Construction Earthwork and Excavation Techniques
Geotechnical Testing and Site Preparation
Geotechnical Soil Testing
Soil testing (geotechnical) is performed through test borings to determine foundation pressure and bearing capacity. Samples are taken across the site and summarized in a comprehensive geotechnical report.
Subgrade Preparation and Layout
Subgrade preparation involves several critical steps:
- Clearing the site and removing organic topsoil.
- Excavating to the required depth.
- Grading and compacting the soil to the proper density.
- Replacing soft or unstable soil with competent material.
Laying out the site uses property pins and setbacks. Surveyors stake corners and offsets (typically a 3–5 foot overbuild of the pad) and establish final pad elevations.
Soil Compaction Principles
Soil compaction mechanically increases density to improve stability, enhance load-bearing capacity, and prevent future settlement. Compaction effectiveness depends on soil type, moisture content, and the required density.
Compaction Methods
Compaction is achieved using two primary forces:
- Static Force: Machine weight compresses the upper layers of soil.
- Vibratory Force: Engine-driven blows compact deeper layers.
Compaction Equipment
Equipment used includes:
- Smooth or sheepsfoot rollers
- Vibrating plate compactors
- Rammers
- Grid rollers
- Front-end loaders
Soil Classification and Earth Filling
Soil Classification by Grain Size
Soil is classified by grain size and characteristics:
- Cohesive (Clay, Silt): Small particles; plastic when wet, hard when dry. Requires vibration for compaction and is generally undesirable for foundations.
- Granular (Sand, Gravel): Good drainage and little plasticity. Stable and preferred for foundations.
- Organic (Topsoil, Compost): Poor material for building due to instability and decomposition.
Cutting, Filling, and Fill Types
Cutting removes vegetation and soil to reach required elevations, while filling raises elevations or replaces unsuitable soil. Filling is done in compacted lifts (usually 6 inches or less).
Types of fill include:
- Common Fill: Inexpensive, often clay-heavy.
- Select Fill: A controlled clay-sand mixture.
- Sand/Gravel: Can be placed in thicker lifts (up to 12 inches) but must be contained by foundations or retaining structures.
Moisture Control and Field Density Testing
Moisture is vital to achieving proper compaction. Too little moisture prevents particle bonding, while too much moisture creates water-filled voids and weakens the soil structure.
Laboratory Density Tests
The Proctor Test and the Modified Proctor Test determine the maximum achievable density and the optimal moisture relationships for a given soil type.
Field Density Tests
Field density tests confirm that the required compaction has been met:
- Sand Cone Test: Measures the volume of the excavated hole using calibrated sand.
- Balloon Densometer: Measures the volume of the hole using a fluid-filled balloon.
- Shelby Tube: Extracts an undisturbed soil sample for laboratory analysis.
- Nuclear Gauge: Uses gamma rays to quickly determine both density and water content in the field.
Excavation Equipment and Support Methods
Common Excavation Equipment
- Excavators: Large machines capable of 360-degree rotation.
- Backhoes: Smaller machines, typically limited to about 200 degrees of rotation.
- Skid Steers: Compact and versatile machines with various attachments, available with tracks or wheels.
- Chain Trenchers: Specialized equipment used to cut narrow, deep trenches.
Excavation Support Systems
Support methods are necessary to prevent trench or pit collapse:
- Sloped or Benched Excavation: The cheapest method, but requires significant space.
- Soldier Piles and Lagging: Uses steel H-sections (piles) driven into the ground, with timber or concrete panels (lagging) placed between them.
- Sheet Piling: Interlocking walls made of timber, steel, or concrete driven into the ground.
- Cross-Bracing: Internal support using horizontal walers and diagonal rakers.
- Tiebacks: Steel cables drilled into stable soil or rock behind the excavation, grouted, and tensioned.
- Soil Mixing: Cement and water are injected to strengthen soil columns before excavation begins.
- Slurry Walls: Bentonite slurry is used to prevent trench collapse during digging; the slurry is later replaced with concrete.
Dewatering and Cofferdams
Dewatering removes groundwater or rainwater using pumps to maintain a dry work area. Cofferdams are watertight enclosures constructed around wet areas, which are then pumped dry to allow construction below the water line.