Lecture 11 - Soil continued

Question 1

Influence of Hilly and mountainous areas (Topography) on Clay Mineral Formation

Answer 1

Good/free drainage, vertical seepage, soils with good engineering properties

Question 2

Influence of Flat and low lying areas (Topography) on Clay Mineral Formation

Answer 2

Poor drainage, little/no vertical seepage. Problems with poor engineering properties

Question 3

Types of transportation

Answer 3

Aeolian (wind), fluvial (rivers/streams), glacial, volcanic

Question 4

What are the properties of volcanic soils?

Answer 4

Highly variable, layers = separate volcanic events, may be weathered after deposition. Freq high water content, v sensitive, drain v quickly

Question 5

Transported soils

Answer 5

Stress history important, sorting tends to produce homogeneous deposits, may originally be residual soils

Question 6

What are residual soils?

Answer 6

Formed by weathering in place

Question 7

What are organic soils?

Answer 7

Formed mainly in situ by growth/decay of plants (peat/mosses)
Accumulation of fragments of inorganic skeletons/ shells of organisms

Question 8

Soil gradation

Answer 8

Classification/behavior based on particle size. Granular soils = gravel + sand. Fines = silt + clays

Question 9

Granular soil shape characterization

Answer 9

Shape can affect engineering behavior, angular sands more interlocking = more str

Question 10

Clay minerals behavior

Answer 10

Behavior dominated by interactions with water, small amounts of clay can dominate overall soil behavior. Hydrophilic - low str, highly compressible, swelling , low hydraulic conductivity. Complex behavior

Question 11

Building blocks of clay

Answer 11

Silicon tetrahedron, alumina octahedron

Question 12

Clay synthesis

Answer 12

Forms sheets (tetrahedral or octahedral) which are stacked in ionic and covalent bonding to form layers. 1:1 or 2:1 semibasic units stacked differently

Question 13

Common clay mineral- Kaolinite

Answer 13

1:1, sheets stack to form aggregates, common in sedimentary/ residual soils
Hexagonal flakes, lower activity

Question 14

Common clay mineral- Illite

Answer 14

2:1 octahedral between 2 silica sheets

Less ordered stacking, higher activity than kaolinite

Question 15

Common clay mineral- Bentonite

Answer 15

2:1 clay mineral
Disordered sheets
extremely high activity
H20 and cations in inter-layer regions

Question 16

Volcanic clays (eg. allophane)

Answer 16

Not organised into sheets
Unique behavior
Greasy feel between fingers
Halloysite = weathered allophane

Question 17

What’s soil fabric

Answer 17

Arrangement of particles
Structure = fabric + composition + inter particle forces
Mineral particles no preferred orientation but becomes anisotropic under overburden

Question 18

What are cohesion-less soils?

Answer 18

Forces b/t soil particles arise from friction between these particles

Question 19

What are cohesive soils?

Answer 19

Main forces from electric repulsion from absorbed water layer between clay particles.