1 Introduction soil mechanics

INTRODUCTION TO SOIL MECHANICS

1.0 Geotechnical Engineering and Soil Mechanics

What is Geotechnical Engineering ? • • • •

What is Soil? List anything that you know /heard. What is Mechanics? How do they relates? What do you think that you will learn in this subject and HOW?

Instruction: On your own – answer all above Q’s (1-2 mins) Compare and discuss your answer with the person next to you (2 - 3 mins)

The Father of Soil Mechanics Said Terzaghi (1948):

"Soil Mechanics is the application of laws of mechanics and hydraulics to engineering problems dealing with sediments and other unconsolidated accumulations of solid particles produced by the mechanical and chemical disintegration of rocks regardless of whether or not they contain an admixture of organic constituent “

Karl Terzaghi in 1951

1.1 Term and Definition Example of Soils • Gravel, sand, silt, clay

Engineering Terms • Soft deposits in between top soil and base rock • BS 892 – any soft or loose deposits that exit naturally, forming part of earth crust and formed from weathering or disintegration of rocks or degradation of vegetation. • BS 1377 : part 1 : 1990 - An assemblage of discrete particles in the form of deposit, usually of mineral composition but sometimes of organic origin, which can be separated by gentle mechanical means and which includes variable amounts of water and air (and sometimes of gases). A soil commonly consists of a naturally occurring deposit forming part of the earth crust but the term is also applied to made ground consisting of replaced natural soil or man made materials exhibiting similar behaviour, e.g. crushed rock, crushed-blastfurnace slag, fly-ash.

1.1 Term and Definition Usage and Practicality • Do geologist use the same definition and term – how do they differ • How do you confirm • What is the problem if they are different – give example

Instruction: On your own – answer all above Q’s (1-2 mins) Compare and discuss your answer with the person next to you (2 - 3 mins)

1.2 Soil Formation GEODe II Interactive CD Presentation -Recap on rock cycle - Weathering

1.2 Soil Formation ROCK

WHEATHERING

PHYSICAL/MECHANICAL Agent – wind, temperature, water, frost, glacier Product – boulders, cobbles, gravel, sand and rock flour Particle props.- similar chemical props. to parent rock, bulky form; might be angular, subangular or rounded, cover wide range of sizes.

COARSE-GRAINED SOIL

CHEMICAL/BOILOGICAL Agent –water, acid, alkali, carbon dioxide Product – silt, clay minerals Particle props.- chemical props. differ to parent rock due to chemical reaction forming crystalline ptcl. of colloidal size (< 0.002 mm) with shape of either plate or needle like having high specific surface. Electrochemically active.

FINE-GRAINED SOIL

Product of Physical Weathering

Particle shape

Boulders (> 200 mm)

Cobbles (60 - 200 mm)

Jigsaw – G1

Product of Physical Weathering Coarse gravel (20-60 mm)

Coarse sand (0.6 – 2 mm)

Fine gravel (2 -6 mm)

Medium sand (0.2 – 0.6 mm)

Fine sand (0.06 – 0.2 mm) Jigsaw – G1

Product of Physical Weathering Particles arrangement Structural arrangement depends on minerals, how its been transported and deposited. The engineering properties depends on interaction between neighbouring particles.

single grain

Strength (shear) – friction between soil particles. Shear failure- mainly due to soil particles roll and slide into new structural arrangement without particles break into smaller pieces. Jigsaw – G1

Product of Chemical Weathering Particle –cannot be seen with naked eye- fine like flour Particle shape – under high power microscope

Plate-like shape

Long needle-shape

Properties – cohesive and plastic, having high specific surface, electrochemically active, consistency depends on water Strength – depends on cohesion Jigsaw – G2

1.5 Clay Minerals BASIC UNITS Alumina Octahedron

Silica tetrahedron

a) One silica sheet consist of 6 units of silica tetrahedron b) silica sheet symbol

a) b)

One alumina sheet is made of 4 units of alumina octahedron Alumina sheet symbol Jigsaw – G2

Structure of Clay Minerals

Kaolinit

illit

(15 m2/g)

(80 m2/g)

montmorillonit (800 m2/g)

Activity Jigsaw – G2

1.4 Properties of Clay Minerals The surface of clay mineral particles – negatively charged due to 1. Isomorphous subtitution of Al or Si by atoms of lower valency. 2. Dissacociation of hydroxyl ion 3. ‘broken bonds’ at the edges of the particles In dry condition The negative charges are balanced by exchangeable such as Ca2+, Mg2+, Na+ and K+ which are held by electrostatic attraction. When water is added Cations and a few anions float around the clay particles. Cations may exchange. Jigsaw – G2/G3

Double-layer When water is added / in wet condition • Cations float and attracted to the negative surface of clay particle • Cations tend to move away from each other because of their thermal energy – resulting in diffuse layer of cations called double layer. • The cation concentration decreasing with increasing distance from the surface until the concentration become equal to that of normal water in the void space. • The thickness of this layer depends on valency and concentration of cations. The higher the valency and concentaration, the lesser the thickness of this layer Jigsaw – G3

Adsorbed water What is adsorbed water?

CLAY

Water layer that formed around clay particles.

Water layer

How does it formed?

or

How does clay attract water (1) / water attracted to clay particles (2). (1) – look at clay properties – the role of negative charges and double layer (explained) (2) – need to investigate the properties of water molecule.

Jigsaw – G3

Formation of adsorbed water water dipolar

Clay particle

Negative charges of clay surface, double layer

Formation of adsorbed water

Jigsaw – G3

Identification of clay minerals • • • •

X-ray diffarction Differential thermal analysis (DTA) Atterberg limit – Plasticity test to find Liquid Limit (LL) and Plastic Limit (PL). Plasticity Index (PI) = LL - PL

Locations of common clay minerals in Casagrande Plasticity Chart (Holtz and Kovacs, 1981)

1.4 Clay Structure

• • • • • •

Clay structures (a) dispersed, (b) flocculated, (c) bookhouse, (d) turbostratic, (e) example of natural clay Forces of attraction and repulsion act between adjacent clay mineral particles Repulsion occurs between like charges of the double layer. Attraction is due to short range van der Waals forces (independent of double layer characteristics) and decrease rapidly with increasing distance between the particles. It is the net interparticle forces influence the structural form Net repulsion – face-to-face orientation – dispersed structure Net attraction – edge-to-face/edge-to-edge – flocculated.

1.5 Transportation and Deposition of Weathered Materials Transportation agent – water, wind, gravity, glacier etc.. • Residual soil • Glacial Soils • Alluvial Soils • Lacustrine and Marine Soils • Aeolian Soil • Colluvial Soils • Organic Soils

Transported Soil

Distribution of Various Soil Types in Semenanjung Malaysia

Residual Soil • Top soil / laterite • Product of chemical and biological attack not transported elsewhere and remains in its original place. • Characteristics – the types of soil depend on the parent rock, boulders, bedding planes • Granite – sandy residual soil, • Igneous and metamorphic – silty and gravelly residual soil • Sedimentary – clayey residual soil

(a and b) Residual soil profile

Jigsaw – G4

Residual Soil

Residual soil classification based on degree of weathering

Jigsaw – G4

Residual Soil

Jigsaw – G4

Laterite in Malaysia • Mainly from weathered igneous rock due to rain and temperature • Cemented with iron oxide (reddish brown) which gives it a high dry strength. • Strength ranging from poor to good and generally improve with depth • Boulders still undergoing weathering process that might be unstable especially in sloping surface. • Clay minerals – kaolinite, geothite, hematite and gibbsite

schematic diagram deposits of granitic material around Bukit Antarabangsa (The Star 21.11.2002) Jigsaw – G4

Bukit Lanjan, 6-12-2003

Occurring of hard materials in thick weathering profile (mainly residual soils) – ripping & blasting method may be expensive (lacking of sufficient information during SI)

Problems associated with residual soil

Problems during excavation

Transported Soil • Weathering product transpoted and deposited elsewhere. • Tranporting agent – water, glacier, wind, etc • Characteristics – material chemical composition differ from parent / surrounding rock, particle shape – rounded and less angular, uniform particle distribution, laminated soil mass, surface profile normally not parallel with the profile of parent • Types include - glacial, alluvial, lacustrine, marine, aeolian dan colluvial soils

(a) Transported soil profile

(b) alluvial deposits at river mouth Jigsaw – G5

Transported Soil Glacial soil • Soil being transported due to melting glaciers (huge mass of ice). • Glacier’s movement resulting in weathering product being grounded and transported to a long long distance. • Characteristics – more rounded particle shape, wide range of particle size an hardness, complex and heterogenous soil coming from different sources. • 3 categories (Till,

Glasiofluvial and Glasiolacustrine) Jigsaw – G5

Transported Soil Alluvial / Fluvial Soil • Transported by rivers and streams • Materials deposited depends on velocity of the streams • Rapid streams forces silts and clays in suspension and finally deposited downstream, sand, gravel and boulders are deposited upstream. • Slow water movement may cause deposition of fines before reaching downstream. • The slowdown of the water velocity resulting in alluvial fan

Jigsaw – G5

Transported Soil Marine and Lacustrine Soils • Lacustrine – those deposited beneath lakes – primarily silt and clay having poor to average bearing capacity. • Marine – those deposited underwater except they formed in the ocean eg. Deltas (river met ocean) forming flat terrain. Primarily silt and clay and very soft. • Both types are rather uniform and consistently poor and hence more predictable. Jigsaw – G5

Transported Soil Aeolian Soils dust

• Those deposited by wind. • Poorly graded soil (narrow range of soil particles) because of the strong sorting power of wind, generally very loose hence fair engineering properties.

coarse sand

• May form horizontal strata or hill (sand dunes) along beaches or desert. Tends to migrate downwind • Prone to erosion and often have deep gullies. Jigsaw – G5

Sand dunes

Jigsaw – G5

Transported Soil Colluvial Soils • Transported downslope by gravity • Downslope movement either – rapid or slow

Jigsaw – G5

Organic Soil • • • • • •

Soil that contain > 20% organic materials Consist of degraded vegetation and other organic matter Peat soil > 75% organic material Top soil ≤ 500 mm, contains high organic materials Not suitable for engineering work hence need to be removed before engineering work begins Engineering properties – low shear strength and highly compressible

1.6 General Information

Relationship between agricultural soil and engineering soil

Tropical Residual Soil – Classification and Recommended Use

Classification of Tropical Residual Soil by degree of weathering (Little, 1969)

Tropical Residual Soil