**Gravity Dam**

A gravity dam is a type of dam that is constructed using concrete or masonry and relies on its own weight to resist the horizontal thrust of the water it holds back. It is a popular choice for dam construction due to its simplicity and effectiveness. In this response, we will discuss the four statements provided and determine which one is correct.

**1) Build on weak foundation**

This statement is incorrect. Gravity dams should ideally be built on a strong and stable foundation. The foundation is essential for the stability and longevity of the dam. If a dam is built on a weak foundation, it can lead to settlement, cracking, and even failure of the structure. Therefore, it is crucial to conduct thorough geotechnical investigations and analysis to ensure the foundation can support the weight of the dam.

**2) More suitable in steep valleys**

This statement is correct. Gravity dams are more suitable in steep valleys or narrow canyons where the natural topography provides a favorable location for the construction of a dam. The steep valley sides can provide additional support to the dam structure, reducing the need for extensive excavation and construction of abutments. The natural confinement of the valley also helps to contain the reservoir behind the dam effectively.

**3) Benefit to cost ratio is low**

This statement is incorrect. Gravity dams generally have a high benefit to cost ratio. The initial construction cost of a gravity dam can be relatively high due to the substantial amount of materials and labor required. However, gravity dams have a long lifespan and require minimal maintenance, resulting in lower operational costs over their service life. Additionally, gravity dams are energy-efficient and can provide multiple benefits such as flood control, water storage, and hydroelectric power generation, which contribute to their overall economic viability.

**4) Initial construction cost is low**

This statement is incorrect. The initial construction cost of a gravity dam is generally high. The dam requires a significant volume of concrete or masonry, which can be expensive. Additionally, the construction process for a gravity dam involves precise formwork, placement, and curing of the concrete, requiring skilled labor and specialized equipment. However, it is important to note that while the initial cost may be high, the long-term benefits and low maintenance costs make gravity dams a cost-effective choice over their lifetime.

In conclusion, the correct statement for a gravity dam is that it is more suitable in steep valleys. The other statements provided are incorrect as gravity dams should be built on a strong foundation, have a high benefit to cost ratio, and generally have a high initial construction cost.