Advanced Geotechnical Engineering Groundwater and Seepage flow in jointed rock mass (i) Advanced Geotechnical Engineering EAG 442 By: Dr Mohd Ashraf Mohamad Ismail

Properties of joint networks Rock mass properties: Filling Wall strength Block size Joint set Roughness Persistence Aperture Joint spacing Dip and dip direction Seepage Rock type Weathering grade

Fluid flow through RM “This topic concerns with fluid flow through rock mass which is one of the most difficult topics facing practicing engineer” “The basic idea is rock mass contains discontinuities and discontinuities are preferential flow paths”

What is groundwater? Rainfall that soaks into the ground and moves downwards into pore spaces and cracks in the rocks becomes groundwater. The rocks that store groundwater are aquifers. The study of groundwater is called hydrogeology. The study of groundwater requires an understanding of the hydrological cycle as it pertains to the recharge and discharge of groundwater and The study of the geological formations that store groundwater is essential for a comprehensive understanding of groundwater.

Key aspects of Groundwater Groundwater occurs as a result of specific geological conditions in combination with specific hydrological conditions. Aquifers exhibit porosity – the capacity to store water. Aquifers exhibit permeability – the capacity to transmit water Groundwater flows down the hydraulic gradient – from high head to low head (m). Flow occurs as seepage / matrix flow or fracture flow or both. Groundwater flows from recharge areas to discharge areas. Aquifers may be unconfined or confined – open to atmospheric pressure or sealed by an overlying impermeable layer. Groundwater chemistry changes naturally due to rock-water interactions in the aquifers. Groundwater management requires reliable aquifer characterization.

Interaction between SF and GW

Hydrogeological diversity Unconsolidated rocks: Primary Porosity Large storage Locally high permeability Consolidated rocks: Secondary fracture porosity Small storage Low permeability Consolidated rocks: Karsts (enlarged fractures) Moderate storage High permeability

Porosity and Permeability Porosity: the percentage of rock or sediment that consists of voids or openings Permeability: the capacity of a rock to transmit a fluid such as water or petroleum through pores and fractures Porous: a rock that holds much water Permeable: a rock that allows water to flow easily through it Impermeable: a rock that does not allow water to flow through it easily

Primary and secondary permeability Fluid flow through intact rock and fractured rock Intact rock: small scale (Rock matrix) Low K Fluid in Fluid out Rock mass: Large scale High K Fluid in Fluid out (Discontinuities)

Table of content Simulation model for GWF in JRM Parallel Plate model & Cubic law Actual Flow Observation in JRM & its simulation Permeability value of rock mass How water will affect rock mass

Table of content Review of Darcy’s Law and exercise on flow through discontinuity networks Next Tuesday 30 / 4 / 2013

Table of content Simulation model for GWF in JRM Parallel Plate model & Cubic law Actual Flow Observation in JRM & its simulation Permeability value of rock mass How water will affect rock mass

Simulation model for JRM (1) Continuum model (Equivalent Porous media) Model assuming an equivalent porous rock containing a discontinuity plane (2) Discontinuum model(Network Model) Assumed to exist in the water path discontinuity in the rock surface, forms a network of pathways of groundwater models

Simulation model for JRM Extremely Porous Rock Porosity CO2 Injection Pumice Permeability Scoria Oil and Gas reservoir

Geometrical Modelling Roughness orientation filling aperture size Number of joints 2-dimensional model 3 dimensional model

Mechanical modelling of rock mass Equivalent Continuum Discontinuum Input: properties of elements Input: properties of discontinuities Output: averaged stress in elements, displacement of nodal points Output: force between blocks displacement of rock blocks

K1 K2 K3 K4 Hydraulic modelling of rock mass K1 K2 K3 K5 K6 K7 K8 K4 Equivalent Porous Media Network Input: properties of elements Input: properties of discontinuities Output: averaged velosity in elements, head of nodal points Output: actual velocity through flow path head of discontinuities intersection points

EPM model Good points ●Easy to constitute ●Can consider vast number of joints Weak point ●Can not grasp the actual velocity ●Applicable only to the elements of REV

Network model Good point ●Actual velocity ●No need to assume the REV Weak point ●Accurate model is required ●Large computation

Table of content Simulation model for GWF in JRM Parallel Plate model & Cubic law Actual Flow Observation in JRM & its simulation Permeability value of rock mass How water will affect rock mass

Darcy’s Law Henry Darcy (or D’Arcy?) (1803-1858), Hydraulic Engineer. The ”discoverer” of Darcy’s Law, 1856. His law is a foundation stone for several fields of study including ground-water hydrology, soil physics, and petroleum engineering.

Hydraulic Conductivity Medium(cross-sectional area,A) Darcy’s Law Q = K A ー DHL Hydraulic Gradient Q Hydraulic Conductivity L DH Q Medium(cross-sectional area,A)

Hydraulic conductivity of the aperture Kf Parallel Plate Model vf= gt2 12n i Gravity：g Kinematic viscosity：n The simplest model of flow through a rock fracture is the parallel plate model (Huitt, 1955; Snow, 1965). This is the only fracture model for which an exact calculation of the hydraulic conductivity is possible; this calculation yields the well-known .cubic law. (Witherspoon et al., 1980). t vf Hydraulic conductivity of the aperture Kf tKf gt3 12n transmissivity Cubic Law

Results of applying the parallel plate model – 8　–7　–6　–5　–4　–3　– 2 2 1 -1 -2 -3 -4 Log(measured K [cm/s]) Log(calculated K[cm/s]) 1：1

Seepage condition in the in-situ rock mass

Spacing of disc opening = aperture Radial pipe flow model Spacing of disc opening = aperture

Result of applying the radial pipe flow model -2 -3 -4 -5 -6 -7 -8 1：1 Log(calculated K[cm/s]) – 8　–7　–6　–5　–4　–3　– 2 Log(measured K[cm/s])

Examination result of flow model Insitu Seepage flow experiment ？

Table of content Simulation model for GWF in JRM Parallel Plate model & Cubic law Actual Flow Observation in JRM & its simulation Permeability value of rock mass How water will affect rock mass

OBJECTS of Pathways Surveys Single Joint Super-Conductive Joints Joint System “One of the most intensive in-situ test to characterize the channeling flow in fractured rock mass”

Experimental area (Kagawa prefecture, Japan) Okayama Prefecture Shodoshima Wells Island Uno City Toyoshima Naoshima Yashima Geology: granite Aji Quarry Takamatsu City Highest quality of Japanese Tombstone Kagawa Prefecture

METHOD of Pathways Surveys Super Fine Grout Injection Removal of the Rock Blocks Observation of Grout Lumps on Both Joint Surfaces

Pathways Survey Method 1. Single Joint sealed

Pathways Survey Method 2. Super Conductive Joints

Pathways Survey Method 3. Joint System sealed sealed

Pathways Survey Method 3. Joint System water chamber concrete side front side front water pipe Rock Block Specimen

Pathways Survey Result Single Joint IN OUT

Pathways Survey Result 2. Super Conductive Joints Borehole

Pathways Survey Result 2. Super Conductive Joints

Pathways Survey Result 3. Joint System Removal

Pathways Survey Result 3. Joint System OUT

Notable Points Pathways in joints are what is called “channeling” pathways. Area ratio of pathways – joint plane is varied and normally distributed. Aperture(grout thickness) is log-normally distributed in a joint.

Historical perspective Fractures as parallel plates Constant width

Historical perspective Cubic Law of Fractures Aperture half width Fracture length

Historical perspective Fractures cannot be assumed as parallel plates. w

Historical perspective Fractures cannot be assumed as parallel plates. A real fracture surface is rough and tortuous.

Historical perspective The flow through a fracture follows preferred paths because of the variation in fracture aperture. Witherspoon (1980) Iwai (1976) Neuzil(1980) Tracy (1980)

Historical perspective The friction associated with the rough fracture surface affects the flow performance. Tsang&Tsang(1988) Brown (1987)

Historical perspective Stochastic aperture simulations Experimental support Aperture Width ? Effect of friction in fracture flow simulations

Historical perspective How do we obtain fracture aperture width? How do we simulate flow through fractures effectively? Application of water-resource research technology into petroleum engineering

Experimental Analysis The approach Experimental Analysis Aperture width, qm, qf Fracture simulation Simulation Aperture distribution Stochastic Analysis

Experimental Analysis The approach Experimental Analysis Aperture width, Qm, Qf Fracture simulation Simulation Aperture distribution Stochastic Analysis

The approach Information from experiments? Fracture permeability Fracture aperture Matrix and fracture flow contributions How these properties change with overburden stress

The approach Apertures measured physically Sand grains Impermeable surface Flow experiments Sand grains Apertures measured physically

The approach The fluid flow through a fractured rock mass will depend on: The aperture of the fractures which in turn will depend on, The normal stress acting across the fractures which in turn will depend on, The depth below the ground surface

The approach 500 psi 1000 psi 1500 psi To quantify the change in aperture with overburden pressure

km Experimental setup Core : Berea Accumulator Graduated Cylinder Pump CORE HOLDER Permeameter Accumulator Graduated Cylinder Pump Hydraulic jack Matrix L=4.98 Cm A=4.96 Cm2 Core : Berea

Experimental Analysis The approach Experimental Analysis Aperture width, qm, qf Fracture simulation Simulation Aperture distribution Stochastic Analysis

From experimental analysis The approach From experimental analysis waperture Is it possible to create an entire aperture distribution from a single value of mean aperture? Yes !

Apertures distributed log-normally The approach Apertures distributed log-normally Log-Normal Mean Log-Normal Deviation Variable ( Aperture )

Through a mean and a variance The approach Through a mean and a variance

Smooth fracture surface The approach Smooth fracture surface

Slightly rough fracture surface The approach Slightly rough fracture surface

Highly rough surface fracture The approach Highly rough surface fracture Larger Aperture Size

Creation of the aperture map The approach Creation of the aperture map Lag distance Co- variance Variogram Kriging

The approach Outcome of Kriging 3D 2D

Not the real picture but effective The approach Comparison Not the real picture but effective

Experimental Analysis The approach Experimental Analysis Aperture width, qm, qf Fracture simulation Simulation Aperture distribution Stochastic Analysis

Table of content Simulation model for GWF in JRM Parallel Plate model & Cubic law Actual Flow Observation in JRM & its simulation Permeability value of rock mass How water will affect rock mass

Primary and secondary permeability Permeability: Ability of the medium (either rock matrix, discontinuities) to transmit water Hydraulic conductivity: Ability of water (or any fluid) to passing through (transmit) between the medium

Primary and secondary permeability

Primary and secondary permeability

Table of content Simulation model for GWF in JRM Parallel Plate model & Cubic law Actual Flow Observation in JRM & its simulation Permeability value of rock mass How water will affect rock mass

Effect of water to rock mass Deterioration of rock Excessive amount of seepage inflow in confined/limited space area (underground excavation) Reducing effective strength in discontinuities

Effect of water to rock mass Deterioration of rock due to water causes instability of the excavation and rock slope. In the tropical region, such as Malaysia, Indonesia, the weathering process of rock is more extensive, which causes the rock deteriorate is accelerated. Very important subject to understand about the deterioration mechanisms of rock strength due to water, an increase in stability of the rock mass and how to improve the safety and performance of excavation.

Effect of water to rock mass The presence of moisture or water (at atmospheric pressure) can have several effects on the discontinuities’ behaviour. If the water is migrating, there can be a mechanical action in which softer material, such as clay or other decomposition, is washed out from the joints or faults. Chemical action may dissolve some of the more soluble mineral components, resulting in a change in the mechanical properties of the rock. Chemical hydration (gain or loss of water by hydration) in certain rocks may cause a volumetric change when accompanied by a change in mechanical properties. The alternation of wetting and drying will cause some rocks to expand and contract and will thus affect their properties. Some rocks will suffer to separate on bedding planes or, in more extreme cases, to completely disintegrate. Hence, in an underground operation where the humidity is seasonable or otherwise variable, the deterioration of surface rocks may be expected, especially in sedimentary rocks containing clay or other decomposition products. Major accidents, due to falling rock or the roof falling, have been reported particularly in underground works. Most cases are caused by the presence of underground water which decreases the rock mass strength. In coal mining operations where the roof and floor are mainly composed of clay-mineral bearing rock, the deterioration of the rock strength may cause the roof to fall and the floor to heave. Some rocks, which may be categorized as clay-bearing rocks such as shale and mudstone, will break without applying any compression to them. These rocks are characterized by a wide variation in their engineering properties, particularly by their resistance to short-term weathering by wetting and drying. For example in underground coal mining operations, shales are normally found in the roof and the floor of the roadway. The problem arises when the strength of shale is deteriorated due to the presence of water/moisture.

Thank you very much for your attention