12.2 Hydraulic stabilization (dewatering, preloading, vertical drains)

Dewatering and preloading are key ground improvement techniques. They remove water from soil and apply temporary loads to increase stability. These methods make construction sites safer and more workable, especially in soft or waterlogged areas.

Vertical drains and hydraulic stabilization further enhance soil properties. By speeding up water drainage and injecting stabilizing materials, these techniques transform weak soils into strong foundations. They're crucial for building on challenging sites.

Dewatering Techniques for Ground Improvement

Principles and Methods of Dewatering

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• Dewatering removes groundwater or surface water from construction sites improving soil stability and facilitating excavation or construction activities
• Lowers groundwater table below excavation level reducing hydrostatic pressure and improving soil strength
• Common techniques include wellpoints, deep wells, eductor wells, and horizontal drains suited for different soil types and project requirements
• Selection of appropriate method depends on factors such as:
  • Soil permeability
  • Excavation depth
  • Groundwater conditions
  • Environmental constraints
• Requires careful design and monitoring to prevent:
  • Excessive settlement
  • Soil erosion
  • Adverse effects on nearby structures

Applications and Effectiveness Assessment

• Applications encompass foundation construction, tunneling, open-pit mining, and environmental remediation projects
• Effectiveness assessed through:
  • Piezometric measurements tracking changes in water pressure
  • Flow rate monitoring quantifying water removal
  • Settlement observations during construction process (using settlement plates)
• Successful dewatering results in:
  • Improved workability of excavated areas
  • Enhanced soil stability for construction activities
  • Reduced risk of hydraulic failure or bottom heave in deep excavations

Preloading for Soil Stabilization

Process and Mechanics of Preloading

• Ground improvement technique applying temporary surcharge load to accelerate soil consolidation and improve engineering properties
• Process involves:
  1. Placing fill material (soil, rock, or water in geotextile containers) on area to be improved
  2. Maintaining load for specified period determined by soil's consolidation characteristics
  3. Monitoring settlement through settlement plates and piezometers
• Induces settlement by forcing water out of soil voids increasing soil density and shear strength
• Particularly effective for soft, compressible soils (clays and organic soils) where conventional compaction methods may be ineffective
• Can be combined with vertical drains to accelerate consolidation process and reduce required preloading time

Benefits and Applications

• Reduced long-term settlement minimizing future structural issues
• Increased bearing capacity allowing for higher structural loads
• Improved soil stability enhancing overall site conditions for construction
• Applications include:
  • Preparation of building sites on soft soils
  • Highway embankments over compressible ground
  • Reclamation projects in coastal areas
  • Industrial facilities on marginal land
• Preloading duration varies from weeks to months depending on soil characteristics and project requirements

Vertical Drains for Soil Consolidation

Function and Design of Vertical Drains

• Geosynthetic materials (prefabricated vertical drains or wick drains) installed vertically in soft soils to accelerate consolidation
• Provide shorter drainage paths for pore water reducing time required for excess pore water pressure dissipation
• Typical composition:
  • Plastic core allowing water flow
  • Geotextile filter preventing soil particle clogging
• Spacing and depth determined based on:
  • Soil properties (permeability, compressibility)
  • Desired consolidation time
  • Project requirements (load, allowable settlement)
• Often used with preloading to significantly reduce consolidation time (from years to months)

Installation Methods and Performance Assessment

• Installation methods:
  • Mandrel-driven installation using hollow mandrel to push drain into ground (most common)
  • Vibratory methods for denser soils
• Performance assessed through:
  • Pore water pressure measurements using piezometers
  • Settlement monitoring during and after installation using settlement plates or extensometers
• Effectiveness factors:
  • Proper drain spacing ensuring optimal drainage
  • Adequate installation depth reaching full thickness of compressible layer
  • Quality of drain material preventing clogging or damage during installation

Hydraulic Stabilization Effectiveness Analysis

Hydraulic Stabilization Techniques

• Use water or other fluids to improve soil properties through modification of soil structure or chemical composition
• Common techniques:
  • Jet grouting creating large-diameter soil-cement columns (effective in cohesive soils)
  • Permeation grouting injecting low-viscosity grout (suited for granular soils with higher permeability)
  • Compaction grouting using high-viscosity grout to densify surrounding soil
• Effectiveness highly dependent on:
  • Soil permeability
  • Grain size distribution
  • In-situ stress conditions
• Site conditions influencing selection and implementation:
  • Groundwater levels
  • Soil contamination
  • Proximity to existing structures

Evaluation and Environmental Considerations

• Success evaluated through:
  • In-situ testing (cone penetration tests, standard penetration tests)
  • Laboratory testing of treated soil samples
  • Field load tests on treated areas
• Environmental considerations:
  • Potential for groundwater contamination from grout materials
  • Disposal of spoil material generated during grouting process
  • Impact on local hydrogeology and groundwater flow patterns
• Monitoring requirements:
  • Grout take and pressure during injection
  • Ground heave or settlement in surrounding areas
  • Water quality testing in nearby wells or water bodies