Pre loading technique of ground improvement:
In 1940’s the technique of pre loading was developed and is mainly was used in the projects connected to highways. But now a days this technique was found very useful and is used for a variety of projects such as buildings, storage tanks, flood control structures and airfield etc. the draw back of this technique is that it does not provide favorable results for the structures having heavy concentrated loads.
But this treatment is very effective for the soils having high moisture contents, high compressibility and low shearing strength. Another advantage is it removes the settlements that should otherwise be done after the completion of the construction.
- Methodology of pre loading:
Before starting the proposed construction the weak soils are improved by applying pre loading technique. The magnitude of the pre load pressure usually rages from 1.2 to 1.3 times of the actual structural pressure or is slightly greater than the maximum pressure that is generated by the proposed structural load. Earth fills, water lowering, vacuum under impervious membrane are some of the techniques used for applying pre-load. Once the settlement under the pre load is completed, the preload is removed is removed and the construction of the structure is started. By the use of vertical sand drains the consolidation process can be increased which reduces the time of pre loading. In general the area of the final structure is lass than the pre loading area because the stresses produced at any depth in the foundation soil are uniform; it should be equal or greater than the final stress produced at that location.
- Merits and demerits of pre loading:
Pre loading contains many advantages and disadvantages. If we are provided with the material for generating the pre loading at low costs and if the sufficient time is available for the pre loading to occur then following merits are achieved:
- Particularly for the soils having heterogeneous characteristics the reduction in the post construction settlement is achieved.
- For the general grading of the site the pre-load fills are usually used. They usually reduce the considerable cost of the pre loading process, it is important because usually pre loading processes are quite expensive.
- One of the best merits of this technique is that it is almost free of noise and vibration problems that are faced in other technique. Therefore it is preferable in those places where environmental restrictions are involved.
Some of its demerits are:
- There could be an unexpected cost increment.
- Further improvement in the project requires the consideration of the pre loading programme.
- Their may be a unexpected time delay.