Soil Compaction and Improvement Techniques: A Comprehensive Guide
Compaction
Compaction is a mechanical process that reduces voids within the soil mass, forcing solid particles into closer contact. It increases soil resistance and decreases compressibility. The required maximum density should be 95% of modified proctor density, and layers typically do not exceed 30 cm.
Factors Determining Compaction Degree
- Effect of Water: Increased moisture content and compaction energy result in higher density, as water acts as a lubricant.
- Compaction Energy: Increasing compaction energy for the same soil dry weight increases volume and decreases optimum moisture.
- Soil Type: Characteristics such as specific gravity, texture, particle shape, and plasticity influence compaction.
Compressibility
Compressibility defines the stress-strain characteristics of soil. Applying effort to soil leads to volume changes and movement, which can cause foundation settlement.
Densification
Densification is the in situ compaction of soils, primarily granular, to increase density.
Consolidation
Consolidation is the process of reducing the volume of fine cohesive soils (clays and plastic silts) due to stresses (loads) over time. Vertical subsidence can occur in buildings if consolidation is significant.
Settlement
Settlement is similar to consolidation but occurs in granular soils.
CBR
CBR (California Bearing Ratio) measures shear strength under specific compaction and moisture conditions. It involves determining moisture and density, swelling properties, and penetration resistance.
Chemical Stabilization
Chemical stabilization aims to enhance soil strength and durability, prevent water ingress, reduce volume change potential, and improve soil management.
Types of Chemical Stabilization
- Portland Cement Stabilization: Used in all organic soil less.
- Lime Stabilization: Effective for clay soils, reducing liquid limit and plasticity index, increasing compressive strength and CBR.
- Bituminous Stabilization: Used in granular soils to form cohesive structures and waterproofing, but excessive use can lead to unstable mixtures.
- Stabilization with Salt: Improves compressive strength and reduces compaction optimum moisture in thin soils.
Soil Improvement (Stabilization)
Soil improvement involves modifying soil properties to enhance its behavior. It can be temporary or permanent and may include:
- Increasing density
- Adding materials for chemical or physical changes
- Lowering the water table
- Removing or replacing unsuitable soils
Mechanical Stabilization
- Compaction
- Drainage
- Soil stabilization burden
Chemical Stabilization
- Injections
- Addition of lime or cement
- Asphalt products
- Stabilization with salt
- Inorganic acid
- Organic enzymes for cohesive soil stabilization
- Additives
Compaction Equipment
Kickstand Roller (Compaction by Kneading)
Suitable for cohesive soils, it mixes and can be towed by a tractor. Benefits include energy distribution, clay breaking, good adhesion between layers, and mixing.
Smooth Roller (Compaction Pressure)
Used for relatively clean sands and gravels, it compacts from top to bottom. It reduces compaction effect in expanded clays and silts, and works in thicknesses from 10 to 20 cm.
Pneumatic Roller (Compaction Pressure)
Suitable for sandy soils with thin plastic and plastic silts. It ensures good adhesion between layers and compacts thicker layers faster. Tires allow uniform stress distribution.
Dynamic Rollers (Compaction by Vibration)
Used in granular soils, they combine vibration and pressure to reduce internal friction and settle particles.
Drains
Drains protect structures from water damage by gravity, pumping, or external load compression.
Vertical Sand Drains
Used in foundations, they accelerate soil consolidation and increase resistance to cutting.
Wick Drains
Linked to soil preloading, they quickly remove water and accelerate consolidation.
Preload
Preload is a static compaction method that applies static loads to overconsolidate soil for future loads. It reduces settlement and secondary consolidation phenomena, but can be time-consuming.
Vibro Flotation (Sand)
Effective for compacting areas with low carrying capacity, it combines ground vibration and saturation. It reduces settlement and increases resistance to liquefaction.
Injections
Injections aim to cement soil or rock to increase mass resistance or fill pores and cracks to reduce compressibility and permeability.
Jet Grouting
Forms soil columns enhanced by injection, introducing a binder material at high speed. It can create walls and seals and improve sealing on screens or piles.
Soil Surveys
Soil surveys determine the basic data of the terrain that will carry structural loads. They provide information on water table location, stratification, layer thickness, and soil properties.
Field Study Methods
- Recognition: Visual inspection, aerial photography, geological reports, previous work data
- Scanning: Geophysics, electrical, wells, sampling and testing, drilling
Field Tests
- Penetration test
- Vane test
- Groundwater level and pore pressure measurement
- Pump test
- Loading
- Compaction
Sample Types
- Altered Samples: Vary in density and moisture, used for geological and geotechnical classification.
- No Change Samples: Represent natural soil conditions, retaining density and structure.
Recognition Techniques
- Creating Pits: Simple and economical, but limited to shallow depths.
- Boreholes: Extract samples from affected and unaffected strata, identifying strength and properties throughout the survey.
- Penetrometer: Field tests to estimate carrying capacity, but do not provide information about strata.
In Situ Tests
- Pinwheel Vane: Measures cutting force in soft or altered clays.
- Plate Load: Determines resistance-strain characteristics, soil capacity for settlement, and load-strain curve coefficients.