mod5 Flashcards
1
Q
in situ testing serves four main purposes
A
- site investigation
- measurement of a specific property in the ground
- control of construction works
- monitoring of performance and back analysis
2
Q
why do in situ testing
A
most common reason to undertake insitu testing is associated with soil disturbance
3
Q
sensitivity
A
- 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
4
Q
quick clay
A
a clay that becomes completely fluid on remoulding
5
Q
St
A
sensitivity ratio = undisturbed Cu / remoulded Cu
6
Q
in situ testing on granular soils
A
- 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
7
Q
SPT
A
Standard Penetration Test
- very popular and relatively economic test to undertake
- carried out in boreholes during a site investigation, typically every 1-2 m
8
Q
SPT Method
A
- 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)
9
Q
greatest variation in SPT method
A
- 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”
10
Q
relationship of N
A
- 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
11
Q
SPT in NZ
A
- 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
12
Q
CPT
A
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
13
Q
CPT Method
A
- 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)
14
Q
advantages of cpt method
A
i) can give virtually continuous logged data (hence good at identifying soil layers)
ii) does not disturb the soil unduly
15
Q
Types of CPT
A
- Dutch (mechanical) cone:
- Electric friction cone
- Electric piezocone
- seismic cone
16
Q
dutch (mechanical) cone
A
- early mechanical device with moveable parts
- pressure alternately applied to central axis or outer rods
17
Q
electric friction cone
A
- strain gauges incorporated to measure qc and qs
18
Q
electric piezocone
A
- modification to allow measuring of pore pressure at cone tip
19
Q
seismic cone
A
- further modified to include a vibration sensor, allowing shear modulus of soil to be measured
20
Q
CPT in NZ
A
- 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
21
Q
FVST
A
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
22
Q
FVST Method
A
- 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)
23
Q
Assumptions in FVST
A
1) Isotropy: Cu(h) = Cu(v)
2) uniform distribution of shear stress with r
24
Q
caution with FVST
A
can overestimate Cu as used in design, particularly as the plasticity of the soil increases
25
Pressuremeter
- cylindrical device, designed to apply uniform radial pressure to sides of cylindrical hole
- measurement of permeability also incorporated
26
Pressuremeter allows the determination of:
- initial tangent shear modulus (in situ)
- undrained shear strength, Cu, of clays
- peak friction angle of sands
27
two basic types of pressuremeter
- Menard Pressuremeter
| - Self-boring pressuremeter
28
Menard Pressuremeter
- lowered into a preformed borehole
29
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
30
Advantages of pressuremeter
- little to no empiricism required to understand the results (analysis based on cylindrical cavity expansion theory)
31
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
32
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
33
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
