What type of particle should be avoided in the final layer of grade for the tank bottom?

• A. Sand
• B. Small gravel
• C. Gravel and large particles
• D. Clay

Answer: (C)

The reason for avoiding gravel and large particles in the final layer of the tank bottom is primarily related to stability and settling characteristics. The bottom of a welded tank for oil storage requires an even, stable base to support the tank structure and provide effective load distribution. Large particles, such as gravel, can create voids beneath the tank if they settle unevenly or if water permeates the base layer, leading to instability.

A smooth and compact substrate is essential for minimizing potential movement, which could cause stress on the tank's sides and bottom, potentially leading to structural failures or leaks. Smaller particles, such as sand, or specially graded materials that ensure compaction can facilitate a more stable, uniform support layer. Using a finer material at this level aids in proper drainage and prevents the accumulation of water, reducing corrosion risk.

The other options, while having their advantages in different contexts, do not provide the level of compaction and uniformity that is crucial for the final layer of the tank bottom. Therefore, the avoidance of gravel and large particles is a critical consideration in the design and construction of welded tanks as specified by API 650.