Coastal Structures Design Questions and Answers

Answer: A

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: a partly built mound with its core exposed and no armour placed is far more vulnerable than the completed structure, and shortening the exposure reduces the chance of meeting a damaging storm. Statement 3 is wrong - construction is checked against a separate and usually much lower return period condition, chosen for the season and duration of the works.

Answer: B

Correct answer: 2 - 4 - 1 - 3

The mound is built from the inside outward and from the bottom up. Core material is placed first, usually end-tipped from an advancing haul road; the underlayer follows over the exposed core, protecting it from washout; the primary armour is placed on the underlayer; and the crown wall is cast last, along the finished crest. Placing armour before the underlayer would leave the core to be lost through the voids.

Answer: C

Correct answer: single layer interlocking concrete armour units

A single layer system develops its stability from interlock, and interlock only exists if the units are placed at the specified density, orientation and pattern - so placement is surveyed unit by unit and verified against the design grid. Rock armour placed at random is far more forgiving, and core and filter layers more forgiving still.

Answer: D

Both are true and the reason gives the necessity, and positioning is often checked unit by unit. Randomly placed rock demands nothing comparable in the way of control. The two systems demand very different supervision.

Answer: A

Correct answer: chloride induced corrosion of any reinforcement present

The splash zone is the worst environment for reinforced concrete: wetting and drying concentrate chloride at the surface, and the abundant oxygen supports the corrosion reaction once the chloride has broken down the passive film on the steel. The rust occupies more volume than the steel it replaced, and spalls the cover off. The remedies are thick cover, low permeability concrete and, where warranted, stainless or coated reinforcement.

Answer: B

Extra water increases permeability and shortens the life of the concrete. The other three each reduce chloride ingress to the reinforcement within. Permeability is the property that matters most. Water content governs permeability directly.

Answer: C

Chlorides accumulate through repeated wetting while oxygen remains freely available for the reaction. Permanently submerged concrete corrodes far more slowly for want of that oxygen. Exposure zone is a design classification in itself.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. Corrosion needs both chloride and oxygen; below the water line the concrete is saturated and oxygen reaches the steel only by slow diffusion through the pore water, so the cathodic reaction is starved and corrosion is very slow. In the splash zone both are abundant, which is why that band, and not the submerged part, governs the durability design.

Answer: A

A sacrificial anode or an impressed current supplies the electrons instead. It is normally combined with a protective coating so that the anodes last considerably longer. Coating and protection are complementary measures.

Answer: B

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: cathodic protection makes the structure the cathode by supplying electrons from a sacrificial anode or an impressed current, and it is normally combined with a coating so that the current is needed only at coating defects. Statement 3 is wrong - the method requires an electrolyte, so it works only where the steel is immersed or buried, and the splash and atmospheric zones must be protected by coatings alone.