Insitu Testing

Question 1

What are the three types of testing to know?

Answer 1

1) Standard penetration test (SPT)
2) Cone penetration test (CPT)
3) Pressuremeter

Question 2

What are the advantages of in-situ tests? (9)

Answer 2

1) Tests can be carried out and interpreted quickly
2) Appropriate tests can test a large volume of soil/rock
3) They can be used as part of ground profiling
4) The test the ground in its in-situ state, under in-situ stresses
5) Test can be carried out in materials that can’t be sampled
6) They avoid the difficulties of sample disturbance
7) They may be used to measure some soil properties directly
8) They may be correlated with lab tests to deriv soil properties (Cu and angle of friction)
9) They may be correlated with engineering behaviour at full scale

Question 3

What are the disadvantages of in-situ testing? (2)

Answer 3

• You do not have control over boundary stresses and strains

- Insertion of the device may actually disturb the ground before the test is carried out

Question 4

What is the basic procedure of an SPT test?

Answer 4

• 65kg hammer free falling at 0.76m increments
• count blows for each 0.075m increment
• N value is the sum of blows for increments 3,4,5,6
• discount first 2 blow counts, its just getting embedded into the soil

Question 5

What is a split-spoon sample?

Answer 5

Used in SPT testing, give sample from borehole for BH log.

Question 6

What are the difficulties of SPT testing? (6)

Answer 6

1) Non-standard equipment (hammers and rods)
2) Non-standard technique (poor tightening of screwed rods) reducing energy at depth
3) Disturbance of soil below BH (e.g. boiling)
4) Effect of overburden stress/depth
5) Effect of water in fine soils
6) Borehole diameter ( might not be penetrating completely vertically)

Question 7

SPT test conclusions

Answer 7

• SPT is a widely used test
• when carried out in accordance with the standard it gives a useful indication of relative density, stiffness and strength of soils
• test data should be corrected for non-standard equipment and procedures and for depth/overburden stress
• various correlations between N value and Rd, E’ and Cu and liquefaction have been developed. These should be calibrated against local experience

Question 8

Describe a cone penetration test

Answer 8

a truck parks up and pushes a probe into the ground. basically a massive rod with a cone on the end of it.

The test is carried out by pushing continuously at 20mm/sec, with the loads on the cone and friction sleeve being continuously recorded. With modern setups the data processed immediately and a plot of cone resistance (qc), sleeve resistance (fs) and friction ratio (Rf) is produced in the field. Also measures pore pressure

Question 9

What are the objectives of a CPT test

Answer 9

• to identify the likely stratigraphy from measured profiles of qc fs and u using derived parameters and classification charts
• for cohesive soils, use correlations to estimate the OCR, UD shear strength. Determine coeff of consolidation from dissipation tests
• for cohesionless soils, use correlations to estimate the relative density, friction angle and stiffness. Assess liquefaction potential, use results directly in pile design and settlement analysis
• with additional sensors the CPT can be used to measure seismic velocities, resistivity and many other parameters

Question 10

Sources of error in CPT tests (7)

Answer 10

1) equipment wear
2) temperature compensation
3) improper calibration
4) the presence of gravel
5) use of the wrong capacity cone
6) inadequate saturation of the piezometer
7) failure to note scale change in results

Question 11

What is a pressuremeter? And what are the two types?

Answer 11

The pressuremeter is a cylindrical device designed to apply a uniform radial pressure to the side of a borehole in which it is placed. 2 basic types:

1. Menard - lowered into a preformed borehole
2. Self-boring - forms its own borehole (less disturbance to the surrounding soil

Question 12

What happens in a pressuremeter test?

Answer 12

A known pressure is applied to the device to expand the BH in a radial direction. The applied pressure and resulting soil deformation is interpreted using cavity expansion theory (CET).

i.e. it doesn’t reply on empirical correlations

Question 13

What corrections must be made during pressuremeter tests? (3)

Answer 13

To account for:

• compressibility of fluid and pipe network
• differences in elevation between pressuremeter and pressure transducer
• stiffness of the rubber membrane

Question 14

What do SPT tests measure?

Answer 14

Density, stiffness and strength of soils