Compaction of soils is one of the most popular means of mechanical stabilization of soil. The primary aim of this process is to increase the bearing capacity of soil and reduce soil settlement. This can be made possible by the reduction of pore spaces in the soil through the expulsion of pore air, rearrangement of soil particles, densification of soil, and reduction of volume. Compaction is employed in the construction of buildings, roads, airfields/runways, wharves, earth dams, retaining walls, etc. Field compaction of soil can be achieved through the use of various mechanical equipment broadly classified into:
- Impact compactors
- Roller compactors
- Vibratory compactors
Some compactors have both rolling and vibrating capability while others may have impact and vibrating capability. There are special situations where each class of compaction is suitable but in road construction, roller compactors are the most common. Roller compactors include smooth-wheeled rollers, sheepsfoot rollers, and pneumatic-tyred rollers. Diagrams of some types of rollers are shown below.
Laboratory Compaction of Soils
Laboratory compaction is usually carried out on soil before the field compaction process and it serves as the control for the field compaction process. The laboratory compaction process is usually employed because it is easier to control and can give more accurate results when compared to the field compaction process. The aim of the laboratory compaction process is the determination of the maximum dry density (MDD) and the corresponding optimum moisture content (OMC). It is expected that when the soil in the field is moistened uniformly at the OMC and compacted using one of the suitable compaction equipment, the MDD achieved in the field would be within +/-5% of the MDD achieved in the laboratory. Laboratory compaction can be achieved using reliable methods such as the standard Proctor method (SPM), modified Proctor method (MPM), British Standard Light (BSL), British Standard Heavy (BSH), and West African Standard Compaction (WASC). There are other less popular methods.
Methods of Insitu Dry Density Test
The control of the field density of soil is done through in-situ dry density test. There are several methods to achieve this which include:
- Core cutter method
- Sand replacement method
- Rubber/water balloon method
- Radiation method and
- Submerged density method
The core cutter method is common but its results are usually affected by soils having coarse-grained particles. In this situation, the sand replacement method or sand cone method is more acceptable. The basic principle of the sand replacement method is to measure the in-situ volume of the hole from which the material was excavated from the weight of the sand with known density filling the hole. The in-situ density of the material is given by the weight of the excavated material divided by the in-situ volume. This method is adopted in Nigerian General Specifications for roads and bridges. The reason could be that laterite and stone bases that are used in pavement layers contain coarse-grained particles. I would describe how to carry out this test for a given soil. Nigerian specifications for roads and bridges gave standard specified limits for in-situ density tests for different highway materials.
Table 1: Specification limits for in-situ dry density tests in Nigeria for different highway materials
Prior to carrying out the test properly on site, the cone to be used for the test should be calibrated. The aim of the calibration is to
- Determine the density of the sand to be used in the experiment.
- Determine the weight of the sand occupying the cone of the sand-pouring cylinder.
For accurate results, it is necessary that
- The height of the sand column in the cylinder should approximately be equal to that in the calibration test.
- The depth of the hole should be equal to the depth of the calibrating cylinder.
The apparatus for the test include:
- Field balance
- Calibrated sand (measured sand of known density was washed and passed through sieve No. 30 (0.600 mm) and retained on sieve No. 50 (0.300 mm).
- Chisel, hammer, measuring tape,
- Sand of known weight
- Sand cone
- No 40 sieve (0.425 mm)
- Speedy moisture tester
Procedure for In-situ Dry Density Test using Sand Cone Method
- Dig a hole of 150 mm for compacted laterite and 200 mm for compacted stone base.
- Take out the dug-up material in a bowl and weigh it.
- Immediately determine the moisture content of the same soil using a speedy moisture tester
Speedy Moisture Tester
Speedy moisture tester often known as the calcium carbide method works on the principle that when water reacts with calcium carbide (CaC2), acetylene gas (C2H2) is produced.
CaC2 + 2H2O = C2H2 + Ca (OH)2 (1)
From this reaction, the moisture content of the soil is determined indirectly from the pressure of the acetylene gas formed.
- Place about 6 g of ground and pulverized wet soil in a sealed container containing acetylene gas.
- If the soils are plastic or cohesive, add steel balls in the pressure container to disaggregate them.
- Shake the pressure container with soil and steel balls vigorously for about 5 minutes to enable a complete reaction of the water in the soil with calcium carbide.
- From the calibrated scale of the pressure gauge, read off the water content (m1) based on the total mass of the soil
- The water content (w) based on the dry mass of the soil can be determined by the following equation
w = m1/ (1-m1) (2)
To complete the sand cone process after having determined the moisture content
- Place the sand cone on top of the hole and release the sand inside the hole
- Weigh the remaining side inside the hole
Given that the following results were obtained from laboratory compaction tests on a given lateritic soil and in the calibration of the sand cone, determine the in-situ dry density of the compacted soil and the degree of compaction as well.
Maximum dry density (Ydmax) = 2290 kgm-3
Optimum moisture content (wopt) = 7.6 %
Density of sand (Ys) = 1465 kgm-3
Weight of calibrated sand in cone (Wsc) = 0.825 kg
The following data below were obtained from the site
Weight of wet soil from the hole (Wws) = 8.944 kg
The moisture content of soil (by speedy moisture tester) (w) = 7.0%
The initial weight of sand in the bottle (Wisb) = 12.030 kg
The final weight of sand in the bottle (Wfsb) = 6.128 kg
Weight of sand in hole + cone (Wshc) = Wisb – Wfsb = 12.030 – 6.128 = 5.902 kg
Weight of sand in hole (Wsh) = Wshc – Wsc = 5.902 – 0.825 = 5.077 kg
Volume of sand in hole (Vsh) = Wsh/Ys = 5.077/1465 = 0.003466 m3
Bulk density of wet soil (Y) = Wws/Vsh = 8.944/0.003466 = 2580.85 kgm-3
Insitu dry density of soil (Yid) = Y/ (1+w) = 2580.85 / (1+ 0.07) = 2412.01 kgm-3
Degree of compaction = (Yid/Ydmax) x 100 = (2412.01/2290) x 100 = 105%
This value is quite high though it is the boundary of acceptable limits for some soils.
Insitu dry density below 95% shows under-compacted soils while insitu dry density above 105% shows over-compacted soil. Both are not good and should be avoided in field compaction of soils. Over-compaction causes the breakdown of natural soil grains which leads to poor grading of the soils and weakens soil strength.