Introduction

Soil mechanics is the branch of engineering mechanics that describes the behaviour of soil under stress, water flow and loading. Almost every civil structure rests on, in, or is built of soil, so its design depends on soil behaviour.

Fields of application

• Foundations: finding the bearing capacity and settlement of footings, rafts and piles so a building, bridge or tower does not fail or settle too much.
• Earth structures: design of earth dams, embankments, canal banks and road/rail embankments, using soil as a construction material (compaction, permeability, strength).
• Retaining structures: earth pressure on retaining walls, sheet piles, basement walls and braced excavations.
• Slope stability: natural hill slopes, cuttings, and embankment slopes, which is vital in the hilly terrain of Nepal where landslides are common.
• Pavements: subgrade strength (CBR), drainage and frost/swelling effects for highways and airfields.
• Seepage and groundwater: flow below dams and weirs, uplift pressure, piping, dewatering of excavations.
• Underground structures: tunnels, culverts, pipelines and shafts, which need lateral earth pressure and ground movement estimates.
• Ground improvement and earthquake engineering: compaction, drainage and stabilisation; liquefaction of loose saturated sands during earthquakes.

Without soil mechanics, structures would be designed by guesswork, leading to excessive settlement, tilting, slope failure or collapse.

Soil is formed by the weathering (breakdown) of parent rock by physical, chemical and biological agents. The rock breaks into smaller particles that are either left in place or carried away and deposited elsewhere.

1. Physical (mechanical) weathering

Rock is broken into smaller pieces without change in its mineral composition. Causes:

• Temperature changes (expansion and contraction, daily heating and cooling).
• Freezing of water in cracks (frost wedging).
• Abrasion by wind, running water, glaciers and waves.
• Plant roots growing in cracks, and unloading (release of pressure).

This produces coarse-grained soils such as gravel, sand and silt.

2. Chemical weathering

Minerals of the rock are altered into new minerals by chemical reactions: oxidation, hydration, carbonation, hydrolysis and leaching. Feldspar, for example, changes to clay minerals (kaolinite, illite, montmorillonite). This produces fine-grained clayey soils.

3. Biological action

Lichens, bacteria, burrowing animals and roots add organic acids and break rock, adding organic matter to the soil.

Fate of the weathered material

• Residual soils stay at the place of formation over the parent rock.
• Transported soils are carried by water (alluvial), wind (aeolian), ice (glacial) or gravity (colluvial) and deposited elsewhere.

Parent rock -> Weathering -> Soil particles
                 |-> stays in place: residual soil
                 |-> moved by agents: transported soil

Geotechnical problems are failures or difficulties arising from the poor behaviour of soil, rock or groundwater under or around civil works. Common problems and their usual solutions are listed below.

A proper site investigation (boring, sampling, laboratory and field tests) must come first so the correct solution is selected.

Residual soils are formed by weathering of rock and remain at the place of formation. Transported soils are weathered particles carried by an agent and deposited away from the parent rock.

Solutions to soil problems

• Settlement: compaction, preloading, drains, or deep foundations.
• Low strength: stabilisation, soil replacement, reinforcement with geosynthetics.
• Slope failure: drainage, retaining structures, slope flattening.
• Seepage: cut-off, filters and sheet piles.
• Expansive soil: lime treatment, replacement, and deep foundations.

Soil is an unconsolidated natural collection of mineral particles, with or without organic matter, produced by weathering of rock and having voids filled with water and/or air. To a civil engineer, soil is the material that supports or forms structures.

Soil engineering problems

1. Bearing capacity failure of foundations on weak soil.
2. Excessive or differential settlement due to compressible soils.
3. Slope instability and landslides in cuts, fills and natural hills.
4. Lateral earth pressure on retaining walls, basements and sheet piles.
5. Seepage and piping under dams, weirs and in excavations.
6. Liquefaction of loose saturated sands during earthquakes.
7. Swelling and shrinkage of expansive clays.
8. Frost heave and erosion problems.
9. Poor subgrade conditions under pavements.

Each problem is studied using the principles of soil mechanics (strength, compressibility, permeability) and solved by proper design, ground improvement or construction methods.

Soil mechanics is the application of the laws of mechanics and hydraulics to engineering problems involving soil. It deals with the physical properties of soil and its behaviour under stress, deformation and water flow. (Terzaghi called it the "application of mechanics to soils".)

Why study it

• All structures transfer their load to the ground, so safe and economical foundations need knowledge of soil strength and settlement.
• Soil is the construction material for dams, embankments and roads.
• Many failures (building tilting, dam piping, landslides) result from lack of understanding of soil.
• It helps choose a suitable foundation type, depth and ground improvement method.

Solutions to different soil problems

Soil mechanics

Soil mechanics is the branch of engineering mechanics that deals with the behaviour of soil under stress and strain and with the flow of water through it. It describes strength, compressibility and permeability of soil masses.

Significance of fluid mechanics in soil mechanics

Soil is a three-phase system of solids, water and air, so fluid mechanics principles are needed in many places:

1. Permeability and seepage: Darcy's law, $v = ki$, and Bernoulli's equation are used to find flow through soil, flow nets, and seepage quantity below dams and weirs.
2. Pore water pressure: hydrostatic pressure and buoyancy are needed for effective stress, $\sigma' = \sigma - u$.
3. Consolidation: squeezing out of pore water from saturated clay under load is a fluid flow process (Terzaghi's theory).
4. Uplift and piping: hydraulic gradient, critical gradient and quick condition ($i_c = (G-1)/(1+e)$) come from fluid mechanics.
5. Capillarity: surface tension and capillary rise in fine soils.
6. Dewatering and drainage design: wells and drains use flow principles.

Thus fluid mechanics is the basis for water-related behaviour of soils.

Both are types of weathering: the breakdown of rock into soil.

Example: granite breaking into sand-size quartz by frost is physical; feldspar of granite turning into kaolinite by hydrolysis is chemical.

Soil has been used since ancient times for foundations, dams and roads, but a scientific approach began only in the 18th century.

• Ancient period: Egyptian, Indian, Chinese and Roman engineers built dams, canals, temples and roads using experience only, with no theory.
• 1773 - Coulomb: gave the theory of earth pressure on retaining walls and the shear strength equation $\tau = c + \sigma\tan\phi$.
• 1856 - Darcy: law of flow of water through sand, $v = ki$.
• 1857 - Rankine: earth pressure theory for cohesionless soil.
• 1900s - Atterberg: consistency limits (liquid, plastic, shrinkage limits).
• 1915 - Petterson and Fellenius: slip circle method for slope stability (Sweden).
• 1925 - Karl Terzaghi: published Erdbaumechanik, the first book on the subject; developed the principle of effective stress and consolidation theory. He is called the Father of Soil Mechanics.
• 1927 - Proctor: compaction test (1933 publication).
• Later: Casagrande (soil classification, plasticity chart), Skempton (pore pressure parameters), Meyerhof and Hansen (bearing capacity), Bishop, and Peck contributed. The International Society of Soil Mechanics was formed in 1936.

Today the field includes computer modelling and ground improvement methods.

Based on formation

• Residual soil: formed and left at the same place (laterite).
• Transported soil: carried by agents. Types: alluvial (water), aeolian/loess (wind), glacial (ice), colluvial (gravity), lacustrine (lakes), marine.

Based on grain size

Gravel (> 4.75 mm), sand (4.75 - 0.075 mm), silt (0.075 - 0.002 mm), clay (< 0.002 mm).

Based on cohesiveness

• Cohesionless (non-cohesive) soils: gravel and sand, with no cohesion and strength from friction only.
• Cohesive soils: clay and clayey silts, with strength from cohesion between particles.
• c-$\phi$ soils: mixtures showing both cohesion and friction.

The characteristics of a residual soil depend on:

1. Parent rock: type and mineral composition (granite gives sandy soil; basalt gives clayey soil).
2. Climate: temperature and rainfall control the rate of weathering; warm humid climates cause deep chemical weathering.
3. Topography: gentle slopes keep the weathered material in place; steep slopes remove it.
4. Time: the longer the weathering, the deeper and finer the soil.
5. Drainage and groundwater: leaching removes soluble minerals.
6. Vegetation and biological activity: add organic matter and acids.
7. Rate of removal: weathering rate must exceed the erosion rate for residual soil to build up.