NOTE:- This was wrtten in early part of A.D. 1970 and published in the April 1970 issue of my employer's house journal then known as GBJHP News.
A new method of improving the soil bearing capacity is being employed at Haldia. This is the sandwicking process about which many have heard and are consequently curious about it. What follows is an attempt at explaining it to the layman.
As you all know, every structure needs a foundation. These foundations are placed on the ground. When the ground is strong as in the case of rock, very heavy structures can be placed on it without any elaborate foundations; whereas when it consists of a weak soil, costly foundations have to be provided.
The soil at Haldia is very weak. It can not support a load of more than half to one kilogram per square centimetre in most places. In such soils, we usually provide pile foundations. Piles are made of concrete, wood, metal etc. Concrete is however the most popular material for piles. These piles are driven into the ground till they refuse to go anymore. Then on top of a number of liles, a concrete "cap" is cast. On the pile cap, the structure is built. The piles provide strong foundations and do not permit any appreciable settlement of the structure founded on them. They are however a costly business. Therefore, whenever we can avoid them, we naturally try to do so. While equipment foundations do not tolerate any settlement, our storage tanks can take a certain amount of settlement, though not as much as one metre which is the expected settlement of untreated soil at Haldia as per a recent soil survey. Thus we can avoid piling under storage tanks and sandwicking which takes care of most of this settlement in advance appears to be a good solution. We hope to save about Rs. three and a half crores by adopting sandwicking.
The process of sandwicking is as follows :-
First, a hole is made into the ground with the help of a small drilling rig similar to ones used for making small bore wells. The sandwick which is a long and narrow jute hosepipe (about 80 m.m. in diameter) is then inserted in this hole. Such sandwicks are placed at an interval of two and a half metres both ways under the tank foundations. the ground is then "loaded" with sand. A heap of sand of pre-determined height is made over the foundations. The weight of this sand is equal to 100 % to 150% of the maximum load of the tank. As you know water is heavier than oil. Therefore the tank carries its heaviest load when it is filled with water during testing for leaks etc. This is the condition taken into account while determining the height of the sand heap. The heap of sand is allowed to remain in place for a period varying from 12 to 17 weeks. This is called preloading. During this period, an interesting phenomenon takes place. The ground under the heap of sand begins to settle. How does this happen ?
The soil consists of a number of layers of different materials. Some layers are pervious i.e. they allow passage of water and some do not in other words, they are impervious. The sandwick penetrates through both pervious and impervious layers. All these layers contain water particles. Due to preloading the water is sqeezed out and the sandwick (made from specially selected coarse variety unlike the sand used for preloading which can be of even fine variety) provides a channel for carrying away the water particles from the soil by capillary action. As the water is squeezed out, the soil contracts in the same way as when a ROSSOGULLA is squeezed. If the sandwicking is not done, the tanks would keep on settling down for a long period of time, giving perpetual headaches to our maintenance people. By adopting this process, we are allowing the settlement in advance and in a short period. We are also avoiding another scourge of the maintenance staff. This is one of an uneven settlement, causing the tank to tilt sideways and spill some of its contents. The bearing capacity of the soil is improved uniformly under the tank and it can take greater loads than before.
In the case of tanks of smaller diameter, what we do is to drive the sandwicks and prepare tank pad consisting of earth or fine sand of about the same height as the expected settlement. On this the tank itself is is erected and gradually filled with water. This is called hydraulic preloading. Here also, the preload is kept for 12 to 17 weeks during which the settlement takes place. The settlement is measured by means of settlement stakes. The water is then gradually pumped out. In the case of sand preloading also, the sand is removed after 90 % of the full settlement has taken place. This sand is now taken to another tank foundation and used there for prelosding. To minimise the quantity of preloading sand, a ring like a fortress made up of sand bags is built around the peripherry of the tank foundation and sand is then filled in this fortress. The preloading sand heaps' slopes would have hindered work in adjoining areas. As you know Haldia site has difficult access to it and bringing a huge quantity of sand to the site would have posed more problems. Sandbag walls have helped to save about 20% of the preloading sand.
You would be interested to know that the sandwicking process has been developed by an Indian engineer and besides Haldia, it is also being used inthe salt lake area of Calcutta (Kolkata).