# Glossary (p. V)

**Take:** see *Grout take*

**Thixotropy:** the property of a material that enables it to stiffen in a relatively short time on standing, but upon agitation or manipulation to change to a very soft consistency or to a fluid of high viscosity, the process being completely reversible, i.e. the viscosity of thixotropic fluids decreases with increasing shear rate (loading) and returns to its initial value after a time of regeneration. Fluids that show an increase in apparent viscosity with time are called thixotropic. Thixotropy is common in non-Newtonian grouts

**TPA method:** TPA stands for "Transient Pressure Analysis". The method, used in rock grouting, is based on the information obtained from observation of the pressure development after shut-in of the grout after a pass. This is done by stopping the pump deliberately and observing and plotting the ensuing pressure drop against time

**Transmissivity:** the transmissivity *T* [m^{2}/s] is the rate of flow of water through a vertical strip of ground which is 1 meter wide and extends the full saturated thickness of the ground when the hydraulic gradient *i* = 1. It can be expressed as the product of the hydraulic conductivity *K* [m/s] and the thickness of the aquifer. A customary unit of transmissivity of the rock mass is the Lugeon. The transmissivity of the rock mass in Lugeon units is defined as the seepage flow in litres/minute per meter length of borehole under a pressure of 1 MPa

**Tremie grouting:** see *Gravity grouting*

**True solution:** one in which the components are 100 % dissolved in the base solvent

**Tube-à-manchette:** see *Sleeve pipe*

**Unconfined compressive strength:** the load per unit area at which an unconfined prismatic or cylindrical specimen (height = 2 × width) of material will fail in a simple compression test without lateral support

**Upstage grouting:** a grouting procedure, usually in rock grouting, where the grout hole is drilled to its full depth and thereafter grouting takes place in stages from the base of the hole upwards

**Viscosity:** the internal fluid resistance of a substance which makes it resist a tendency to flow. A distinction is made between kinematic viscosity, *v*, and dynamic viscosity, η, for which: *v* = η/Δ, where: Δ = density. Apparent viscosity is equivalent to dynamic viscosity, but considers point events:

shear

The apparent viscosity, µ_{app}, measured in [Pa·s] represents the ratio of shear stress, τ, and shear rate, . For most solutions, the viscosity is a function of the shear stress and depends on the agitation of the substance. If subjected to rapid agitation, the viscosity of a substance reduces and tends towards a minimum value called plastic viscosity. For some substances, notably Newtonian liquids, the viscosity is independent of the shear stress and the ratio, , becomes a constant, the absolute (dynamic) viscosity, η. Hence, for Newtonian fluids such as the true solutions used during grouting, the concept of plastic viscosity does not apply. The kinematic viscosity, *v*, measured in m^{2}/s is a function of the density of the material *v* = µ/ρ

**Void filling:** see *Bulk filling*. Term not recommended

**Watertable:** the phreatic surface at which the pore water pressure equals atmospheric pressure, i.e. the standing water level which establishes in a hole dug in the ground

**W/C ratio:** the water-cement ratio is the ratio by weight of water to the dry cement content of a grout

**Water content:** see *Moisture content*

**Water retention capacity:** the ability of a suspension to retain water if not subjected to pressure

**Yield point (Yield stress):** the lowest shear stress value at which there is a sudden drop in the value of the applied stress and from where continuing extension occurs at more or less the same stress value. The yield point, yield stress or cohesion τ_{0} is the shear stress at which plastic deformation occurs and a Bingham fluid begins to flow. The yield stress, τ_{0}, of a Newtonian fluid is zero

**Yield strength:** the stress at which a material exhibits a specific deviation from proportionality of stress and strain

Key

- 1 Shear stress τ [Pa]
- 2 τ
_{0}yield point = yield stress - 3 c cohesion
- 4 Apparent viscosity µ
_{app} - 5 Shear rate [s
^{–1}]