Foundation under Special Conditions

For some structures or for soil conditions the routine method of providing foundations may not be suitable. Such foundations under special conditions are dealt below.

1. Grillage Foundation

Steel columns may be founded on concrete footings or footings of steel. Many a times the latter approach is preferred. This primarily consists of steel beams arranged in layers at right angles to one another and the beams are connected with each other by bolts in order to form a rigid unit. The entire assembly is embedded in concrete. This is known as Grillage foundation (Fig. 15.8).

The steel grillage foundations are adopted for structure having concentrated loads. Hence they are employed for the foundations of buildings such as theatres, factories, town halls, clock towers, etc.

1. In this method the depth of foundation is fixed to 1–1.5 m and the width is increased to satisfy the safe bearing capacity and permissible settlements.
2. The beams are R.S.Js which are fully embedded in concrete so as to protect them from atmospheric actions.
3. The bed of concrete should have a minimum thickness of 15 cm and nowhere the depth of concrete is less than 80 mm. The concrete filling does not carry any load but it maintains the R.S.Js in proper position.

Sometimes timber beams are used for temporary grillage foundation.

Figure 15.8 Typical grillage footing for steel columns

2. Stepped Foundation

In a sloping ground, it is uneconomical to provide the conventional type of foundation. In such cases, stepped foundation may be provided as given in Fig. 15.9.

Figure 15.9 Stepped foundation

The following points are to be taken care of:

1. The overlap between two layers of foundation concrete should be greater of the depth of foundation concrete or twice the height of the step.
2. In order to protect from weathering action a minimum depth of 80 cm should be provided at all points.
3. The depth of foundation concrete should be in even number.
4. The distance of the sloping surface from the lower edge point should be at least 100 cm for soils and 60 cm for rocks.
5. Stability of slope has to be checked if heavy load is expected on the foundation.

3. Foundations near Adjacent Structures

The horizontal location of a footing is often affected by adjacent structures and property lines. The existing adjacent structure may be damaged due to construction of new foundation because of vibration and shock due to blasting, caving in due to nearby excavation, lowering of water table or increasing stress.

The Indian Standards (IS: 1904, 1986) recommends the following for footings placed adjacent to a sloping ground or when the bases of footings are at different levels.

When the ground surface slopes downwards adjacent to a footing, the sloping surface should not encroach upon a frustum of bearing material under the footing, as shown in Fig. 15.10(a) and (b) for granular soils and clayey soils respectively.

The following norms have to be adopted to avoid any damage to the existing structure:

1. The footing should be founded at least at a distance S from the edge of the existing footing where S is the width of the larger footing.
2. The line from the edge of the new footing to the edge of the existing footing should make an angle of 45° or less.

Figure 15.10 Footings at different levels (Source: IS: 1904, 1986)

1. When a new footing is constructed lower than an old footing, the excavation for the foundation must be carefully done with a suitable bracing system so as to prevent damage to the existing structure (Fig. 15.11).

Figure 15.11 Footings for old and near structures (Source: IS: 1904, 1986)

4. Under-reamed Piles

Under-reamed piles are of bored cast-in-situ and bored compaction concrete piles with enlarged base. The enlarged base is termed as a bulb or under-ream. An under-reamed pile may have one, two or more bulbs. Accordingly, they are called as single-, double-, or multi-under-reamed piles. The bulb provides adequate bearing or anchorage. Under-reamed piles are used for a variety of field conditions, viz.,

1. To obtain adequate capacity for downwards, upward, and lateral loads and moments, e.g., transmission tower foundation.
2. To take the foundation to deeper structure in order to prevent the effect of seasonal changes, e.g., in expansive soils (black cotton soils)
3. To take the foundation, below scour level, e.g., in piers.

In deep deposits of expansive soils, the minimum length of piles (irrespective of any other factors) should be 3–5 m below ground level. In weak soil structure or in recently filled grounds, the pile should pass through such soils and be seated in strong bearing strata, Fig. 15.12 (IS: 2911-Part 3, 1980).

Figure 15.12 Single and double under-reamed Piles (IS: 2911–Part 3, 1980)