Question 508976
Here's a clue.
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This problem involves two sub-problems. The first sub-problem is how do you find the volume of a circular pile if you know the diameter and the depth? The second sub-problem is what is the answer in metric dimensions?
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Let's begin by tackling the first sub-problem. The volume of a cylinder is found by multiplying the circular cross-section area by the length (in this case the depth) of the cylinder. The area of the circular cross section is found by squaring its RADIUS and multiplying by pi (Greek symbol {{{pi}}} and pronounced 'pie') whose value is, according to my $8 calculator, 3.141592654. That is certainly more accuracy than is needed for concrete estimates. 
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So if we let A equal the area and R equal the radius, we can use the equation:
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{{{A = pi*R^2}}}
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to calculate the area of the cross section. Then all we have to do is to multiply that by the height of the pile, call it H, and we have the volume of a single pile. 
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This method works as long as the dimensions are all in the same system of units. In the U.S. concrete is normally sold by the cubic yard, so we would normally find the radius in terms of a yard and the height (or depth in this case) in yards and substitute these values into the Area equation and then the Volume equation. Since a cubic yard and a cubic meter are almost equivalent in size, I would presume that you would want the answer in terms of cubic meters instead of cubic yards.
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The process of finding the area of the circular cross-sectional and then multiplying by the depth of the pile work no matter what units you are in. You just have to be consistent. Use the same units for the radius as you do for the depth. So we want to convert the given diameter (in inches) in two ways. First divide it by 2 to find the radius and then convert that radius to meters. Then we will convert the 8-foot depth of the column to meters. Finally we will calculate the volume of the pile in cubic meters.
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If you do a lot of this type of work, you may want to get a copy of Pocket Ref by Thomas J. Glover. It is nearly 800 pages (pocket size) of very useful information for math, plumbing, electrical, steel, wood, chemical information. The cost is likely to be in the range of $10 to $15 US. (Worth every penny.) I got mine from Grizzly Industrial at 1-800-523-4777 or on the web at grizzly.com. The cover says Model 63382, but that was probably 5 years or more ago.  Enough for the unsolicited ad.
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Anyhow, the 16 inch diameter gets divided in 2 to get the radius of 8 inches. According to the conversion tables in my Pocket Ref, to convert inches to meters, multiply the inches by 0.0254. When you multiply the 8 inches by 0.0254 you get that the radius is 0.2032 meters. Next we convert the 8 foot depth using the same method. According to Pocket Ref, to convert feet to meters multiply the number of feet by 0.3048. So 8 feet times 0.3048 tells us that the depth in meters is 2.4384 meters.
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Now we're set. First substitute the radius in meters into the circular Area equation:
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{{{A = pi*(0.2032)^2}}}
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For {{{pi}}} substitute 3.141592654 to get:
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{{{A = 3.141592654*(0.2032)^2}}}
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If you square the radius on a calculator you get 0.04129024 which simplifies the equation to:
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{{{A = 3.141592654*0.04129024}}}
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And multiplying these two values on a calculator we find that the Area of the circular cross section is 0.129717114 square meters.
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We multiply this by the depth which is 2.4384 meters and we get that the volume of a single piling is 0.316302212 cubic meters. 
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Finally we multiply the volume for the single piling by the 8 pilings that we need and we get that the total volume for the 8 pilings is 2.53 cubic meters.  Just to be on the safe side, you may want to think in terms of 3 cubic meters ... just in case.
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Hope this helps you out. Check my math ... it's a little late here to be working math problems, but I think this is correct. I worked it out in US units, got an answer in cubic yards and converted that answer to cubic meters and got the same answer for a single piling as we found above.