Question

Table 3 and 4 present the particle size and index properties of soils. Use at least two methods to qualitatively identify the swell potential of the samples. Conclude on the effect of sand addition.

Table 4. Percentages of fines passing 75um size sieve.

Sample	Silt-size fraction (%)	Clay size fraction (%)	Colloidal particles
Natural Bentonite	22	78	57
33% Sand	15	56	48
50% Sand	16	34	28
75% Sand	7	18	16.5

Table 3. Atterberg limits of natural soil and mixtures with sand.

Sand (%)	Bentonite (%)	PL (%)	PI (%)	LL (%)
0	100	79	40	119
33	67	49	29	77
50	50	30	27	57
75	25	24	8	32

Answer 1

Ans) Method 1 - By using Van der Merwe formula

Swell potential of soil in terms of K , mathematically given as :

(P_clay -0.73K)(PI -0.16P_clay K^0.4) - K = 0

where P_clay = fraction of clay

PI = Plasticity Index

Putting values,

1) Natural Bentonite

(78 - 0.73K)(40 - 0.16(78)K^0.40) - K =0

3120 -973.44 K^0.40 -29.2 K - 9.11 K^1.40 - K =0

On solving,

K = 18

Since,

When K < 16 low swell potential

16 < K < 27 medium swell potential

27 < K <37 high swell potential

K > 37 very high swell potential

For K = 18 , medium swell potential

2) 33 % sand

(56 - 0.73K)(29 - 0.16(56)K^0.40 - K = 0

Again K = 18 , hence medium swell potential

3) 50 % sand

( 34 -0.73K)(27 - 0.16(34)K^0.40 - K = 0

On solving K = 52

Hence , very high swell potential

4) 75% sand

(18 - 0.73K) ( 8 -0.16(18)K^0.40 - K =0

On solving K = 12

Hence , low swelling potential

Method 2) According to Holtz and Gibbs,

% Colloid	Plasticity Index	Shrinkage Limit	Liquid Limit	Swelling Potential
<15	<18	<15	<39	Low
13 -23	15 -28	10 -16	39 -50	Medium
20 -31	25 -41	7 -12	50 -63	High
> 28	>35	>11	>63	Very High

1) Natural Bentonite

According to above table , very high swell potential

2) 33% sand

Very high swell potential

3) 50% sand

High swell potential

4) 75% sand

Low swell potential

From above results we can see that addition of sand will reduce the swelling upto great extent. Sand content as high as 75% will reduce the swelling potential to very low. Therefore to reduce swelling sand treatment is effective way