Field unit weight of compaction by sand cone method

This article represents the sand cone method which used to determine the field compacted dry unit weight of soil, in order to insure that compaction has been done properly and that the soil meets the specifications required for construction. .Soil compaction occurs when the porosity between the soil will decrease and so the strength of ability to resist the failure will increase.

The soil in the field before compaction

The standard procedures for determining the field unit weight of compaction are sand cone method, rubber balloon method , and nuclear method. In this experiment the sand cone method will be discussed. Here the sand cone was calibrated in the lab, and the sand volume and weight were measured to find its unit weight. A hole of about 15 cm in radius and 15 cm in depth was made in the required field outside, and its filling which is the field sample was retained. Sand was allowed to fill the hole through the sand cone whose weight is known. Therefore, sand filling the whole had a weight and unit weight that are easily measured, and so the volume of the hole (volume of field sample) was calculated. Using these values, the dry unit weight in the field was thus calculated.


γ of the sand = M_c/V


In the field calculations:
Let the mass of the sand to fill the hole in the field with the cone = M₃
So the mass of sand to fill the hole (M_H) = M₃ – M₁
The volume of the hole is denoted by V_H = M_H / density of the sand

For the calculations of the water content in the soil sample:

ω% = [(W₂ – W₃)/(W₃ – W₁)]*100%

Where:
W1: mass of the pan
W2: mass of the pan with the moist sample of soil
W3: mass of the pan with the dry sample of soil

And from the data upwards, the moist unit weight of the soil sample can be calculated as:

γ moist = weight of moist sample / volume of the hole

Dry unit weight of the soil sample:
γ dry = γ moist / [1+ (ω%/100)]

Equipment:

1. Sand cone apparatus which consists of a one-gallon plastic bottle with a metal cone attached to it.
2. Metal tray (base plate).
3. Tools for excavating a hole in the ground,(chisel, hummer, and spoon).
4. Air dry sand.
5. Balance sensitive to ± 1 g.
6. Weighing tray.
7. Nylon bag, for carrying wet excavated soil from field to the lab
8. Drying oven.
9. A cylindrical mold with known volume (used for calibration).

Procedure:

The calibration of the cone
The plastic container with the metal cone was filled with an amount of sand sufficient to fill the cone. The apparatus was weighed and the weight before use was recorded (W₁). Then the cone was placed on the table over the base plate and the valve was opened so that sand flew and filled the cone. When the flow of sand into the cone stopped completely, the valve was closed and the apparatus was weighed again. After that, the final weight was recorded (W₂).

After the calibration of the sand
The plastic container with the metal cone was filled with sand enough to fill the cone and the calibration cylinder, then the apparatus was weighed and the weight before usage was recorded. The cone was placed on a flat surface over the base plate above the calibration cylinder and the valve was opened so that sand flew and filled the cone and the cylinder, when the flow of sand stopped, the valve was closed and the apparatus was weighted again and the final weight after use was recorded, later on, the sand cone apparatus was filled with sand and the weight before usage was recorded.


After all calibrations (Field`s work)
An area was chosen in the university parking and a hole about 20 cm was dug in the ground (20 cm not more not to reach the beneath layer) , and the base plate was fixed by standing on its edges since no nails were provided, the hole diameter is similar to the diameter of the base plate which is 15cm, then a hummer and a chisel were used to dig the hole deep vertically below the opening of the base plate, then a spoon were used to move any lose material inside the hole into a nylon bag(any materials with a diameter more than 20 mm was returned to the hole not to result in an increase in the volume, (those materials were arranged in the hole such that the minimum volume of voids was kept), after the burying was done, and care was taken not to lose any material, the nylon bag into which the field sample was inserted was sealed so that nothing is lost. The sand cone apparatus was placed over the hole and the valve was opened so that sand flew in the hole and cone. When the flow ended, the valve was closed.

Returning to the lab 
The weight of the gallon, remained sand, and the cone was determined. The wet soil in the nylon bag was weighed as the Weight moist soil from the field. Then the sample was put in the oven for 24 hours, after which the weight of the sample was taken as the Weight of dry soil after 24hours.

Data and calculations:

Calculations:

calculate the Weight of sand to fill the cone
➧ W3 = W1 – W2 = 8836 – 7177 = 1659 g

calculate the Weight of sand filling cylinder
➧ W4 = Wa – Wb – W3 = 7177 - 1956 - 1659 = 3562 g


calculate the Volume of the calibrating proctor mold is equal to
➧ V = πr²h = π (7.52)² (15) = 2650.719 cm


calculate γd (sand) of the sand
➧ γd (sand) = W4/V = 3562/2650.719 = 1.344 g/cm


calculate the Weight of sand to fill the hole
➧ W7 = W5 –W6 – W3 = 7980 – 2514 – 1659 = 3807 g


calculate The volume of the hole
➧ V2 = W7 / γd sand = 3807 / 1.344 = 2832.6 cm³


calculate the moist unit weight of the soil sample:
➧ γmoist = W8 / V2 = 6357 / 2832.6 = 2.24 g/cm³


calculate the water content in the soil sample:
➧ ω% = [(W8 – W9)/(W9)]*100% = [(6357 – 6202)/(6202)]*100% = 2.49%


calculate Dry unit weight of the soil sample:
➧ γ dry = γ moist / [1+ (ω%/100)] = 2.24 / [1+ (2.49/100)] = 2.18 g/cm³

Results and conclosion:

The main result:

γ dry = 2.15 g/cm³

It is hard from this type of experiments to find a reference in which to compare the results with a theoretical value of the dry density, but there are some aspects that an engineer has to consider when performing the sand cone method test.

For an engineer who is working on a road construction site, and wants to determine the value of the dry density to see if the compaction level has been attained or not, it is important for him\here that the density reaches a value which should be considered to be of a success to the project. In order to achieve this, the engineer has to take all the soil that was excavated from the hole with no more because that increases the density and with no loss because that decreases the density. So as engineers, the credibility of our values has to be taken into consideration so as to achieve the maximum performance and conditions with the least costs.

Another aspect that needs to be corrected is the aspect talking about the large parts of the soil which gives a term of virtual density of the soil. These parts are highly compacted throughout the time, and increase the density of the soil sample taken from the excavation of the hole which makes a virtual moist or dry density of the sample of the soil tested. So it is important to remove such large parts of the soil to get the credible and not virtual measurement of the density.

In addition to that, it is important to note that the height of the hole should be more than 12 cm so as to reach the area where the entire load will be concentrated on. Moreover, it is important to test the degree of compaction of the soil for that depth mentioned upwards.

There are some sources of errors that might affect our results in the determination of the dry unit weight of the soil sample in-site such as:

➧ Errors in the measurements of masses in each trial of the experiment.
➧ Vibration or shaking may affect our results since it may compact the soil.
➧ Existence of water in the sand so that the sand used might not be purely dry.
➧ From the excavation hole, the soil might have rock fragments which might give a virtual density.
➧ The height of the hole dig might not be fair enough to attain the perfect conditions of our results.
➧ I doubt that there was some error in the weight of the dry soil from the field after 24 hours. Some soil maybe added to it by error.

Engineering use:

The compaction of soils is of great importance in the construction of almost every significant structure built on soil. Compaction is used to improve the strength of the soil, giving it the ability to withstand heavy and important structures. Not only that, but a well compacted soil reduces seepage of water, soil settlement, and provides greater stability. However, once compaction has been done in the field, it is not guaranteed that the soil has been compacted properly and according to specification set. Therefore, performing a simple test such as the sand cone method which gives the dry unit weight of the compacted soil in the field aids significantly in determining wither the soil is compacted properly or not, since compaction is determined by the dry unit weight of the soil.

References:

DAS, Principles of Geotechnical engineering, third edition .