METHODS OF IMPROVING SAFE BEARING PRESSURE OF SOILS.

Sometimes, the safe bearing pressure of soil is so low that the dimensions of the footings work out to be very large and uneconomical. In such a circumstance, it becomes essential to improve the safe bearing pressure, which can be done by the following methods : (i) increasing depth of foundation (ii) compacting the soil (iii) draining the soil (iv) confining the soil (v) grouting and (vi) chemical treatment.

1. Increasing depth of foundation. It has been found that in granular soil, the bearing capacity increases with the depth due to the confining weight of overlying material. However, this is not economical since the cost of construction increases with the depth. Also, the load on the foundation also increases with the increase in the depth. The method is useful only when better bearing stratum is encountered at greater depth.

2. Compaction of soil. It has found that compaction of natural soil deposits (loose) or man-made fills results in the improvement of bearing capacity and reduction in the resulting settlements. Compaction of soil can be effectively achieved by the following means:

(a) Ranmining moist soil. The foundation, soil is moistened and then compacted with the help of hand rammers or mechanically operated frog rammers or vibratory rollers. The voids of the soil are very much reduced, resulting in the reduction in settlements.

(b) Rubble Compaction into the soil. A Iayer of 30 to 45 cm thick well graded rubble is spread over the foundation level (Fig. 2.26) and weIlrammed. If this layer of rubble gets buried in the soil (specially when it is very loose) another Iayer of 15 cm thick rubble is spread and well rammed manually. This results in an increase in the bearing value of the soil

FIG. 2.26  RUBBLE COMPACTION INTO THE SOIL.

(c) Flooding the soil. The bearing pressure of very loose sands can be increased by flooding the soil. The method is very effecilve in improving the safe bearing pressure of dune sands, which cannot otherwise be effectivety compacted. The Author has an experience of improving the bearing power of desert soils by this method at  many locations where it was required to support heavy loads.

(d) Vibration. Heavy vibratory rollers and compactors may compact a Iayer of granular soils to a depth of 1 to 3 m. If the method of flooding and then vibration is used, sandy soil can be very effectively compacted, resulting in increased safe bearing power and decreased settlements when super-structure loads come on the soil. After flooding the soil, so that moisture penetration is al least 1 to 2 m, form vibrators or platform vibrators (about 1 m x 1.5 m base area, with a pair of eccentrically loaded motors) can be silded on the sand surface with the help of two labourers. A large area can be covered by this process, without the help of sophlsticated vibrating equipment.

(e) Vibroflotation. It is a commercial method which combine the effect of vibration and jetting. A heavy cylinder, known a vibroflot is inserted in the ground (soil) while the cylinder vibrates due to a rotary eccentric weight. A water jet on the tip of the vibro flot supplies a large amount of water under pressure. As the vibro flot sinks, clean sand is added into a crater that develops on the surface. The method is very useful when foundation is required to support heavy loads spread over a greater area.

(f) Compaction by pre-loading. This method is useful when he footing is founded on clayey solis which result in long term settlements. Pre-loading results in accelerated consolidation, so that settlements are achieved well before the actual footing is laid. The load used for this process is removed before the construction of the footing.

(g) Using sand piles. This method is very useful in sandy soil or soft soils. Hollow pipes are driven in the ground, at close interval. This results in the compaction of soil enclosed between the adjacent pipes. These pipes are then gradually removed, filling and ramming sand in the hole, resulting in the formation of sand piles.

3. Drainage of soil. It is a weIl known fact that presence of water decreases the bearing power of soil, specially when it is saturated. This is because of low shearing strength of soil in presence of excess water. Drainage results in decrease in the voids ratio, and improvement of bearing power.

4. Confining the soil. Sometimcs the safe bearing pressurc of the soil is low because of settlements resulting due to the lateral movement of loose granular soil. Such a tendency of lateral movement can be checked by confining the soil, outside ihe peritneter of foundation area, by driving sheet piles, thus forming an enclosure and confining the soil.

5. Grouting. This method is useful in loose gravels and fissured rocky strata. Bores holes in sufficient numbers are driven in the ground and cement grout is forced through these under pressure. The cracks, voids and fissurcs of the strata are thus filled with the grout, resulting in the increase in the bearing value.

6. Chemical treatment. In this method, certain chemicals are grouted in the place of cement grout. The chemical should be such that it can solidify and gain early strength.