Studying the impact of granular features on triaxial behaviour in sand – combine micromechanics and advanced lab testing

Introduction
Developments of offshore renewable energy are mostly located near shore where sandy sediments are often encountered at shallow depths below the seafloor. Many of these sandy sediments have special granular features (i.e. mineralogy, angularity, crushability), which can govern the sand response (Figure 1).

The classic geotechnical parameters in sand (i.e. relative density, friction angle) cannot effectively correlate with the granular features. Moreover, the current design standards provide very little information on the impact of granular features in sand. Detailed investigations are thus needed to help relate the granular features with geotechnical parameters for offshore foundation design (i.e. monopile, pile group, suction anchor), which can form the basis for further design optimization in sand.

Triaxial test is a widely used advanced laboratory test to measure shear strength and stress-strain response of soil. In a typical triaxial test in sand, one can obtain shear strength and friction angle as geotechnical parameters. The sample granular features, preparation methods and testing conditions (i.e. drained/undrained, isotropic/anisotropic) can largely affect the sand response. The impact of preparation methods and testing conditions have been studied widely, however, references about sample granular features on a triaxial response in sand are scarce. This is mostly because all measurements in a triaxial test do not contain information detailed enough at a granular scale.

A computed tomography (CT) scanner can capture detailed 3-D information of sand samples at a granular level in the field of micromechanics. Granular features and inter-granular behaviour of sand have been studied widely using CT scanning and imaging technique. Measuring triaxial response in sand and granular interactions by performing a small-scale triaxial test in a CT scanner provides an opportunity to visualize and quantify the granular features, and allow for detailed study of the correlation between granular features and triaxial element response in sand (Figure 2).
 

Image may contain: Rock, Text, Pebble, Gravel, Organism.
Figure 1 a) Common seen silica sand, b) offshore carbonate sand with high organic content. Click here for a bigger version of the picture. 
Image may contain: Yellow, Line, Text, Font.
Figure 2 Simulation of sand response in a triaxial test.
Click here for a bigger version of the picture.


Aim of the thesis

The aim of the thesis is to study the impact of granular features on the triaxial element response in sand.

After the study, the student should have a good understanding of basic sand characterisation and triaxial testing, and have a good overview of high particle angularity on the triaxial behaviour in sand.


Objective

  1. Perform basic soil characterisation on a carbonate sand sample based on the procedure from reference standards
  2. Make use of CT scanning to quantify the granular features of the carbonate sample
  3. Perform the small-scale triaxial test in CT scanner
  4. Post-processing of CT triaxial test result
  5. Explore the links between the observed triaxial behaviour and granular features of the carbonate sample


Research method and dataset

The student will work on a carbonate sand sample from Offshore Australia. Basic lab testing, advanced lab testing and CT scanning will be performed at NGI. Appropriate training and safety course for the NGI lab will be given before performing relevant tests.

 

The student will learn

  • The procedure of sand characterisation used in the industry and for research development.
  • Basic to advanced geotechnical laboratory testing essential for offshore foundation design.
  • Python programming and image analysis technique

Contact

Yining Teng (yining.teng@ngi.no)

Published Oct. 7, 2020 1:48 PM - Last modified Oct. 7, 2020 1:50 PM

Scope (credits)

60