mod5

Question 1

in situ testing serves four main purposes

Answer 1

1. site investigation
2. measurement of a specific property in the ground
3. control of construction works
4. monitoring of performance and back analysis

Question 2

why do in situ testing

Answer 2

most common reason to undertake insitu testing is associated with soil disturbance

Question 3

sensitivity

Answer 3

• 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

Question 4

quick clay

Answer 4

a clay that becomes completely fluid on remoulding

Question 5

St

Answer 5

sensitivity ratio = undisturbed Cu / remoulded Cu

Question 6

in situ testing on granular soils

Answer 6

• 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

Question 7

SPT

Answer 7

Standard Penetration Test

• very popular and relatively economic test to undertake
• carried out in boreholes during a site investigation, typically every 1-2 m

Question 8

SPT Method

Answer 8

• 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)

Question 9

greatest variation in SPT method

Answer 9

• 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”

Question 10

relationship of N

Answer 10

• 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

Question 11

SPT in NZ

Answer 11

• 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

Question 12

CPT

Answer 12

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

Question 13

CPT Method

Answer 13

• 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)

Question 14

advantages of cpt method

Answer 14

i) can give virtually continuous logged data (hence good at identifying soil layers)
ii) does not disturb the soil unduly

Question 15

Types of CPT

Answer 15

1. Dutch (mechanical) cone:
2. Electric friction cone
3. Electric piezocone
4. seismic cone

Question 16

dutch (mechanical) cone

Answer 16

• early mechanical device with moveable parts

- pressure alternately applied to central axis or outer rods

Question 17

electric friction cone

Answer 17

• strain gauges incorporated to measure qc and qs

Question 18

electric piezocone

Answer 18

• modification to allow measuring of pore pressure at cone tip

Question 19

seismic cone

Answer 19

• further modified to include a vibration sensor, allowing shear modulus of soil to be measured

Question 20

CPT in NZ

Answer 20

• 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

Question 21

FVST

Answer 21

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

Question 22

FVST Method

Answer 22

• 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)

Question 23

Assumptions in FVST

Answer 23

1) Isotropy: Cu(h) = Cu(v)

2) uniform distribution of shear stress with r

Question 24

caution with FVST

Answer 24

can overestimate Cu as used in design, particularly as the plasticity of the soil increases

Question 25

Pressuremeter

Answer 25

• cylindrical device, designed to apply uniform radial pressure to sides of cylindrical hole
• measurement of permeability also incorporated

Question 26

Pressuremeter allows the determination of:

Answer 26

• initial tangent shear modulus (in situ)
• undrained shear strength, Cu, of clays
• peak friction angle of sands

Question 27

two basic types of pressuremeter

Answer 27

• Menard Pressuremeter

- Self-boring pressuremeter

Question 28

Menard Pressuremeter

Answer 28

• lowered into a preformed borehole

Question 29

self-boring pressuremeter

Answer 29

• 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

Question 30

Advantages of pressuremeter

Answer 30

• little to no empiricism required to understand the results (analysis based on cylindrical cavity expansion theory)

Question 31

Disadvantages of pressuremeter

Answer 31

• 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

Question 32

SWS

Answer 32

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

Question 33

SWS Method

Answer 33

• 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