This diameter d is determined by the soil type. Clay Suppose the upwind piers are in medium clay. On the other hand the tower itself provides windload proportional to half of the square of the height h times the cross sectional area per unit height. Sand Suppose the upwind piers are in dense dry sand. We have oversized.
Mixed soil Suppose the upwind legs are in mixed soil.
Read the horizontal lines of Figure 4 to find the percentage of clay versus sand. Layered soil Sum up the resistance from each layer. Careful design would necessitate an on-site shear vane test in clay soil or a standard penetration test in sand. This would significantly reduce the effective wind load, yielding an additional safety factor of perhaps 6. R.B. Peck, W.E. Hanson, and T.H. Thornburn, Foundation Engineering, 2nd.