Introduction to site investigation Flashcards
Site investigation objectives
- 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
Investigation outcomes
- Soil classification
- Elementary soil definitions/properties
- Shear strength
- Coefficient of permeability
- Other properties applicable to other geotechnical engineering applications
Defining Properties - direct
properties measured directly using method in the feild
Defining Properties - empirical correlations
- test in the field is related to soil properties
- correlations developed from testing data bases
Defining Properties - sampling and laboratory testing
- soil samples extracted in the field
- transported to lab for testing
Investigation types
- Hand Methods
- Test Pits
- Borehole Methods Probe Methods Geophysical
- Methods
Direct soil classification
hand auger, test pit, borehole
Hand auger
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
Test Pits
By hand or with machinery (digger), Shallow Investigations, Disturbed soil removed, If entering pit excavation support needed, Dewatering needed if below water table
Boreholes
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
Disturbed sampling
mechanical, cores, split spoon samplers
classification and visual identification
Undisturbed sampling (less disturbed)
push tubes, dames and moore, gel push
can be used for lab testing
how can sample be disturbed
Volume displacement of sampling tube, Friction on sides of collection device, Changes in water content,
Handling and transportation effects, Quality of personnel, Environmental effects
Hand methods
Shear vane, Scala, Sand Cone, Nuclear Densometer, No large equipment involved
Shear vane
- 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
Scala
- 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
Direct measurements of k in the field
- constant head
- falling head
- well pumping
What details do borehole logs summarise
- Borehole method (how borehole was constructed)
- Soil classification (visual classification based on sampling)
- Testing details (testing undertaken as borehole is advanced)
Common methods for investigating deeper into soil profile (probe methods)
CPT, DMT
Cone Penetration Test
- 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
Dilatometer Test (DMT)
- Pressure to develop known displacement of surrounding soil
- 200 mm test depth intervals
- material index
Scope of investigations
- available data and experience
- geotechnical risk (hazard, likelihood, consequences)
- Objectives
- Cost savings
Extent of investigations
- number of locations
- depth
- more detailed investigations
- use multiple different methods at each site
Impact of uncertainty
Unforeseen ground conditions can add significant costs to the construction process. There is correlation between low spending on site investigation and construction costs overrun.
Range of methods can be used to investigate soils
Soil types, Soil properties, Groundwater characteristics
Range of methods available to characterise properties
Direct, Correlations, Sampling to send to lab for testing
