Monitoring of small and medium embankment dams on permafrost in a changing climate

Rudolf V.Zhang

Melnikov Permafrost Institute,Siberian Branch of the Russian Academy of Sciences,Yakutsk 677010,Russia

1 Introduction

Recent climate change which began in the second half of the 20thcentury is a fact–– warming has occurred and continues to occur (Zubakov,2005).However,some studies demonstrate,based on air temperature frequency-amplitude analysis and observational data,that the rate of warming has recently slowed in north-European Russia,northeastern Canada and eastern Mongolia,with some areas beginning to show a cooling trend (Pavlov and Malkova,2005).Moreover,some researchers (e.g.,Balobaevet al.,2009;Sapunov,2011) strongly advocate that the warming period will end soon and a cooling phase will return.Similar situations have repeatedly occurred in Earth’s history (Shpolyanskaya,2008).Climate change is driven by the so-called climatic cycles close to solar activity cycles,as well as by orbital and planetary factors related to the position of the Earth and other planets in the Solar System relative to the Sun.Climate change is accompanied by atmospheric warming,increased wind strength,rising ocean temperature,increased geological activity in the Earth’s crust,changing precipitation patterns and increasing precipitation leading to extensive floods.There is widespread interest in climate change,since it has an impact on every aspect of human life,including economy and ecology.Risks associated with resource development are especially high in the vast permafrost zone which occupies about 65% of Russia and is becoming the epicenter of accelerated economic development.

Table 1 is provided to illustrate the magnitude of recent warming in permafrost regions of Russia(Melnikovet al.,2007).

Observed climate warming has altered the thermal regime of the upper permafrost which provides an infrastructure foundation.It should be noted however that structural-stability effects of temperature change vary with the type of geotechnical system.It is common knowledge that engineering properties of frozen soils depend on temperature.For example,an increase in temperature from-4 °C to-1 °C results in a 2.5-fold reduction in adfreeze shear strength of frozen fine-grained and sand soils (Gosstroy USSR,1990),potentially threatening the integrity of civil and industrial structures.For frozen core dams,temperature requirements are not so stringent,because the hydraulic properties remain virtually unchanged in the said temperature range.The frozen mass is impermeable to water as long as its ice exists.The presence of ice in the soil pores is certainly dependent on the complex thermodynamic conditions in the system.In practice,however,the frozen mass will remain impervious within a wide temperature range,from near 0 °C and below.

Table 1 Increase in mean annual air temperature (°C) and its trends in northern regions of Russia over the period from 1965 to 2005

To understand the effects of climate change on permafrost,long-term monitoring observations are conducted in various parts of the Russian permafrost zone.These studies indicate that the response of permafrost varies across regions.In some areas,opposite processes have been found to occur within short distances were thaw depths increase at one location and decrease at others.Climate change effects have also been detected within the layer of annual temperature fluctuation.For example,in western Yamal,upper permafrost temperatures have warmed by 0.1 to 1.0 °C,while in the vicinity of Yakutsk deviations for particular summer months were 1.0–1.5 °C,although on a seasonal basis the deviations were close to zero.As for deeper permafrost horizons,measurements in Yamal have detected that changes extend to depths of 80–110 m (Melnikovet al.,2007).

Thus,upper permafrost temperatures have generally increased in response to recent climatic warming.Although the general increase is not significant,in some years the deviations are great enough to potentially trigger,in combination with other processes,permafrost degradation with all the ensuing consequences.

Hydraulic structures are complex natural-technical systems.Some researchers (e.g.,Krivonogova,1996)classify them as hydro-geotechnical systems,a special type of natural-technical system.Today,monitoring of hydraulic structure performance is more important than ever when the main tasks of performance assessment for new and existing projects were to obtain new knowledge,to verify design solutions and to improve hydraulic engineering standards and regulations.At present,maintaining and preserving existing structures is in the forefront of priority concerns.The problem is enhanced by the fact that many hydraulic structures have been deserted after closing down of large mining enterprises and water development agencies.Land reclamation structures,in particular,have been in abeyance for several decades.

In permafrost regions,every dam is unique.Design and operating conditions can be similar,but never the same.Therefore,monitoring of dam performance should not be limited to a standard set of observations but must be scientific in character.Our studies of small dams have shown that the engineering properties of fill and foundation materials continue changing over a long period,often comparable with the design life of the structure.By virtue of the structural and operational peculiarities,dams on permafrost require a special approach to organization of field observation,investigation and condition assessment.Suffice to say,formation of the hydrothermal regime takes many years and is associated with changes in the static behavior of the dam and foundation,as well as with drastic modification of the seepage regime (Zhang,2002).

Safe operation and performance of dams is one of the key issues in permafrost regions.Intensive geocryological processes (e.g.,thermokarst,thermal erosion of embankment slopes and reservoir shorelines,frost heaving,suffosion,and concentrated seepage along the voids left by melt ice) begin to develop at the early stages of construction.These processes are hazardous both in the upstream and the downstream areas.It should be noted that large Class I–II dams,50 m or more in height (primarily for hydropower production) have a lower catastrophic failure rate compared to medium (20 to 50 m in height)and small (20 m or lower) Class III and IV dams.International Commission on Large Dams statistics show that the failure rate for small embankment dams accounts for approximately 36% or higher.In Russia,half of the Class III and IV dams,90% of which are under the jurisdiction of the Federal Ministry of Natural Resources and Environment,are identified as unsafe (Zoteevet al.,2003).Dam failures can have disastrous consequences for population and property in the downstream areas,especially where development occurs in the low floodplains in violation of all existing building norms and regulations.

In permafrost regions,the percentage of dam failure is higher due to the unique environmental and geotechnical conditions,as well as to global climate change observed since the mid-twentieth century.In Central Yakutia,for example,about 90% of small embankment dams built for land reclamation fail within the first few years of operation (Zhang,1969).Over 40% of medium dams for domestic and industrial water supply are subject to failure,about 50% of which occur in the first three years of operation and 31%–35% occur between the third and fifth years(Pridorogin,1979).

At present,the surviving dams are 35–40 years old and they are reaching their design life.It is appropriate to mention here that not a single water project has been completed in Russia over the last half a century.The only projects to be completed are the Boguchany (construction period 1974–2013) and Bureya (construction period 1978–2013) hydroelectric power stations.In this context,rehabilitation becomes important to prolong the useful life of the dams that are adequately operating.

It is known that ageing and degradation of the hydraulic structure as a whole and its materials begin from the moment of placement of the material(Mirtskhulava,2008).These processes are even more intensive in harsh permafrost environments due to large thermal and moisture gradients and the resulting complex thermal stress-strain state in the structures(Rasskazov and Vitenberg,1972;Panov,1977;Zhang,2002).All this suggests that without monitoring dam safety,reliability cannot be controlled.

In summary,dams on permafrost are complex natural-technical systems which contain frozen materials in the embankment and foundation and a tremendous heat source provided by reservoir water.Therefore,geocryological monitoring is not only mandatory,but it must be carried out from pre-construction through construction and operation periods.

2 Geocryological monitoring of dams

Geocryological monitoring of natural and natural-technical systems is an integral component of permafrost monitoring,which in turn is part of geological monitoring.Geocryological monitoringis the system of observations of geocryological processes occurring in natural-technical systems to obtain information for evaluating the performance of structures and for predicting the permafrost processes that will affect the stability of structures and adjacent terrain.

Geocryological monitoring should represent a system of periodic or continuous observations processed by software packages that automatically consider temporal changes in material properties and process trends occurring in dams and appurtenant structures.This system must include the following components:observation;data collection,processing,analysis,evaluation and storage;and prediction and response.

For adequate assessment of processes occurring in the dam and appurtenant structures,monitoring should be established during the construction period.Designing of instrumentation and observation programs should be guided by the Building Code SNiP 2.06.05-84* (Gosstroy,1991),Guidelines for Design and Construction of Embankment Dams for Industrial and Domestic Water Supply in Far Northern and Permafrost Regions (Vodgeo Institute,1976) and Small Dams on Permafrost in Yakutia:Guidelines for Design and Construction (Melnikov Permafrost Institute,2012).

In permafrost regions,monitoring of dams and adjacent terrain focuses on the effects of cryogenic and post-cryogenic phenomena as dominant controls of structural and terrain stability.Monitoring activities should be initiated simultaneously with environmental site investigations because they are closely related.In fact,geocryological monitoring is a primary portion of investigations to determine project environmental impact.Therefore,similar to environmental investigations,monitoring is implemented in four phases:planning,design,construction and operation (Alekseevet al.,1999).

At the "planning" phase,the need for geocryological monitoring is established,structure and type of observations are defined,and technical instrument requirements are determined.A permafrost terrain survey of the damsite and adjacent areas is performed,producing large-scale maps,plans and profiles.Data from these surveys are presented in graphic and text forms in accordance with standard requirements.Consideration should be given to the environmental impact of the project and rehabilitation plans must be outlined.All data collected at this stage should be assembled into a report.

At the "design" phase,monitoring sites are selected and instrumentation justified based on the permafrost terrain survey and the structural design of the dam and appurtenant structures.A geocryological monitoring plan is prepared which defines:

-purpose and justification of types of observations;

-selection of instrument locations,parameters to monitor,and observation procedures;

-definition of purpose of instruments and equipment;

-data recording forms;

-schedules;

-software programs;

-emergency action plan;

-personnel;

-funding justification;

-cost estimate.

At the "construction" phase,instrumentation begins with the start of engineering site preparation.Activities during this phase include:

-drilling and instrumentation of temperature and piezometric boreholes,installation of frost meters,heave gauges and piezometers;

-establishment of a surveying network;

-establishment of sites for frost weathering observations at the dam and surrounding environment;

-establishment of a network to monitor snow levels at the dam and water levels and ice conditions in the reservoir.

At the "operation" phase,routine observations and investigations are conducted following the geocryological monitoring program.

Once construction activity has been completed,all dam structures must be accepted by a commissioning team appointed by the owner as required in the Building Code SNiP 3.01.04-87 (Gosstroy USSR,1987).Designers and builders should handover to the operator a location plan of observation points (boreholes,bench marks,markers,and other types of instrumentation),observation records obtained during site investigation and construction phases,and a program of further field observations.

A special operation and maintenance unit should be established by the operator with qualified personnel,equipment,mechanisms and premises.A permafrost geotechnical group set up during construction is integrated with this unit.

3 Scope,methodology and management of geocryological monitoring

The scope and frequency of observation,as well as the number of instrumentation depend on the dam importance class,its design and size,and the subsurface and permafrost conditions.The parameters to be monitored include (Zhang,2001):

-geocryological and hydrogeological conditions in the adjacent terrain;

-restoration of disturbed lands at the construction site and borrow area;

-ground temperatures in the embankment,foundation,abutments,spillway-embankment contact zones,and reservoir shore and floor;

-seepage through and beneath the dam,around the spillway,and at abutments;

-reservoir water temperatures near the dam,including the sub-water slope and abutments;

-hydrogeological regime of suprapermafrost water in the active layer and of groundwater in the river taliks beneath the reservoir,dam and abutments;

-seasonal freezing-thawing at the crest and along the downstream slope;

-settlement of the embankment,spillway,and shores.

-landscape conditions in the adjacent terrain;

-snow conditions;

-icings;

-water chemistry in the reservoir and downstream from the dam;

-frost heaving and thaw settlement of the embankment,foundation,spillways,and shoreline;

-frost weathering;

-structural stability of the project features (e.g.,buildings,freezing systems,drainage,power and communication line supports,earth canals).

Temperature observationsduring the construction and operation of the project are carried out to determine the following characteristics that influence long-term structural stability:

-the thermal and ice regimes in the reservoir,including its sanitary shoreline area to the depth of thermal effect;

-the hydrological regime of the reservoir;

-changes in temperature and moisture regimes in the embankment and foundation;

-local temperature regimes in the natural talik below the stream,in the spillway and outlet works area,as well as in the damsite areas with adverse permafrost conditions (e.g.,natural thermokarst,solifluction,icing,frost heaving,and frost cracking);

-changes in seasonal thaw/freeze in the embankment,foundation and abutment areas;

-development of geocryological features.

Locations for temperature boreholes should be selected based on the following considerations:

-Boreholes should be placed in areas of potential local seepage that can lead to permafrost degradation in the dam embankment and foundation,as well as within natural taliks preserved in the foundation below the downstream toe.

-At least three boreholes must be established within the embankment,one at the maximum height of the dam and two near the abutments.Borehole depth and plan location are determined based on thermal analysis,taking into account the geocryological structure of the foundation.Each instrumentation point should consist of a coupled borehole and piezometer system,so that error checking could be made by comparing temperature and water level readings.

-In the embankment incorporating a frozen core,temperature boreholes should be installed between the cooling devices and along the line perpendicular to the dam axis.

-A high spur berm is recommended to be made on the upstream sideslope for placing a temperature borehole to control thawing in the foundation beneath the upstream slope in front of the frozen cutoff.The borehole within the frozen cutoff should be at least 5 m deeper than the core.Temperature sensors should be spaced at 1 m interval.

At the spillway,temperature boreholes should be placed in the fore apron before the sheeting,in the center of the stilling basin,at the end of the stilling basin behind the sheeting and at the terminal structure.

-Temperature boreholes should be made of weld-free steel or plastic pipes 80–120 mm in diameter with their joints sealed.They must have insulating and protecting heads.It is advisable that boreholes in the reservoir floor continue the row of embankment boreholes.

Thermal and piezometric observations,as well as salt detection should be made using instruments manufactured by specialized organizations.Digital sensors connected to dataloggers can be used for primary observations.

To monitorseasonal freezing-thawingat the crest,along the downstream slope and at the abutments,electric frost gages are used.The frost gages should be inserted to a maximum depth of 5 m.In unfrozen dams,seasonal freezing and thawing should be prevented in the impervious earth cores,which can result in thaw weakening and increased seepage.In frozen dams,thaw penetration into the dam crest should not extend below the maximum upstream water level.The observation procedure is described in the Methodological Guidelines for Studying Seasonal Soil Freezing and Thawing (PNIIS,1986).

Monitoring offrost heave and thaw settlementas a hydrothermal indicator of ground movement is performed with heave and settlement gauges placed in the dam during construction.Observations are conducted following the procedure developed at the VSEGINGEO Institute (Grechishchev and Nevecherya,1979).

Thermokarstmonitoring includes observations of the rate,magnitude and nature of surface deformations.Thermokarst development is dangerous at the abutments of the embankment and spillways,as well as on the river valley sides downstream.Rapid thermokarst development is caused by surface disturbance during the dam and reservoir construction.Thermokarst observations should be made using the Methodological Guidelines for Thermokarst Study in Engineering Site Investigations in Permafrost Regions (PNIIS,1969b)and the Methodological Guidelines for Monitoring Investigations of Cryogenic Physical-Geological processes (Grechishchev and Nevecherya,1979),as well as modern GIS technologies (Fedorov and Torgovkin,2006).

Solifluctionmonitoring is performed using electrometric and mechanical techniques (PNIIS,1969a).Fundamentally new methods have recently been developed for solifluction measurements which use digital triaxial inclinometers and temperature sensors.Models obtained allow early detection of landslides from changes in the temperature field and stress-strain state of the soil mass.

Icingmonitoring is conducted at the embankment,abutments,and downstream and upstream areas.Icings may be particularly dangerous on the downstream slope of the dam and along the spillway channel.Large icings along the reservoir shoreline and in the downstream river channel are easily identified on aerial and satellite images with subsequent computer processing.Direct size (area and volume) measurements can be made with modern land-surveying instruments.Special attention should be given to the chemistry of icings to infer its origin and assess aggressiveness in construction materials.It is also important to study ice structure,icing stratification,and mineral inclusions to judge piping processes in the embankment and foundation.Observations of icing dynamics are conducted with mandatory temperature control of its body and underlying ground,because this information allows one to judge the stability of the project as a whole:stability of dam slopes,abutments,river banks in the downstream area,and reservoir shorelines.For these observations,the existing guidelines should be used(Piguzova and Shepelev,1975;Alekseev and Sokolov,1980;Sokolov,1984).

Thermal erosion and thermal abrasiondevelop during the warm season.Thermal erosion poses a threat for the embankment,abutments and downstream areas.Thermal erosion is initiated by flows produced by rapid snowmelt and rainwater.Thermal erosion beneath the spillway and drainage structures leads to piping.Lateral and bottom erosion is determined by surveying methods accompanied by soil temperature,moisture and thaw progression measurements.Thermal abrasion is also determined by the position of stakes and marks in relation to reference datums.Aerial and satellite images provide good visual information.To predict reservoir shoreline erosion,the procedures developed by VNIIG (1975) and the Permafrost Institute (Are,1985) are recommended.At present,geophysical methods are widely used,along with conventional methods for monitoring erosional scours,sinkholes and slumps.

Frost crackingis one of the key elements in dam monitoring programs.Frost cracking results from the complex thermally stressed strain state which develops in the embankment and appurtenant structures upon their interaction with the environment.Of special concern are transverse cracks in the embankment and so-called detachment cracks in the spillways.If cracks in the embankment are readily visible,deep-seated detachment cracks between the ground and the spillway substructure are concealed and require special instruments embedded during construction.Frost cracking in the impervious earth cores (central or inclined) may also be of concern.This type of deformation is easily detected with geophysical methods.Prediction of frost cracking in the embankment dams is performed using the procedure developed by Grechishchev (1972) and Zhang (2002).

Frost weatheringof embankment soils and rocks is a new component which is recommended to be included in dam monitoring programs.This process is important in that it leads to changes in gradation and hence in thermal properties of fill materials,thus affecting the thermal state of the embankment and foundation.Frost weathering occurs in rockfill shells,resulting in fines that fill the void space.Surface and subsurface weathering should be distinguished.Surface weathering is caused by ambient climatic factors,while subsurface weathering occurs within the layer of seasonal freeze-thaw mainly due to phase change of water and hydrochemical processes.The basic methods for studying frost weathering are visual observations in the field and gradation analysis in the laboratory (VNIIG,1989).Depths to which the soils and rocks are affected by weathering are determined by borings and test pits.Geophysical methods are also recommended.

Control temperature measurement cycles should be made during the year:

-first (spring) – prior to spring flood and after ice melt in the reservoir;

-second (summer) – after the onset of consistently positive air temperatures;

-third (autumn) – during the freeze-up period;

-fourth (winter) – from December to March.

During visual observations and surveying for frost action,deformations and seepage,special attention should be given to:

-location,depth and width of frost and detachment cracks,rates of crack growth or decay;frost heaving;conditions of the sideslopes (thermal erosion,thermal solifluction);

-snow depth and accumulation dynamics on the crest and the upstream and downstream slopes;thermal abrasion of the reservoir shoreline;thermokarst at the abutments;

-location,size and timing of icings and frost blisters at the embankment toe;

-condition of the spillway and outlet works and their readiness to safely pass flood flows;

-condition of the instrumentation systems.

An instruction should be prepared for all monitoring activities as an annex to the Operation and Maintenance Manual.This instruction must be reviewed annually in light of the actual dam performance and observation results.Observations and data processing and analysis are recommended to be made by specialized organizations at least twice a year,comparing them against the design.

Deformation (settlement and displacement) measurements should be performed in accordance with the existing regulatory documents (Gidroproekt,1980).Reference datums must be isolated from the influence of frost heave,thaw settlement and other frost-related processes.

For frozen dams,safe performance parameters for which threshold values should be specified in the design include:

-minimum thickness of the frozen cutoff,including an average for the cutoff and a value at midpoint between two adjacent thermosyphons;

-maximum depth of thaw in the permafrost foundation beneath the upstream shell in front of the frozen cutoff;

-temperature and natural freezing rate of the talik beneath the downstream shell behind the frozen cutoff;

-maximum depth of seasonal thaw in the dam crest,sideslopes and abutments;

-mean monthly ground temperatures at the contact with buried elements of the freezing system which create a frozen cutoff in the embankment,foundation and abutments,as well as at the embankment-spillway(outlet) contacts;

-water levels in non-freezing piezometers equipped with automated data recorders and located before and behind the frozen cutoff;

-surface temperature of the upstream sideslope of the dam below pool level and bottom temperature of the reservoir;

-ground temperature in the river talik before the frozen cutoff;

-operation schedule for the freezing systems (start and end of the winter operation cycle;shutdowns);

-temperature field in the permafrost foundation within and along the predicted zone of thermal effect from the reservoir.

The instrumentation system should include geophysical tools and equipment as the most effective means of studying frost action and seepage processes.Geophysical methods in hydraulic engineering are regulated by STO 17330282.27.140.003 – 2008 (Unified Energy System of Russia,2008).

Operational difficulties posed by severe climatic and environmental conditions,as well as stringent requirements enacted by the Russian Law on safety of hydraulic structures (State Duma of the Russian Federation,1997),call for the application of advanced systems for timely data collection and processing.An automated remote monitoring system developed by the Center for Extreme Situations Research,Moscow,and tested and adapted at the Alrosa Co.facilities has been concluded by the Yakutniproalmaz Institute specialists to be very effective.The system can operate in harsh environments (e.g.,long and cold winters,aggressive waters,limited accessibility) in automated mode(manual data collection is also possible).It provides data acquisition and processing with data loggers in real time or at specified time intervals.It also provides remote communication via high-speed cabling or mobile radio.

There are more advanced systems,such as GLONASS-based Geolink technologies,but they are very expensive.Similar,but less expensive systems have been developed at the Institute of Cosmophysical Research and Aeronomy and Melnikov Permafrost Institute in Yakutsk,which are presently being tested in the field.

4 Summary

Monitoring of dams constructed on permafrost is becoming extremely important.There are numerous cases when the dams which have been designed to rely on frozen ground for stability and seepage containment change,for various reasons,to the thawed mode.This issue is very complicated and requires in-depth studies to understand,for example,the contribution of reservoir effects under changing climatic conditions and the role of cryogenic processes in alteration of the geological and geocryological environments.Conclusions that the permafrost response to climatic change is not strong have been inferred from observations in middle and high latitudes of the permafrost region,but not in its southern margin.Moreover,these observations refer to naturalconditions.

Geocryological monitoring of dam projects in permafrost regions is fundamentally important,because it is the only way to understand current trends of processes,predict future conditions and take timely and effective measures to ensure dam stability.On the other hand,stability problems resulting from faulty design,substandard construction,or poor maintenance should not be attributed to climate warming.

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