Introduction to site investigation

Question 1

Site investigation objectives

Answer 1

• Characterise the thickness and extent of soil strata (layers) (possibly depth to bedrock)
• Characterise the properties of relevant strata
• Assess ground water conditions
• Understand the subsurface in order to inform engineering decisions and design

Question 2

Investigation outcomes

Answer 2

• Soil classification
• Elementary soil definitions/properties
• Shear strength
• Coefficient of permeability
• Other properties applicable to other geotechnical engineering applications

Question 3

Defining Properties - direct

Answer 3

properties measured directly using method in the feild

Question 4

Defining Properties - empirical correlations

Answer 4

• test in the field is related to soil properties
• correlations developed from testing data bases

Question 5

Defining Properties - sampling and laboratory testing

Answer 5

• soil samples extracted in the field
• transported to lab for testing

Question 6

Investigation types

Answer 6

• Hand Methods
• Test Pits
• Borehole Methods Probe Methods Geophysical
• Methods

Question 7

Direct soil classification

Answer 7

hand auger, test pit, borehole

Question 8

Hand auger

Answer 8

Rotate auger at ends of rods to create small hole, Incremental removal of disturbed soil from each depth, Shallow investigations, Cheap and easy, Measure level of water in hole

Question 9

Test Pits

Answer 9

By hand or with machinery (digger), Shallow Investigations, Disturbed soil removed, If entering pit excavation support needed, Dewatering needed if below water table

Question 10

Boreholes

Answer 10

Advance a hole into the ground, Bring soil to the surface to understand soil stratigraphy, Can investigate to most depths of engineering interest, Range of methods used to do this

Question 11

Disturbed sampling

Answer 11

mechanical, cores, split spoon samplers

classification and visual identification

Question 12

Undisturbed sampling (less disturbed)

Answer 12

push tubes, dames and moore, gel push

can be used for lab testing

Question 13

how can sample be disturbed

Answer 13

Volume displacement of sampling tube, Friction on sides of collection device, Changes in water content,
Handling and transportation effects, Quality of personnel, Environmental effects

Question 14

Hand methods

Answer 14

Shear vane, Scala, Sand Cone, Nuclear Densometer, No large equipment involved

Question 15

Shear vane

Answer 15

• Direct measure of shear strength in the field (fine grained soils, undrained shear strength (su), sensitivity)
• Torque required to fail cylinder of soil around vane
  (undisturbed, remoulded)
• Test at base of hand auger hole at regular intervals

Question 16

Scala

Answer 16

• Dropping weight known height to drive cone into the ground
• Count number of weight drops to drive cone into ground for set depth increments
• Coarse method to identify “good ground”
• Guided by experience

Question 17

Direct measurements of k in the field

Answer 17

• constant head
• falling head
• well pumping

Question 18

What details do borehole logs summarise

Answer 18

• Borehole method (how borehole was constructed)
• Soil classification (visual classification based on sampling)
• Testing details (testing undertaken as borehole is advanced)

Question 19

Common methods for investigating deeper into soil profile (probe methods)

Answer 19

CPT, DMT

Question 20

Cone Penetration Test

Answer 20

• An instrumented cone is pushed into the ground at constant rate (effectively provides continuous data)
• Measures multiple variables: load cells measure cone resistance, qc and sleeve friction, fs, Transducer to measure pore water pressure, u2, via a saturated filter
• Does not measure soil properties directly
• Not possible in stiff gravel deposits

Question 21

Dilatometer Test (DMT)

Answer 21

• Pressure to develop known displacement of surrounding soil
• 200 mm test depth intervals
• material index

Question 22

Scope of investigations

Answer 22

• available data and experience
• geotechnical risk (hazard, likelihood, consequences)
• Objectives
• Cost savings

Question 23

Extent of investigations

Answer 23

• number of locations
• depth
• more detailed investigations
• use multiple different methods at each site

Question 24

Impact of uncertainty

Answer 24

Unforeseen ground conditions can add significant costs to the construction process. There is correlation between low spending on site investigation and construction costs overrun.

Question 25

Range of methods can be used to investigate soils

Answer 25

Soil types, Soil properties, Groundwater characteristics

Question 26

Range of methods available to characterise properties

Answer 26

Direct, Correlations, Sampling to send to lab for testing