The construction of the leaning Tower of Pisa (in today’s Italy) which started in the year 1172 is often a case study for the relationship between a structure and the ground on which it stands (or leans in this case)

The tower site was soft ground composed mostly of clay, fine sand, and shells (the city sits between the Arno and the Serchio rivers). By the time the builders finished the second floor in 1178, the tower was beginning to lean. It was a lucky break that local wars stopped construction for almost a century; modern analysis says that the tower definitely would have toppled had construction continued without time for the soil to settle. When work on the tower resumed again, it was leaning 0.2 degrees north of vertical, but by the time the workers started on the seventh floor, the tower was tilting one degree south. Over the next century, construction continued intermittently, several remedies were tried to correct the lean but it only got worse. The tower was officially completed circa 1370. Since then Engineers and Architects from the word over have constantly monitored and tried to correct the lean. In 2008, 70 tonnes of soil was removed from below the tower on the North side and for the first time in its history, the tower stopped moving. The lean now is about 4 degrees and analysts say it will be stable for a couple of centuries.

Read on to understand a building’s relationship with the soil on which it stands

FIRM FOUNDATIONS

There is immense truth in the saying ‘You can’t build a great building on a weak foundation’.

When we look at a building, we rarely see the foundation or base (which is usually located below ground) on which the building is standing. We see only the ‘superstructure’ The load of the superstructure is transmitted to the soil by the foundation, which is that part of the structure that is in direct contact with the ground. The foundation’s job is to reduce load intensity and spread the load of the structure over a larger area, distributing that load evenly and providing an even surface on which the structure can be built upon.

The bearing capacity of soil (which is primarily a mixture of solid particles, moisture and air) is the key metric which a structural engineer uses to design a foundation. Simply stated, it is the capacity of the soil to resist deformation when a load is applied. For a very small and light building a presumptive analysis can be made based on knowledge of the terrain or of building foundations adopted in the locality. For accurate data however, the soil has to be properly tested. The Standard Penetration Test (SPT) is the most commonly used because of its simplicity and versatility SPT uses bore hole samples of the soil and a drop hammer to measure penetration. The test results can be used to determine the relative density, bearing capacity, and settlement of granular soil. Standard Penetration Resistance or ‘N VALUE’ of the soil is key reference for designing the ideal foundation.

Modern foundations are designed by professional structural engineers, based on the loads from the building, the design and end use of the structure and the bearing capacity of the soil on which it will stand. Locational factors like type of structures in the neighbourhood, wind, proximity to the sea, seismic profile etc. are also taken into consideration.

If the soil has good bearing capacity, small buildings like independent villas can still built on rubble foundations and tied together with concrete at the plinth level. Good Rubble Masons are sadly, a dwindling tribe. The strength of the foundation will depend on the way in which the stones broken to shape and laid with proper bedding and bonding, requiring the skills of an experienced stone mason.

For larger buildings, different types of Reinforced Cement Concrete (RCC) pile foundations are invariably used. The piles are capped off with ‘Pile Caps’ (often multiple piles capped with a single pile cap) and connected with ‘plinth beams’ thus forming a rigid grid. RCC columns rise from the centre of the pile caps.

The chart below gives you a basic idea of the different types of pile foundations.

I hope this article has given you a basic idea about soil and building foundations.

If you have any questions about anything at all regarding building construction or interiors, do reach out to us at www.fromthegroundup.in. We would love to discuss your goals and projects, and help you start out on a firm footing.

Note: We use the professional services of TLINE INFRA, based out of Cochin for all our Pile foundation needs. To know more about their services, visit www.tlineinfra.com

Cheers

Zachariah Abraham

Managing Partner, A & T

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