Foundation Soil Improvements

Soil improvement (ground improvement) is the process of improving the strength, stiffness or permeability of poor soil, or reducing its settlement and liquefaction potential, so that it can carry loads safely and economically.

Conditions or purposes

• To increase the bearing capacity and reduce settlement of foundation soil.
• To control permeability and seepage (dewatering, grouting).
• To reduce liquefaction potential in earthquake zones.
• To stabilise slopes, excavations and embankments on soft ground.
• To improve soil under roads, runways and airfields.

Methods

1. Compaction: surface rolling, dynamic compaction (heavy tamping), vibroflotation, compaction piles; for granular and loose soils.
2. Preloading (surcharge) with vertical drains: sand drains, prefabricated vertical drains (PVD) accelerate consolidation of soft clay.
3. Replacement and excavation: removal of poor soil and replacement with compacted granular fill.
4. Stone columns and sand compaction piles: densify and reinforce soft or loose soil.
5. Grouting: injection of cement, bentonite or chemical grout to fill voids, reduce permeability and increase strength.
6. Chemical (admixture) stabilisation: lime, cement, fly ash, bitumen mixed with soil.
7. Dewatering and drainage: well points, deep wells, electro-osmosis.
8. Reinforced soil / geosynthetics: geotextiles, geogrids, reinforced earth walls.
9. Soil nailing and anchoring.
10. Thermal methods: ground freezing (temporary) and heating.

Preloading is a soil improvement method in which a temporary load (surcharge), usually an earth fill, is placed on soft compressible soil before construction, so that most of the settlement takes place before the structure is built.

Method

1. The site is cleared and instrumented (settlement plates, piezometers).
2. A fill equal to or greater than the future structure load (a surcharge, often 1.2 to 1.5 times) is placed over the area.
3. The load is left in place until the required consolidation (generally 90%) is reached. Settlement is monitored, and the surcharge is removed, after which the structure is built with small additional settlement.
4. In thick clay layers the drainage path is long, so vertical drains (sand drains or prefabricated band drains) are installed to shorten it; the time of consolidation is $t = \dfrac{T_vH^2}{c_v}$ and reduces with the square of the drainage path.

   Surcharge fill
   /////////////////
  =================  sand blanket
   | | | | | | |     vertical drains
  ~ soft clay ~~~~

Advantages: simple, cheap, and increases the strength of the clay. Limitation: takes a long time, and needs space and a fill material.

Sand compaction piles are columns of compacted sand formed in loose sandy or soft clayey soil. A casing is driven or vibrated into the ground, filled with sand, and withdrawn in stages while being re-driven to compact the sand. The surrounding soil is densified and the sand pile increases the strength and acts as a drain, reducing settlement and liquefaction potential. They are also used in soft clay.

Stone columns are vertical columns of compacted crushed stone or gravel (size 20 to 75 mm) installed in soft clay or loose silty soil, usually by vibro-replacement (a vibrator is jetted into the ground and the hole is backfilled with stone in lifts) or by a driven casing (rammed stone column). Diameters are 0.6 to 1.0 m, spaced at 1.5 to 3 m in a triangular or square grid.

Functions: they increase the bearing capacity, reduce settlement, speed up consolidation by acting as drains, and reduce the liquefaction potential. The capacity of a column depends on the lateral confinement of the soil: $q_{ult} \approx \sigma_{r}K_p$ where $\sigma_r$ is the limiting radial stress in the soft soil (bulging failure).

Soil improvement techniques include compaction, preloading with vertical drains, stone columns, grouting, chemical stabilisation, replacement, dewatering, geosynthetic reinforcement and soil nailing. Two of them are explained below.

1. Compaction (dynamic compaction and vibroflotation)

Compaction densifies loose granular soil by reducing its voids using mechanical energy, which increases the strength and reduces settlement and liquefaction potential.

• Surface compaction: rollers (smooth, sheepsfoot, vibratory) compact thin layers of fill to a specified density at optimum moisture content.
• Dynamic compaction: a heavy weight (10 to 40 t) is dropped from 10 to 30 m onto the ground in a grid, which compacts the soil to depths of 5 to 10 m.
• Vibroflotation: a vibrating probe (vibroflot) is sunk by water jet into the sand; the sand around it is densified and the hole is backfilled with sand/gravel.
• Suitable for loose sand and gravel with little fines. Quality is checked by SPT or CPT after compaction.

2. Preloading with vertical drains

For soft saturated clay, a surcharge is placed over the area to force the water out of the clay and speed up the consolidation settlement before the structure is built.

• A sand blanket is laid on the surface, and vertical sand drains or prefabricated band drains are installed at 1.5 to 3 m spacing to shorten the drainage path, so consolidation is completed in months rather than years.
• The fill (surcharge) is left until about 90% consolidation, monitored by settlement plates and piezometers, and then removed.
• It also increases the shear strength of the clay, and is used under embankments, roads, airport runways, tanks and warehouses.

Admixture stabilization is the improvement of soil strength, durability and volume stability by mixing a suitable additive (admixture) into the soil and compacting the mixture. The additive changes the soil properties by chemical reaction, cementing action or by filling voids.

Common admixtures and their action:

• Cement: hydration forms a cementing gel that binds particles. Best for sands, gravels and low-plasticity silts (soil-cement, 5-12% cement by weight).
• Lime: cation exchange and flocculation reduce plasticity; pozzolanic reaction gives long-term strength. Best for clays (usually 3-8% lime).
• Fly ash: a pozzolana, used with lime or cement to give extra strength.
• Bitumen: waterproofs and binds granular or fine-grained soils.
• Chemicals (calcium chloride, sodium silicate): reduce water loss or bind particles.

Procedure: pulverise the soil, spread the admixture, mix with water to optimum moisture content (OMC), compact to maximum dry density, and cure.

Results: higher strength and CBR, lower plasticity, swelling and compressibility, and better resistance to water. It is widely used in subgrades, road bases and under light foundations.

Soil stabilization is the process of improving the engineering properties of soil (strength, bearing capacity, permeability, volume stability) by mechanical or chemical means.

Mechanical methods

1. Mechanical (granular) stabilization: blending two or more soils of different gradation, such as coarse sand or gravel with clay, to get a well-graded mixture with higher density and strength.
2. Compaction: densifying the soil by rolling, tamping or vibration in layers at OMC. Rollers (smooth wheel, sheepsfoot, pneumatic) are used for surface layers.
3. Deep compaction: dynamic compaction, vibroflotation and compaction piles are used for deep loose sands.
4. Preloading, drainage and reinforcement: surcharge loading, sand drains, and geotextiles or geogrids.

Chemical methods

1. Cement stabilization: cement hydration binds particles. It is suitable for sandy soils.
2. Lime stabilization: lime reduces plasticity and swelling and gives pozzolanic strength gain. It is suitable for clays.
3. Bituminous stabilization: bitumen or cutback/emulsion waterproofs and binds the soil particles.
4. Chemical admixtures: calcium chloride, sodium silicate and fly ash are mixed for special effects.
5. Grouting: cement or chemical grout is injected into voids or fissures to bind and seal the soil.

Mechanical methods change the physical arrangement of particles, while chemical methods change the bonding between particles.

Soil stabilization improves strength and reduces settlement of weak soils. Two common deep-improvement methods are given below.

Preloading

Preloading means placing a temporary load (surcharge fill) on the site before construction, so most of the consolidation settlement of soft clay occurs beforehand.

• The fill is kept in place until the expected settlement is achieved, then removed and the structure is built.
• Vertical sand drains or prefabricated vertical drains (PVD) are often provided to shorten drainage paths and speed consolidation.
• Settlement is monitored with settlement plates and piezometers.
• Suitable for soft clays, silts and organic soils where time is available. Disadvantage: needs time and space.

Stone columns

Stone columns are vertical columns of compacted crushed stone (gravel) installed in soft clay or loose silty soil.

• A hole is made by a vibrating probe (vibro-replacement) or by driving a casing, then filled with stone in lifts and compacted.
• The stiff columns carry most of the load and also act as drains.
• They increase bearing capacity, reduce settlement and reduce liquefaction potential.
• Typical diameter is 0.6-1.0 m at 1.5-3 m spacing.

Bituminous stabilization is the improvement of soil by mixing it with bituminous material (bitumen, cutback or emulsion) so that it becomes waterproof and cohesive.

• In cohesionless soils (sand, gravel): the bitumen coats the particles and binds them together, giving cohesion and strength.
• In cohesive soils (clay): the bitumen waterproofs the soil clumps, so they do not absorb water and lose strength or swell.

Procedure: pulverise the soil, add the bitumen (about 4-7% for sand and 5-10% for fine-grained soils), mix thoroughly, spread, compact and allow volatile solvents to evaporate (curing).

Materials used: cutback bitumen, bitumen emulsion, tar.

Advantages: waterproofing, good durability, and suitable for road bases and subgrades. Limitation: not economical for deep or large works and is affected by high temperature.

Grouting is the injection of a fluid material (grout) under pressure into soil voids, fissures or cavities. The grout then sets or gels, binding the particles, reducing permeability and increasing strength.

Types of grout:

• Cement grout: cement-water mixture for coarse sands, gravels and fissured rock.
• Clay or bentonite grout: for sealing and filling voids.
• Chemical grout: sodium silicate, acrylamide or resins, which have low viscosity and penetrate fine sands.
• Bituminous grout: used for sealing.

Method: holes are drilled or pipes driven to the required depth, and grout is pumped in stages at controlled pressure until refusal, so that the grout permeates the soil without fracturing it.

Advantages:

1. Increases bearing capacity and reduces settlement.
2. Reduces permeability, so it controls seepage under dams and in excavations.
3. Can be done in confined spaces, with little vibration or disturbance to existing structures.
4. Underpins existing foundations without excavation.
5. Fast and works at any depth.

Dynamic compaction is a ground improvement method where a heavy weight (10-40 t) is repeatedly dropped from a great height (10-40 m) on the ground surface in a grid pattern. The impact energy densifies loose granular and fill soils to considerable depth.

Mechanical compaction (surface): smooth-wheeled rollers, sheepsfoot rollers, pneumatic-tyred rollers, vibratory rollers, and rammers or tampers.

Dynamic (deep) compaction: dynamic compaction (heavy tamping), vibroflotation, vibro-compaction, blasting, compaction piles, and vibro-replacement (stone columns).

Vibroflotation is a deep compaction method for loose clean sands in which a cylindrical vibrating probe (vibroflot) densifies the soil to depths of up to about 20-30 m.

Procedure:

1. The vibroflot is lowered into the ground with the help of water jets at its tip, which loosen the soil and make a hole.
2. After reaching the required depth, the jets are shut off, and the vibrating action starts.
3. Sand or gravel is added from the top to fill the hole, while the vibroflot is withdrawn slowly in steps.
4. The vibration rearranges the grains into a denser state, forming a compacted column of about 2-3 m diameter.
5. Points are repeated in a grid pattern.

It is suitable for sands with less than about 15-20% fines. It increases relative density, bearing capacity and resistance to liquefaction.

The purposes of foundation soil improvement are:

1. To increase the bearing capacity and shear strength of weak soil.
2. To reduce total and differential settlement of structures.
3. To reduce permeability and control seepage.
4. To control the swelling and shrinkage of expansive soils.
5. To reduce liquefaction potential of loose saturated sands during earthquakes.
6. To improve slope stability and reduce lateral earth pressure.
7. To make the site suitable for shallow foundations, avoiding costly deep foundations.