Preservation, storage and transportation of undistured soil samples for geotechnical work are critical to actual laboratory simulation of the field conditions and experiences.

Although the total characteristics of undisturb soil samples before core recovery cannot be entirely guarrantee since the mechanical tools engaged and the driving of the thin-walled tubes, sometimes referred to as Shelby tubes, during recovery will induce some amount of stresses into the soil but the degree of disturbance can be substantially minimised during core extraction depending on the adopted method of recovery.

The degree of disturbance can also be calculated as a percent value of the area ratio, that is, ratio of disturbed area to total area of soil. This can be used to critically judge the level of disturbance for a soil sample based on the method of core recovery and when the calculated value is less than or equal to 10%, the sample is considered as undisturb. This can also be compesated for, with a factor of safety if disturbance is slightly above 10%.

Preservation and transportation of soil samples for geotechnical work has been critically guided by the international code of practise, ASTM D4220, and BS EN ISO 22475, which provide for perfect wax selant for shebby tube after recovery and the avoidance for vibrations or any undue external effect that is capable of particle distortion or material shrinkage for the transported samples which shall only be conveyed in a moisture proof container.
                
 ...................Credit: Engr. Femi Olomola

The practise for a geotechnical soil sampling is limited to the nature and type of soil conditions within the boring interface and the intended type of geotechnical test to be carried out. 

The sampling processes for an undisturb soil samples that requires compressibility, moisture content, density and shear strenght test is most critical to the entire geotechnical line of action and analysis and therefore requires indepth understanding of the associated activities as these parameters will require to be adequately provided for in the laboratory under a good simulation of the field model. 

The international code of practise ASTM D1587, BS EN ISO 22475, had provided detail and accepted guildiances as to how this is carried out coupled with expertise and individual experience of the field personnel.

The sampling method engaged for an intact cohesive soil will be different for that engaged for a cohesionless porous soil. Intact cohesive soil can be adequately recovered by shelby tube which is usually attached to a drill rod. This practice is limited to fine-grained soils that can be penetrated by the shelby tube. 

The shelby tube sampling method may not be adequate for soils containing coarse sand, or gravel, or very hard soils as the process of breaking through might distort the particle arrangement and therefore render the recover core as not usageble. Other suitable sampler that will drive through easily without much disturbance to the soil structure can be engaged. 
           
 ...................Credit: Engr. Femi Olomola

Continuous calibration of the testing equiment is essential and crucial to achieving very accurate result as this provide a way to monitor the equipment against malfunctioning. Calibration is done against an accurate standard which has been verified and accepted for such level of accuracy.

The following measuring parameters in a summary are required to be calibrated as guided by the international code of practise for every six month to two years depending on the type of instrument involve and the detail provision of this had been provided in the code as stated above:

1. Force measurement 
2. Pressure measuremnt
3. Volume change
4. Displacement measurement
5. Local strain measurement      

Having known this, all our geotechnical testing equipment had been caliberated and certifield during manufacture and have been debugged during software development. 

Nevertheless, our laboratory manual and quality plan had incorporated the need for a periodic check and monitoring, along with instrument calibration.


           
   ................Credit: Engr. Femi Olomola