What mixture of components do I need for my grout? 

1. Introduction

This knowledgebase article looks at the consistency required when mixing grout for installations of Piezometers and Inclinometer casing.

2. About grout mixing

The grout mix is traditionally a mix of water, Bentonite powder and cement, with proportions of water to cement mainly determining the ultimate grout strength, the Bentonite provides some plasticity to the mixture and provides a suspension medium to stop the cement settling within the grout column so much, before hardening.

The theory is that you try to replace the borehole natural material with grout of similar permeability and strength. As this varies with depth, it is difficult to do, so a compromise grout is designed for use throughout the length of the borehole. It must not be too strong in soft clays or too soft in hard rock.

If there are very distinct boundaries between hard and soft geological materials (soft clays on to rock), then a hard grout maybe placed in the rock and a softer grout placed in the clay.

Therefore you need to consider the geology strengths and then find a grout mix to match. When the grout is set it should move the casing with the surrounding grounds movements, it is key that the set grout is not significantly stronger than the surrounding soil.

As cement and Bentonite qualities vary globally, it would be advisable to conduct some tests with varying proportions of locally available materials to determine the correct mixes for your boreholes.

Manual 171 (linked below) goes into further technical specifications of strengths of certain mixtures 

Link: http://www.itmsoilsupport.com/manuals/Man171_Bentonite_Pellets.pdf

Checked MO'R v1.0 13/05/15