Cutoff Trench Research Articles

FINITE ELEMENT METHOD OF SEEPAGE AND STABILITY ANALYSIS OF SAURA-KEFFI EARTH DAM

Finite element method (FEM) based analysis using the SEEP/W and SLOPE/W sub-programme options of a GEOSTUDIO software was used to analyze Saura-Keffi earth dam in Nasarawa State, Nigeria with the view to analyze the dam seepage and its stability. The created dam geometry was assigned material properties and boundary conditions at reservoir water levels of: minimum (3 m), normal (6 m) and maximum (7 m). The analysis was conducted under the cases that the dam was without and with seepage measures (SCM). The results of the analyzed semi-permeable homogenous dam revealed a percentage reduction inseepage flux to 28.3% when the dam was considered with a central core and complete cut-off trench along the dam crest axis. see figure 7. The pore-water (PWP) revealed a decrease in value from (12 to 11.2) kPa at minimum water head with SCM. The PWP further increased by 18.9% as water rises from 6 m to 7m with SCM. The exit gradient showed an insignificant increase in value of 1.2% with the construction of SCM which indicates less flow at the exit end of the dam leading to increase in stability. The horizontal filter drain at the downstream lower and kept the phreatic line within the dam body to allow for adequate embankment and foundation drainage as shown in figure (5&6). The F.S at the maximum water level revealed an increase in value from 1.27 to 1.52 when a horizontal filter was considered. The Morgenstern-priced method of slope stability was used to estimate an acceptable minimum F.S of 1.52 at 0.5 lambda value with the introduction of horizontal filter.

Process Water Management and Seepage Control in Tailings Storage Facilities: Engineered Environmental Solutions Applied in Chile and Peru

In the past thirty years many mining projects in Chile and Peru have used: (i) polymeric geomembranes and (ii) design-and-build cutoff trenches, plastic concrete slurry walls, and grout curtain systems to control seepage at tailings storage facilities (TSFs). Geosynthetics are a viable alternative at a TSF dam for clay cores or impermeable materials, mainly because of their marked advantages in cost, installation, and construction time. This article describes the use of geosynthetics liners and cutoff trench–plastic concrete slurry walls–grout curtain systems in TSF dams in Chile and Peru mining, with the objective to decrease seepage to the environment, considering different dam material cases such as: cycloned tailings sand dams, borrow dams, and mine waste rock dams. Finally, this article discusses aspects of geosynthetic technology acceptance in the local regulatory frameworks, lessons learned, and advances. It focuses on the use and implementation of geosynthetics in TSFs in Chile and Peru, which have some of the highest TSF dams in the world, as well as a wet environment, dry environment, extreme topography, and severe seismic conditions. These conditions constitute a challenge for manufacturers, engineers, and contractors, who must achieve optimal technical solutions, while being environmentally aware and economic.

A levee breach induced by internal erosion in Western Australia

This case study presents a levee breach induced by piping erosion under cyclonic conditions in 2019. The levee is a 2.5 m-high, 500 m-long, mass earth-fill embankment; with no cutoff trench, core or ancillary structures. Located near Port Hedland on the north coast of Western Australia, its purpose is diversionary: to redirect cyclonic surface water away from the nearby Great Northern Highway. The levee was founded directly on alluvium in 1987; it is formed of locally sourced clayey sandy gravel. In 2003, the levee was partially excavated to enable the placement of a buried pipeline through the levee. Following a cyclonic event in 2019, a 27 m length of the levee breached, resulting in significant scour of the foundation and downstream soil. A site visit and investigation were conducted shortly thereafter, where in situ testing and laboratory soil tests on the levee and foundation materials were conducted. Analysis of the site observations and laboratory testing data led to the probable failure mechanism being theorized as having been initiated within the foundation by piping erosion within sand-rich beds of alluvium. The large quantity of water ponding upstream of the levee then caused a progressive washout and breach of the levee. Thematic collection: This article is part of the Role of water in destabilizing slopes collection available at: https://www.lyellcollection.org/cc/Role-of-water-in-destabilizing-slopes

Toddbrook Reservoir cut-off trench site observations

Significant damage to the auxiliary spillway at Toddbrook Reservoir resulted in the evacuation of 1500 residents from Whaley Bridge for a period of 7 days. During this period, emergency works were carried out to minimise the risk of the embankment failing and allow residents to return to their homes and livelihoods. Since the emergency works, the Canal & River Trust have completed an interim project which includes a number of measures to improve the resilience of the reservoir until the spillway is replaced. This paper presents the site observations made during the construction of the in situ concrete cut-off trench located along the crest of the auxiliary spillway.

An Investigation of Excessive Seepage from the Al-Fulaij Recharge Dam, Oman

The Al-Fulaij recharge dam is located on the Al Batinah coast in Oman and was constructed in 1992. The dam is about 3.3 km long and 7.7 m high with a storage capacity of 3.7 million cubic meters of water. It is an earthfill dam with silty, sandy gravel fill in the embankment. Excessive seepage of between 5,000–12,500 m3/day was observed during floods in 1993, and several sinkholes were noticed close to the upstream toe. Remedial work consisting of an upstream blanket and a cutoff trench wall was performed in 2000. However, these remedial measures failed and almost the same seepage was noticed again during the impoundment. This paper investigates possible causes of the seepage using a finite element model. The input data for the model were collected from site investigations and field records during the construction and monitoring of the dam. The study reveals that the most probable cause of the excessive seepage is the presence of a permeable soil layer underneath the dam due to the dissolution of the gypsum material.

Construction of the Cut-Off Wall for a 330mw Hydropower Project at Kishanganga in J&K

Cutoff Wall indicated is a thin watertight wall of clay or concrete (concrete in this case) built up from a cutoff trench to reduce seepage through any natural water passages-joints and cleavage planes in the rock massif or passages between ground constituents such as boulders and cobbles and through existing granular material such as sands and gravels and deepen the flow lines to make them pass beyond the dam area. It is also known as core wall. The use of diaphragm walls as cut-off or seepage barriers or as structural elements has been on the civil engineering and geotechnical scene in a relatively widespread manner since just after the Second World War.

New solutions in designing mechanized hump yards

For the reduction of investments in the construction and operational costs, associated with the breaking-up and making-up of trains on the mechanized hump yards, the new approach to the design of the plan of yard necks of classification yards was proposed. Transfer of the park brake position to the straight segments within the curved beyond the last railroad switches allows to reduce the length of the calculated path of rolling of cuts and consequently to reduce the cost of the humping track and braking of cuts. Thus, the required brake power of retarders of the drain part of the hump yard and the park brake position is reduced. New design solutions on the placement of retarders of park brake position on tracks with minimal inter-track space were developed, namely the allocation of retarders of one bunch in the general cut-off trench with the arrangement of steps in each inter-track space on both sides of the trench; the displacement of the control equipment of retarder on the inter-track space axis for compliance with the loading gauge and structure gauge, and the corresponding displacement of the high-pressure air pipelines for the switching-on of retarders

Tube-a-manchette grouting at Upper Rivington reservoir

The Yarrow embankment was built in 1857 and comprises one of two embankments that form the Upper Rivington reservoir. The embankment is approximately 300 m long, 12 m high and is of Pennine type construction with a thin central puddle clay core extending 2 m into a cut-off trench. Following a major leak in 2002 and remedial grouting works, quantitative assessments were carried out by United Utilities (UU) using adapted methodologies referenced in the ‘Risk analysis for dam safety’ document. The assessment identified a significant risk of failure by internal erosion caused by erosion of the clay core within the fissured rock foundation. UU invested in a remedial solution comprising tube-a-manchette (TAM) grouting to supplement the previous 2002 works and methodologies. The grouting was undertaken within 2012 and comprised TAM holes along the crest at 1 m spacing both upstream and downstream of the clay core. During the works significant fracturing and erosion of the bedrock was recorded at various points and was accompanied by significant interconnectivity between the upstream and downstream TAM. The areas of high grout takes indicate a good correlation with the underlying geological conditions and the previous geophysical survey.

The Silent Valley dam: setting the record straight

The Silent Valley dam and associated reservoir are situated in the Mourne Mountains in County Down in Northern Ireland. The reservoir has been a major source of water for Belfast since 1933. The dam was first proposed in 1891 but its implementation was initially delayed by the client, the Belfast Water Commissioners, and subsequently by the 1914--1918 war so that construction did not commence until 1923. During construction, a major problem occurred in the excavation for the cut-off trench and arbitration was sought by the contractor. Resulting from the arbitration, which took place in December 1926, a proposed method to overcome the problem was explored and found to be successful. Construction of the dam was then satisfactorily completed in 1933. The paper provides brief details of the project, focuses on the engineers and lawyers involved in the arbitration and lays bare certain untruths that have been allowed to circulate unjustly for over 80 years.

Analysis and Design of Vibration Isolation System Using Open Trenches

Field vibration tests were carried out at a proposed site for the vibration testing room, and 2D numerical analysis using finite difference tool FLAC 5.0 was performed to suggest effective vibration isolation systems. In the analysis, the numerical model is first calibrated with respect to material properties, damping value, and boundary conditions to obtain the output comparable to the field test results. The calibrated model was further used to perform a parametric study by (1) providing vibrating input motions from vibrating machines to be operated; (2) using two depths of cutoff trench; and (3) providing gravel bed, gravel bed with rubber pad, and gravel bed with rubber pad and cutoff trench to study the isolation effects. Comparing the results from the parametric studies with the human perception level of vibration, a decision on the isolation system was determined.

BDS conference visit to Katrine waterworks

On 25 June 2010 a technical visit by about 60 delegates took place to the Katrine waterworks scheme as part of the BDS 2010 conference. They saw the Mugdock and Craigmaddie reservoirs, which had been substantially unchanged since they were built in 1859 and 1892, until the drawoff arrangements were changed for construction of the Milngavie water treatment works in 2008. Both dams are earthfill with puddle clay cores and cutoff trenches. Craigmaddie dam has a cutoff trench over 200 ft deep in places. The reservoir was completely emptied in order to layer submerged pipes across the floor. The 240 Ml/day water treatment works had been neatly landscaped into a peninsula between the reservoirs. Delegates were shown features of both reservoirs and the water treatment works.

Gypsum-karst problems in constructing dams in the USA

Gypsum is a highly soluble rock and is dissolved readily to form caves, sinkholes, disappearing streams, and other karst features that typically are also present in limestones and dolomites. Gypsum karst is widespread in the USA and has caused problems at several sites where dams were built, or where dam construction was considered. Gypsum karst is present (at least locally) in most areas where gypsum crops out, or is less than 30–60 m below the land surface. These karst features can compromise on the ability of a dam to hold water in a reservoir, and can even cause collapse of a dam. Gypsum karst in the abutments or foundation of a dam can allow water to pass through, around, or under a dam, and solution channels can enlarge quickly, once water starts flowing through such a karst system. The common procedure for controlling gypsum karst beneath the dam is a deep cut-off trench, backfilled with impermeable material, or a close-spaced grout curtain that hopefully will fill all cavities. In Oklahoma, the proposed Upper Mangum Dam was abandoned before construction, because of extensive gypsum karst in the abutments and impoundment area. Catastrophic failure of the Quail Creek Dike in southwest Utah in 1989 was due to flow of water through an undetected karstified gypsum unit beneath the earth-fill embankment. The dike was rebuilt, at a cost of US $12 million, with construction of a cut-off trench 600 m long and 25 m deep. Other dams in the USA with severe gypsum-karst leakage problems in recent years are Horsetooth and Carter Lake Dams, in Colorado, and Anchor Dam, in Wyoming.

Cambering and valley bulging, periglacial solifluction and LateglacialColeoptera at Dowdeswell, near Cheltenham

Engineering works at Dowdeswell Dam, Gloucestershire, where the River Chelt emerges from the Cotswold scarp, enabled investigation of some of the local Quaternary features. The log of the dam cut-off trench (1884) shows the presence of a hitherto unreported valley bulge. It is of the type known as a crumple, exhibiting two sharp anticlinal folds superimposed upon a much gentler valley anticline. The former bulge had, as usual, been eroded away by the river. A trial pit at the foot of the northern valley side showed 2.86 m of silty clay head over a thin Oolitic breccia resting on the eroded surface of the Charmouth Mudstone Formation. Resting on the breccia was a thin discontinuous layer of organic clay. This was found to contain 19 taxa of Coleoptera, of which 11 could be identified to species or species group. The assemblage indicates that cold conditions obtained, with an open, barren landscape and a mean annual air temperature of −3°C. The deposit is assigned to the Younger Dryas period. The head mantling the valley slopes, at least in the area of Trial Pit 1, was probably emplaced predominantly during this cold period. The valley bulging was much earlier, evidence from other sites indicating an age of about oxygen isotope Stage 12.

Groundwater pollution due to a tailings dam

In 1982, the Oman Mining Company (OMC) commenced copper mining and smelting operations in the area of Sohar (Wadi Suq), Sultanate of Oman. Seawater was used for mining operations until 1993. During this period, 11 million tonnes of tailings had been deposited behind an unlined tailings dam. This has resulted in a major groundwater pollution problem. This paper presents results from investigation of groundwater pollution. Data pertaining to various parameters such as pH and TDS were obtained from the OMC. Most of the data were for a period of 12 years since 1984 and were gathered from an extensive network of field monitoring wells covering a distance of 14 km downstream of the tailings dam. The trends of groundwater pollution are presented with explanation. The issue of the existing remedial measures (cut-off trench for containment of the pollution source) is addressed briefly.

Analysis of progressive failure and cracking in old British dams

Three sections, representative of existing old puddle core embankments, were analysed using non-linear finite element techniques. They had shoulders of granular fill, puddle clay cores and strong foundations. The analyses showed that progressive failure was negligible during drained construction, operation and undrained draw-down. The cores were vulnerable to cracking if they were subjected to full reservoir pressure on their upstream boundary. However, very slow, drained impounding caused mean total stresses to rise so that fracture was not indicated. Hydraulic fracture was predicted in a deep cut-off trench filled with puddle clay. Cyclic drawdown and impounding increased stresses in the upstream side of the core, so increasing the resistance to cracking. Total stresses increased and decreased with reservoir level, as has been observed in the field. Les auteurs ont utilisé des techniques des éléments finis non-linéaires pour analyser trois sections types de vieux barrages en enrochements à noyau en argile compactée comprenant des massifs de remblai granulaire, un noyau étanche en argile compactée et des fondations solides. Les analyses montrent que la rupture progressive est négligeable pendant la construction drainée, l'exploitation du barrage et la vidange non drainée. On constate que les noyaux sont vulnérables à la fissuration si leur parement amont est soumis à la pleine pression du lac de retenue. Toutefois, un remplissage très lent et draé fait augmenter les contraintes totales moyennes, de sorte qu'aucune fracture West indiquée. La fracturation hydraulique a é prédite dans une tranchée d'étanchéité profonde remplie d'argile compactée. Un cycle de vidange et de remplissage a fait augmenter les contraintes exercées sur la paroi amont du noyau, augmentant ainsi la résistance à la fissuration. Comme le montrent les observations sur le terrain, les contraintes totales augmentent et diminuent en fonction du niveau Wean dans le lac de retenue.

Landfills and associated leachate in the greater Durban area: two case histories

Abstract Two waste disposal sites in the greater Durban area used for codisposal of solid and liquid wastes are considered. One site is still operational and the other has been closed for a number of years. An investigation of Site 1 revealed that much of the area is covered by Berea Red Sand. However, the clay content of much of this formation means that it generally has a low permeability. The sandstones and tillite which occur beneath the sand also are of low permeability or are virtually impermeable. This is necessary when the character of the leachate produced is considered. The very high values, for example, of chemical oxygen demand are attributable to a large extent to the disposal of hop waste. Accordingly the leachate is conveyed from a sump at the toe of the landfill to a sewerage works. At site 2 very little has been done since its closure to ensure that leakage from the landfill does not pollute the surrounding ground or surface waters. It was suspected that this was occurring. Small non-perennial and perennial streams occur in the area of the site. The site itself is underlain by sandstones and tillite. These rocks have been weathered to form a mantle of residual soil of variable thickness which is overlain by colluvium on the valley slopes and alluvium in the valley floor. An investigation was undertaken to determine the extent of the pollution and revealed that a plume extended some 300 m downstream of the leachate pond. Two main remedial measures were proposed. First, a subsoil drain should be constructed immediately downstream of the landfill site. This should be located in bedrock, its purpose being to intercept seepage from the landfill. The drain should lead into a pump chamber that would remove polluted water which would then be conveyed to the sewer system. Secondly, to make doubly sure that the pollution was contained, a cement-bentonite cut-off trench, founded in bedrock, should be constructed 3 m downstream of the subsoil drain.

Comments on the teton dam failure: problems with the use of loess material in earth dam structures, by I. J. Smalley and T.A. Dijkstra

The premise of the paper by Smalley and Dijkstra is that loess is not suitable for use as a core material in an earth dam because it does not have the mineralogy and properties required for this application. As noted by Smalley and Dijkstra, the Independent Panel pointed out several design flaws in the Teton dam: (a) complete dependence for seepage control on a combination of deep key trenches filled with loess and a grout curtain; (b) geometric configuration for the key trench that encouraged arching, cracking, and hydraulic fracturing; (c) reliance on special compaction of the loess as the only protection against piping of materials into open joints in abutment rock; (d) inadequate provisions for collection and safe discharge of seepage, which inevitably would occur through the foundation rock and cutoff systems. The highly erosive character of loess is well known (Gibbs, 1962). The prevention of piping by a cracked core constructed of loess requires a proper downstream filter material. It has been documented that the use of concrete sand as a drain for loess will prevent piping (Sanchez, Strutynsky and Silver, 1983). Therefore, whenever loess is used as a construction material in an embankment dam (core, cutoff trench, etc.) a proper drain material must be provided. Many dams, for example Ririe Dam located about 20 miles from Teton Dam, have been constructed with a loess core and properly designed drains (US Army Engineer District, Walla Walla, 1969). Therefore, the authors' premise that loess is not suitable for use as a core material in an earth dam because it does not have the mineralogy and properties required for this application is debatable. Loess is an acceptable material when proper drains are provided.

Lessons from the Teton Dam failure

The Teton Dam failure is one of the most important single events in the history of dam engineering. Teton Dam is the highest dam which has completely failed. It was designed and constructed under the supervision of the U.S. Bureau of Reclamation which had built many major dams over the previous 70 years, and which was widely reputed to be a leader in the field. This paper is one of a group on the same subject for the International Workshop on Dam Failures, Purdue University. The objective is to determine whether concentrated re-examination of the failure details after about 9 years have elapsed, and most of the emotional heat generated by the disaster has dissipated, will allow any better understanding of the lessons learned for the profession. No attempt is made here to describe all the project features or review the failure details. The reader is assumed to be already familiar with the main reports prepared by the two eminent review panels (Independent Panel, 1976; U.S. Dept. of the Interior Group, 1977, 1980). A good summary of the main failure details is given in Seed and Duncan (1981). Soon after the failure I spent several weeks studying the problem, for my own edification. This effort included discussions with the responsible engineering staff of the USBR in the Denver headquarters. In Denver also I studied all the main documents, and had conferences with the geologists who worked on the job during the design stage, and others who had mapped the detailed geology exposed during the dam construction. Subsequently during September 1976 1 spent about ten days at the site. During this time the embankment remnant on the right abutment was being slowly and carefully excavated, exposing the rock foundation and rock walls of the cutoff trench as part of the failure cause investigation. The Resident Engineer and inspection staff which had controlled construction were still on the site and they spoke frankly about all the main points, although they were still somewhat shell-shocked. Later, during 1979--81, I was employed to study the subsequent events by a group of insurance companies who had paid flood damages and were petitioning the U.S. Government (unsuccessfully) for reimbursement. During

Dissecting a Dam

To determine the source of seepage from a Soil Conservation Service (SCS) high hazard flood prevention dam in New York, a geologic and engineering investigation was made which showed unexplained high permeability in the foundation and embankment and a possibility of voids in the trench drain system. The decision was made to immediately breach this 65 ft high earth fill structure. During the breaching a detailed study was made of the embankment, cutoff trench and drains. The embankment contained some oversize material but was in excellent condition. Unexplained high permeability rates measured in the embankment appeared to be the result of hydraulic fracturing during drilling operations. The drains were intact and were not overloaded; however, they had relatively low permeability. An ancient landslide was discovered in the lower left abutment which was not recognized during construction; therefore, the cutoff trench had not been extended through the slide material. The seepage appearing near the toe of the dam was moving through and under the slide material. The dam was redesigned and reconstructed with higher capacity drains. The cutoff trench in the left abutment was extended well below the slide zone and the embankment was replaced to its original design.

Expect the Unexpected: Foundations for Dams in Karst

Dam foundations in karstic terrain encounter unique problems and solutions, ranging from simple to complex. Two dams designed and constructed by the Tennessee Valley Authority (TVA) and founded on Ordovician limestones are discussed, starting with initial exploration and continuing through excavation, treatment and final construction, The foundation at the Columbia Dam site had two major problems—a fault zone beneath the east flood plain, and a continuous water-bearing seam that occurred approximately 35 ft below the top of rock and extended into each abutment. The Normandy Dam foundation is located across a minor syncline which exhibits two distinct sets of high-angle joints. Solution cavities associated with these joints necessitated a 40-ft deep cutoff trench in rock and major treatment in a localized area to a depth of 150 ft below the top of rock.