Soil Exploration & Stabilization

Objectives

Soil exploration (site investigation) determines the subsurface conditions needed for safe, economical design:

• sequence, thickness and extent of soil and rock strata;
• ground water level and its variation;
• engineering properties — strength, compressibility, permeability — from samples and in-situ tests;
• suitable type and depth of foundation, bearing capacity and settlement;
• construction problems (excavation support, dewatering, aggressive chemicals) and suitability of borrow materials.

Stages

1. Reconnaissance — desk study of geological maps, aerial images, previous records; site visit to see topography, drainage, existing structures and cracks, vegetation, cuttings, wells.
2. Preliminary exploration — a few borings and tests to establish general stratigraphy.
3. Detailed exploration — enough borings, sampling and tests for design; sometimes construction-stage checks.

Planning

• Spacing of borings depends on variability and structure type; for example about 15–30 m apart for multi-storey buildings and closer for heavy or sensitive structures; along roads and pipelines at larger intervals (hundreds of metres).
• Depth of exploration — until the stress increase from the structure becomes negligible (about 10% of the applied pressure or 20% of the effective overburden), commonly about 1.5 to 2 times the width of isolated footings or the loaded area; for pile foundations, well beyond the pile tips (at least about 1.5 times the pile-group width below the tips); into rock where rock is met, to prove it is bedrock and not a boulder (a few metres of coring).
• Critical zones — expected compressible layers, weak seams, water table.

Methods of exploration

Samples and samplers

• Disturbed samples — structure destroyed but mineral composition and (if sealed) water content preserved; used for classification, index tests, compaction.
• Undisturbed samples — structure and water content substantially preserved; needed for shear strength, consolidation and permeability tests. Truly undisturbed sampling is impossible; disturbance is minimised by design.

Rock Quality Designation (RQD): sum of lengths of core pieces ≥ 100 mm divided by total length of the core run × 100%. RQD < 25% very poor; 25–50 poor; 50–75 fair; 75–90 good; 90–100 excellent.

Standard Penetration Test (SPT)

Corrections

1. Overburden pressure correction (granular soils) — N at shallow depth is too low because confining stress is low. A common form (Peck, Hanson & Thornburn):

2. Dilatancy correction (very fine or silty sand below the water table, apply after overburden correction when the value exceeds 15):

3. Energy (hammer efficiency) correction to $N_{60}$ is used in modern practice.

Correlations

N also correlates with $\varphi$ of sands (roughly 28° at N = 5 to about 40° at N = 50) and is used in IS 6403 / IS 8009 bearing capacity and settlement charts.

Why improve ground

Where soil is too weak, compressible, permeable or variable, engineers can replace it, bypass it with deep foundations, or improve it in place. Ground improvement modifies soil to:

• increase shear strength and bearing capacity;
• reduce compressibility and settlement (and speed up consolidation);
• reduce permeability (seepage control) or increase drainage;
• reduce swelling/shrinkage and liquefaction potential;
• improve workability and durability of subgrades and fills.

Soil stabilization (mainly for roads, airfields and fills)

Mechanical stabilization

Mixing soils of different gradations to obtain a well-graded, dense mixture, then compacting at OMC. Proportioning is by grading limits (e.g. Rothfutch's graphical method) and plasticity limits of the fines. Example: adding clay binder to a clean sand or gravel to form a stable soil–aggregate base.

Cement stabilization (soil–cement)

• Portland cement (typically about 5–15% by weight) mixed with pulverised soil, compacted and cured.
• Cement hydrates and binds particles → strength, durability, reduced plasticity.
• Best for granular and low-plasticity soils (sandy soils); less effective in highly plastic clays (difficult mixing) and organic soils (retard hydration).

Lime stabilization

• Hydrated lime (typically about 2–8%) mixed with clayey soils.
• Immediate effects: cation exchange and flocculation → reduced plasticity index, improved workability, reduced swell.
• Long-term effects: pozzolanic reaction between lime and clay silica/alumina → cementing compounds → strength gain over months.
• Ideal for expansive black cotton soils and wet clay subgrades.

Bituminous stabilization

Bitumen (cutbacks or emulsions) coats particles, providing cohesion in sands and waterproofing in cohesive soils.

Fly ash, chemicals and others

• Fly ash (with lime or cement) — pozzolanic; used in embankments and subgrades (also reuses waste).
• Calcium chloride / sodium chloride — retain moisture, reduce dust on unpaved roads, lower frost point.
• Sodium silicate, polymers, enzymes, resins — special applications.
• Thermal stabilization — heating (reduces plasticity of clays) or ground freezing (temporary support/water cut-off during excavation).

Densification of granular soils

Improvement of soft clays

Clays cannot be densified by vibration; their improvement relies on consolidation or reinforcement.

Preloading (precompression)

Apply a surcharge (earth fill) equal to or greater than the future structural load before construction, so that consolidation settlement occurs in advance. The surcharge is removed before building. Slow in thick clays — so combined with drains.

Vertical drains

Clay consolidates mainly by radial flow to closely spaced drains, reducing the drainage path from half the layer thickness to half the drain spacing:

• Sand drains — boreholes filled with sand (commonly 200–450 mm diameter at 1.5–5 m spacing).
• Prefabricated vertical drains (PVDs / wick drains) — plastic core wrapped in filter fabric, about 100 mm × 4 mm, installed by mandrel at close spacing (about 1–3 m); faster and cheaper than sand drains.
• Designed with Barron's radial consolidation theory; combined degree of consolidation $1-U=(1-U_v)(1-U_r)$.
• Smear zone (disturbance around the drain) and well resistance reduce efficiency.

Vacuum consolidation

A sealed membrane over the drains with vacuum pumps applies atmospheric pressure as preload — no heavy fill, no stability problem of a high embankment.

Electro-osmosis

Direct current drives pore water towards cathodes in fine-grained soils; used for special cases (e.g. slope stabilization in silts).

Reinforcement and inclusions

• Stone columns (granular piles) — vertical columns of compacted crushed stone (commonly 600–1000 mm diameter) formed by vibro-replacement or rammed methods in soft clays and loose silty sands; they reinforce (carry more load due to stiffness), act as drains (speed consolidation) and reduce liquefaction risk. Designed per IS 15284; capacity limited by bulging of the column in the top few diameters.
• Lime columns, deep soil mixing (cement/lime columns), jet grouting columns.
• Micropiles for underpinning.

Grouting

Injection of fluid materials into soil or rock voids and fissures:

Groutability ratio (for granular soils) $=\frac{D_{15}\text{ (soil)}}{D_{85}\text{ (grout)}}$ — should be more than about 25 for successful permeation.