Compaction Techniques
There are basically two ways to compact soil: the static application of heavy loads or the repeated dynamic application of smaller loads at high frequencies. Most compaction equipment utilizes both types of forces. These forces are in turn each divided into two separate techniques for applying these forces:
- Static
1. Pressure
2. Kneading
- Dynamic
1. Impact
2. Vibration
Static force is simply the operating weight of the compaction machine applied as a dead load to the soil underneath, relying on applied pressure and/or kneading of the soil to achieve compaction. Heavier machines have greater static loads. Compaction equipment operating on relatively steep slopes applies proportionally less direct dead weight depending on the angle of the slope to the horizontal. Static load is measured as pressure, the weight of the machine over the area of the parts of the machine that are in direct contact with the soil (pneumatic tires, steel drum, padded foot, etc.). The applied pressure, especially when delivered to many small areas, has the effect of kneading the soil. Static force is simple to apply but does not usually affect the soil to a significant depth. A static sheepsfoot and a static roller are examples of machines that primarily deliver pressure. Kneading of soil can be accomplished with scrapers and pneumatic tires.
Dynamic force is created not by dead weight but by a live engine that moves a mass in a reciprocating motion that delivers multiple impacts to the soil in a short period of time, relying on vibration and impact to get the job done. In keeping with the laws of physics, the resultant force is proportional the mass of the impact object times the square of its velocity. So while the weight of the impact mass doesn’t change, the force it translates to the soil can be greatly increased by increasing the speed of its impact. The repeated shock waves passing through the soil cause compaction to occur at greater depths than static force application. Rammers and dropped weights are examples of machines that use heavy impact for compaction. Vibratory sheepsfoot, vibratory roller, and vibratory plate are examples of dynamic compaction machines utilizing vibration.
So what method works best on which soils? As there are two main methods of compaction, there are two broad categories of soils: granular soils and cohesive soils. Granular soils consist of large particles from fine sands to large cobbles but are divided into two subcategories of sand and gravels. Cohesive soils consist of smaller particles of either clays or silts that are densely compacted and adhere together. Granular soils are best for foundation support, embankment shell construction, and roadway pavement subgrades. Cohesive soils are typically used to construct embankment cores as well as low-permeability liners for liquid containment in ponds, impoundments, and landfills.
Each type of compaction equipment is suitable for different kinds of soil:
- Sheepsfoot rollers using both dynamic vibration and static pressure applied over multiple small contact points are useful for compacting cohesive soils (100% to 50% clay).
- Pneumatic wheels using static kneading action are used for mixed cohesive and granular soils (75% sand/25% clay to 25% sand/75% clay).
- Rollers using both dynamic vibration and static pressure applied over a single large contact area can be used on granular soils (50% sand to 100% sand).
- High-speed rammers and tamping plates using dynamic impact are used on either cohesive or granular soils.
Measuring Compaction
So how much compaction is enough? That depends on the soil and its moisture content as well as the compaction application and the acceptance criteria. Soil can be compacted to provide a stable base for constructing a building foundation, a roadbed, a structural embankment, or an impoundment liner. Each application has a preferred level of compaction, soil type, and moisture content.
Detailed information on a soil’s physical state can be derived from a relatively simple series of laboratory tests. Two of the most common are the Standard and Modified Proctor tests. These tests determine the relationship between a soil’s density and moisture content and a set compaction effort. Each soil sample is subdivided into smaller test samples that are wetted to varying moisture contents. Each test sample is compacted into a standard mold measuring 57.6 cubic inches. In the Standard Proctor test, a hammer weight of 5.5 pounds is dropped on each of three lifts of soil from a height of 12 inches (resulting in compactive energy of 12,375 foot-pounds per cubic foot). The Modified Proctor increases this energy to 56,250 foot-pounds per cubic foot by dropping a 10-pound hammer from a height of 18 inches on each of five lifts of soil. The Proctor test was originally developed to control and measure the compaction of soils used as the foundation for highways and airfields. The standard test simulates the compaction of soil in the field with medium-sized compaction equipment, while the modified test approximates compaction with the heaviest compaction equipment.
The results of the compaction effort are measured by first determining the water content of each compacted sample and its dry unit weight. These values are plotted on a graph with the water content as the x-axis and the dry unit weight as the y-axis. Three sample points are usually used to create a compaction curve that resembles an upside-down parabola. The first sample is compacted at field water content. The second sample is compacted at a water content two percentage points wetter than the field sample. Depending on the outcome of the send test, a third sample is compacted at either two percentage points drier than the first sample or two percentage points wetter than the second sample. The top of the resultant parabola curve represents the soil’s optimum water content. That is, the specific moisture content that results in the highest dry unit weight.
Soil is typically compacted to either an acceptable moisture/density zone based on shear strength criteria (for embankment and foundation construction) or to an acceptable zone based on hydraulic conductivity (for lining ponds and landfills). To determine these zones, soil samples are compacted and the results plotted for three different curves resulting from modified, standard, and reduced compaction efforts. Along the upper right-hand side of these curves is plotted another curve representing the soil’s density and moisture characteristics at a zero air voids situation representing a completely saturated condition. This represents an ideal upper bound condition and is calculated from the previously determined compaction data. Acceptable zones are determined by the zero air voids curve, the lower bounds that represent the moisture and density relationships that result in the soil having the minimum required shear strength of the maximum allowable hydraulic conductivity.
Field tests can also be performed on in-situ compacted soils. They can all be done relatively fast with reasonable to high degrees of accuracy. Aside from applying a pocket penetrometer to the soil with hand pressure, a field measurement of soil usually involves the removal of a large sample with hand shovels. The resultant hole is then filled with material of known volume, such as sand, which is poured into the hole to measure its volume (also known as the sand cone test). Meanwhile, the excavated soil is weighed and compared with the volume of the hole it was extracted from to determine its in-place density. A more measured volume sample can be extracted with a Shelby tube. Direct measurement of a soil’s density can be made by using a nuclear gauge to measure the radiation from a gamma ray source driven into the soil to a fixed depth.
