Lecture Hydrogeological 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

Water cycle The level of water is seasonal and, in coastal regions, tidal. Ground water can erode subsurface materials by solution and dispersion of fines. fl.water.usgs.gov/Orlando/outreach/cycle.gif

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 1 Hydrogeological 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?

Hydrogeological investigation 2 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 3 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).

Methods to record the water level The water level in the ground can be measured via a piezometers. This is the simplest devise for hydrogeological investigation. Water levels can be recorded via: Standpipes / piezometers / observation wells Standpipe piezometer / Casagrande piezometers

Standpipe 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 piezometer 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.

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 (hydraulic conductivity) 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 = 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)

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) lab in-situ ‘impermeable’

Permeability tests Various 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 In all these tests, permeability is determined based on the Darcy law: and the Bernoulli equation:

Open borehole tests flow rate, q Constant head: water is poured to maintain H constant. casing H to increase accuracy, measurements need to be repeated for various q (and H) wt. Falling head: Time = t0 H0 t Ht f is a shape factor that varies according to the casing arrangements in the borehole – see Barnes p.53 for details. wt. d area A

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

Single packer test: q h l Constant head source Packer q Surface h 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. Borehole and test pipe Water table l Test zone After Waltham (2002)

Double packer test: q h l d Constant head source Surface Borehole and test pipe Water table Test zone Packer h q l d After Waltham (2002) Use falling head source for rocks with low permeability. 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

Pumping out tests for unconfined problem 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. q Old w.t. w.t. w.t. w.t. h1 h2 impermeable r1 r2 for unconfined problem see Barnes p.63 for derivation of the equations

Pumping out tests for confined problem q Old w.t. w.t. w.t. h1 h2 impermeable w.t. permeable l impermeable r1 r2 for confined problem 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 Test Disadvantages 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 & Reading BS5930:1999. 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.