CEG 404: Ground Improvement Introduction

• CEG 404 is a ground improvement course taught by Engr. Darwin E. Manalo

Agenda

• Introduction to Ground Improvement
• Improvement in Soil Behavior
• Overview of Ground Improvement Techniques
• Importance of Construction
• Potential Problems

Introduction to Ground Improvement

• Ground Modification is not a new concept.
• Wattle and daub has been used to provide tensile reinforcement to clayey materials in buildings for thousands of years.
• Adding straw to clay and baking improved the clay's strength, creating a building material used for thousands of years.
• The Romans used timber as a base layer for roads.
• Geogrids and geotextiles are commonly used for modern-day ground improvement.
• Ground improvement techniques improve soil characteristics to match project results, such as:
  • Increased density and shear strength for stability
  • Reduced soil compressibility
  • Controlled permeability to reduce groundwater flow
  • Rapid consolidation rate
  • Improved soil homogeneity
• Ground improvement alters site foundation conditions or project earth structures to provide better performance under design and/or operational loading conditions (Schaefer et al., 2017).
• First-order improvements like compaction increase soil density.
• Second-order effects include increased strength and decreased compressibility.
• Third-order effects are increased bearing capacity and reduced settlement/improved liquefaction resistance.
• Alternatives to ground improvement:
  • Avoiding the site
  • Removing and replacing soil
  • Transferring load to deeper strata
  • Designing the structure accordingly

Alternatives to Ground Improvement

• Owners/developers can find an alternative site or area as a viable option regarding the proposed facility's location.
• Excavating unsuitable soils and replacing them with more suitable materials like crushed stone is a common choice when a localized fill is encountered.
• Deep foundations, such as piles or drilled shafts, transfer loads through stiff structural members to competent bearing materials at deeper depths when unsuitable materials are near the surface.
• Specific sites and structures may lend themselves to structural redesign to accommodate site conditions, and also incorporate construction joints, allowing some differential settlement without causing distress.

Improvement in Soil Behavior

• Ground improvement may increase the allowable bearing value of a footing supported on soils beneath a structure.
• Ground improvement alternatives are evaluated for their ability to increase bearing capacity and decrease settlement.
• Increasing soil stiffness decreases settlement.
• Increasing soil shear strength increases bearing capacity.
• Decreasing soil property variability decreases differential settlement.
• Densifying granular or consolidating cohesive materials can increase soil strength and stiffness

Fundamental Soil Behavior Characteristics

• Shear strength can be increased using ground improvement techniques that decrease the void ratio and/or add a cohesive (cementing) component.
• Benefits include increased bearing capacity, improved slope stability, and reduced liquefaction potential.
• Compressibility is not a unique value; it depends on the load application and soil's initial stress state.
• Soil stiffness can be increased, reducing compressibility, by reducing the void ratio or adding a cohesive/cementing component via compaction and consolidation.
• Improved ground has reduced permeability to control detrimental effects of groundwater.
• Flow beneath a dam can lead to soil particle movement and instability.
• Construction dewatering is required for projects frequently below grade and the water table.
• Loose granular materials below the groundwater level can be subject to liquefaction upon the application of a dynamic load, such as during an earthquake.
• Loose granular soil deposits decrease in volume during shaking.
• Mitigation aims to densify soils, reducing their liquefaction potential.
• Other techniques include groundwater control and in-situ mixing.
• Soils containing smectitic clays undergo substantial volume changes with wetting and drying cycles.
• The shrink/swell behavior can damage buildings or cause retaining walls to fail.
• Physical and engineering properties of soils are naturally variable, affecting the performance of planned structures.
• Variability in compressibility can lead to excessive differential settlement
• Ground improvement modifies subsurface material to provide more uniform performance.

Overview of Ground Improvement Techniques

• Four defining principles, in order of increasing complexity, are:
  • Control of water: removal or control of groundwater
  • Mechanical modification: rearrangement of soil or water particles
  • Modification by additives: addition of chemicals
  • Modification by inclusions or confinement: system-behavior modification via rigid or flexible element inclusion or soil confinement.
• Important principles, engineering considerations, and construction methods in ground improvement
  • Compaction: Shallow and Deep Methods
  • Soil mixing and injection methods
  • Stabilization and Solidification
  • Grouting
  • Dewatering
  • Consolidation

Importance of Construction

• Designing a ground improvement program requires a full understanding of construction means and methods.
• Credit for development of ground improvement techniques largely lies with innovative contractors.
• Many experts in ground improvement are or were contractors.

Problems

• Water content and saturation significantly impact certain ground improvement techniques' efficacy.
• The effect of water content may be related to building sandcastles on the beach.