GEOTECHNICAL ENGINEERING I
(Question no. 1.1, 1.2, 1.4, 1.5, & 1.9 ..Al-Khafaji textbook)
1.1 Describe the main difference between kaolinite and montmorillonite clay minerals.
- Kaolinite consists of a series of tetrahedral and octahedral layers alternate (two-layer lattice).
- Kaolinite crystal is relatively stable and water cannot enter between the sheets to expand or to shrink the unit cells.
- In the presence of water some hydroxyls dissociates and lose hydrogen atoms, leaving the Kaolinite crystal with small residual negative charge.
- Linkage between sheets is H-bond + secondary valence.
- Length of particle = 0.2-2.0 mm and thickness = 0.005-0.2 mm.
- Specific surface = 10-20 m2/g.
- Approximate exchange capacity = 5 meq/100g.
- Montmorillonite consists of an octahedral sheet sandwiched between two silica sheets (three-layer lattice).
- The octahedral sheet may contain Al, Fe, Mg. or some combination.
- Some of silica atom in tetrahedral sheets may be replaced by Al atom (<15%).
- The bonding between units is the mutual attraction for the (exchangeable) cation plus weak Van der Waals forces.
- In the presence of water, the montmorillonite minerals exhibit to expand due to weak bonding.
- Length of particle = 0.1-1.0 mm and thickness = 0.001-0.1 mm.
- Specific surface = 800 m2/g.
- Approximate exchange capacity = 100 meq/100g.
1.2 Explain why the flat surfaces of a clay mineral have a negative charge.
Clay minerals having a negative charge in the surfaces due to the dissociation of of the hydroxyls and lose the hydrogen atoms in the presence of water leaving the crystal to have the negative charge, for other kind of clay may hav the negative charge due to isomorphous substitution (replacing silica atoms in the tetrahedral sheets by aluminum atoms).
1.4. Explain why some clay swells when water is available.
Some clays for example, montmorillonite, expands in the presence of the water due to weak bonding (mutual attraction and Van der Waals) between units of structure so that water can enter between sheets and causing to expand. More water can enter because the hydrogen (+) will be attracted by the negative charge of montmorillonite particle and the oxygen (-) will reject/push the particle away, so that the minerals exhibit high swelling when water is available.
1.5 For each of the following sedimentary soils identify the main transportation agent: (a) sand dunes, (b) beach sand, (c) alluvium, (d) glacial till, (e) loess, (f) talus.
(a) Sand dunes = agent is wind
(b) Beach sand = agent is water (sea)
(c) Alluvium = agent is water
(d) Glacial till = agent is ice
(e) Loess = agent is wind
(f) Talus = agent is water (colluvial soil)
1.9 For areas containing limestone with sinkholes and/or solution cavities, discuss problems related to (a) foundations (b) dams.
Limestones are characterized by their solubility and potential for cavity development, and also erratic behavior in sustaining external load.
(a) Foundation: The foundation has to satisfy safety factor means tolerable settlement and definite bearing capacity which limestone as the soil cannot satisfy the safety, certainty/assurance in bearing capacity due the erratic behavior and easy to be dissolved by other chemical solution that may enter the soil.
(b) Dam: The potential for cavity development of the limestone and the existing sinkhole will deteriorate the condition (sustaining load and seepage/intrusion resistance) of the dam because the water or water contains chemical solution can enter (seepage/intrusion) beneath the dam and cause the dam to collapse.