Question

foundation design

- Soil investigations in the past have shown that the soil at the site of the project consists of soft clay over a founding stratum of dense sand. The decision has been made to use axially loaded piles with end bear- ing in the sand. Describe the techniques you would employ to obtain the thickness of the clay layer and the relative density of the sand.

Answer 1

Relative Density of is comparison of the porosity or void ratio of the given soil with that of the same soil in its loosest and densest possible state and hence the term, relative density is introduced.

Relative Density = (e$^{_{min}}$ - e)/(e$^{_{max}}$ - e$^{_{min}}$)

Determination of Relative Density:

Apparatus Required for determination of Relative Density are as follows:

1) Relative Density Apparatus

2) Mixing Pan (Suitable size are 60 x 90 cm and 10 cm deep and 40 x 40 cm and 5 cm deep)

3) Weighing Scale (100 kg capacity with sensitivity of 20 g)

Calibration of mould:

1. Measure inside diameter of mould at different depths using a bore gauge and take the average diameter.

2. Keep the mould on a flat surface or flat plate. Measure the height at different positions and take the average (accuracy = 0.025 mm).

3. Calculate the volume of the mould.

4. Fill the mould with distilled water till over flowing takes place.

5. Slid thick glass plate over the top surface of mould.

6. Weigh the water filled mould.

7. Note the temperature of water.

8. Obtain density of water for the above temperature from physical tables.

9. Calculate the volume of the mould which is weight of water filled mould /density of water.

Preparation of the Sample:

1. Dry the soil sample in a thermostatically controlled electric oven.

2. Cool in the sample in a desiccator.

3. Segregate soil lumps with out breaking individual particles

4. Sieve it through the required sieve size.

Minimum Density:

The mould is weighed accurately (W). Pour the dry pulverized soil into the mould through a funnel in a steady stream. The spout is adjusted so that the free fall of soil particle is always 25 mm. While pouring soil the spout must have a spiral motion from the rim to the centre. The process is continued to fill up the mould with soil upto about 25mm above the top. It is then leveled, with the soil and weight is recorded (W$^{_{1}}$).

Let Volume of Mould = V cc.

Mass of Dry Soil, M$^{_{s}}$ = (W$^{_{1}}$ - W) gm.

Now, ($\gamma$$^{_{d}}$)$^{_{min}}$ = M$^{_{s}}$/V gm/cc

e$^{_{max}}$ = G.$\gamma$$^{_{w}}$/($\gamma$$^{_{d}}$)$^{_{min}}$-1

Maximum Density:

Weigh the empty mould (W). Put the collar on top of the mould and clamp it. Fill the mould with the oven dried soil sample till 1/2 or 2/3 of the collar is filled. Place the mould on the vibrating deck and fix it with nuts and bolts. Then place the surcharge weight on it. The vibrator is allowed to run for 8 minutes. Then mould is weighed with the soil and weight is recorded (W₂).

Let Volume of Mould = V cc.

Mass of Dry Soil, M$^{_{s}}$ = (W$^{_{2}}$ - W) gm.

Now, ($\gamma$$^{_{d}}$)$^{_{max}}$ = M$^{_{s}}$/V gm/cc

e$^{_{min}}$ = G.$\gamma$$^{_{w}}$/($\gamma$$^{_{d}}$)$^{_{max}}$-1

Natural Density:

Weigh the mould with dry soil. Knowing the volume of the mould and weight of dry soil natural density, $\gamma$ $^{_{d}}$, can be calculated.

e = G.$\gamma$$^{_{w}}$/($\gamma$$^{_{d}}$)-1

Relative Density = (e$^{_{min}}$ - e)/(e$^{_{max}}$ - e$^{_{min}}$)