The first step in planning to build a house is the selection of a suitable site. N. UNNIKRISHNAN and ARVEE SUJIL JOHNSON discuss the next step, building the foundation.
The break-up of the traditional joint family system, coupled with the migration of young families to job centers, has resulted in the need for more dwelling units. This, in turn, has triggered a spurt in the demand for residential sites.
In all major towns and suburban areas, house sites with soil that allows quality foundations are becoming scarce. In Karalla, where the rural-urban dichotomy is practically non-existent, the problem is ubiquitous.
The first step in planning to build a house is the selection of a suitable site. The buyer is often ignorant of the previous use or history of the site offered to him. In many places, wetlands are reclaimed and ‘developed’ by planting trees, laying roads and subdividing into ‘plots.’ Only when the new owner starts the excavation for the foundation does he realize that the subsoil is a weak unconsolidated fill extending to considerable depths. If the house is built with the traditional shallow foundations on such soil, the building will eventually settle, develop cracks and result in functional, or even structural, failure. It will ruin the owner both financially and mentally.
A soil test is strongly recommended before the design and construction of a house. A good foundation engineer can design an appropriate and economical foundation for any structure for any subsoil condition.
There have been reports about residential buildings sinking up to 2 m below the ground level. Such sinking may or may not be accompanied by cracks in the building. A building that is situated on uniform clay may settle uniformly at all the locations if the building is loaded uniformly. A qualified engineer can ensure uniform distribution of load.
For example, if a building is constructed in such a way that the upper storey completely covers the lower floor, the loading intensity may be almost uniform. However, if the loading is not uniform, even this may result in uneven settlement. Such uneven settlement results in tilting of the structure, often accompanied by unsightly cracks and structural distress.
Even if the load distribution is fairly uniform, buildings can settle unevenly, if the soil properties are not uniform. For example, in a situation, where part of the building is located over a filled-up pond, that portion may settle more.
Proper selection of the foundation type and its design will avoid such mishaps. The selection of a suitable foundation depends on a number of factors.
These include the types of subsoil at different depths, their composition, strength and compressibility characteristics, the loads being transferred to the soil from the building, position of the water table, the general nature of the terrain, the past use of the land and the stress history. The use, function and the importance of the structure are significant.
A ‘soft’ foundation soil may be treacherous, whereas a hard medium such as late rite will be inherently strong. Compact sand is a good foundation material.
However, loose sand may cause excessive deformation leading to unacceptable levels of settlement.
Traditionally, the foundation for a residential building is constructed by laying a bed of plain cement concrete (PCC) of a few centimeters thickness and constructing a random rubble or dry rubble masonry over the PCC. On reaching the plinth level, the walls are constructed. In engineering parlance, this is called a load- bearing wall foundation. To adopt such a foundation, a soil stratum capable of supporting superimposed loads should be available at shallow depths. It should also extend to sufficient depths depending on the imposed stress levels.
Another alternative is to use a column-beam construction with the columns supported by independent footings.
This construction is called a ‘framed’ system and is sensitive to settlements. The choice should be based on expert advice. The footing again has to rest on a soil layer of sufficient strength, while not causing any unacceptable settlement. If such a hard stratum is available only at somewhat deep levels, the excavation for the footing may be surrounded and supported by ordinary concrete rings (as used for open wells) usually filled with sand, graded aggregates or PCC.
Another type of foundation that is supported near the ground level is “raft” or “mat” foundation. As the name indicates, the raft or mat is a slab that covers the entire floor area of the building.
Pile foundations are used if a strong soil stratum is not available at shallow depth. To some extent, the strength of pile foundation also depends upon the method of installation. Concrete piles for tall buildings are installed using a chiseling method with direct mud circulation (DMC). This is a system which needs mechanical power and is capable of reaching the desired soil stratum or bed rock. Another method of installation of concrete piles is using hand auger.
Timber piles are used to support buildings in some situations.
Ground having poor strength can be improved to acquire more strength by suitable means. Improvement methods depend upon the type and composition of the soil constituents. Densification techniques have found to produce improvements in loose sands.
Sand piling is a widely adopted densification technique.
The depth of improvement and the distribution of such sand piles should be based on expert guidance.
The authors are lecturers in the Department of Civil Engineering, College of Engineering Thiruvananthapuram.