Correct Answer: D. Under reamed piles
Solution:
Expansive soils, like black cotton soil, swell when wet and shrink when dry, causing significant ground movement. Under reamed piles are ideal for these conditions. They are bored cast-in-situ concrete piles with one or more 'bulbs' or 'under-reams' at their base. These piles are anchored in a stable soil layer below the zone of seasonal moisture variation, and the bulbs provide strong resistance against upward pull from the swelling soil, ensuring the foundation's stability.
Correct Answer (Incorrect Statement): C. The bearing capacity of soils should be very high to place a grillage foundation.
Solution:
This statement is incorrect. A grillage foundation is specifically designed to distribute heavy, concentrated column loads over a wide area. It is used precisely when the soil has a low bearing capacity, as it effectively reduces the pressure on the soil to a permissible level. The other statements are all correct characteristics of a grillage footing.
Correct Answer: B. square
Solution:
While various shapes can be used, the most common and generally used shape for pre-cast concrete piles is square. This shape is straightforward to form and cast, provides good resistance to driving stresses, and offers a consistent surface area for skin friction.
Correct Answer: D. All of the above
Solution:
All the listed methods are effective for improving soil bearing capacity. Increasing footing depth places it on stronger, more compacted soil. Draining water reduces pore pressure and increases soil strength. Ramming granular material (compaction) increases the soil's density and shear strength. Therefore, the correct answer is All of the above.
Correct Answer: A. Bridge
Solution:
Pile foundations are ideal for supporting structures with very heavy, concentrated loads, especially when surface soils are weak or when building over water. Bridges fit this description perfectly, as their piers concentrate massive loads onto small areas, often in challenging soil or riverbed conditions, making pile foundations the most suitable choice.
Correct Answer: D. Strip foundation
Solution:
A strip foundation, also known as a continuous footing, is a long, narrow strip of concrete that supports load-bearing walls. By definition, its length is significantly greater than its width, running along the entire length of the wall it supports.
Correct Answer: A. Shallow foundation
Solution:
Spread foundations (which include footings like pad, strip, and raft) transfer building loads to the earth at a shallow depth. They are characterized by having a depth that is less than their width. This places them in the category of Shallow foundation.
Correct Answer: A. Cased cast in situ concrete pile
Solution:
A Raymond pile is a proprietary type of pile where a thin, corrugated steel shell (casing) is driven into the ground with a collapsible steel mandrel. The mandrel is then withdrawn, and the shell is filled with concrete. This process makes it a Cased cast in situ concrete pile.
Correct Answer: B. Strip / Pad foundation
Solution:
This question describes a method for anti-termite treatment. A perforated pipe system is laid along the foundation to periodically inject chemicals into the soil, creating a barrier against termites. This treatment is typically applied along the perimeter and under the floor of shallow foundations like Strip / Pad foundations.
Correct Answer: D. Pad footing
Solution:
A spread footing that supports a single, individual column is most commonly known as a Pad footing or an isolated footing. Its purpose is to 'spread' the concentrated load from the column over a larger area of soil.
Correct Answer: D. Analytical method
Solution:
Analytical methods for determining soil bearing capacity involve using theoretical formulas based on soil mechanics principles. Rankine's theory, which analyzes earth pressure, is a fundamental component of these analytical approaches, alongside theories from Terzaghi, Meyerhof, and others.
Correct Answer: D. 2 - 3
Solution:
In geotechnical engineering, a factor of safety is applied to the ultimate bearing capacity to determine the allowable bearing capacity. This accounts for uncertainties in soil properties and load calculations. A typical factor of safety for foundations ranges from 2 to 3.
Correct Answer: D. grillage footing
Solution:
A grillage footing is specifically designed to support heavy, concentrated loads from steel columns, especially when the soil has low bearing capacity. It consists of one or more tiers of steel beams used to spread the load over a wide area, making it the most suitable option among the choices.
Correct Answer: D. 100 mm
Solution:
To ensure proper load transfer and to protect the steel beams from corrosion, grillage foundations are encased in concrete. Standard practice requires a minimum concrete cover of 100 mm (or 4 inches) around the steel beams, including beyond their edges.