Engineering Geology and Geotechnics - Lecture 7: Soil Improvement

Recap - Settlements

• Calculation of total settlement $(s<em>T, \rho</em>T)$.

  • ${s<em>T = s</em>0 + s<em>1 + s</em>2}$

  • $\rho<em>T = \rho</em>i + \rho<em>c + \rho</em>s$

  • $s<em>0$, $\rho</em>i$ = 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.

  • $s<em>1$, $\rho</em>c$ = 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.

  • $s<em>2$, $\rho</em>s$ = 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):

  • $s<em>0 = \mu</em>0 \mu<em>1 \frac{q</em>n B}{E_u} (1-\nu^2)$

    • $q_n$ − 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

    • $S<em>c = \frac{C</em>c}{1+e<em>0} H</em>0 log \frac{\sigma<em>0' + \Delta\sigma'}{\sigma</em>0'}$

• 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: $\sigma<em>0' + \Delta\sigma' \leq \sigma</em>c'$

  • $S<em>c = \frac{C</em>s}{1+e<em>0} H</em>0 log \frac{\sigma<em>0' + \Delta\sigma'}{\sigma</em>0'}$

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

  • $\sigma<em>0' \rightarrow \sigma</em>c'$

  • $\sigma<em>c' \rightarrow \sigma</em>0' + \Delta\sigma'$

  • $S<em>c = \frac{C</em>s}{1+e<em>0} H</em>0 log \frac{\sigma<em>c'}{\sigma</em>0'} + \frac{C<em>c}{1+e</em>0} H<em>0 log \frac{\sigma</em>0' + \Delta\sigma'}{\sigma_c'}$

Recap - Secondary settlement

• $C<em>\alpha = \frac{\Delta e}{log \frac{t</em>2}{t_1}}$

  • $\Delta e$ 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)

• $s<em>2 = \frac{H C</em>\alpha}{1 + e<em>0} log \frac{t</em>2}{t_1}$

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):

  • $\frac{R}{A'} = q<em>u = cN</em>c s<em>c i</em>c b<em>c + \gamma</em>1 D N<em>q s</em>q i<em>q b</em>q + \frac{1}{2} \gamma<em>2 B N</em>\gamma s<em>\gamma i</em>\gamma b_\gamma$

• Bearing capacity equation (undrained):

  • $\frac{R}{A'} = q<em>u = (\pi + 2) c</em>u s<em>c i</em>c b<em>c + \gamma</em>1 D$

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.