Geotechnical probesObtaining high-quality undisturbed samples is one of the biggest challenges faced in the field of geotechnology. Poor quality samples may result in non-representative results being obtained in lab tests, the use of which may give rise to faulty designs.

Igeotest has developed a methodology for the extraction of undisturbed samples, as well as a specific device which offers a new perspective of an activity hitherto ill-considered in our country notwithstanding its vital importance when undertaking a geotechnical characterisation campaign.

The choice of sampling equipment, in terms of both the project needs and the type of soil, is key to obtaining high quality, representative samples.

The following table shows the sampling techniques available and the recommended applications for each, together with their advantages and disadvantages:

 

PERFORATIONS

DRIVE SAMPLERS
Open drive Thick wall Cohesive soft to firm soils; Granular soils without gravel Low cost
Robust
Easy to use
High level of ground disturbance L=600
d=85-98
Thin wall Shelby (ASTM D 1587-74) Cohesive soft to firm soils Low cost
Easy to use
Ground disturbance
Boca corte Fácilmente deteriorable
L=600
70-80-90
Fixed piston FIXED (Osterberg) Very soft to soft soils Sample quality Hard to use
High cost
77 mm
Continuous Begemann
link with Beguemann further down
Very soft to soft soils Very high sample quality
length of samples up to 19 m
Perforation by pressure
High cost
Hard to use
Static perforation
35-65

 

Undisturbed samples    Undisturbed samples   Undisturbed samples

 

Bore diameter

The bore diameter should be adjusted according to the needs of each specific project. The following values may be used as guides for deciding bore diameter.

  • No cladding Ds > 6-19 mm Dt
  • Cladding Ds > 6-25 mm Dr
  • Special cases Ds > 12-19 mm Dt
  • Ds bore diameter
  • Dt sample diameter
  • Dr cladding diameter

 

Perforation method

METHOD RECOMMENDATON
Rotation Sampling surveys
Percussion Hard to perforate gravels and bolos
Destroza Cavity detection
Rapid advance
Registration of perforation parameters
Wire line Surveys of great depth (>100 m)

 

Sample conservation

After extraction, a correct and careful procedure must be followed to preserve sample integrity. The following is a list of minimum precautions which should be observed:

  • Careful extraction always in a vertical position
  • Sealing of extremities and filling of gaps in the extremities.
  • Labelling
  • Placement and transportation in special containers kept vertical at all times
  • Isolation from external contamination

 

See special CPT samplers (MOSTAP, Delft continuous soil sampler).