A new development is to be constructed in Mexico City. Due to the large subsidence, the elevation at the proposed site must be raised prior to construction. The total volume of soil needed to raise the site is 100,000 cubic yards. Sand earth fill is available from three different borrow sources. The soil must be compacted to a relative density of 75 percent at the site. Tests on the three different sands gave the following results.
Borrow Site In-Situ Void Ratio Minimum Void Ratio Maximum Void Ratio
1 0.80 0.48 0.94
2 0.79 0.51 1.07
3 0.49 0.38 0.78
The estimated total cost per cubic yard (based on its in-situ void ratio at each borrow site) to import the soil and construct the fill as follows:
Borrow Site Cost per yd3
1 $10.50
2 $12.00
3 $11.00
Note: The 100,000 yd3 is the volume of the fill compacted to a relative density of 75%, while the cost of the fill is given per yd3 of soil having a relative density corresponding to the listed in-situ void ratio at the borrow site.
a) What volume of soil needs to be excavated from each of the borrow soil in order to have 100,000 yd3 of compacted fill having Dr = 75%?
b) Which borrow site is the most economical? Please list the total estimated costs for each site.
c) For borrow site 2, what is the in-situ total unit weight in pcf of the borrow soil (i.e., prior to excavation)? Assume the degree of saturation of the sand in-situ is 65 percent, and Gs = 2.65.