download-manuals-ground water-manual-gw-volume4fieldmanualgeo-hydrologypartii

Government of India & Government of The Netherlands

DHV CONSULTANTS &DELFT HYDRAULICS withHALCROW, TAHAL, CES,ORG & JPS

VOLUME 4GEO-HYDROLOGY

FIELD MANUAL – PART II

DRILLING OF LITHOSPECIFIC PIEZOMETER

Field Manual – Geo-hydrology (GW) Volume 4 – Part II

Geo-hydrology March 2003 Page i

Table of Contents

GENERAL 1

1 STEPS IN THE CONSTRUCTION OF PIEZOMETERS 1-1

1.1 PIEZOMETER LOCATION 1-11.2 FINALISATION OF PIEZOMETER LOCATION 1-11.3 REPORTING OF FIELD INVESTIGATIONS 1-21.4 APPROVAL FOR PIEZOMETER CONSTRUCTION 1-21.5 DISCUSSION AND INTERACTION WITH LOCAL COMMUNITY 1-3

2 CONSTRUCTION PROCEDURE 2-1

2.1 PREPARATORY WORK 2-12.1.1 SITE PREPARATION 2-12.1.2 SUPERVISION OF DRILLING 2-12.1.3 ESSENTIAL TOOLS FOR FIELD HYDROGEOLOGIST 2-12.1.4 FIELD NOTES 2-22.1.5 DESCRIPTION OF DRILL CUTTINGS 2-3

3 DRILLING 3-1

3.1 SELECTING THE APPROPRIATE DRILLING TECHNIQUE 3-13.2 DECIDING THE DEPTH OF PIEZOMETERS 3-13.3 DIAMETER OF PIEZOMETER 3-23.4 ACTIONS TO BE TAKEN PRIOR TO DRILLING 3-33.5 PIEZOMETER CONSTRUCTION IN UNCONSOLIDATED FORMATIONS 3-33.6 SAMPLING PROCEDURES DURING DRILLING 3-43.7 DOWNHOLE INSPECTION 3-63.8 PIEZOMETER COMPLETION 3-6

3.8.1 PIEZOMETER DESIGN 3-63.8.2 SCREEN LENGTH 3-63.8.3 DESIGN OF GRAVEL SIZE AND SCREEN SLOT SIZE 3-73.8.4 ANNULAR SEALS 3-83.8.5 SURFACE SEAL 3-83.8.6 PROTECTIVE COVER 3-83.8.7 DEVELOPMENT 3-93.8.8 PUMPING TEST 3-10

4 PIEZOMETERS CONSTRUCTION IN CONSOLIDATED FORMATIONS 4-1

4.1 DTH DRILLING CHARACTERISTICS 4-14.2 SAMPLING PROCEDURES FOR CONSOLIDATED ROCKS 4-1

4.2.1 REMOVAL OF FINES DURING DRILLING 4-24.2.2 PIEZOMETER COMPLETION 4-2

4.3 DOCUMENTATION 4-2

5 REFERENCES 5-1


Geo-hydrology March 2003 Page 1

GENERAL

The Field Manual on Geo-Hydrology comprises the procedures to be carried out to ensure properexecution of design of the groundwater water level monitoring network, operation and maintenance ofobservation well and piezometers. The operational procedures are tuned to the task descriptionsprepared for each Hydrological Information System (HIS) function. The task description for each HIS-function is presented in Volume 1 of the Field Manual.

It is essential, that the procedures, described in the Manual, are closely followed to create uniformityin the field operations, which is the first step to arrive at comparable hydrological data of high quality.It is stressed that water level network must not be seen in isolation; in the HIS integration of networksand of activities is a must.

• Volume 4 of the Field Manual deals with the steps to be taken for network design andoptimisation as well as for its operation and maintenance. It covers the following aspects.

• Part I deals with the steps to be taken for network design and optimisation. Furthermore, siteselection procedures are included, tuned to the suitability of a site for specific measurementprocedures.

• Part II details with piezometer construction procedure with details of the different elements andthe significance of different elements in the piezometer construction

• Part III comprises the preparatory activities and procedures for carrying out aquifer tests. Theprocedures to be adopted for analysis of pumping test data is briefly discussed

• Part IV comprises the testing and installation of DWLR’s. Procedures to be followed forprocurements and installation are outlined in Volume 4 of the reference manual.

• Part V deals with the need for carrying out Reduced Level Surveys and the procedures incarrying out the survey are outlined.

• Part VI deals with the standardised procedures to be adopted for manual collection of water leveldata from open wells and piezometers.

• Part VII deals with the standardised procedures to be adopted for retrieval of data from DWLRand integration with the software.

• Part VIII, deals with procedures to be adopted for regular inspection and maintenance ofpiezometers and DWLR’s.

The procedures as listed out in this manual are in concurrence with the ISO standards as far asavailable for the various techniques and applicable to the conditions in Peninsular India.


Geo-hydrology March 2003 Page 1-1

1 STEPS IN THE CONSTRUCTION OF PIEZOMETERS

In this chapter an overview of steps in the construction of litho-specific piezometers is presented.Piezometers are purpose built water level and water quality monitoring system. These need to be ofsmall diameter sufficient enough to accommodate the water level measuring device and the water-sampling pump. In unconsolidated formations, piezometers are to be provided with screens tappingthe zone of interest; where as in the consolidated rocks, piezometers need to be left open ended(uncased) beneath the loose soil/loose over-burden where the hole has to be provided with a casing.

Up-gradation/strengthening of the observation well network need to be continuously carried out byreplacing non performing open wells with dedicated piezometers as well as constructing deeppiezometers to cover aquifers that have not been previously monitored. Improvement in the density ofthe network would also arise with time. All this would involve construction of many piezometers. Thepiezometer construction procedure should follow standard procedures with incorporation of site-specific elements.

1.1 PIEZOMETER LOCATION

The piezometers should record harmonized natural behaviour of groundwater rather than local micro-trends. To ensure this, the site chosen has to ensure that:

• The selected location shows no impact to any external sources such as from canal, tank,perennial river and irrigation return flows, except in special cases where interest is the study ofthe influence of these parameters on groundwater system.

• The site should not fall within the radius of influence of a well, which is under pumping; but itshould be capable of recording the effects of the pumping as a regional phenomenon.

• The piezometric head/water quality at the site should not be influenced by local recharge/pollutantsources.

There could be many general as well as area-specific logistical considerations such as:

• No other agency is considering constructing a piezometer tapping the same aquifer, in thevicinity.

• The site is approachable by the rig and support vehicles.

• Adequate space is available at the site for setting up drilling equipment, digging mud pit anddraining the discharge, while the site should be clear of trees, overhead electric cables, underground cables/ pipelines/ drainage lines etc.

• The ownership of the site is clear and agreements have been made for drilling the piezometerand for continued monitoring.

• The site should be safe from vandalism, as a costly DWLR will be installed.

• The site should be neither too close nor too far off from the road.

1.2 FINALISATION OF PIEZOMETER LOCATION

Based on all the studies and keeping in mind the logistical and safety considerations the potential sitehas to be identified. Where more than one site is considered then a joint team of hydrogeologistsshould visit the area and identify the most favourable location. The site selected should be verified forits true representation of the area specific lithology and regime system. The interference frompumping wells, surface water sources, polluting sources, seepage from return flows should beavoided at all costs.



1.3 REPORTING OF FIELD INVESTIGATIONS

Based on the field investigations the field data collection format has to be filled and the feasibilityreport has to be prepared. The following information must be documented as a file giving details of theprocedures followed in deciding the piezometer site. The report should include:

• A sketch showing the identified site and important landmarks in the vicinity. The sketch shouldincorporate the north direction and the distance of the site from the landmarks.

• Locate the site on the toposheet of 1:50,000 scale. Record its longitude, latitude and the reducedlevel as read from the toposheet. Use the hand held GPS wherever available for getting thegeographical co-ordinate values.

• A narrative describing the regional lithologic, stratigraphic, structural, and hydrologic settings ofthe area should be given.

• A narrative must be provided which describes field procedures used to characterize geologic andhydrologic conditions of the site. Standardized field procedures may be referenced. Details of thesite-specific geology and hydrology based on data collected should be explained. The narrativemust describe the proposed piezometer design. Interpretations of results must be presented in aclear and concise manner. All published information sources used in the compilation of thehydrogeologic investigation must be listed.

• Appendices of the report must include:

- Compiled logs of all borewells and piezometers.- The raw data for any and all tests (e.g., geophysical survey, bore hole logging, water quality

analysis, pumping tests).- Water level hydrographs of monitoring wells in the neighbourhood- Water table elevation contour maps- Hydrometerological data of the area- All additional information that may facilitate the clearance of the proposed site.

Lithologic cross-sections must be constructed or inferred. At least one cross-section must beconstructed parallel to groundwater flow. The subsurface conditions of the site must be illustrated inthese cross-sections. Where more than one interpretation may be reasonably made, conservativeassumptions must be used. A clear picture has to be given of the thickness, depth and lateral extentof the aquifers in the area with a clear definition of the aquifer to be monitored and the geo-hydrologicconditions. The type of monitoring required and the need if any for a DWLR and sampling pumpshould be brought out. The report should clearly bring out the need for the Piezometer at theproposed site with a justification for the expenditure to be made in establishing and running thenetwork. The utility of the information emerging from the piezometer should be highlighted.

An estimate should also be prepared which should include site preparation, drilling, casing/ screeninstallation, gravel pack, sealing, development, pump test, platform and well head construction.

The recommended location should be marked on the ground with paint and the same described in thereport.

1.4 APPROVAL FOR PIEZOMETER CONSTRUCTION

The site selection report from the field offices should be forwarded to the head quarters for approvaland clearance. It is expected that the justification for the construction of the piezometer would bereviewed by a committee of senior officers at the head quarters, who will look at the requirement froma national/state perspective as well as from a local perspective. The location of the piezometer shouldbe superposed on the existing network and its utility assessed. The aquifer to be monitored has to beverified on the cross section. The added value from the new piezometer should be verified from atechnical, management and financial perspective. On complete satisfaction of the utility of thepiezometer the financial estimate has to be examined. While standard rates should be the norm,deviations should also be considered on case by case basis. The sanctions for depth of drilling,



casing depth, screen position should be based on the field report, which should come up forratification after completion of drilling. In the case where drilling contractors are to be hired for drillingthe piezometers the procedure for hiring drilling contractors should follow the established norms. Thetender document for inviting the drilling contractors should clearly mention that a qualifiedHydrogeologist should be part of the drilling team and his/her CV should be part of the enclosures.The utility of hiring more than one contractor when the piezometer locations are far part should beexamined seriously. Drilling Contractors when used the terms of the contract should clearly specifythe obligations of the contractor as well as the department. Drilling being a seasonal task theprocedures for selection of contractors should not be cumbersome. Acceptance of State GovtApproved rates can reduce the process of selection. Since rain, water and mud are major hindrances,it is normally recommended that the most difficult holes be drilled first if they are accessible, savingthe most convenient holes for last or to drill when the others can't be reached.

1.5 DISCUSSION AND INTERACTION WITH LOCAL COMMUNITY

On obtaining the clearance for construction of the piezometer from head quarters, a meeting shall beconvened in the village where the piezometer site is proposed. The invitees should include the villageelected representatives, village officials, elders, farmers, women, schoolteachers and youth. Theservices of NGO groups active in the area should be used for conducting the meeting. The meetingshould address the local groundwater issues and the need for groundwater monitoring. The proposedplan for establishment of the piezometer and the most favourable site location identified need to bediscussed. Any suggestions from the community should be considered and animated in detail. Theagency should also promise the community to make available the interpretations of the data collected.As a follow up to the discussions, an agreement should be obtained from the community to makeavailable the required co-operation for safeguarding the piezometer as well as upkeep of the area.



2 CONSTRUCTION PROCEDURE

2.1 PREPARATORY WORK

Successful piezometer construction requires careful advance planning to be conducted in the mostexpedient manner. Proper drill site selection and preparation are essential to avoid drilling at wrongsite, minimise wastage of drill time and other associated costs. Land clearance is an essential itemthat cannot be taken lightly or ignored. Disputed lands can result in a tremendous litigation and liabilityto the department. The following are some detailed items to consider prior to commencing drilling ofpiezometer.

2.1.1 SITE PREPARATION

Drilling sites need to be prepared prior to arrival of the drilling rig. The site has to be levelled in orderto drill a vertical hole. Inclined bores considerably reduce the diameter and depth calculations becomeerroneous. Prior to extensive site work, the driller must visit the site and clearly place hisrequirements. Overhead area must be clear of obstructions. Sites with trees and overhead power lineshould be avoided. If it is necessary to work closer to power lines, the drill crew should inform theelectrical authorities either to shut down the power supply or to make the working environment safe.Underground laid infrastructure such as water lines, sewer lines, electrical/telephone cables, if any,should be checked before commencing work. Roots are a major problem; they force their way into thepiezometers. In such areas proper preventive care should be taken by increasing the casing depth oridentifying the root path and treating them.

It has to be ensured that the drilling rig has access to the site upon arrival. Problems have arisen inthe past from hostile villagers and uncooperative landowners, which can be avoided if the villagemeetings are conducted and local communities are taken into confidence. Bridges/culverts to becrossed must be inspected to check whether they have the required width/ soil strength and have thecapacity to take the weight of the rig, along with the spares.

2.1.2 SUPERVISION OF DRILLING

It is important to monitor the drilling and ensure that all procedures adopted should help inconstructing a quality piezometer. The piezometer on completion should be providing the true pictureof the water level and water quality without any bias. The drilling of the piezometer, geophysical downhole logging, development and pumping test need to be carried out under the supervision of an onsite hydrogeologists. Where the work is subcontracted to a drilling contractor, the drilling contractorshould be responsible for employing the site hydrogeologist who will be available at all times till thepiezometer construction is complete. The site hydrogeologist shall be responsible to record the drillingdetails, examine and interpret the drill cuttings, describe and record the physical and lithologicalcharacteristics of the geological material, supervise the well design, well development, measure thedischarge and collect the water samples.

2.1.3 ESSENTIAL TOOLS FOR FIELD HYDROGEOLOGIST

Field tools assist the field hydrogeologist in giving an accurate description of the drill cuttings. It isrecommended the field hydrogeologist have these basic items (see Figure 2.1) which include:

• Pocket knife to cut the samples for testing hardness and exposing fresh surfaces.

• Millimeter scale and grain size indicator to determine the size of the particles

• Dilute hydrochloric acid to aid in recognizing calcium carbonate materials such as limestone,chalk, or dolomite

• Magnifying glass (a 10x) to make a better identification of materials by enabling closer inspection



Figure 2.1:Field tools for drill cuttings examination

2.1.4 FIELD NOTES

Field logs and notes on drilling should be prepared at the drill site itself.

The field description of drill cuttings should be simple and orderly so that the use of the terminology isuniform. A good field description of the drill cuttings is very important for the design and preparation ofvertical sections. The site hydrogeologist and the drill crew are the only people who witness thedrilling and the material obtained. Therefore a reasonable amount of accurate information must belogged. At a minimum, the field hydrogeologist must, in the field, note on a descriptive log thefollowing: The field hydrogeologist must make sure to note the following on descriptive log:

• Start and stop times for drilling

• Names of field personnel

• Drill cuttings details-Colour, Texture , shape, mineral assemblage, rock type

• Diameter of drill bits

• Depth at which water encountered (water "struck" level) and discharge variations with depth

• Drilling rate

• Casing depth

• Drill completion depth

• Screen position

• Gravel pack position

• Well completion depth

• Water bearing zones

• Development time

• Discharge after development

• Water quality details pH, EC

• Depth to water upon completion (water "rest" level)

A Standard data collection format should be adopted. All field data should be computerisedsystematically as soon as the drilling is complete and the field data brought to the District/RegionalData Centre.



2.1.5 DESCRIPTION OF DRILL CUTTINGS

The descriptions of the drill cuttings should be as simple as possible (see table 2.1). Every smallvariation does not necessarily warrant description on the log. The description should include:

Principal constituent: First determine the major constituent in the sample. If a significant portion(greater than five percent) of a secondary material is present then describe and identify it.

Colour: Describe the primary color and restrict description to one colour. If one main colour does notexist in a sample, make a simple description of the multicolouration.

Texture: Mention the texture of the primary material under three to four main categories such asCoarse-grained, medium grained, Fine-grained, Highly organic etc.

Shape: Categorise the most dominant shape of the drill cuttings under rounded, sub rounded orangular.

Hardness: should be mentioned with respect to Mohs 'Hardness Scale

5.5 - 10: Rocks that will scratch the knife: Sandstone, Chert, Schist, Granite, Gneiss, someLimestone

3 - 5.5: Rocks that can be scratched with the knife blade: Siltstone, Shale, most Limestone

1 - 3: Rocks that can be scratched with fingernail: Gypsum, Calcite, Evaporites, Chalk, someShale

Cementation: Identify the degree of cementation if any.

Descriptive adjectives: Use any descriptive adjectives that might further aid in the understanding.

Log form: To promote consistency, use the standard log form, which is consistent with the data entrysystem.

Depth to(m)

Lithological description Colour Texture Shape Remarks

0.2 Laterite red hard subangular-subrounded

6.5 Laterite verigated/ wuggy red medium subangular to angular

17.1 Lateritic clay red fine rounded

17.5 Basalt weathered black medium subangular-subrounded

29.5 Basalt weathered/ fractured black coarse subangular to angular

51 Basalt hard black fine subangular-subrounded

52 Clay black fine rounded

83 Basalt hard black fine subangular-subrounded

83.9 Basalt weathered/ fractured black coarse subangular to angular Water toucheddischarge 0.2cum/hr

86 Clay Ash fine rounded

87 Sand White fine subangular-subrounded Grainsize: fineto medium

Table 2.1: Sample description of drill cuttings during the construction of a piezometer



3 DRILLING

The purpose of constructing a lithospecific piezometer is to obtain complete lithological data and notjust to drill a monitoring well. In order to obtain data of maximum accuracy, the field hydrogeologistmust work closely with the driller and consult with him whenever changes are noticed in penetrationrate, slow returns, change in colour of samples, reduction in discharge etc. The hydrogeologist mustrecognize the reasons for such changes. The difficulties in drilling, such as caving, boulders, caverns,etc. Whenever encountered, must be clearly recorded.

Construction of lithospecific piezometers must ensure that the piezometers meet the design criteria forwater level and water quality monitoring. Factors to be considered for piezometer construction shallinclude the following: aquifers to be monitored, nature of materials that make up and overlie theaquifer (for example, unconsolidated or consolidated materials; if consolidated materials whetherfractured or have cavities caused by dissolution); the depth to water, the type of drilling equipmentrequired; access to the site; well casing and screen materials, length, and diameter, and cost. Inunconsolidated deposits, the piezometer design, including the well screen, casing, annular space,back fill, gravel and surface seals.

Specific aspects of design however, can vary depending on specific requirements to meet localvariations, site conditions encountered, and the drilling method used.

3.1 SELECTING THE APPROPRIATE DRILLING TECHNIQUE

Drilling technique for construction of piezometer will depend upon the type and nature of formationslikely to be encountered below at the selected site. The technique to be adopted for soft andunconsolidated sediments shall be rotary, with bentonite mud or any other biodegradable mud as thedrilling fluid. In the hard rocks, DTH drilling rigs are best suited. The DTH drilling technique uses air tobring the cuttings to the surface, as well as cleanses the hole. Availability of high-pressurecompressors makes drilling very fast. In such situations the fines get deposited in the fractures. Thedrilling in such cases should be followed up systematic development. In the soft rocks, with pooraccessibility and in river alluvium, hand rotary drilling can be adopted as in parts of Orissa, TamilNadu and Andhra Pradesh. In hard rocks, with heavy overburden having boulders the drilling has tobe done using a combination of rotary and DTH rigs.

The drilling should ensure that it is capable of recording faithfully the harmonized areal behaviour ofgroundwater of the targeted aquifer in the area, instead of a local micro trend. The piezometer shouldnot be effected by wrong drilling techniques which can bring in external contaminants such as, poorquality water used in the mud pit, thick bentonite mud, drilling oil etc.

3.2 DECIDING THE DEPTH OF PIEZOMETERS

The depth and diameter of piezometers are two important factors, which not only decide their best-suited design, but may also affect the cost/economics of the piezometer installation.

In the unconsolidated formations, the aquifer horizon for construction of piezometer has to be basedon good understanding of the different vertically distributed aquifers, and the specific aquifer ofinterest. In case all the aquifers need to be monitored, piezometer nests have to be constructed.When constructing piezometer nests, different screen zones must be sealed perfectly using cementgroup.

Systematic collection of drills cuttings and recording of drill time log, followed by electrical logging ofthe borehole, is very important in delineating the exact thickness of the aquifer. In the event ofconstruction of nests, the deepest aquifer should be drilled first. The identified zones should be



correlated with the regional aquifer system, distributed in the sub-basin or basin, and accordingly thepiezometer depth is then decided.

In crystalline rocks, the depth of the piezometer should be decided on the basis of the occurrence ofaquifer(s) to be monitored in a given hydrogeological environment. Three typical situations arediscussed

Case i: Weathered zone is made up of quartz and the fractured rock immediately underlying it. Theweathered zone acts as a good storage zone with its inter-granular connection, while the underlyingfractured zone forms the main flow/conduit zone. In such a case the overlying permeable zonerecharges the fractured zone and hence the two zones can be considered as part of the same aquifer.The piezometer should be then drilled down to the fractured zone.

Case ii: Fractured zone is overlain by clayey weathered zone. The weathered and fractured zoneexhibit different permeabilities. In such situations both the weathered and fractured zone are to beconsidered as independent zones. The monitoring should be done independently for the weatheredas well as the fractured zone. The shallow weathered zone can be monitored using an existing opendug well while the fractured zone is monitored by constructing a piezometer.

Case iii: Weathered zone is clayey and impermeable, the recharge to deeper fracture zone may befrom a distant recharge area. In such case the piezometer has to be installed against the fracturedzone only. The extent and thickness of the fractures do not follow a systematic fashion, hence theneed for greater care in identifying the fractured zone by thoroughly monitoring the drilling.

In case of basaltic rocks, occurrence of multiple aquifers is common. Generally, the upper weatheredzone in such rocks is totally clayey and impervious and the first aquifer in such formations may occurat different depth as vesicular zones. Each vesicular flow should be tapped by an independentpiezometer. In areas where more than one vesicular flow has to be monitored, piezometer nests or agroup of piezometers within a limited area (village, watershed) need to be installed, tapping differentaquifers. Care has to be taken in properly sealing the upper aquifers while tapping the deeper zones.Typically, contractors who drill drinking water wells are not the best suited for drilling suchpiezometers. Departmental drilling rigs should be mobilised for taking up such drilling.

In the case of hard sedimentary rocks, good understanding of the stratigraphy is critical inunderstanding the different potential aquifers. Sandstone, shale and limestone occur in sequences.The sandstone in many cases have copious supplies. The limestone rocks possess both primary andsecondary porosity in the form of fractures, solution cavities and cavernous zones. Shale have limiteddischarge. Good understanding of the stratigraphy, combined with judiciously used geophysicalsurveys and profiling, the depth of the aquifer to be monitored can be inferred. Confined aquiferswhen met with produce artesian free flowing wells, should be, anticipated at the design stage itself.Methods to monitor the pressure changes should be part of the design.

The depth has to be accurately measured after the piezometer construction is complete by using aweighted tape. The measurement should also be compared with the total number of drill rods usedduring the piezometer construction.

3.3 DIAMETER OF PIEZOMETER

A piezometer is a non-pumping well and ideally needs to be as small in diameter as possible. Thediameter should be such that it shall facilitate measurement of water table using a variety ofmeasuring devices and collection of water sample. The diameter will also be dictated by the diameterof the measuring device, such as the probe of the Digital Water Level Recorder, diameter of the waterquality sampling pump. Piezometers having a diameters of 100 mm are the most suitable. The utilityof the piezometers, to carry out pumping tests, geophysical down hole logging and hydrofracturingshould also influence the diameter of the piezometer. In the case of deep tube wells (>100 m) in the



alluvial areas, the cost will be a major consideration in deciding the diameter of the piezometer. Insuch situations telescopic design of 100-150mm down to 30 m followed by 50 mm diameter till thebottom should also be seriously considered. Inclined piezometers can reduce the diameterconsiderably and cause major problems during lowering of DWLR probe or the sampling pump etc.

The diameter of the hole is often critical and is recorded based on the diameter of the drilling bit. Thehole diameter is best measured using a calliper log.

The piezometer is intended to be vertical, however it does not always stay vertical but drifts fromverticality. To understand the drift use of a mirror should be made to reflect sunlight down the hole toenable a visual check on the straightness of a hole. Visibility of half hole is an indication of loss ofverticality. The exact point of deviation can be checked by measuring the depth with a tape, whilereflecting light down the hole.

3.4 ACTIONS TO BE TAKEN PRIOR TO DRILLING

• Confirm landowner's/concerned government agencies, permission to enter the property fordrilling.

• If the location is within a school/office/hospital discuss with the authorities to confirm theappropriate time when the drilling can be carried out without disturbing their functioning.

• Check the marking at the site and confirm with the geographical co-ordinates.

• Locate any subsurface power lines, waters lines, telephone cables, sewer etc.

• Locate water sources for drilling purposes and secure permission for their use.

• Prepare the drainage channel for draining of water.

3.5 PIEZOMETER CONSTRUCTION IN UNCONSOLIDATED FORMATIONS

Unconsolidated formations in peninsular India are largely localised to coastal tracts composed of bedsof sand and clays, and sedimentary beds of Gondwana and Tertiary formations made of alternatelayers of sandstone and shales. Piezometer construction in these areas is through rotary drilling. Inthe unconsolidated formation, rotary drilling has to be adopted. Rotary drilling makes use of viscousbentonite mixed fluid as medium of drilling. The mud fluid acts as coolant to the rotating drilling bit aswell as a medium for bringing out drill cuttings outside the borehole. Use of bentonite clay has beenbanned for water well drilling in many countries, as they are not bio-degradable. Organic materials likeguar gum are replacing bentonite clay as popular bio-degradable drilling fluid.

The main components of a piezometer in an unconsolidated formation are (see Figure 3.1):

Borehole: This is the primary component of a piezometer and acts as a host to the othercomponents.

Well assembly: This is essentially the hardware of the piezometer and is accommodated in theborehole and also protrudes above the ground. Depending upon location of the aquifer in the verticalsection, it may comprise one or more of the following parts:



Figure 3.1:Piezometer components in unconsolidated rocks

Blank casing pipe: A blank casing pipe is provided to serve one or more of the following objectives:

• To prevent caving-in/sloughing of the drilled formation.

• To prevent a hydraulic connection between the piezometer and the drilled formation other thanthe aquifer to be monitored.

• To collect the fines entering into the screen. As debris sump.

Screen: A screen provides a hydraulic connection between the piezometer and the aquifer to bemonitored.

Gravel pack and seal: Gravel is provided in the annular space between the borehole and the wellassembly around the screen and beyond, extending preferably over the entire thickness of the aquiferto be monitored. The gravel pack serves the following purposes:

• inhibits the entry of the fines into the screen.

• enhances the hydraulic connection between the piezometer and the aquifer

Cement seal: is provided just above and just below the gravel pack to pre-empt any hydraulicconnection between the piezometer and the overlying/ underlying formations, through the gravel packand screen perforations.

Sanitary seal: A 50cm thick concrete seal is provided at the ground surface to prevent the entry ofsurface water into the piezometer. The seal should be in the form of a cone around the casing to drainthe water away from the well. The seal is underlain by a clay fill/packing for a more effective isolationof the aquifer to be monitored.

3.6 SAMPLING PROCEDURES DURING DRILLING

Examination of drill cuttings is very critical part of piezometer design in the un-consolidatedformations. Some formations are better aquifers than others. Grain size have to be interpreted duringthe examination of the lithology (see Figure 3.2).



Clean gravel have large pores and hold large quantities of water.

Sand and gravel mixture are very good aquifers. When percentage of gravel to sand is very high theaquifer will permit copious discharges.

Coarse sand are potential aquifers.

Figure 3.2:Grain size classification

Fine sand are poor aquifers.

Clays hold lot of water but cannot flow. In some situations the clays when tapped can yield poorquality water.

Sandy aquifers when overlain by thick impermeable clay and when penetrated by the piezometer canresult in flowing wells.

Standardised sampling procedures have to be adopted by all agencies:

• Collect the samples for every meter.

• Lay the samples in succession, as obtained, and mark the depth

• Dry the samples for accurate identification and classification.

• Describe the samples precisely before and after washing and record any additional information.

• Look out for fossils and identify them

• Compare all samples with previous samples.

• Place the samples in plastic wrap and label legibly for any future identification/test.

• Sample boxes with pigeon hole windows are best suited to transport and for preserving thesamples.



3.7 DOWNHOLE INSPECTION

In order to take a decision on the design the piezometer assembly, downhole geophysical loggingneeds to be carried out. Logging should be used for providing additional information than gained fromexamination of drill cuttings. The details to be collected shall include the formation penetrated,formation characteristics modified as a result of drilling, condition of the hole, the exact depth andthickness of the aquifers and water quality of the aquifers. The standard probes to be used shall beelectric, SP, Gamma, calliper, temp and fluid conductivity. The geo-physical logging, examination ofdrill cuttings and the objective of monitoring should form the basis for finalising the piezometer design.

3.8 PIEZOMETER COMPLETION

Piezometer completion should ensure that the hydraulic head measured in the piezometer is that ofthe aquifer of interest. Ensures that only the aquifer of interest contributes water to the piezometerand prevents the annular space from being a vertical conduit for water and contaminants. Suchcompletion steps are critical to the long-term goals of groundwater monitoring. It has to beremembered that the investments made in the construction of piezometers are part of networkmonitoring programme that have to last for decades. Well completion in unconsolidated deposit rocksconsists of installing the well casing and screen, filling and sealing the annular space between the wellcasing and piezometer wall.

3.8.1 PIEZOMETER DESIGN

Good design and careful well construction can only ensure good hydraulic flow characteristics in theaquifer. The screens should be lined up exactly with the permeable portion of the aquifer. The screensshould provide the same hydraulic conductivity of the aquifer. The design should prevent entry of finesand sand particles into the piezometer. The piezometer should completely seal the aquifer which arenot to be monitored. The well assembly should be able to withstand any corrosion or physicaldamages during pumping and logging.

Unconfined aquifer For monitoring the piezometric head of an unconfined aquifer, the piezometeressentially comprises a cement seal at its bottom followed by the well assembly, resting on the seal,comprising of (starting from the bottom) a bail plug, screen and finally a watertight casing pipeextending above the ground surface.

Confined/leaky-confined aquifer: For monitoring the piezometric head of a confined/leaky-confinedaquifer, the piezometer essentially comprises a borehole drilled through the overlying formation andthe entire thickness of the aquifer, into the lower formation to accommodate a cement seal at itsbottom. The well assembly, resting on the seal, comprises (starting from the bottom) a bail plug,screen and watertight casing pipe extending above the ground surface.

3.8.2 SCREEN LENGTH

The well screen should be long enough to ensure that the piezometer records the vertically integratedpiezometric head of the investigated aquifer. Thus, there must be a perfect hydraulic connectionbetween the piezometer and the aquifer over the entire aquifer thickness. Ideally, this requires a fullypenetrating piezometer, that is, the screen provided over the entire thickness of the aquifer.

In case of thin aquifers, a fully penetrating piezometer may be provided. However, in case of thickeraquifers, a fully penetrating piezometer may not be economically feasible, and as such, a partiallypenetrating piezometer may have to be provided. But even a partially penetrating piezometer canprovide an almost perfect hydraulic contact, if it is surrounded by a fully penetrating (that is, extendingover the entire aquifer thickness) gravel pack of large enough thickness and hydraulic conductivity.The length of the screen, in such a case must be large enough to ensure a free inter-flow of water



between the piezometer and the aquifer through the gravel pack. A screen length of two meterssurrounded by a fully penetrating gravel pack may provide the necessary hydraulic contact andensure the free inter-flow.

The gravel pack should have a greater grain size than that of the aquifer material in the vicinity of thescreen. The gravel pack grain size and gradation should be so designed to stabilize the aquifermaterial adjacent to the screen and permit only the finest grains to enter the screen duringdevelopment, finally providing sediment-free water into piezometer (see figure 3.3). The gravel packmust not intersect multiple aquifers and should not cross-confining units, otherwise they wouldestablish vertical, hydraulic connection along the annulus between the two aquifers, thus defeating thewhole concept of piezometers monitoring single aquifers.

Figure 3.3:Idealised arrangement of gravel around the filterassembly for increasing porosity and hydraulicconductivity

Specific details of completion require consideration of several hydrogeologic factors, including thedepth to water, to the top of the aquifer of interest, and to the zone in the aquifer to be monitored;

• the nature of materials that make up the aquifer to be monitored and that overlie the aquifer

• expected water-level fluctuations

• expected direction of the vertical head gradient--down ward,

• whether the aquifer is confined or unconfined

3.8.3 DESIGN OF GRAVEL SIZE AND SCREEN SLOT SIZE

Particle sizes are to be determined in the field after sieve analysis of the aquifer material (see figure3.4). Before sieve analysis the samples need to be dried and weighed. The standard sets of requiredsieves need to be placed one above the other in the order of increasing sieve diameter. The sampleshould be placed in the top sieve and shaken to separate the various grain sizes. The weight of thematerial retained in each sieve should be measured and expressed in percent of the initial weight.



Figure 3.4:Standard sets of sieves

The percentage weight should be plotted against the sieve size, on a logarithmic scale. The resultantcurve that is obtained gives information about the uniformity of the aquifer material. Use of screenhaving a median size of the aquifer material is generally preferred. Since piezometers are notpumping wells the main concern should be top have a good hydraulic connection while at the sametime preventing any entry of fine material into the piezometer.

The slot size of the screen should be so designed that the aquifer material does not enter into thepiezometer. Assuming that fractions greater than or equal to the d60 of the aquifer material are to beretained, a slot size of d60 may be provided. (d60/d10) gives the uniformity coefficient. The higher theuniformity coefficient, the higher would the efficiency and vice versa. Thus, depending upon theuniformity coefficient and the extent of the expected well development, the usually recommended slotsize is d40 to d60 of the aquifer material.

The average size of the gravel should be 4 to 6 times the d50 size of the aquifer. The gravel shouldbe as uniform as possible to avoid segregation during the placement.

3.8.4 ANNULAR SEALS

Annular seal(s) are to be installed from above the gravel pack to near land surface, in order to sealthe annular space between the casing and borehole wall. These seals should prohibit vertical flow ofwater between aquifers and prevent mixing and cross-contamination of aquifers. They also protectagainst infiltration of water and contaminants from the surface.

3.8.5 SURFACE SEAL

The surface seal prevents surface runoff down the annulus of the well and, in situations in which aprotective casing around the well is needed, holds the protective casing in place. The depth ofinstallation of a surface seal can change from area to area. The surface seal should be a mixture ofcement and gravel.

3.8.6 PROTECTIVE COVER

A protective cover should be installed around the piezometer to prevent unauthorized access, housethe measuring device as well as to protect the piezometer from damage. The protective cover shouldbe installed at the same time as the surface seal and should extend to below the ground. Many



designs of protective casing are already available. Essentially it should be a large diameter casing ora GI sheet with locking protective cover and weep hole, which permits condensation to drain out.

3.8.7 DEVELOPMENT

The development of the piezometer, is primarily aimed at ensuring an efficient hydraulic connectionbetween the aquifer and the piezometer. The development is very crucial since the drilling mud, whichinevitably sticks to the walls and invades into the aquifer inhibits the hydraulic connection between theaquifer and the piezometer. The invasion of the drilling mud and thickness of the cake depends uponthe hydraulic conductivity of the aquifer. The higher the hydraulic conductivity, the higher is the mudinvasion and mud cake thickness. The development should completely remove the invaded/stickingmud and also the fines (see Figure 3.5). Under-developed piezometers will fail to provide the trueinformation of the aquifer being monitored and the water level data emerging from such piezometerscan lead to wrong understanding of the system.

Figure 3.5:Effective development bypumping water under pressurethrough the screens

The mud cake around the screen should be dissolved using sodium tripolyphosphate. Sufficientvolume of solution of sodium tripolyphosphate should be made and circulated to displace mud aroundthe screen area as well as a portion of the casing for disaggregating the clays. The polyphosphatesolution should be allowed to act for at least 24 to 36 hours. The solution should be circulated throughthe well screen that effectively acts on the mud cake. This should be followed by washing.

The development should be carried out through air compressor by alternatively surging and pumpingwith air. The air should be injected into the piezometer to lift the water. As the water level reaches thetop of the casing, air supply should be shut off allowing the aerated water column to fall. Use ofeductor lines is recommended when the static water level is deep.

High velocity jetting is another development technique that consists of a jetting tool fitted to the bottomof the drill string. The jetting tool should be lowered and washed all along the screen length usingfresh water. This should be followed by airlift. Careful jetting of the screened area is required. Jettingcombined with airlift should be continued till pumped water is free from fine sand and bentonite, andthe discharge from the piezometer stabilizes.

Development can also be done through back washing. In back washing, there is a reversal of flowthrough screen opening, which agitates the sediments and leads to the removal of the finer fractionand rearrangement of the formation particles. As a part of back washing the water column should bealternatively lifted and allowed to fall back. The pump should initially be started at a reduced capacityand gradually increased to full capacity.

Mechanical surging needs to be carried out at times using surge blocks attached to drill rods. Thesurge block forces water into and out of the screen similar to a piston in a cylinder. The surging



process at times forces fine material back into the screens and hence the fines should be removedbefore taking up surging.

3.8.8 PUMPING TEST

A pumping test is conducted with constant discharge or variable discharge with constant head forestimating hydraulic parameters of the aquifer tapped in the piezometer. The test involves monitoringof the time variation of drawdown in one or more observation wells in response to a pumping at aknown discharge, from the piezometer. The observation must be in the vicinity of the piezometer andmust be tapping the same zone. If no such observation well is available, the drawdown can bemonitored in the piezometer itself. Details of pumping test procedures is enclosed as Part III.



4 PIEZOMETERS CONSTRUCTION IN CONSOLIDATEDFORMATIONS

The drilling of piezometers in consolidated formations is different from the construction ofunconsolidated and semi-consolidated formations. The groundwater occurrence in the consolidatedrocks is in the weathered zone and fractured zone. The consolidated rocks have negligible primaryporosity and it is only the secondary porosity, like fracturing and weathering, that provides the porosityand permeability necessary for the storage and flow of groundwater.

Groundwater discharges are largely dependent upon the rock type. In granite, gneiss and khondaliteshighly productive groundwater zones are found in the vicinity of large lineaments, fractures and deepweathered areas. The lava flows are mostly horizontal and occasionally gently dipping and as such,groundwater occurrence is controlled by the water bearing properties of the vesicular zones. Incarbonate rocks like limestone, marble and dolomite, solution cavities serve as large repositories ofgroundwater. In all these rocks the drilling is usually carried out by the Down The Hole (DTH) drillingtechnique or a combination of DTH and rotary drilling.

For monitoring the piezometric head of an unconfined aquifer, the design should be a cased boreholedrilled through the top collapsible/weathered rock zone, overlying the unconfined formation to bemonitored and is hard enough to stand on its own without the casing. The casing should stand abovethe ground by 0.3 to 0.5 m.

For monitoring the piezometric head of semi-confined aquifer, which has different permeability fromthe top weathered zone then the design should be a cased hole, drilled through the entire weatheredrock zone, overlying the fractured/hard formation to be monitored. The depth of drilling should be suchthat it taps the most productive part of the fractured zone. Geophysical resistivity surveys shouldprovide the value for the depth of drilling. DTH drilling is very fast and completion of one piezometer of100-m depth takes only 12-18 hours.

4.1 DTH DRILLING CHARACTERISTICS

The drilling being very fast, supervision of DTH drilling becomes very important. The sitehydrogeologist has to ensure that the compressor is in good condition to deliver the required airpressure and that the drill bit is of the required diameter. The site hydrogeologist has to ensure thatthe drilled hole is constantly cleaned of the drill cuttings. During the change of the drill rod as well aswhen a water bearing zone is met, the well should be adequately developed and the dischargemeasured using a V notch. The drill cuttings should be collected and studied continuously. At the endof drilling to the desired depth, the well should be cleaned for at least two hours. The cleaning shouldlead to de-clogging of all the fractures drilled through, and removal of all fines and cuttings.

4.2 SAMPLING PROCEDURES FOR CONSOLIDATED ROCKS

The drill cuttings should be sampled for every one-meter frequency and whenever there is a changein lithology. The samples obtained in the DTH drilling are due to the action of the drill bit, which shouldbe kept in mind while examining the sampled cuttings. Further, the depths of the formations asrevealed by the cuttings may not always be accurate - though they can be generally relied upon. Thedrill cuttings have to be classified on the basis of megascopic observations using hand lens, both fortexture and mineral constituents. The description should identify the rock, colour, grain size, shape,fossils, trace minerals, etc. The drill cuttings should be dried, packed in polythene bags, marked withwell number and depth interval, date and time. The samples should be stored in a box with numberedcompartments. A correct procedure for collection and storage of drill cuttings ensures good correlationbetween the drillers log, VES interpretation, downhole logging and samples collected. The recordeddrilling data should include the following:



• A drill log (time taken for drilling each meter of the drilled depth)

• A description of drill action (such as nature of drilling noise and motion of the rig)

• Depths at which moisture is struck

• Depth at which water flows

• Depths at which discharge increases

• Colour, pH and EC of the water

4.2.1 REMOVAL OF FINES DURING DRILLING

Air drilling causes plugging of fractures and crevices with fines of drill cuttings. The clogged materialuntil removed the hydraulic conductivity with the aquifer cannot be established. The water levelmeasurements from such piezometers give erroneous data and the interpretations of such data givesa wrong picture about the groundwater reservoir. Cleaning and development of the drilled hole shouldbe part of drilling activity and should be carried out simultaneously with the drilling operations. Afterchange of every drill rod, cleaning and flushing of the hole should be carried out. On completion ofdrilling to the desired depth a final development should be done by running the compressor of the rigtill the water is free of cuttings and the water is clear. The development should be carried out usingeducator pipes if the piezometer is very deep. Jetting should be carried out where the depth of drillingis large, the discharge is low and the drilling speed is very high.

4.2.2 PIEZOMETER COMPLETION

Piezometer completion should ensure that the hydraulic head measured in the piezometer is that ofthe aquifer of interest and also should ensure that only the aquifer of interest contributes water to thepiezometer, and that the annular space is prevented from being a vertical conduit for water andcontaminants. Such completion steps are critical to the long-term goals of water level monitoring. Thiscan assure that the piezometer installed can be safely used for water level-quality sampling fordecades. Well completion should include installing the well casing and screen, and filling and sealingthe annular space between the well casing and borehole wall, construction of platform and protectivewell head.

Specific details of well completion should include the depth to water, depth of top of aquifer of interest,the zone in the aquifer to be monitored, the nature of materials that make up the aquifer to bemonitored and that material that overlie the aquifer, expected water-level fluctuations, expecteddirection of the vertical head gradient--down ward, upward, or fairly uniform with depth, whether theaquifer is confined or unconfined, the design of the piezometer.

The reference level (RL) of ground level, description of RL, platform design and dimensions, top ofcasing, measuring point details, details of protective casing.

4.3 DOCUMENTATION

The procedure of piezometer construction are to be documented systematically initially as a fieldreport and finally in the electronic format. Careful and complete documentation aids in interpretation ofground-water data and provides historical reference for future use of the well. Types of constructioninformation include drilling information, lithological description, well design, well development,discharge measurements, water quality measurements, and personnel involved. A paper manuscriptand electronic piezometer site file should be maintained for each piezometer. HIS policy requires thatall of the data recorded during construction must be stored in the computer files of the HydrologicalInformation System.



Chronological series of documentary photographs of each piezometer site would provide a visualrecord of land use near the piezometer, which can aid in the explanation and interpretation ofanalytical results, and can aid in locating the site in the future. When changes occur at or near the sitethat might affect hydrologic interpretation of data from the piezometer, a new set of photographsshould be taken to document those changes.



5 REFERENCES

• Driscoll, F.L., 1986Groundwater Wells.Johnson Division, St. Paul, Minnesata.

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