mod5 Flashcards
in situ testing serves four main purposes
- site investigation
- measurement of a specific property in the ground
- control of construction works
- monitoring of performance and back analysis
why do in situ testing
most common reason to undertake insitu testing is associated with soil disturbance
sensitivity
- undrained shear strength of clay soils greatly reduced after remoulding
- caused by the destruction of the clay particle structure that was developed during original sedimentation
quick clay
a clay that becomes completely fluid on remoulding
St
sensitivity ratio = undisturbed Cu / remoulded Cu
in situ testing on granular soils
- mechanical behaviour derives from their structure or “fabric”
- “aged” granular deposits behave differently (usually sitffer and stronger) than newer deposits
- laboratory test samples made by different methods can behave differently
SPT
Standard Penetration Test
- very popular and relatively economic test to undertake
- carried out in boreholes during a site investigation, typically every 1-2 m
SPT Method
- split barrel sampler attached to the end of a series of rods and driven into the soil at the bottom of a bore hole to a standard depth of 150 mm using a falling weight (drop hammer)
- sampler driven an additional 300 mm while number of blows to drive this distance is recorded (CPT blowcount)
greatest variation in SPT method
- result of the different energies imparted to the soil by each blow
- a function of the hammer arrangment (weight and friction ) and rod length
- test is by no means “standard”
relationship of N
- increases linearly with overburden pressure at constant relative density
- increases with relative density squared (Dr^2)
- at a given Dr and stressv, N is higher for sands with larger particles
SPT in NZ
- routinely carried in NZ as result of the variable nature of soils and ease of testing
- close supervision of SPT is always required by the engineer
- SPT results notoriously unreliable if not carried out by skilled technicians and well-maintained equipment
CPT
Cone Penetrometer Test
- simple test widely used in place of SPT
- used particularly where there are soft clays, soft silts and fine/medium sands
- not well adapted to gravel or stiff/hard clay deposits
CPT Method
- standard cone pushed into the ground at a rate between 10 and 25 mm/s while the resistance is recorded
- record: tip resistance (qc), side friction (qs), depth (z)
advantages of cpt method
i) can give virtually continuous logged data (hence good at identifying soil layers)
ii) does not disturb the soil unduly
Types of CPT
- Dutch (mechanical) cone:
- Electric friction cone
- Electric piezocone
- seismic cone
dutch (mechanical) cone
- early mechanical device with moveable parts
- pressure alternately applied to central axis or outer rods
electric friction cone
- strain gauges incorporated to measure qc and qs
electric piezocone
- modification to allow measuring of pore pressure at cone tip
seismic cone
- further modified to include a vibration sensor, allowing shear modulus of soil to be measured
CPT in NZ
- not particularly common in NZ until Canterbury earthqukes
- becoming more routine in site investigations because CPT very good at identifying possible liquefaction prone deposits
- In ChCh thousands of CPT probes undertaken to determine soil conditions with respect to liquefaction potential and identification of landslide mechanisms
FVST
Field Vane Shear Test
- relatively quick and reliable means to estimate the insitu undrained shear strength of medium stiff to very soft and often sensitive fine grained soils
FVST Method
- vane consists of four thin equal sized steel plates welded to a steel torque rod
- test performed by inserting the vane into the soil and applying a torque T at constant angular speed
- cylinder of soil will resist the torque until the soil fails
- can be inserted at the bottom of a bore hole or in hole from vane sheath
- torque measured during the rotation and correlated to the vane shear strength
- larger vane used in softer soils (since it is more sensitive)
Assumptions in FVST
1) Isotropy: Cu(h) = Cu(v)
2) uniform distribution of shear stress with r
caution with FVST
can overestimate Cu as used in design, particularly as the plasticity of the soil increases
Pressuremeter
- cylindrical device, designed to apply uniform radial pressure to sides of cylindrical hole
- measurement of permeability also incorporated
Pressuremeter allows the determination of:
- initial tangent shear modulus (in situ)
- undrained shear strength, Cu, of clays
- peak friction angle of sands
two basic types of pressuremeter
- Menard Pressuremeter
- Self-boring pressuremeter
Menard Pressuremeter
- lowered into a preformed borehole
self-boring pressuremeter
- forms its own borehole and hence causes much less disturbance to the soil prior to testing
- described as allowing the “near perfect” testing of undisturbed soil
Advantages of pressuremeter
- little to no empiricism required to understand the results (analysis based on cylindrical cavity expansion theory)
Disadvantages of pressuremeter
- interpretation very different to SPT (relies on understanding of mechanics)
- ALWAYS carried out by specialist testing companies
- tend to be specified only where very high quality in situ soil data is required
- rarely carried out in NZ
SWS
Swedish Weight Sounding Method
- introduced in NZ as result of 2010 Chch earthquake
- highly portable, suitable for obtaining CPT-like data in confined areas with difficult access
- suitable for characterising soft to medium-dense soils to a depth of ~9m
- manually controlled - need min of 3 people to operate successfully
SWS Method
- number of half-turns required for each penetration of 250 mm
- when equipment is turned into the soil it emits a sound
- occasionally, if the soil is very soft, the point will fall through the soil under the gravity load of the weights alone (this is recorded as zero turns for the depth penetrated)
- testing continued until all equipment used or excessively stiff/impermeable layer is encountered
