IDTENTIFICATION OF SEEPAGE PATH BY TRACER METHOD

By

Prashant Kumar Ojha

(1EW11CV049)

Under the Guidance ofMr. Nagaraja Gupta MS

Associate ProfessorDept. of Civil Engineering

East West Institute of TechnologyBangalore-560091

CONTENTS1. ABSTRACT2. INTRODUCTION3. ESTIMATION OF PERMISSIBLE SEEPAGE IN MASONRY DAM4. TRACER TECHNIQUES5. METHODOLOGY6. CASE STUDIES7. CONCLUSIONS8. REFERENCES

CONTENTS

ABSTRACTLeakage is a major safety issue that, if left

unchecked, may result in dam failure by various mechanisms.

In the last few decades, a series of new hydrological techniques have been developed to help in the assessment of leakage and seepage in dams.

These techniques allow detection of dam leakage, including leakage evaluation, analysis, design, construction, and post-construction verification of repairs.

ABSTRACT

INTRODUCTION Dams have been a part of the economic development

model of almost all nations of the world. At some stage of their development, most countries with water resources that can be economically exploited have built dams for energy, irrigation, and drinking water.

The hydraulic structures viz. dam, reservoir, canal etc. are although designed not to seep or leak, still minimum seepage or leakage occurs through these hydraulic structures when founded on pervious foundations.

Generally, large quantity of seepage in dam occurs through(i) the abutments i.e. through geological inhomogeneities

occurring in the vicinity of the structure,(ii) the structure - foundation interface and(iii) the body of the dam itself.

INTRODUCTION

ESTIMATION OF PERMISSIBLE SEEPAGE IN MASONRY DAM

Central Water Commission, New Delhi has evolved a method for estimating permissible seepage in masonry dam where drains are provided. The methods assume:1. No water escapes to downstream portion past the

drainage pipes.2. Masonry is permeable having high coefficient of

permeability in horizontal direction.

Seepage in body of dam is a measure of hydraulic head. When reservoir is at its full level, the seepage is maximum.

ESTIMATION OF SEEPAGE

Cross section and upstream elevation of the dam.

Cross section and upstream elevation of the dam.

By Darcy’s law Q = k*i*AThe notations used are:-

k= permeability of masonry and h/d = i = hydraulic gradient of flow (linear pressure variation

is assumed).

Taking an elemental rectangle dl X dh at height ‘h’ below reservoir level and distance ‘l’ away from left abutment

dQ=k*i*dh*dldQ=k*(h/d)*dl*dh

Darcy’s law

Darcy’s law

TRACER TECHNIQUES The tracer is a certain substance added to a

material in a chemical, biological or physical system to mark that material for study, to observe its progress through the system or to determine its final distribution.

Tracer technique is adopted by injecting a predetermined quantity of tracer into borehole located near to the suspected seepage zone of the hydraulic structure and observing either the dilution of the tracer in the injection bore holes itself or by detecting its arrival in the observation bore holes located near the injection boreholes in the probable direction of seepage.

TRACER TECHNIQUES

The major objectives of using tracer’s techniques in geotechnical studies are to determine :-

(1) seepage in dams, reservoirs and canals,(2) location of seepage entry zones, delineating

seepage path, assessing the efficiency of remedial measures, examination of soundness of bedrock etc.

(3) hydraulic parameters of subsurface flow or seepage through hydraulic structures

(4) interconnection between solution cavities,(5) seepage losses through irrigation canals etc.

TRACER TECHNIQUES

TYPES OF TRACERS

TYPES OF TRACERS

ISOTOPES TRACERS

STABLE ISOTOPES

UNSTABLE

ISOTOPES

CONVENTIONAL TRACERS

CONVENTIONAL TRACERS

Conventional tracers are strong electrolytes like Sodium Chloride (NaCl) and organic dyes like Sodium Fluoroscene, Rhodamine-B, etc.

Chemical and dye tracers are generally affected by absorption and adsorption processes.

CONVENTIONAL TRACERS

ISOTOPES TRACERS

ISOTOPES TRACERS

Isotope tracers can be subdivided into stable and unstable isotopes.

E.g. 3H, 51Cr, 60Co, 82Br, and 131I Stable isotopes include environmental isotopes

while unstable isotopes are radioactive

ISOTOPES TRACERSEnvironment

Isotopes Tracers They are ideal

geochemical tracers of water because their concentrations are usually not subjected to change by interaction with the aquifer material.

e.g.- Iodine, Cobalt

Radioactive (Unstable) Isotope

Tracers Radioactive tracers have

an optimum half-life period.

Ease and speed of measurement are the most significant advantages of the radioactive tracer technique

e.g.- Radon Gas, Tritium

ISOTOPES TRACERS

METHODOLOGY

TRACER TECHNIQUE

SINGLE WELL OR POINT DILUTION TECHNIQUE MULTIWELL TECHNIQUE

METHODOLOGY

Single Well Technique

The aim of the method is to obtain a direct measurement of filtration velocity.

Multiwell Technique

The method involves injecting a predetermined quantity of tracer in the form of a solution in one of the boreholes and monitoring its appearance in a number of boreholes located at the downstream, in the anticipated direction of flow.

METHODOLOGY

Conceptual model showing the field setup of tracer injection at dam site

CASE STUDIES Delineating path of seepage in the masonry at

Pawana Dam, Maharashtra Ascertaining seepage path in the foundation at

Nagarjunasagar Dam, Andhra Pradesh Delineation of Seepage path through galleries in

Koyna Dam, Maharsahtra Detecting path of seepage using tracer method in

Bhama-Askhed Irrigation Project, Maharashtra Delineation of path of seepage in the downstream

toe region of Salauli Irrigation Project, Goa

CASE STUDIES

Pawana Dam

Delineating path of seepage in the masonry atPawana Dam, Maharashtra

Excessive seepage observed at drainage gallery and downstream of the dam. Seepage was observed like water jets on the downstream slope of Left Hand Side (LHS) of masonry.

The appearance of Potassium Permanganate dye tracer was monitored at downstream observation points at every 10 minutes interval for about two hours after injection.

CASE STUDIES

Nagarjunasagar Dam

Ascertaining seepage path in the foundation atNagarjunasagar Dam, Andhra Pradesh

Three Nx size boreholes viz. Nx-1, Nx-2 and Nx-3 were drilled on the upstream side of the cavity area to facilitate injection of tracer.

Tracers used were sodium fluoresceine, Rhodamine-B and common salt.

The tracer studies revealed that there was a hydraulic interconnection between the foundation rock and the toe.

High seepage velocity ranging between 3m/day to 7.8 m/day has also been observed.

CASE STUDIES

SALAULI DAM

Delineation of path of seepage in the downstream toe region of Salauli Irrigation Project, Goa

An excessive seepage was noticed in the downstream toe of the earthen dam through the foundation and dam abutment.

For tracer studies, Rhoda mine-B, sodium chloride and sodium flouroscein were selected as tracer.

Point dilution tracer method was conducted to measure the filtration velocity in the water bearing formations.

Seepage velocity was determined by knowing the arrival of the peak in the concentration versus time curve and the distance between the injection and observation holes.

The studies helped in finalizing design and construction of diaphragm wall to stop excessive seepage.

CASE STUDIES

CONCLUSIONS Tracer techniques play a prominent role in the

investigations required for different stages of hydraulic structures such site selection, site investigations, watershed studies, dam design construction, leakage investigations, sediment control improving the longitivity of hydraulic structures

Tracer technique is a definite, reliable and economic tool for finalizing the design and construction of diaphragm walls, cut-off-walls, grouting and guiniting of the dams.

CONCLUSIONS

REFERENCES Tree Plantation for Intercepting Canal Seepage and Controlling Watertable, Pakistan Journal of Water Resources, Vol.11

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Radio-tracer Techniques for the estimation of seepage losses from unline irrigation channels. Workshop on Nuclear Technology in Hyderabad.

  “The future of applied tracers in hydrogeology”, Hydrogeology Journal, 13:255-258. 

“Radioactive Tracers in Hydrology”, Elsevier Sci., New York.

“Borehole Dilution Technique, A critical Review”, IAEA.

Guidebook on Nuclear Techniques in Hydrology.

“Tracing Technique in Geohydrology”, A. A. Balkema, Brookfield, Vt.

“Application of Tritium Tracer to canal seepage measurements” Proc. IAEA Symp.

Isotope Tracers in Catchment Hydrology. Elsevier Science, Amsterdam.

Applications of Uranium- and Thorium-Series Radionuclides in Catchment Hydrology Studies.

Isotope Tracers in Catchment Hydrology, Elsevier, Amsterdam.

“Radioactive tracer techniques used in solute transport studies in a field soil”, Int. Agrophysics.

“Injected radiotracer technique in hydrology” Proceedings India Academy Science (Earth Planet Science), Vol 93.

Ground Water Hydrology, John, Wiley & sons, New York

REFERENCES

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