CEG 404 Ground Improvement Techniques

• CEG 404 is named Ground Improvement
• Prepared by Engr. Darwin E. Manalo

Agenda

• Introduction to ground improvement
• Improve soil behavior
• Overview of ground improvement techniques
• Importance of construction
• Potential problems

Introduction to Ground Improvement

• Altering the ground is not new
• Wattle and daub have provided tensile reinforcement to clayey materials for buildings, used for thousands of years
• Adding straw to clay and baking it in the sun improves the clay's strength properties to create a building material
• The Romans used timber as a base layer for roads
• Modern inclusions, such as geogrids and geotextiles, are commonly used for ground improvement

Treatise on Soil

• All ground improvement techniques seek to improve those soil characteristics that match project's desired results
• Ground improvement can increase density and shear strength to aid stability problems
• Ground improvement can reduce soil compressibility
• Influencing permeability can reduce and control groundwater flow
• Ground improvement can increase the rate of consolidation and improve soil homogeneity
• According to Schaefer et al. (2017), ground improvement is "the alteration of site foundation conditions or project earth structures to provide better performance under design and/or operational loading conditions"
• Compaction will increase the density of soil
• Density effects can lead to increased strength and reduced compressibility
• Second order improvements lead to increased bearing capacity and reduced settlement and/or improved liquefaction resistance

Common Alternatives to Ground Improvement

• Avoid the site or area
• Remove and replace
• Transfer load to deeper strata
• Design structure accordingly
• Owner/developer has the viable option of finding an alternative site or a different area of the same site
• If the unsuitable materials are limited in aerial and/or vertical extent, the best option may be to simply excavate and replace them with more suitable materials having more predictable properties like crushed stone
• This is also common when a localized fill is encountered
• Deep foundations like piles or drilled shafts have long been the choice where unsuitable bearing materials are near the ground surface
• Deep foundations result in load transfer via stiff structural members placed between the structure and competent bearing materials found at deeper depths
• Some sites lend themselves to structural redesign to accommodate the site conditions
• Structures can incorporate construction joints, allowing some differential settlement without causing distress

Improvement in Soil Behavior

• Ground improvement examined from the perspective of system performance
• Improve ground to increase the allowable bearing value of a footing supported on the soils beneath a structure
• Alternatives would be evaluated for their ability to increase bearing capacity and decrease settlement
• Allowable Bearing Value is increased by:
  • Increasing the stiffness of the soil which decreases settlement
  • Increasing the shear strength of the soil, which increases bearing capacity
  • Decreasing soil property variability, which decreases differential settlement
  • Densifying granular materials or consolidating cohesive materials can increase soil strength and stiffness

Fundamental Soil Behavior

• Shear strength, compressibility, hydraulic conductivity, liquefaction potential, shrink and swell behavior and reduction in variability all affect soil behavior
• Shear strength can be increased through ground improvement techniques that decrease the void ratio and/or adding a cohesive (cementing) component
• Improved shear strength causes increased bearing capacity, improved slope stability, and reduced liquefaction potential
• Compressibility depends on the nature of the load application and the initial stress state of the soil so compressibility is not a unique value
• Soil stiffness can be increased or compressibility decreased via improvement techniques that reduce void ratio or add a cohesive or cementing component
• Cohesive soil stiffness can be increased by compaction and consolidation
• Improved ground is modified to produce a zone of reduced permeability to control groundwater
• Flow beneath a dam leads to soil particle movement (piping) and/or instability.
• Construction projects require construction below grade and below the water table so construction dewatering is needed because it lowers hydraulic conductivity
• Loose granular materials are subject to liquefaction during earthquakes
• During shaking, loose granular soil deposits generally decrease in volume
• Risk of liquefaction can be mitigated by densifying the soils to reduce their liquefaction potential, groundwater control and in situ mixing
• Smectitic clays undergo substantial volume changes in response to cycles of wetting and drying
• Shrink/swell of expansive soils can damage building or a retaining wall

Reduction in Variability

• Soils' engineering properties are innately variable
• Variability affects planned structure, like excessive differential settlement
• Ground improvement can modify subsurface materials to provide a more uniform performance.

Ground Improvement Techniques

• Four defining principles in order of complexity
  • Control of water via removal or control of groundwater
  • Mechanical modification through rearrangement of soil or water particles
  • Modification using additives
  • Modification by inclusions or confinement which means system behavior modification through rigid or flexible element inclusion or soil confinement

Important Principles for Ground Improvement

• Compaction: Shallow Methods, Compaction: Deep Methods
• Soil mixing and injection methods
• Stabilization and Solidification
• Grouting and Dewatering
• Consolidation

Importance of Construction

• A ground improvement program needs full understanding of the construction means and methods
• Credit for the development of ground improvement techniques lies largely with innovative contractors

Problems

• Water content and degree of saturation significantly impact the efficacy of certain ground improvement techniques