Sand-cone equipment is used to determine an inplace unit weight (field density test) on a compacted earth fill. Ottowa sand is used in the cone and is known to have a bulk density of 100 pcf. (a) From the information given below, determine the in-place dry unit weight of the tested soil, and the water content. Soil sample dug from test hole, wet weight 4.62 lb Dried weight of soil sample 4.02 lb Weight of Ottowa sand (sand cone) to fill test hole 3.60 lb (b) Determine the percentage of compaction of the tested soil if the laboratory moisture–unit weight curve indicates a dry unit weight of 115 pcf and an optimum moisture content of 13 percent
The Correct Answer and Explanation is:
To solve this sand cone test problem, we will follow standard procedures used in geotechnical engineering to determine the in-place dry unit weight and water content of compacted soil, and then compute the percentage of compaction based on lab maximum dry density.
(a) In-Place Dry Unit Weight and Water Content
Given:
- Wet weight of soil from test hole = 4.62 lb
- Dry weight of soil = 4.02 lb
- Weight of sand used to fill test hole = 3.60 lb
- Bulk density of Ottowa sand = 100 pcf
Step 1: Compute Volume of Test Hole
The sand that filled the test hole weighs 3.60 lb and has a bulk density of 100 pcf. So, Volume of test hole=Weight of sandDensity of sand=3.60 lb100 pcf=0.036 ft3\text{Volume of test hole} = \frac{\text{Weight of sand}}{\text{Density of sand}} = \frac{3.60\ \text{lb}}{100\ \text{pcf}} = 0.036\ \text{ft}^3
Step 2: Compute In-Place Wet Unit Weight
γwet=Wet weight of soilVolume=4.62 lb0.036 ft3=128.33 pcf\gamma_{\text{wet}} = \frac{\text{Wet weight of soil}}{\text{Volume}} = \frac{4.62\ \text{lb}}{0.036\ \text{ft}^3} = 128.33\ \text{pcf}
Step 3: Compute Water Content
w=Wwet−WdryWdry×100=4.62−4.024.02×100=14.93%w = \frac{W_{\text{wet}} – W_{\text{dry}}}{W_{\text{dry}}} \times 100 = \frac{4.62 – 4.02}{4.02} \times 100 = 14.93\%
Step 4: Compute In-Place Dry Unit Weight
γdry=γwet1+w100=128.331+0.1493=128.331.1493=111.65 pcf\gamma_{\text{dry}} = \frac{\gamma_{\text{wet}}}{1 + \frac{w}{100}} = \frac{128.33}{1 + 0.1493} = \frac{128.33}{1.1493} = 111.65\ \text{pcf}
(b) Percentage of Compaction
%Compaction=Field dry unit weightMaximum dry unit weight×100=111.65115×100=97.09%\% \text{Compaction} = \frac{\text{Field dry unit weight}}{\text{Maximum dry unit weight}} \times 100 = \frac{111.65}{115} \times 100 = 97.09\%
Final Answers:
- In-place dry unit weight = 111.65 pcf
- Water content = 14.93%
- Percentage of compaction = 97.09%
Explanation
The sand cone method is a field test used to determine the in-place density of soil, essential for ensuring proper soil compaction during construction. This test helps verify that the compacted soil meets specifications derived from laboratory Proctor compaction tests.
In this problem, the weight of the sand required to fill the test hole was 3.60 lb. Given that the Ottowa sand has a known density of 100 pcf, we computed the test hole volume as 0.036 ft³. The in-place wet unit weight was then calculated using the wet weight of soil removed from the hole (4.62 lb) and the hole volume, resulting in 128.33 pcf.
To determine the moisture condition of the soil, we used the difference between wet and dry weights of the excavated soil. The water content was calculated to be approximately 14.93%, indicating that the soil contains more moisture than the optimum (13%), possibly reducing its strength.
The dry unit weight is crucial, as it represents the weight of soil solids per unit volume. This was found by correcting the wet unit weight for moisture, resulting in a dry unit weight of 111.65 pcf.
Finally, to evaluate whether the soil compaction meets specifications, we compared the field dry density with the maximum dry density obtained from lab Proctor tests (115 pcf). The field soil achieved 97.09% compaction, which is generally acceptable, as most standards require a minimum of 95% compaction.
This analysis confirms that the compaction is nearly optimal, but slightly above the optimum moisture content, which may warrant minor adjustments in the field.
