SEDIMENT SAMPLING

or Digging in the Dirt

 

Subsurface sampling of sediments or alluvium can be conducted in very unsophisticated (and inexpensive) ways and in very sophisticated (and expensive) ways. The main purpose is to obtain a sample for lithological description (grain size, mineralogy, etc.) and for laboratory testing (sieve analysis, permeameters, contaminant monitoring). Many techniques can be used to obtain disturbed samples. Disturbed samples have been mixed or moved in such a way that the original orientation of sediment particles has been disturbed. Intact samples are collected in a way to minimize disturbance and retain the orientation, packing and fabric of the material.

Shovel Technique

PROS:

CONS:

HAND AUGER TECHNIQUE

An auger has a cutting end (bit) which advances the device through the subsurface as it is turned. A disturbed sediment sample is collected (typically half a foot or less in length) and lifted within the auger to the surface.

Shown below is a hand auger with a section of rod attached and a padded handle for easy turning. More sections of rod can be attached for deeper augering.

The auger bit:

PROS:

CONS:

Machine-Driven Auger Drilling

Gasoline-powered augers can be rented but typically have a limit of less than 10 ft drilling depth.

The standard technique for environmental drilling is hollow-stem auger drilling. Similar to hand augering, you can add numerous sections or flights of augers (typically 5 ft long each) to reach a range of depths. In hollow-stem augering, once the sampling depth is attained, the inside rods and plug are removed - leaving the outer augers holding the borehole open and the undisturbed sediments below the borehole available for sampling.

In addition to sampling through the hollow-stem auger, cuttings are brought to the surface by the spiral fluted edges on the outside of the auger flights. These disturbed auger cuttings should be assessed and logged although the exact depth of their occurrence is unknown.

Auger Cuttings Pros:

Auger Cuttings Cons:

 

INTACT OR "UNDISTURBED" SAMPLES

Intact or undisturbed samples are obtained by driving a tube into the subsurface and retrieving the sample within the tube (coring). There are many differing coring techniques which differ by tube design and coring technique.

 

CORING SAMPLE TUBE DESIGN

DRIVING TECHNIQUES FOR CORE SAMPLER TUBES

HAND-DRIVEN CORING

Heavy hammer device which slides up and can be dropped down to drive rods and sampler into subsurface.

Sampling system with multiple core tube design

Another method of driving a sampler into the ground had students using a jackhammer.

 

Basket retainers are used at the bottom of the core sampling tube. They are also called "core catchers". They aid in retaining noncohesive sediments such as saturated sands within the core tube while it is being raised to the surface.

 

Sediment-Sampling with Hollow-Stem Auger Drill Rig

 

Sampling through the borehole of the hollow-stem augers:

Standard Hammer Technique or Standard Penetration Test (SPT)

Core samples are opened and logged in the field to produce a borehole geologic log. Subsamples are taken for laboratory analyses.