PPP Notes - Trees and Plants

PLANTING OF TREEES AND SHRUBS

 (These notes are compiled from AGS)

TREES PLANTS AND GROUND COVERS

 

• The physical environment of the site, the design needs of the project, and the design character of the trees are all factors that must be considered in selecting trees and preparing a landscape plan for a building.
• Soil conditions (acidity, porosity) at the site, the amount and intensity of sunlight and precipitation, and the seasonal temperature range in the area create the physical environment in which trees must be able to survive.
• In addition, it is essential to consider how the location and topography of the site will direct the wind, resulting in cold winds and cooling breezes that can affect the health of trees.
• Trees can be used to address the design needs of a project by directing pedestrian or vehicle movement, framing vistas, screening objectionable views, and defining and shaping exterior space.
• Trees can also be used to modify the microclimate of a site and to help conserve building energy use from heating, cooling, and lighting systems.
• The design character of the trees themselves plays a part in which species are best suited for a particular application.
• The shape of a tree can be columnar, conical, spherical, or spreading, and the resulting height and mass will change over time as the tree matures.
• Some trees grow quickly, and others more slowly, and their color and texture varies from coarse to medium to fine, affecting their character.
• The appearance of deciduous trees changes with the seasons, while the effect of an evergreen remains relatively constant.

 

PHYSICAL CHARACTERISTICS

 

• CROWN: The head of foliage of a tree
• LEAVES: The foliage unit of a tree that function primarily in food manufacturing by photosynthesis.
• ROOTS: Anchors a tree and helps hold against soil erosion.
• ROOT HAIRS: Absorb minerals from the soil moisture and send them as nutrient salts in the sapwood to the leaves.
• HEARTWOOD: Nonliving central part of a tree, gives strength and stability.
• ANNUAL RINGS: Reveal the age of a tree by showing yearly growth.
• OUTER BARK: Aged inner bark that protects tree from desiccation and injury.
• INNER BARK (PHLOEM): Carries food from leaves to branches, trunk, and roots.
• CAMBIUM: Layer between xylem and phloem where cell-adding growth occurs, new sapwood to inside and new inner bark outside.
• SAPWOOD (XYLEM): Carries nutrients and water to leaves from roots.

 

GLARE PROTECTION

 

• Trees protect viewer from glare of surfaces such as water, paving and glass.
• The vertical angle of the sun changes seasonally; therefore, the area of a building subject to the glare of reflected sunlight varies.
• Plants of various heights can screen sun (and artificial light) glare from adjacent surfaces.

 

AIR INFILTRATION

 

• Large masses of plants physically and chemically filter and deodorize the air, reducing air pollution.
• Particulate matter trapped on the leaves washes to the ground during rainfall.
• Gaseous pollutants are assimilated by the leaves.
• Fragrant plants can mechanically mask fumes and odors.  Also, these pollutants are chemically metabolized in the photosynthesis process.

 

WIND PROTECTION

 

• Shelterbelt (providing trees) wind protection reduces evaporation at ground level, increases relative humidity, lowers the temperature in summer and reduces heat loss in winter, and reduces blowing dust and drifting snow.
• The amount of protection afforded is directly related to the height and density of the shelterbelt.

 

SHADE PROVISION

 

• In summer, trees obstruct or filter the strong radiation from the sun, cooling and protecting the area beneath them.
• In winter, evergreen trees still have this effect, whereas deciduous trees, having lost their leaves, do not.

 

SOUND ATTENUATION

 

• A combination of deciduous and evergreen trees and shrubs reduces sound more effectively than deciduous plants alone.
• Planting trees and shrubs on earth mounds increases the attenuating effects of a buffer belt.

 

RUNOFF REDUCTION

 

• Leaves and branches are coated with thin films of water, holding it from running off.
• Branch structure channels water to dry area under the tree to be absorbed.
• Roots absorb water runoff from branches.
• Mature trees absorb or delay runoff from stormwater, and general design considerations are the topics addressed in this section.

 

PLANTING DETAILS

 

• Planting details for trees and shrubs, tips on soil improvement and general design considerations are topics to be addressed by all design professionals.

 

TREE PLANTING DETAILS

 

• These three guidelines will aid in the successful planting of trees:
• For container-grown trees, use fingers or small hand tools to pull the roots out of the outer layer of potting soil; then cut or pull apart any roots circling the perimeter of the container.  Incorporate commercially prepared mycorrhiza spores in the soil immediately around the root ball at rates specified by the manufacturer.
• During the design phase, confirm that water drains out of the soil; design alternative drainage systems as required.
• Thoroughly soak the tree root ball and adjacent prepared soil several times during the first month after planting, and regularly throughout the following two summers.
• Note that the planting process is similar for deciduous and evergreen trees.

 

SHRUB PLANTING DETAILS

 

• For successful shrub planting, follow these guidelines:
• For container-grown shrubs, use fingers or small hand tools to pull the roots out of the outer layer of potting soil; then cut or pull apart any roots that circle the perimeter of the container.  Incorporate commercially prepared mycorrhiza spores in the soil immediately around the root ball at rates specified by the manufacturer.
• Confirm that water drains out of the soil during the design phase; design alternative drainage systems as required.

 

SOIL IMPROVEMENT

 

• The quality of soil available for planting varies widely from site to site, especially after construction activity has occurred. 
• The nature of construction results in compaction, filling, contamination, and grading of the original soil on a site, rapidly making it useless for planting.
• Previous human activity at a site can also affect the ability of the soil to support plants.
• During the design phase, assumptions must be made regarding the probably condition of the soil after construction is complete.
• The health of existing or remaining soil determines what types of soil preparation will be required and the volume of soil to be prepared.
• Conditions will vary from location to location within a project, and details must be condition-specific.
• For large projects or extreme conditions, it is useful to consult an expert experienced in modifying planting soils at urban sites.
• To ensure good soil health at a project site, follow these guidelines:
• Whenever possible, connect the soil improvement area from tree to tree.
• Always test soil for pH and nutrient levels, and adjust these as required.
• Loosen soil with a backhoe or other large coarse-tilling equipment, when possible.  Tilling that produces large, coarse chunks of soil is preferable to tilling that results in fine grains uniform in texture.
• Make sure that the bottom of planting soil excavations is rough; to avoid matting of soil layers as new soil is added.  It is preferable to till the first lift (2 to 3 in.) of planting soil into the subsoil.

 

CONSTRUCTION AROUND EXISTING TREES

 

• Great care should be taken not to compact, cut, or fill the earth within the crown area of existing trees.
• Most tree roots are located in the top 6 to 18 in. of the soil, and often spread considerably farther than the drip line of the tree.
• Compaction can cause severe root damage and reduce the movement of water and air through the soil.
• To avoid compacting the earth, do not operate equipment or store materials within the crown spread.
• Before construction begins, inject the soil within the crown area of nearby mature trees with commercially prepared kelp-based fertilizer and mycorrhiza fungus developed to invigorate tree roots.
• Prune tree roots at the edge of the root save area, as roots pulled during grading can snap or split well into the root save area.
• Rot and disease that enters dying roots in compacted or filled areas can move into the tree if root pruning has not been carried out.
• Install tree protection fencing and silt protection at the limits of construction activity near trees.
• During construction, apply additional water in the canopy area to compensate for any root loss beyond the crown spread.
• Have all mature trees inspected by a certified arborist before construction begins, to identify any special problems.
• Remove all deadwood, and treat all trees for existing insect and disease problems.
• When possible, begin fertilization and problem treatments at least one full growing season before construction.
• Removal of significant portions of the crown will affect the health of a tree by reducing its ability to photosynthesize in proportion to the mass of its trunk. 
• Younger, healthier trees withstand construction impacts better than older trees.

 

GENERAL RANGE OF SOIL MODIFICATIONS AND VOLUMES

 

• GOOD SOIL (NOT PREVIOUSLY GRADED/COMPACTED)
• Min. Width – 6’ or twice the width of the root ball, whichever is greater.
• COMPACTED SOIL (NOT PREVIOUSLY GRADED)
• 15’
• GRADED SUBSOILS AND CLEAN FILLS WITH 5-35% CLAY
• 20’
• POOR-QUALITY FILLS, HEAVY CLAY SOILS, SOILS CONTAMINATED
• 20’

 

ROOT PRUNE TRENCH

 

• A root prune trench severs roots with a clean cut, protecting remaining roots from cracking, rot, and disease.
• Typically 3” or more in width and 18-24” in depth.
• Root prune trenches are typically cut with rock saws or trenchers, and filled after with soil.

 

UNDERGROUND UTILITY LINES NEAR EXISTING TREES

 

• Fewer roots are severed by tunneling under a tree than by digging a trench beside it.

 

TREE AND ROOT PROTECTION

 

• If construction operations must take place within the crown spread area, install 6 in. of wood chips on top of the soil to protect it.
• Use plywood matting over mulch in areas where equipment must operate. 
• Protect the trunk of the tree with planking loosely cabled around the tree to reduce scarring by equipment.
• Remove planking, matting, and mulch as soon as operations are finished. 
• A barrier such as that illustrated can keep construction equipment and personnel from compacting the soil around tree roots.

 

TREE PLANTING IN URBAN AREAS

 

• Traditional urban designs in which trees are regularly spaced in small opening within paved areas generally result in poor tree performance because such designs generally do not provide adequate soil for root growth, and ignore the fact that trees must significantly increase trunk size every year.
• Moreover, competition for space, both at ground level and below, is intense in urban areas.
• Although it is possible to design uncompacted soil volumes for trees under pavement, this is very expensive and the soil is never as efficient as that in open planting beds.
• Increasing trunk size can only be accommodated by using flexible materials that can change configuration over time.
• Urban designs that have flexible relationships between trees, paving, and planting beds and large areas of open planting soil offer the best opportunity for long-term tree health and lower maintenance costs.
• Areas of dense urban development leave little room for tree roots to develop.
• Large areas of pavement, competition with foundations and utilities for space belowground, and extensive soil compaction and disruption limit the amount of soil available for trees.
• When the area of ground around the tree is open to the rain and sun is less than 400 to 500 sq. ft. per tree, the following design guidelines should be followed to encourage the growth of large healthy trees.
• Five major parts of the tree structure must be accommodated in the design process:
• CROWN GROWTH: The tree crown expands every growing season at a rate of 6 to 18 in. per year.  Once the crown reaches a competing object such as a building or another tree canopy, the canopy growth in that area slows and then stops.  Eventually the branches on that side of the tree die.  As the canopy expansion potential is reduced, the overall growth rate and tree health are also reduced.
• TRUNK GROWTH: The tree trunk expands about ½ to 1 in. per year.  As the tree increases in size, the lower branches die and the trunk lengthens.  Tree trunks move considerably in the wind, especially during the early years of development, and are damaged by close objects.
• TRUNK FLARE: At the point at which the trunk leaves the ground, most tree species develop a pronounced swelling or flare as the tree matures.  This flare grows at more than twice the rate of the main trunk diameter and helps the tree remain structurally stable.  Any hard object placed in this area, such as a tree grate or confining pavement, will either damage the tree or be moved by the tremendous force of this growth.
• ZONE OF RAPID ROOT TAPER: Tree roots begin to form in the trunk flare and divide several times in the immediate area around the trunk.  In this area, about 5 to 6 ft. away from the trunk, the roots rapidly taper from about 6 in. in diameter to about 2 in.  Most damage to adjacent paving occurs in this area immediately around the tree.  Keeping the zone of rapid taper free of obstructions is important to long-term tree health.  Once a tree is established, the zone of rapid taper is generally less susceptible to compaction damage than the rest of the root zone.
• ROOT ZONE: Tree roots grow radially and horizontally from the trunk and occupy only the upper layers (12 to 24 inches) of the soil.  Trees in all but the most well-drained soils do not have taproots.  A relationship exists between the amount of tree canopy and the volume of root-supporting soil required.  This relationship is the most critical factor in determining long-term tree health.  Root-supporting soil is generally defined as soil with adequate drainage, low compaction, and sufficient organic and nutrient components to support the tree.  The root zone must be protected from compaction both during and after construction.  Root zones that are connected from tree to tree generally produce healthier trees than isolated root zones.

 

SOIL VOLUME FOR TREES

 

• There’s a linear relationship between the soil volume required and the ultimate tree size.
• The ultimate tree size is defined by the project size of the crown and the diameter of the tree at breast height.  For example, a 16-in. diameter tree requires 1000 cu ft. of soil.

 

SOIL MODIFICATIONS

 

• To improve the soil’s ability to retain water and nutrients:
• Thoroughly till organic matter into the top 6 to 12 inches of most planting soils.  (Do not add organic matter to soil more than 12 in. deep).  Use composted bark, recycled yard waste, peat moss, or municipal processed sewage sludge.  All products should be composted to a dark color and be free of pieces with identifiable leaf or wood structure.  Recycled material should be tested for pH and certified free of toxic material by the supplier.  Avoid material with a pH higher than 7.5.
• Modify heavy clay or silt soils (more than 40% clay or silt) by adding composted pine bark (up to 30% by volume) and/or gypsum.  Coarse sand may be used if enough is added to bring the sand content to more than 60% of the total mix.
• Improve drainage in heavy soils by planting on raised mounds or beds and including subsurface drainage lines.
• Modify extremely sandy soils (more than 85% sand) by adding organic matter and/or dry, shredded clay loam up to 30% of the total mix.

 

SOIL VOLUME – REQUIREMENT FOR TREES

 

• Soil volume provided for trees in urban areas must be sufficient for long-term maintenance.
• There must be adequate drainage and room to grow for trees.

 

SOIL VOLUME – INTERCONNECTION

 

• The interconnection of soil volumes from tree to tree has been observed to improve the health and vigor of trees.

 

SOIL PROTECTION FROM COMPACTION OR DEGRADATION

 

• Coarse plantings keep pedestrians out of planters.
• Curbs protect planters from pedestrians and deicing salts.
• Underground steam lines must be insulated or vented to protect planter soil.

 

VISUALLY SYMMETRICAL TREES

 

• If visually symmetrical tree planting is required, symmetrical soil volumes are also required to produce trees of similar crown size.

 

TREE GUARDS

 

• Tree guards can protect young trees from trunk damage caused by bicycles.
• If made too small, however (less than 30 in. in diameter), they can damage the tree as it grows and are difficult to remove.
• The high cost and potential harm to trees outweigh the minor protection tree guards afford a trunk.
• They should be used only in areas with particularly high traffic.

 

SELECTING PLANTS FOR ROOFTOP PLANTING

 

• When choosing plants for a rooftop setting, consider the factors outlined here:
• WIND TOLERANCE: Higher elevations and exposure to wind can cause defoliation and increase the transpiration rate of plants.  High parapet walls with louvers can reduce wind velocity and provide shelter for plants.
• HIGH EVAPORATION RATE: The drying effects of wind and sun on the soil in a planter reduce soil moisture rapidly.  Irrigation, mulches, and moisture-holding soil additives (diatomaceous earth or organic matter) help reduce this moisture loss.
• RAPID SOIL TEMPERATURE FLUCTUATION: The variation in conduction capacity of planter materials results in a broad range of soil temperatures in planters of different materials.  Cold or heat can cause severe root damage in certain plant species.  Proper drainage helps alleviate this condition.
• TOPSOIL: Improve topsoil in planters to provide optimum growing conditions for the plants selected.  A general formula calls for adding fertilizer (determined by soil testing) and one part peat moss to five parts sandy loam topsoil.  More specific requirements for certain varieties of plants or grasses should be considered.
• ROOT CAPACITY: Choose plant species carefully, considering their adaptation to the size of the plant bed.  If species with shallow, fibrous roots are used instead of species with a coarse root system, consult with a nursery advisor.  Consider the ultimate maturity of the plant species when sizing a planter.

 

ROOFTOP PLANTING DETAILS

 

• There are five factors when designing rooftop planting:
• SOIL DEPTH: Minimum soil depth in a planter varies with the plant type: for large trees, the soil should be 37 inches deep or 6 inches deeper than the root ball; for small trees, 30 inches deep; for shrubs, 24 inches deep; and for lawns, 12 inches deep (10 in. if irrigated).
• SOIL VOLUME: To determine sufficient soil volume, refer to the linear relationship between soil volume and ultimate tree size (graph provided in text).
• SOIL WEIGHT: The saturated weight of normal soil mix ranges from 100 to 120 pcf, depending on soil type and compaction rate.  Soils can be made lighter by adding expanded shale or perlite.  Soils lighter than 80 pcf cannot provide structure adequate to support trees.
• DRAINAGE FABRIC: Plastic drainage material should be a minimum of ½ in. thick.  Most drainage material comes with a filter fabric attached, but the overlap joints provided are not wide enough for the unconsolidated soils found in planters.  A second layer of woven filter fabric, delivered in rolls greater than 10 ft. in width, should be installed.  Tuck the fabric over the exposed top of the drainage material to keep soil out of the drainage layer.
• INSULATION: Most planters do not require insulation; however, in colder climates planters with small soil volumes located over heated structures may require insulation.  Consult local sources for a list of cold-hardy plants.

PPP Notes - Basement Excavation

BASEMENT CONSTRUCTION

 (These notes are compiled from AGS)

BASEMENT EXCAVATION

 

TRENCHING

 

• Trenches are narrow vertical cuts in soil used to place utilities and to construct continuous foundations.
• For short periods of time, many trenches may appear to be stable, but then collapse suddenly.
• Unsupported trenches can be dangerous to workers, as the early-warning signs of a trench collapse cannot be seen from the bottom of the trench.
• Moreover, in a narrow trench, there is nowhere to escape, if a sudden collapse of the trench wall occurs.
• And, because 1 to 2 cu. ft. of soil weighs as much as 100 to 250 lbs., even a relatively small collapse can severely injure or kill a worker.
• For safety purposes, therefore, OHSA requires trenches more than 5 ft. deep be supported with shoring, or protected with a trench box, before allowing worker access.

 

LATERAL SUPPORT

 

• Walls that extend to depths of 10 to 15 ft. can generally be self-supported or cantilevered and do not require additional lateral support.
• This is accomplished by extending the shoring sufficient depth below the excavation.
• These “cantilevered” walls resist the lateral pressures from the soil and groundwater, although some deflection at the top of the wall should be expected.
• However, walls that extend deeper or have significant surcharge loads, or where deflection is a concern, will require some type of additional lateral support (known as “braced walls”).

 

INTERNAL SUPPORT

 

• The lateral support can be internal to the excavation, in the form of wales, struts, or rakers.
• Internal bracing is very effective but interferes with the construction of the permanent structure.

 

EXTERNAL SUPPORT

 

• External bracing of the excavation is usually in the form of drilled tieback (ground) anchors.
• The use of tieback anchors to support the wall enables a clear and unencumbered excavation, but requires access to areas beyond the building, which may extend outside the property line.
• For this reason, tieback anchors are almost always the most desirable method of lateral support if the owner controls the property around the excavation, or if temporary easements can be obtained from private owners or public municipalities.

 

SOIL STABILIZATION

 

• Soil stabilization is sometimes called soil modification or soil conditioning.
• For deep excavations in very soft soils, the construction of earth support systems can be quite massive and expensive because of the higher lateral soil and hydrostatic pressures.
• Furthermore, in some situations, wall installation can be difficult to accomplish because of space requirements.
• Various techniques have been developed to improve the effective strength of resistance of the soils around and below the excavations.
• Improving the in-situ soil strength or resistance reduces or eliminates the net pressure at the face of the excavation.
• The underground construction industry has developed the following techniques for improving or reinforcing soil.

 

SOIL-MIXING

 

• Soil mixing stabilization is the mixing the soils in the ground with asphalt, cement, lime, fly ash, or lime-fly ash to stabilize and strengthen the soil.
• Often, only a selected portion of soil mass must be treated to provide a significant increase in overall soil strength or resistance.

 

PRESSURE GROUTING SOIL STABILIZATION

 

• Pressure freezing uses the natural moisture in the soil and artificial cooling methods to freeze and harden the soil.
• Soil particles are then “glued” together to form a hardened solid mass of grouted soil.
• The spacing of the injection points and the grout mixture is varied to achieve a specific pattern and grout coverage, depending on the requirements of the project.

 

GROUND FREEZING

 

• Ground freezing uses the natural moisture in the soil and artificial cooling methods to freeze and harden the soil.
• The ambient temperature of nearly all soil is above freezing (except for permafrost regions).
• Lowering the soil temperatures to below freezing causes the water in the pores of the soil to freeze and “cement” the soil particles together.
• This results in a very hard, impermeable condition.
• However, soil has a relatively low thermal conductivity.
• Liquid Nitrogen may be used to freeze the soil in a timely manner.
• A continual flow of a brine solution through a series of embedded refrigeration pipes must be used to maintain the soil in a frozen state.

 

SOIL NAILING

 

• Soil nailing is a method used for both temporary excavation bracing and for permanent retaining walls.
• This technique employs closely spaced, high-strength steel anchors grouted into the soil, and may include a reinforced shotcrete facing.
• The major advantages of soil nail wall construction over more traditional excavation bracing methods include relatively low cost and the ability to construct the wall system from the top down, as the excavation proceeds.
• A typical installation sequence for grouted soil nailing is as follows:
• 1. Excavation begins by exposing a cut about 3 to 5 ft. in depth.
• 2. A borehole (typically 4 to 6 in. in diameter) is drilled into the face of the excavation at a downward angle of approximately 15 degrees to the horizontal.  The length of the borehole is dependent on the height of the cut and the nature of the material exposed in the excavation.  Typical lengths range from 60 to 70 percent of the wall height.
• 3. A high-strength, threaded reinforcing bar is inserted into the borehole, and then the borehole is grouted to the excavation face.
• 4. After the grout has cured, a wire mesh is placed over the exposed face of the cut, and reinforcement is placed to span over the borehole and reinforcing bar.  When groundwater is a concern, a geosynthetic drainage mat is typically placed against the soil face to intercept water and direct it to the base of the wall.
• 5. The exposed excavation face is sprayed with shotcrete, typically 8 to 10 in. thick.
• 6. A bearing plate is fitted over the reinforcing bar, and a nut is screwed into place to tension the soil nail.
• 7. If the method is to be used as a permanent wall, a second application of shotcrete is used to cover the soil nail head and bearing plate.
• 8. After completion of the first level, the excavation extends downward an additional 4 to 6 ft., and the process is repeated.

 

BACKFILL AND COMPACTION

 

FILL AND BACKFILL

 

• Fill is typically used to raise or level site grades.
• Backfill is used to fill in spaces around below-grade structural elements, such as around basement walls.
• The fill must have sufficient strength or resistance and low compressibility to support its own weight and any other overlying structure pavements, floor slabs, foundations, etc, without excessive settlement.
• When soils are excavated, they become loosened and disturbed.
• If they are suitable for reuse as structural fill or backfill, the soils must be placed in thin layers and compacted to achieve the required strength, resistance and stability.

 

COMPACTION

 

• Compaction is the process by which mechanical energy is applied to a soil to increase its density.
• The degree to which soil can be densified depends on the amount and type of compactive effort, type of soil, and moisture content.
• Soil is made up of solids and the void spaces between the solid particles.
• The void space almost always contains some water.
• If the water completely fills the void, the soil is considered to be totally saturated.
• During compaction, the total volume of the soil is decreased by reducing the volume of voids, while the volume of solids remains essentially unchanged.
• If the soil is saturated or nearly saturated during compaction, water must be expelled to decrease the void space.

 

MOISTURE-DENSITY RELATIONSHIP

 

• Nearly all soil exhibits a defined moisture-density relationship for a specific level of compactive effort.
• These relationships can be graphed in a nearly bell-shaped curve, with the maximum density at the apex, corresponding to the optimum moisture content.
• Standard laboratory tests, such as the Standard Proctor (ASTM D 698) and Modified Proctor (ASTM D 1557), use a standard-size mold and a specific level of compactive energy to develop the moisture density curve for a specific soil.
• The maximum density from these curves defines the 100 percent level of compaction for a given soil.
• Compaction requirements for fill and backfill are generally specified as a percentage of the maximum density, typically between 90 and 95 percent, as determined using one of the standard laboratory tests mentioned above.
• The low and high moisture contents are usually represented as a horizontal line connecting opposite sides of the Proctor curve for a given density.
• This represents the range of moisture content within which the soil can be compacted most readily.

 

FIELD DENSITY TESTS

 

• Field density tests are performed on compacted soil to verify that a specific level of compaction has been achieved. 
• There are several methods for determining the in-place density of soil.
• Today, the most commonly used method involves the nuclear density gauge, a radioactive source material to determine the soil density and moisture content.
• The test is performed at the surface without any excavation, and results can be obtained faster than with other test methods.

 

MOISTURE CONTROL

 

• Moisture content of the fill and backfill should be near the optimum moisture content.
• Otherwise, the minimum required field density is very difficult (or impossible) to obtain, no matter how much energy is used for compaction.
• If the soil is too wet, the water in the pores cannot be expelled fast enough to allow for a sufficient decrease in volume.
• If the soil is too dry, the capillary forces around the soil particles are too large to be broken down by the compactive energy.
• Therefore, controlling the moisture of the fill and backfill to within specific limits near the optimum moisture content is necessary to achieve the required level of compaction.

 

SOIL TYPE AND COMPACTION EQUIPMENT

 

• To be most effective, the compaction equipment must match the type of soil to be compacted.
• In general, compaction equipment can be divided into two basic groups: rollers or plates.
• Rollers come in large variations in size, but all use a weighted wheel or drum to impart energy to the soil.
• In addition, some rollers use an electric rotor to vibrate the drum, thereby increasing the energy to the soil.
• Other drums have protrusions called sheepsfoots, which impart a kneading action to the soils.
• Some plate compactors also use vibratory energy to compact the soils, while other plate compactors, called tampers, move up and down, imparting a vertical dynamic load to the soil.
• In general, coarse-grained granular soils such as sands and gravels are more easily compacted than fine-grained soils such as clays and clayey silts.
• Vibratory energy is very effective is very effective in densifying sands and gravels, since the interparticle bonds are relatively weak.
• When the granular soils are vibrated at the correct frequency, the soil particles rearrange themselves into a denser state under their own weight and the weight of the compactor.
• Vibratory steel drum rollers and vibratory plate compactors are considered the most effective compaction equipment for granular soils.
• Fine-grained soils hold more moisture and have higher internal interparticle forces.
• Vibratory energy is much less effective for these soils.
• Clayey soils require more mechanical energy to break down the internal forces during compaction.
• The kneading action of a sheepsfoot roller is very effective in this regard.
• Fine-grained soils generally have a narrower range of moisture contents for optimum compaction.
• Often, the clayey soils are too wet to compact and the moisture content must be reduced.
• Reducing the moisture content in clay is typically done by allowing water to evaporate from the surface of the clay.
• The rate of evaporation is dependent on the ambient temperature and wind conditions.
• However, the drying process can be enhanced by using a process called aeration in which steel discs are used to periodically turn the soil, thus exposing more of the soil to the atmosphere.