48. Read the text carefully paying attention to the terms in italics. Answer the following question.

What is the greatest disadvantage of rock-fill dams?

Text E Rock-fill Dams

Rock-fill dams have characteristics both gravity dams and earth dams.

The rock-fill dam has two basic structural components - an impervious membrane and an embankment which supports the membrane.

The embankment usually consists of an upstream section of dry rubble or masonry and a downstream section of loose rock fill. The rock used should be capable of resisting erosion and strong enough to withstand loads of high intensity even when wet. Rocks may vary from small stones to boulders 10 or more ft in diameter. Large stones of regular shape are used to obtain a flat surface on which membrane can be placed.

The impervious membrane is most commonly constructed of concrete. The membrane is sometimes poured as a monolith without expansion joints but with ample steel reinforcement in both horizontal and vertical directions. Most membranes have expansion joints at intervals of about 50 ft with asphaltic joint filler to minimize leakage. Slab thickness is usually between 6 and 18 in with greater thickness often used near the base of the dam.

Rock-fill dams are subject to considerable settlement, which may result in cracking of the membrane. This is perhaps the greatest disadvantage of rock-fill dams, although in many instances leakage can be controlled by periodic repair of the membrane. A rock-fill dam of good design and careful construction has high resistance to earthquakes. Moreover, much less material is required for a rock-fill dam than for an earth dam. Because of the narrow base width and the possibility of high seepage, foundation requirements for rock-fill dams are more rigid than for earth dams. Rock-fill dams are generally cheaper than concrete dams and can be constructed more rapidly if the proper material is available.

49. Put the sentences in the right order according to the text.

•  It is sometimes poured as a monolith without expansion joints but with ample steel reinforcement.

•  The rock used should be capable of resisting erosion and withstanding loads of high intensity.

•  Foundation requirements for this type of dams are more rigid than for earth dams.

•  The quantity of used material is less for a rock-fill dam than for an earth dam.

•  Slab thickness is usually between 6 and 18 in.

•  Leakage can be controlled by periodic repair of the membrane.

•  The embankment usually consists of an upstream section of dry rubble or masonry and a downstream section of loose rock fill.

•  Rocks may vary from small stones to boulders.

•  A rock-fill dam generally has high resistance to earthquakes.

•  The impervious membrane is most commonly constructed of concrete.

•  Rock-fill dams are subject to considerable settlement.

•  The rock-fill dam has an impervious membrane and an embankment which supports the membrane.