prima facia observation had convinced me further that the locally soil is not unsuitable as a material for road construction. Although it was subjected to detailed investigation but the direction was determined.

This was just a short trip with an objective of getting a feel of the site before the kick-off meeting. During the kick-off I interacted with the bidding team to understand their perception about different aspects of the project, including soil. I also went through various data provided by the client.

Soon the construction team was mobilised and a site laboratory was started in an interim set up. OGL testing started. I requested our senior material engineer, Bala, to conduct a few tests to check field density of the original ground. This was to check the theory which took shape in my mind during my aforesaid interactions and reading. We also dug the soil for more than 1m, observed it and tested it in laboratory.

Before I reveal the conclusion let me tell you some notable points:

·??????The site was very near to sea

·??????MSL was to the tune of 1m below the OGL of the site

·??????Water table was very high and keeps fluctuating

·??????The ground water was hard and contains high levels of chlorides and sulphates

·??????The field dry density of OGL was 70-75% of the MDD of the OGL soil

·??????The soil at 1m below OGL was fully saturated and black in colour

·??????On drying the black colour of soil became grey, that is same colour as that of soil on top

·??????The soil, in general, could be classified as MI-ML (Silt of low to medium plasticity)

·??????Almost 95% of the soil was passing the 75-micron sieve

·??????CBR of the soil varied from 3-7%

·??????MDD was around 1.8 g/cc and OMC around 14%

·??????The black soil from the bottom had similar properties as of top soil.

The above set of data shall be used to interpret this soil in two ways, Soil as Strata and Soil as Material

The strata is weak because

·??????The ground water level is fluctuating

·??????Due to unsuitable quality of water, there is no flora and fauna hence absence of natural consolidation agents

·??????The constituent soil is uniformly graded

But the index properties of the soil were good for road embankment and the strength of the soil excavated from the ground, improved after being compacted.

The black colour of the soil beneath was probably because of its prolonged exposure to the hard ground water.

This observation and study made it possible to use approx. 12.5 lakh (1.25 million) cu.m. of the locally excavated soil in embankment filling. This excavated soil was earlier considered waste. Which means added cost of its disposal along with cost of bringing embankment soil from some borrow area. This led to savings in the tune of ?25 crores (INR 250 million) over the cost estimated during bidding.