The results of laboratory compaction tests are not directly applicable to field compaction. It is because the compaction efforts in the laboratory tests are different, and are applied differently, from those produced by field equipment. Further, the laboratory tests are conducted on soil particles smaller than either 20 or 37.5 mm.
However, the maximum dry densities obtained in the laboratory using the 2.5 kg and 4.5 kg rammers cover the dry densities normally produced by field compaction equipment. A minimum number of passes must be carried out with the selected compaction equipment to give the necessary dry density value. This number of passes depends on the type and mass of the equipment, as well as on the thickness of the soil layer. It is usually within the range 3-12. Above a certain number of passes, no considerable increase in dry density is obtained. Generally, the thicker the soil layer, the heavier the equipment required for producing a sufficient degree of compaction.
Two methods are used to get an acceptable standard of compaction in the field. Those two methods are known as method compaction and end-product compaction.
In method compaction, the type and mass of the equipment, the layer depth, and the number of passes are specified.
In end-product compaction, the required dry density is specified. The dry density of the compacted fill must be equal to or greater than a stated percentage of the maximum dry density achieved in one of the standard laboratory compaction tests.
Field density tests can be carried out to verify the standard of compaction in earthworks.
SPECIFICATION OF COMPACTION IN THE FIELD
After the compaction curve for a given soil is received from laboratory tests, the specification for compaction in the field is prepared.
Relative compaction (R.C.) is defined as;
where γd,field is the specified dry density, that shall be reached in the field, and γd,max is the maximum dry density acquired from the laboratory compaction test.
Since γd,max varies depending on the compaction energy level or test method such as standard Proctor, etc., it shall be noted that R.C. could be more than 100% if the compaction energy in the laboratory was low.
That implies that if a higher R.C. value, >100%, is required in the field, higher field compaction energy than the laboratory energy level is required to achieve the specified requirement.
FIELD COMPACTION METHODS
After the compaction specification is given at the site, contractors are required to achieve its specified unit weight as the minimum in the field with proper equipment.
Several key factors influence field compaction. In addition to the level of applied compaction energy, controlling the water content as close as possible to its optimum water content is a must.
1) Number of passes: In general construction practice, several or more passes of rollers are required to obtain a specified unit weight. The more number of passes is applied, the higher dry density can be obtained. The figure below shows γd versus depth with various numbers of passes from 2 to 45 of a single 2.44 m fill by a 55.6 kN smooth roller. After five passes, a large number of passes is required to reach a significant increase in compaction. In general, it is considered that more than 10 to 15 passes may not be effective and not an economical way to compact fills.
2) Amount of lift: The amount of lift is also significant. The figure below also shows that only the upper section at 0.3-0.5 m deep is effectively compacted.
The lift should be small enough to receive the maximum compaction effect over the entire depth. But it shall not be too small, since the very top portion of the layer also cannot be well compacted due to particle segregation with vibration application. In general construction works, a loose lift is limited to about 0.5 m.
FIELD DENSITY DETERMINATIONS
The final key step in compaction is the field monitoring and inspection of the compaction practice. After the completion of compaction, it is not easy to say whether the site is appropriately compacted or not according to the specification. Therefore, monitoring during compaction and inspection after compaction is needed.
In most cases, the field dry density is measured after the completion of compaction.
There are several methods available, such as the sand cone method, core cutter method, rubber balloon method, nuclear density method, etc. The sand cone method is the widely used test.
Sand Cone Method
This method uses free-flowing sand to fill a field-excavated hole to measure its volume and then calculate the field bulk density as well as the dry density with the measured moisture content of the soil excavated from the pit.
Uniformly graded, clean, dry sand with gradation between 2 mm – passing No. 10 Sieve- and 0.25 mm – retaining No. 60 Sieve – are used for this purpose. Calibration is done in the laboratory to find out the sand’s dry density, γd,sand, before the field measurement.
The field procedure involves the following steps:
1. Before fieldwork, γd,sand shall be calibrated. Several containers with identification numbers for each are filled with dry sand, and their total weights are recorded.
2. At the site selected for field density determination, the surface of the ground is flattened and leveled by the edge of the rigid base plate. The surface level is usually located at a certain depth since the compacted top surface may not represent the true compaction result of the soil layer.
3. Through the circular opening at the center of the base plate, the compacted ground is carefully excavated by using a spoon. All soil from the excavated hole shall be collected in a plastic bag.
4. The jar filled with sand is placed upside down so that the top of the cone is kept into the inner edge of the opening in the base plate. At this stage, the valve in the jar must be in closed position.
5. After the jar with the cone is firmly placed on the base plate, the valve of the jar is carefully opened and sand is allowed to flow freely into the excavated hole and the cone section of the equipment.
6. After the completion of sand flow into the space, the valve is carefully closed. The total weight of the jar and the remaining sand is measured later. This weight is subtracted from the original weight of the jar of sand, and then the weight that filled the space of the hole and cone is obtained as Wsand,cone+hole.
7. The field wet (total) specimens from the excavated holes are weighted and their water contents are determined as Wt,hole,and w, respectively. These measurements could be done either in the field by using a balance and a quick-drying microwave oven or in the laboratory.
The calculation is as follows:
In the foregoing computation of γd, the volume of the cone Vcone and γd,sand shall be calibrated with the sand used.
The values of Wsand,cone+hole, Wt,hole, and w are field-measured properties.
The measured γd value is compared with the specified γd,field. When the measured values do not satisfy the requirement, the site shall be re-compacted.
Core Cutter Method
This method is suitable for cohesive soils free of stones. It is done by driving a steel cylinder with, a hardened cutting edge, into the ground using a specially designed steel rammer and protective dolly. The internal diameter of the cylinder is 100 mm and the height is 130 mm. The cutter is then dug out and soil is trimmed off flush at each end. Since the volume of the cutter is known and contained mass of soil can be found by weighing and the bulk density may easily be determined.
At the same time, small samples of soil are taken from either end from which the water content is determined.
Other Field Density Methods
Another popular field density determination method is the rubber balloon method. It uses a similar principle as the sand cone method. Instead of using a dry clean sand container, it uses a rubber balloon with water to fill the excavated hole. The hole is replaced with water to measure the volume.
In recent years, the nuclear density method has become a popular method to determine the field density and the water content. • It uses gamma radiation for density determination. It measures the scatter of radiation, which is proportional to the density, while the scatter of alpha particles detects water content. Both need prior calibrations to establish empirical correlations. This quick and nondestructive test is handy, but it requires specially trained technicians and careful handling of low-level radioactive materials.