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The word compaction is defined as joined or packed together; closely and firmly united; dense; solid; to make firm or stable.
The process of compacting underlying layers gives us a foundation on which to build. But compaction goes beyond that action. It also involves examination of various factors. These factors include:
• The soil type (clay, sandy, loam)
• Climatic region (moisture, rainfall, freeze-thaw)
• Traffic load (pedestrian, light or heavy vehicular)
• Use of correct materials for sub-base and base
• Will geotextiles be needed to spread/ distribute the load?
• Use of the proper compaction equipment
• Executing proper compacting techniques
• Are density tests needed?
• Will a soil or civil engineer need to be consulted?
• Have underground utilities been marked before excavation begins?
Soil type, climatic region and traffic load will determine the thickness of the base and subbase as well as the use of geotextiles over the subgrade. Weak native soils that do not drain well will require deeper excavation and geotextile use. Compaction of the subgrade, subbase and base should generally be done in lifts of three to four inches. Material should be compacted to 95 to 98 percent modified proctor density. Base material should contain +/-8 percent fines (passing the 200 sieve.) Research has shown that base material with >12 percent fines decreases strength, increases frost heaving and reduces drainage while base material with <6 to 8 percent fines creates voids where bed material will migrate into the base.
Density tests should be done on most installations with special attention being paid to compaction against buildings and walls, curbs, corners, transitions from concrete pavement to unit pavement and around utility structures, as these are the areas that typically settle. Areas not passing should be re-excavated and reworked. It is much less expensive to pay for testing and rework areas in the base, subbase and subgrade than to repair pavement failures.
Bases should be shaped to reflect the surface slope. A 2 percent slope (1/4 inch per foot) is ideal. Time should be spent on each lift shaping the slope.
Bedding sands should never be used to correct deficiencies in the base material. Over time the effort put into and the quality of the base, subbase and subgrade will be reflected in the surface. If one cuts corners below, it will eventually be seen in the finished pavement.
Most of an installer?EUR??,,????'???s work is on these underlying layers. This is the backbone of the system and where the success or failure of the segmental paving system lies.
Once the pavers are set, the pavement is compacted setting the pavers into the sand bed. It is important to protect clay brick pavers and some special finish concrete pavers by using a rubber pad on the bottom of the plate compactor or by placing plywood over the paver surface. Less chipping of the pavers will result and the paver surface will be pristine.
A little homework on the project and attention to installation detail will produce successful results. More information can be obtained from trade organizations such as the Interlocking Concrete Pavement Institute and the Brick Industry Association as well as from manufacturers as to what works well.
Setting the Table: Preparation and Compaction of the Subgrade Preparation of the pavement foundation is one of the most important elements of every project. In terms of the labor component of the project, the subgrade and base preparation consume up to 70 percent yet is 100 percent hidden from the finished appearance. However, if improperly done, the effects become 100 percent visible!
Preparing the pavement subgrade and base can be viewed as setting the table for the rest of the job. The contractor must recognize the soil type, moisture content, grade, the amount of compaction and the type of compaction equipment required. A properly prepared subgrade will support the base above it and allow the base to distribute loads and stresses from the paver surface.
The soil type should be identified before the job starts, i.e. during the bidding stage. Different soils don?EUR??,,????'???t react the same when moist and some take longer to compact than others. Therefore, understanding the soil type during the estimating stage can assist in gaining an accurate estimate of labor hours for excavation and compaction. It can also guide selection of compaction equipment. ICPI recommends that before estimating a job, a soil sample is taken to a depth that approximates the expected total excavation depth.
How does soil type affect the job estimate? Soils range in particle size from coarse to fine grained. Sands are coarse soils, with silts and clays having the smallest or finest particles. Generally the suitability of a soil for use under a pavement decreases with its particle size. In other words, fine grained silts and clays are the least desirable. They are also the most common soil types in North America. Some clay soils hold water particularly well. This means they might be difficult to compact, especially when nearly saturated. These clays have high plasticity, meaning they hold water and drain it slowly.
Compaction mechanically increases the weight per unit volume or density of soil and base. Soils are a combination of sand, silt or clay with water and air in tiny spaces between the particles. When compacted, the air voids are greatly reduced and the particles are arranged so they fit tightly together. Compaction achieves four main purposes. It increases the load bearing capacity of the soil; prevents settlement/rutting; reduces seasonal movement from moisture changes and freeze-thaw; and helps ensure that movement during freeze and thaw cycles is uniform.
The next step after identifying a soil is understanding the optimum moisture at which the soil can achieve the highest density during compaction. Every soil has optimum moisture content. Higher or lower water content than optimum produces lower density during compaction. The optimum moisture in relation to density of a soil is tested in a soils laboratory using a standard Proctor density test. The Proctor test is named after its inventor.
The test method evaluates the density of a soil sample taken from the job site at various moisture contents and densities. The test finds the 100 percent Proctor density at its optimum moisture content. Sandy soils have lower moisture contents and higher densities than silts or clays. Once the Proctor density is established in the laboratory, tests can be done on the job site to compare the density of the compacted soil and its moisture to that established in the laboratory. The nuclear gauge test is used to rapidly check density and moisture content of compacted soil on the job site. This test is done by a technician from a soils testing laboratory.
ICPI recommends that soils and base in residential applications are compacted to a minimum of 98 percent laboratory Proctor density. This shows a typical moisture density curve for a silty clay soil. A range of moisture content is allowable to achieve 98 percent Proctor density. Generally, Proctor density tests are not needed for residential pedestrian applications. However, they are recommended for vehicular applications including residential driveways.
A simple test can find if the soil in pedestrian applications is at the right moisture content for compacting. It is called the drop test. Prior to soil compaction, remove a sample from the newly excavated subgrade surface and press it into a tennis ball sized clump. Hold the ball about two feet above a flat rigid surface and drop it. If the sample breaks into at least three or four equal size particles, it is close to optimum moisture content and ready to compact. If it breaks into many small pieces, it is too dry and water may need to be applied to the soil prior to compacting. If the ball doesn?EUR??,,????'???t break at all, it is too wet and the soil will likely need to dry prior to compacting.
Contractors measure practically everything on the job site except the most critical component of any pavement, the density of the compacted soil and base! Having a laboratory Proctor density test done with field testing on residential driveways is a small price to pay to prevent future settlement and expensive call-backs. Measuring density will help reduce liability. In addition, it will save you time if you are overcompacting. Contractors can promote this as a part of a quality job.
Soils with more than 50 percent clay or silt are called cohesive soils. Clay particles under a microscope often appear flat and elongated. They slide over each other when compacted, especially when compacted with a vibratory plate compactor. The best way to compact these types of soils is with a low amplitude vibratory roller or rammer as they effectively remove air and force the particles closer together. For heavy clays, ICPI recommends the technique of using a minimum 10,000 pounds force (44 kiloNewtons) reversible plate rammer. Adding a thin layer of base material over stable but sticky clay can reduce compaction time.
Non-cohesive soils (sands and sandy gravels) compact best with vibratory plate compactors and vibratory rollers. Under the force of these machines, particles in these soils are rearranged to fit more closely together. Contractors use large plate compactors (at least 8,000 lbf. or 35 kN) or a walk-behind vibratory roller. For much larger jobs contractors will use a ride-on vibratory roller. A ride-on double drum roller compactor with 7,000 lbf to 9,000 lbf. (30 to 40 kN) and a 32 to 36 inches minimum width or a single drum with rear rubber tires are recommended.
Sometimes during compaction soft spots will arise, often in heavy clay soils. In these cases it will be necessary to remove the soil and replace it with suitable base material or stabilize the area with cement. For isolated spots an easy solution is to mix bagged Portland cement into the soil using a pick and shovel to strengthen it. Full remediation or stabilization of soil with cement for an entire site is beyond the scope of this article.
Compaction of the subgrade sets the table for the construction of the remainder of the pavement. Contractors interested in learning more about preparation and installation of soil subgrades and bases should consider purchasing the ICPI Continuing Education unit ?EUR??,,????'??Theory of Base Installation.?EUR??,,????'?? ICPI certified contractors can earn continuing education units by taking this self-taught course. Individual continuing education programs for contractors can be purchased from ICPI.
Raleigh, North Carolina
Francisco Uviña, University of New Mexico
Hardscape Oasis in Litchfield Park
Ash Nochian, Ph.D. Landscape Architect
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