Question 4 (a) Clay minerals can have a 1:1 or a 2:1 structure. Give a brief exp

Question

Question 4 (a) Clay minerals can have a 1:1 or a 2:1 structure. Give a brief explanation of the differences in these structures and how the two different structures deal with wetting. (3 mark) (b) A sample of a clay soil deposit, upon which a structure is to be constructed, has the following properties: Total Volume 100 cm3 Total Mass =170grams Moisture Content (mass basis) 16% Specific Gravity of Solids 2.70 i. Calculate the relative proportions, on a volume basis, of solid, water and air in the sample (9 mark) ii. The sample was then soaked for a day and the total volume increased to 105cm, witha corresponding increase in moisture content (mass basis) to 32%. Is the sample now fully saturated? You have to justify your answer with relevant calculations (3 marks) Note: In your calculation, you can assume density of water = 1 g/cm'

Explanation / Answer

(a) A 1:1 clay mineral will have one tetrahedral sheet linked to one octahedral sheet. An example of this type of clay is Kaolinite.

A 2:1 clay mineral will have an octahedral sheet between two tetrahedral sheets (like a sandwich). An example of this type of clay is Vermicullite

1:1 clay has lesser tendancy to absorb water (wetting) than 2:1 clay

(b) (i) Given that 17% of mass is moisture- hence 16% of 170 grams= 2.72 gms

Volume of water Vw= Mass of water/ density of water= 2.72gms/ 1 gm/cm3= 2.72 cm3

Volume of solid Vs= Mass of solid/ Density of solid = (170-2.72)/2.70= 61.955 cm3

Volume of air= Total volume- (Volume of solid+ volume of water)= 100-(61.955-2.72)= 40.765 cm3

(ii)

Volume change= 5cm3

Moisture content change= 16%

Hence additional 16% moisture increase gives 5 grams of water,

the above calculation stated that 16% of water is 2.72 grams

Hence 32% of moisture is 2.72*2= 5.44 grams

So, the sample is fully saturated

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