site investigation

Question 1

geology vs. geotechnical engineering

Answer 1

geology is the science of how nature operates, and the engineering is the application of this science to solve problems

Question 2

what is the objective of site investigation?

Answer 2

understand subsurface to make engineering decisions

Question 3

Test pits

Answer 3

shallow pits dug by hand or machinery, which allow direct observation of the subsurface. Block sampling is permitted, but the method is limited by the shallow depth.

Question 4

Hand Auguring

Answer 4

rods extend the depth you can reach (several meters). This method is suitable in low strength clays and is cheap and easy

Question 5

Rotary drilling. Why do we need drilling fluid?

Answer 5

apply downwards force to large drill rods and rotate. Drilling fluid must be pumped down the centre of drill rods otherwise the rods will fail and fracture.

Question 6

4 steps of undisturbed sampling procedure

Answer 6

1. drill
2. clean base of borehole
3. attach sampler to drilling rods
4. push sample tube into base of hole

Question 7

why do we want to minimise the disturbance of soil when sampling? How do we minimise it?

Answer 7

disturbance will change soil properties when lab testing. e.g. It might affect the strength of the sample. Minimise disturbance by using thin-walled sample tubes

Question 8

Ar = (area of outer diameter - area of inner diameter) / (area of inner diameter), what should Ar be less than in an undisturbed sample?

Answer 8

Ar < 10%

Question 9

In Situ testing

Answer 9

= In Place methods, alternative to expensive undisturbed sampling.

Question 10

Standard Penetration test (SPT)

Answer 10

In situ method, 65kg hammer is dropped 0.76m, driving a sampler (split spoon sampler) into the ground. Measures the strength by counting the number of blows to penetrate 150mm over 450mm. Operator dependent, therefore, huge variance in results

Question 11

split spoon sampler

Answer 11

disturbed sample (strength measured by number of blows to go 150mm over 450mm. Used in standard penetration test

Question 12

what could you still test on a disturbed sample?

Answer 12

atterburg limits (PL, LL, etc.), colour, consistency

Question 13

Cone Penetration testing (CPT), what is it? benefits?

Answer 13

in situ method. A cone on the end of a rod is pushed into the soil and measurements taken.
Main benefit is the ability to take continuous readings (e.g. computer records readings every 1cm). Also operator independent, so consistent results.

Question 14

If using a Cone Penetration test in seperate samples of clay and sand and the cone resistance was the same, how could you tell these apart?

Answer 14

the sleeve resistance from clay would be higher

Question 15

what 3 things are measured by a Cone penetration test.

Answer 15

1. Cone Resistance
2. Sleeve resistance (friction on sleeve of instrument)
3. Pore pressure (water content)

Question 16

review: hydraulic activity of sand vs clay

Answer 16

higher hydraulic activity in sand

Question 17

disadvantages of CPT, how to overcome them?

Answer 17

• cant go through gravel (SPT can go through more dense soil)
• cant collect sample (do SPT at same time to overcome)

Question 18

does water table vary much?

Answer 18

yes, due to rain, seasons, etc.

Question 19

measuring water table

Answer 19

boring