LectureHydrogeological Investigations

Key issues:• Assessment of hydrogeological regime

is vital prior to the commencement of tunnelling operations

• What hydrogeological data is required? • How can this data be obtained?• Long term monitoring of the

groundwater system may be necessary during construction and once the tunnel is completed.

Hydrogeological Concepts

• Groundwater Table and flow

• Measurements of the Hydrostatic Head

• Measurements of permeability

• Groundwater chemistry

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The level of water is seasonal and, in coastal regions, tidal.

Ground water can erode subsurface materials by solution and dispersion of fines.

Water cycle

Groundwater conditions

Ground surface

Soil belt

Intermediate belt

Capillary fringe

Ground or free water

Bedrock

Saturated zone

Aeration zone(held water)

Phreatic surface / water table

Confined and Unconfined Aquifer

Hydraulic Head

Artesian & Sub-artesian Conditions

Hydrogeological investigation 1Hydrogeological investigation is run to assess the potential groundwater regime in the vicinity of the proposed tunnel. We need to answer the following questions:• Will the tunnel be excavated in dry conditions or is

the ground likely to be saturated?• Are there seasonal fluctuations in the groundwater

regime?• What is the predicted rate of water ingress along

the tunnel length?• Is the chemistry of the groundwater aggressive and

will it impact on the integrity of the proposed tunnel structure?

How to carry it out? An hydrogeological

investigation consists of:

• Measuring groundwater levels during the site

investigation and tunnel excavation (e.g. water

levels are measured at the beginning and end

of each drilling shift).

• Installing separate piezometers for each

aquifer

• Measuring the ground permeability

Hydrogeological investigation 2

We want to: • Plot groundwater level data in a contoured map

(contour lines indicating the ground water level)• Identify the aquifers and relate the mapped

groundwater levels to the identified aquifers• Determine the direction of groundwater flow (900

to contours) from the groundwater contour plot. • Determine the hydraulic gradient from the mapped

groundwater contour lines (calculating the gradient).

Hydrogeological investigation 3

Methods to record the water level

Water levels can be recorded via:

• Standpipes / piezometers / observation wells

• Standpipe piezometer / Casagrande

piezometers

The water level in the ground can be measured via a piezometers. This is the simplest devise for hydrogeological investigation.

It can measure the water level of a single aquifer, but it does not provide the value of the hydraulic head of artesian aquifer.

It is made of:– A plastic pipe installed in a borehole– A perforated or slotted section to allow

groundwater from the ground to flow into the pipe

– The porous element allows water through but prevents fine sediment from entering the piezometer

Standpipe

It can measure the water level of any aquifer and check for the presence of artesian aquifers.

It is made of a standpipe with grout seals above and below the zone of the measurement.

Standpipe piezometer

Water level readings

• Dipmeters are employed to read the water level in the standpipe

• Transducers can permit remote reading of water level

• Good practice to check the water level manually during the shortly after transducer has been installed to ensure that is functioning correctly

Permeability is the ability for a material to transmit fluid through its interconnected voids. It is not the same as porosity!If a fluid, present in a permeable material, is subjected to a difference in head (or potential) it will flow from the greater to the lower head. From Darcy’s law we can show that:q

v kiA

q = quantity of flow (L3/T)

A = cross-sectional area (L2)

v= discharge velocity (L/T)

i = hydraulic gradient

k = coefficient of permeability (L/T)

n = porosity

vs = seepage velocity (L/T)

Permeability (hydraulic conductivity)

Coefficient of permeability

10 10-1 10-3 10-5 10-7 10-9 10-11 10-13 10-15

Coefficient of Permeability (m s-1)

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in-situ

‘impermeable’

Permeability testsVarious tests can be carried out in boreholes to assess the permeability of the ground:

• Borehole test with Variable Head

• Borehole test with Constant Head

• Packer Test

• Pumping Test

qv ki

A

In all these tests, permeability is determined based on

the Darcy law:

and the Bernoulli equation: w

uh z

wt.

wt.

casing

Constant head: water is poured to maintain H constant.

Falling head:

d area A

H

H0

Ht

Time = t0

t

flow rate, q

t

0 ln = HH

fdtA

k

= q

kfdH

f is a shape factor that varies according to the casing arrangements in the borehole – see Barnes p.53 for details.

Open borehole tests

to increase accuracy, measurements need to be repeated for various q (and H)

Intake or Shape Factors• For a borehole open to its base, of diameter D,

and lined to the full depth • Intake Factor (F )= 2.75D.• For open borehole cased through impermeable

soil with its base at the interface with a permeable stratum.

• Intake Factor (F) = 2D

Packer Testing• Borehole packers are pneumatic or mechanical

devices that isolate sections of the borehole by sealing against the borehole walls.

• Single packer test – test interval from base of the borehole to the base of sealed zone.

• Double packer test – test interval isolated by two packers. Can be used for multiple tests after the borehole has been completed

Constant head source

Surface

Borehole and test pipe

Water table

Test

zone

Packer

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q

l

Aft

er W

alth

am (

2002

)

dl

lhq

k2

2ln

Use falling head source for rocks with low permeability.

The usual procedure is to pump water at a standpipe pressure of 10 kN m-2 into a washed-out borehole (typically 150 mm diameter) between inflated hydraulic packers which may be 1 - 5 m apart, depending upon the frequency of fissures.

Single packer test:

Constant head source

Surface

Borehole and test pipe

Water table

Test

zone

Packer

h

q

l

d Aft

er W

alth

am (

2002

)

dl

lhq

k2

2ln

Use falling head source for rocks with low permeability.

Double packer test:

• Ascending test suitable for stable borehole walls which do not require casing.

• Two inflatable packers usually 1.5m to 3.0m apart are installed on the drill rod or pipe.

• Start the tests at the base of the hole

• After each test, the packers are raised and another test performed.

Double Packer

Old w.t.

w.t. w.t.w.t.

r1 r2

h1 h2

q

)(

)ln( =

1 2 22

1

2

hhr

rqk

This test gives an average value for a stratum of soil below the phreatic surface and is effective to a depth of less than 45 m. Both casing and observation wells are perforated to allow free flow.

• Water is pumped out of the central casing

• Change in the water level is recorded in a series of monitoring boreholes

• The observed reduction in water level is called drawdown.

impermeable

for unconfined problem

Pumping out tests

see Barnes p.63 for derivation of the equations

Old w.t.

w.t. w.t.w.t.

r1 r2

l

h2

q

)(

)ln( =

1 2 hhlr

rqk

21

2

for confined problem

permeable

impermeable h1

impermeable

Pumping out tests

see Barnes p.63 for derivation of the equations

Pumping Test

Advantages• Regarded as the most reliable method

of estimating hydraulic conductivity• Permeability (K) over a sizeable section

of the aquifer can be obtained from these tests

Pumping TestDisadvantages• Expensive to carry out as several

boreholes are required – one as the pumped well plus monitoring wells to record drawdown

• Long duration of test, usually takes several days

Pumping Tests• To check that the drawdown around the

pumped well is equal in all directions we can arrange a series of monitoring/observation wells at right angles to each other.

• If the groundwater flow towards the well is isotropic, the drawdown or cone of depression around the pumped well should be circular in plan.

Groundwater Modelling• Once, we have collected the hydrogeological data, several

softwares may be used for the modelling of the water conditions in the ground. For instance, one of the industry standard groundwater software packages is MODFLOW developed by the US Geological Survey, freely downloadable at http://water.usgs.gov/ogw/mfusg/

Groundwater chemical analysis

• Groundwater chemistry varies depending on the nature and mineralogy of the soils and rocks that it passes through.

• Natural processes or ground contamination may produce groundwater that has an adverse affect on the structural integrity of the tunnel and its lining.

• Samples of groundwater analysed for potentially harmful components

Groundwater sampling• Ensure that the water sampled is a fresh influx

into the borehole. It is recommended that at least 3 borehole volumes are removed (purged) from the borehole prior to the sample being taken.

• Groundwater samples should be maintained at the same temperature as the temperature in the borehole

• Samples need to be placed immediately in cool box for transportation to the laboratory.

• Fill containers need to be employed to avoid head space and hence partitioning of the sample.

Sources & ReadingBS5930:1999.  Code of practice for site investigation

BS ISO 14686:2003. Hydrometric determinations. Pumping tests for water wells. Considerations and guidelines for design, performance and use.

Brassington, F.C., 2007. Field Hydrogeology. 3rd ed. Chichester: John Wiley and Sons Ltd.

Todd, D.K. And Mays, L.W., 2005. Groundwater Hydrology. 3rd ed. Hoboken: John Wiley and Sons Inc.

Price, M., 1996. Introducing Groundwater. 2nd ed. Abingdon: Taylor and Francis.

www.waterrauk.com.