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.