Engineering Geology and Geotechnics - Lecture 7: Soil Improvement

Recap - Settlements

  • Calculation of total settlement (sT, \rhoT).

    • {sT = s0 + s1 + s2}

    • \rhoT = \rhoi + \rhoc + \rhos

    • s0, \rhoi = Immediate settlement

      • Occurs rapidly during the application of load.

      • Elastic deformation with no change in water content.

      • Quite small quantity in dense sands/gravels and stiff/hard clays.

    • s1, \rhoc = Consolidation settlement

      • Occurs slowly in low permeability soils.

      • Decrease in voids volume as pore water is squeezed out of the soil.

      • Volume change.

      • Only significant in clays and silts.

    • s2, \rhos = Secondary settlement

      • Occurs very slowly, long after consolidation is completed.

      • Due to gradual changes in the particulate structure of the soil.

      • Most significant in soft organic soils and peats.

Recap - Immediate settlement

  • The average value of immediate settlement under a flexible area carrying a uniform pressure (Janbu et al, 1956):

    • s0 = \mu0 \mu1 \frac{qn B}{E_u} (1-\nu^2)

      • q_n − net foundation pressure

      • B − width or diameter of foundation

      • E_u − undrained modulus of elasticity of the soil

      • \mu_0 − correction factor for depth of excavation

      • \mu_1 − correction factor for thickness of soil layer below excavation

      • \nu − Poisson's ratio

Recap - Consolidation settlement

  • Normally consolidated clays (clays that have never experienced the vertical pressure before).

    • If the initial vertical stress is \sigma_0' and it increases by \Delta\sigma', then

      • Sc = \frac{Cc}{1+e0} H0 log \frac{\sigma0' + \Delta\sigma'}{\sigma0'}

  • Over-consolidated clays (clays that have experienced some vertical pressure in the past).

    • Preconsolidation pressure \sigma_c'

Recap - Consolidation settlement (Over-consolidated clays)

  • Case 1: \sigma0' + \Delta\sigma' \leq \sigmac'

    • Sc = \frac{Cs}{1+e0} H0 log \frac{\sigma0' + \Delta\sigma'}{\sigma0'}

  • Case 2: \sigma0' + \Delta\sigma' > \sigmac'

    • \sigma0' \rightarrow \sigmac'

    • \sigmac' \rightarrow \sigma0' + \Delta\sigma'

    • Sc = \frac{Cs}{1+e0} H0 log \frac{\sigmac'}{\sigma0'} + \frac{Cc}{1+e0} H0 log \frac{\sigma0' + \Delta\sigma'}{\sigma_c'}

Recap - Secondary settlement

  • C\alpha = \frac{\Delta e}{log \frac{t2}{t_1}}

    • \Delta e or \Delta H

    • where

      • t_1 - reference time (e.g. 1 year as construction time)

      • t_2 - time after which settlement required (e.g. design life of structure)

  • s2 = \frac{H C\alpha}{1 + e0} log \frac{t2}{t_1}

Lecture Outline and Shallow Foundations

  • Introduction to Earth.

  • Weathering.

  • Geological mapping.

  • Geological structures.

  • Bearing capacity.

  • Settlement.

  • Soil improvement.

Lecture Outline - Soil Improvement

  • Introduction to soil improvement.

  • Methods for soil improvement.

General form of bearing capacity equation

  • Bearing capacity equation (drained):

    • \frac{R}{A'} = qu = cNc sc ic bc + \gamma1 D Nq sq iq bq + \frac{1}{2} \gamma2 B N\gamma s\gamma i\gamma b_\gamma

  • Bearing capacity equation (undrained):

    • \frac{R}{A'} = qu = (\pi + 2) cu sc ic bc + \gamma1 D

Soil Improvement

  • Even with a good shallow foundation design, the soil properties underneath may not bear the load transferred by the superstructure.

  • Soil improvement relates to the use of techniques and methods to improve the general soil properties.

  • Objectives:

    • Increase shear strength.

    • Reduce compressibility.

    • Reduce permeability.

    • Improve ground water condition.

  • Understanding the ground is crucial to finding the most suitable soil improvement solution.

  • Soil improvement relates to the use of techniques and methods to improve the general properties of the soils.

Techniques and Methods

  • Ground improvement:

    • Surface compaction.

    • Deep dynamic compaction.

    • Compaction grouting.

    • Drainage/surcharge.

    • Increasing soil density.

    • Removing air voids, pore water.

    • Changing soil water properties.

  • Ground reinforcement:

    • Stone columns.

    • Soil nails.

    • Micro piles.

    • Jet grouting.

    • Ground anchors.

    • Geosynthetics.

    • Fiber reinforcement.

    • Soil and reinforcing materials act as a system where reinforcing elements take the majority of loads.

  • Ground treatment:

    • Cement.

    • Fly ash.

    • Lime admixtures.

    • Changing soil properties by adding soil, fly ash, cement, chemicals, etc.

Techniques and Methods for Soil Improvement

  • Removal and replacement.

  • In-situ densification.

  • Preloading.

  • Vertical drains.

  • Grouting.

  • Reinforcement.

  • Stabilization using admixtures.

Removal and Replacement

  • Involves removing and/or replacement of soil.

  • One of the oldest and simplest methods.

  • Soils that will have to be replaced include contaminated soils and/or organic soils.

  • Method is usually practical only above the groundwater table.

In-situ Densification

  • Most effective in sands.

  • Methods used in conventional earthworks are only effective to about 2 m below the surface.

  • In-situ methods like dynamic deep compaction are for soils deeper than can be compacted from the surface.

    • Load: Dynamic (Impact) / Vibration

In-situ Densification - Surface compaction

  • Sand: Smooth-wheel roller, Pneumatic roller.

  • Clay: Sheepsfoot roller.

  • Smooth-wheel roller: Granular.

  • Sheepsfoot roller: Fine-grained.

  • Pneumatic roller: Granular or fine-grained.

In-situ Densification - Dynamic compaction

  • A heavy steel or concrete weight is dropped repeatedly on the ground in a grid pattern.

  • The weight is dropped from a height of up to 10 meters.

  • The imprints left by the weight are filled with granular material.

  • The process is repeated two or three times, depending on the soil type and condition.

In-situ Densification - Shallow/deep compaction

  • Long probe, mounted onto a vibratory pile driver, lowered into the ground.

  • Cohesionless granular soils.

  • Penetration usually helped by water jetting.

  • Backfill is added and compacted while the vibrator is gradually being removed.

In-situ Densification - Vibro stone columns

  • A vibrator penetrates the ground to the desired depth using the vibrator’s weight, vibrations, and air jets located in the tip.

  • The bore is filled with aggregates and compacted to create a dense column.

  • Bottom Feed Process: In bottom-feed process, the stone is fed to the vibrator tip through an attached feed pipe.

  • Top Feed Process.

Preloading

  • Applies a temporary surcharge load to a soil mass to accelerate consolidation.

  • Forces water out of the soil voids, leading to increased soil density and shear strength.

  • Once sufficient consolidation has taken place, the fill can be removed, and construction can take place.

  • Surcharge fills are typically 3 - 8 m thick and generally produces settlement of 0.3 – 1 m.

  • Particularly effective for soft, compressible soils like clays and organic soils.

Preloading - Advantages/Disadvantages

  • Advantages

    • Requires only conventional earthmoving equipment.

    • Long track record of success.

  • Disadvantages

    • Transport of large quantities of soil required.

    • Surcharge must remain in place for months or years, thus delaying construction.

  • Preloading using concrete blocks (after Phipps-Speckman, 2018)

Vertical Drains

  • Vertical drains are installed under a surcharge load to accelerate the drainage of impervious soils and thus speed up consolidation.

  • These drains provide a shorter path for water to get away from the soil.

  • Time to drain clay layers can be reduced from years to a couple of months. (BS EN 15237:2007)

Vertical Drains - Prefabricated vertical drains

  • Geosynthetics installed by being pushed or vibrated into the ground.

  • Most are about 100 mm wide and 5 mm thick.

  • Installation:

    • Driving mandrel + drain

    • Anchoring drain and extracting mandrel

  • Typically spaced 3 m.

Grouting

  • Involves the injection of grout into the soil to fill voids, compact the ground, and mitigate soil settlement.

  • Types of grouts:

    • Cementitious grouts (e.g., Portland cement).

    • Chemical grouts (e.g., synthetic polymers, silicates, or resins).

Grouting Methods

  • Permeation grouting: low-viscosity grout is injected into the soil, filling voids without disturbing the soil structure, e.g., waterproofing underground structures (e.g., basements).

  • Compaction grouting: uses a high-pressure injection of low-mobility cementitious grout to displace and compact loose soil. Resulting in a series of very stiff grout columns surrounded by the soil of increased density, e.g., sinkhole repair.

  • Jet grouting: a high-pressure jet of grout, water, and air is used to break up soil and mix it with grout to form a solidified column, e.g., Creating deep soil stabilization columns.

Grouting Methods (Continued)

  • Fracture grouting (Hydrofracture Grouting): grout is injected under high pressure, creating controlled fractures to improve soil stability, e.g., creating barriers to control groundwater flow.

  • Compensation grouting: grout is injected in stages beneath existing structures to compensate for settlement, e.g., protecting historic or sensitive structures during underground construction.

Reinforcement

  • Soil is stronger in compression than in tension.

  • To improve strength in tension, geosynthetics can be placed in soil. (Leao et al. 2012)

Reinforcement - Soil nailing

  • Involves inserting steel bars (nails) into the soil at an angle and securing them with grout and facing materials

  • Stabilize slopes, embankments, and excavations.

Stabilization using Admixtures

  • Admixtures are additives mixed with soil to improve its strength, durability, workability, and performance.

  • They modify soil properties to make it more stable, less permeable, and more resistant to environmental factors such as moisture and temperature changes.

  • Types of admixtures:

    • Cementitious admixtures (e.g., Portland cement)

    • Chemical admixtures

Techniques and Methods for Soil Improvement

  • Removal and replacement.

  • In-situ densification.

  • Preloading.

  • Vertical drains.

  • Grouting.

  • Reinforcement.

  • Stabilization using admixtures.

Summary

  • All these soil improvement methods mentioned can be applied in combination.

  • Consider using soil improvement techniques when designing foundations as the cost can be much lower than redesigning the foundation of structures.

  • The more you understand about soil and rock behaviour the more creative you can be when designing your next shallow foundation or any other geotechnical structure.