Punching shear is a type of structural failure that occurs when a concentrated load is applied to a slab or footing, causing it to punch through the surrounding material. In the case of loose sand, the density of the sand plays a critical role in determining whether punching shear will occur.

Explanation:

1. Punching Shear in Loose Sand:
Punching shear in loose sand refers to the failure of a slab or footing when a concentrated load is applied, causing it to punch through the loose sand. This type of failure occurs due to the inability of the sand to resist the vertical forces exerted on the slab or footing.

2. Density of Sand:
The density of sand is a measure of how closely packed the sand particles are. It is expressed as a percentage of the maximum density that the sand can achieve. The maximum density of sand is typically achieved when it is fully compacted.

3. Effects of Density on Punching Shear:
The density of loose sand determines its ability to resist the vertical forces applied by the concentrated load. When the sand density is low, the particles are not closely packed, leading to a decrease in its load-bearing capacity. As a result, the slab or footing is more likely to punch through the loose sand, causing punching shear.

4. Optimum Density for Sand:
The optimum density of sand depends on various factors such as grain size, shape, and moisture content. However, in general, loose sand with a density less than 35% is considered to be susceptible to punching shear. This means that if the sand is loosely packed and has a density less than 35%, there is a high likelihood of punching shear occurring when a concentrated load is applied.

5. Importance of Sand Compaction:
To prevent punching shear in loose sand, it is crucial to compact the sand to achieve a higher density. Compaction involves applying mechanical force to the sand, which helps to pack the particles closely together. By achieving a higher density, the sand becomes more resistant to punching shear.

In conclusion, punching shear may occur in loose sand with a density less than 35%. It is important to ensure proper compaction of the sand to increase its density and prevent punching shear failures in structural elements such as slabs and footings.