 This is the second part of a two-part series on the topic of dam foundations. In the first part, Dr. Don Deere discussed significant geologic features and methods of exploration to discover and evaluate those features. In this part, Dr. Deere will discuss methods for remediation of these features. His discussions address dam foundations and dams that are not performing properly due to the existence of a significant geologic feature. Once again, it is my pleasure to present Dr. Don Deere. Let us move to the next topic of Foundation Remedial Works. Once weak features identified, they require that remedial construction be done to improve the foundation. Additional excavation heads the list, in my opinion, of remedial measures for treating weak geologic features. It is often the most practical and cost-effective method. Of course, if the questionable zone is not found until it is uncovered during construction, the extra excavation quantities may result in scheduled delays, additional costs, and claims. One of the principal reasons for the separate excavation contract is precisely to avoid these unpleasant surprises in the main dam contract where the real money is. The geometric form of the unanticipated excavation may range from general deepening in an area of weathered or altered rock to trench excavation along a fault zone, to expensive and slow added or shaft excavation with concrete backfill. Two cases come to mind where a number of concrete blocks of gravity dams had already been founded above weak near-horizontal shear zones. The seriousness of the problem was recognized, and several possible remedial measures were examined to determine the most feasible from the viewpoint of providing an adequate safety factor against sliding, of lowest cost and least schedule interference, and of less interference to the construction of the remaining dam blocks. Decisions were made to excavate a number of tunnels under the existing blocks along the shear zones and backfilling them with concrete to serve as shear keys. The objectives were achieved and the dams now have several years of successful operation. In examining the different solutions, the design team must be resourceful and should include specialists in engineering, geology, rock mechanics, structural mechanics, and construction planning and contract administration. The second remedial work that we will talk about is consolidation and curtain grouting. Grouting is no stranger to any dam engineer or geologist as it is done at every modern dam site. I am bringing it up to discuss a number of recent improvements that have been made in some of the procedures. Drill holes for grouting are now made in the majority of cases with rotary percussion drills rather than with rotary diamond drills. The advance rates are up to five times faster with similar lower cost. Down-the-hole drills of three-inch and four-inch diameters have been used on a number of projects. Grout mixes of neat cement or cement with bentonite additives have now been mostly replaced by thicker and more dense grout mixtures of around 0.75 to 1 water to cement by weight with about 1.5 to 1% of super-plasticizer to reduce the viscosity in yield point. The hardened grout is denser, stronger, and more resistant to chemical and mechanical deterioration. A single grout mix is commonly used throughout the grouting of a project instead of starting with a thin mix and then progressively changing to thicker ones. Lower injection rates as low as 2.5 to a little over 6 gallons per minute that is 600 to 1,000 liters per hour are sometimes specified. That is to give a uniform grouting to the rock. Pressures are often allowed that are higher nowadays than those that would formerly be used and they are controlled in many of the recent dams together with the volume of grout by the Lombardi-Dear-Gen method, GIN, meeting grouting intensity number. The following brief discussion is divided into consolidation grouting and curtain grouting. Consolidation grouting, also called blanket or area grouting, consists of grouting on some predetermined grid pattern to a shallow depth in the range of 20 to 35 feet. The grid spacing for the primary holes may range from about 12 to 24 feet. Secondary holes may be drilled and grouted in the center of the grid as mandatory holes in the specifications or as required if the grouted mission of an adjacent primary hole exceeds a specified quantity. Similarly, tertiary holes may be added as needed where the secondary holes have large grout admissions. Consolidation grouting accomplishes two things. It increases the modulous or stiffness of the rock mass and it reduces the permeability of the rock mass, both good things for the dam foundation. By filling the more open fractures in joints completely or almost completely and the thinner fractures partially, the grouting homogenizes the foundation rock, leading to less settlement of the dam and less undersea beach. Consolidation grouting is required for almost every concrete arch dam for most concrete gravity dams and for some embankment dams depending on the dam height, the foundation type, and the philosophy of the dam designer. On several recent projects, I have recommended the increase of the depth of the consolidation holes by up to 50% or so, just for those two or three lines upstream of the future deep grout curtain, which is a topic of the next section. In preparing the drawings and specifications, the design team must make three decisions about the grout curtain geometry, the depth of the grout curtain, a single line versus a multi-line curtain, and the spacing of the primary grout holes. The primary grout holes are the first holes drilled of the curtain. They are widely spaced and carried to considerable depths. I prefer to drill these with core recovery and with Lejeune permeability test, but only a few of the primary holes, about one of each four or one of each five. These should be drilled to a depth of, I feel, about one H, where H is the future hydraulic head above the foundation grade at the point in question. These primary exploratory holes are grouted, and the results of these, plus the results of any original design exploratory holes in the region or in the immediate area, are examined to arrive at the depths of the remainder of the primary grout holes. Now, the remainder of the primary grout holes would not be core drilled in my recommendation that they not be tested for Lejeune permeability. I think the grouting results give us the information in themselves. The depth of the primary holes other than the exploratory primary holes is frequently in the range of one half to three quarters of the hydraulic head. The depths of the secondary holes will depend on the grouted missions for the adjacent primary holes, but usually it will be somewhat less. Similarly, similar procedures are used for the tertiary and quaternary holes and for the quaternary holes if needed. At most rock foundations, the single-line curtain is employed. Where poor rock quality exists, a triple-line curtain is commonly used to reduce the erosion in the piping potential. Upon occasion, a five-line or seven-line curtain has been constructed with the final two lines of microfine cement and more of hard, durable chemical grout after setting. With respect to the spacing of the primary grout holes, the range is about 20 feet to 40 feet. After the primary holes are finished in an area, the secondary holes are split-spaced between the primaries and the tertiaries are split-spaced between all the others, as are the quaternaries if needed. If quaternary holes were used and if the original primary hole spacing were 40 feet, the final spacing after the quaternaries would be 5 feet. In the areas of quaternary takes or admissions greater than specified or desired, the additional quaternaries would be drilled, resulting in a 2.5-foot final spacing and occasionally even a few additional holes. In scoriae's highly permeable lava, in karstic limestone or in sandstone or other rock types with very open, very permeable joints, my philosophy is to provide a triple-line curtain. The outside lines are grouted first through the primary, secondary and tertiaries to act as barrier lines or confining lines for the grout injected in the central line. The final closure or final tightening of the rock may require in this central line up to quaternary or quaternary holes. My final comment on grouting is to say that I have found the trend of employing field computers to control the grouting rates, quantities injected and grouting pressures to be very welcome and of value to the grouting engineer. Grouting statistics can also best be displayed and stored by the computers. In lieu of grout curtains, cut-offs may be used to reduce seepage below dams where, one, the rock foundation contains so many voids or fractures with clay that the efficiency and permanence of the grout curtain is brought into question. Or two, if a layer of permeable sand and gravel occurs below an embankment dam. Slurry trenches have been used below earth embankments where the dam is not too high and where the permeable layer to be cut off is shallow, say less than 60 feet. A wide trench with sloping sides is excavated by a drag line and is supported by a thin bentonite slurry. The excavated alluvium is mixed at the surface with some additional bentonite and perhaps with some more clay and silt and then is redeposited in the trench by the drag line. The result is a cheap, low permeability, low density cut-off. One can see why this type of cut-off is seldom used below high dams. Concrete panel walls of two and a half to about five feet in thickness of cast in place concrete trimmed into a vertical slot filled with bentonite slurry is a popular type of cut-off below high embankment dams. The excavation of the trench is by an elongated clamshell working between surface concrete guide walls. The wall is excavated in concrete backfield in alternating panels. Depths of alluvium of up to 200 feet or so have been constructed by this procedure. Some designers prefer a very low-strength concrete backfield called a plastic concrete in an attempt to match the modulus of the alluvial foundation so that under the weight of the dam the settlement of the wall and the settlement of the alluvium will be similar. Others prefer a moderate-strength concrete of perhaps 1,500 to 2,500 psi. Good experience has been reported for both types of concrete. Backfield overlapping tunnels have been used a number of times to construct a vertical concrete cut-off wall. The wall may be constructed from the top downward or from the bottom upward. After the first tunnel is excavated it is backfilled with cast in place concrete and the second tunnel, either above it or below it, is begun backfield, etc. The type of cut-off is used in poor rock or dry alluvium or other soil. Excavation of the tunnels may be by hand, by small mechanical road headers or in some cases by light blasting. Trenches may be excavated in sections by the more exotic means of hydrofrase or high-pressure water jets. The CCAN overlapping method has also been used to construct vertical cut-off walls. In this method, alternate drill holes are drilled in concrete backfield. The infilling drill hole is then drilled between the first two holes with care and difficulty to assure several inches of overlap. The drill hole diameters may be in the range of 18 to 30 inches. My own experience with cut-off walls includes concrete panel walls through alluvium with boulders, three cases. Overlapping tunnels constructed both from the bottom up and from the top down, three cases. One in friable sandstone, another in caustic limestone, and one in a complex volcanic sand, scoria, and weathered andesite. And secant wall, one case in scoria and weathered lavas. Moving on to the topic of drainage. Drainage control in the foundation is an essential for dams. Concrete dams customarily have one or more grouting galleries close to the rock foundation for grouting the rock. The grout holes are drilled from the upstream part of the floor of the gallery with the drill holes angled of 10 to 15 degrees in an upstream direction. The gallery also serves as a drainage gallery with the drain holes drilled with the holes angled at 5 or 10 degrees in a downstream direction. The drain holes may be from 3 to 4 inches in diameter typically and are spaced at 10 feet to 25 foot distances. The lengths of the drain holes are similar to those of the grout holes or somewhat shorter. The holes should be provided with a rigid plastic pipe outlet with fittings with a valve to be closed and the water pressure to be measured when desired. The drain should also have reverse goosenecks so that the air cannot enter into the drain hole and promote a precipitation of minerals primarily iron but also other oxides. Drain may also be provided with a double perforated casing and the inner pipe with a filter fabric in zones of possible erosion and piping of fines such as you might get in a weathered rock or in a fault zone. To prevent the seepage from exiting the abutment area just downstream of the dam that might cause piping of fines or saturation of the slope special methods may be taken. These include covering the area with an inverted filter drain or better yet to drill drain holes into the abutment. Failure to control the drainage has led to slope instability with potential problems to the edge of the dam abutment. Upon occasion, the drainage system has been enhanced by excavating drainage addits from which drainage holes could then be drilled and peaceometers install into the water pressures in the abutment. Would any of the panel members like to comment on the methods of remediation that I have covered? I know that a lot of you have good experience in this. Don, in addition to the consolidation and curtain grouting that you mentioned I think it's probably important to talk a little bit about the contact between the foundation and the dam to be built and the need to treat that surface with minimal concrete. I know there's lots of examples of problems that have arisen because that area didn't get properly addressed when the dam started being built. Well, I think this is certainly true, but I also have a little hesitation about the slurry grouting of the surface. I think sometimes that those are so thin that they are very, very weak as the material is being placed and compacted above them if this is an earth embankment, for instance, that they might loosen and I do recall on one large dam where the consulting board was visiting the project and we saw water flowing out at the contact between the earth and underneath this little thin layer of grout so we had them excavate several trenches so we could look at it as a problem. They just simply had not been able to get the nice ceiling that they wanted and have it to be preserved. My own particular feeling about placing a covering is to use concrete sufficiently thick that it does allow soil to be placed above it and the dam to be started if it's an embankment dam and there are a number of projects called concrete. We don't call it dental concrete because that gets bid at too high a price. We simply call it a smoothing concrete let's see another term that's been used a homogenizing concrete a leveling concrete it's to put in so that when you walk away from it you have a minimum of 6 inches and in lots of areas up to 12 or even 24 inches and you have improved the surface the contact is going to remain as a contact but you can now place the rest of the dam in rapid high speed mode and some of these excavations through the weather rock down to the foundation level ends up with extremely irregular topography which is almost impossible to do a good job of placing the thing but by placing the concrete we can achieve the thing. Thank you for your point and I'd like to move on and see if Jim has a comment on this. I have a question. You mentioned your preference for grouting do you have a preference of percussion drilling versus core drilling for drainage? I would say the answer is yes I probably have gone with percussion drill holes more often with ones that have been done by rotary drilling and the reason is they are so much cheaper you can put 5 times the number of drain holes if you do it by percussion drilling so you can actually bring the spacing down to 5 feet if you wish or 3 feet now in a number of dam sites the agencies have a policy of putting their drain holes in with rotary drilling and I think they are probably getting a superior job but I really wonder if they will get the same benefits from the more closely spaced percussion drills obviously it's a little bit on what the contractor has available particularly if you are coming in to do some remedial work and you have to get into a drainage and you need a drill or something that you can get the hole in but I think your point is well taken and it is certainly one that engineers differ a little bit on that subject Frank do you have comments on this? In your discussion you mentioned that they are intending to use higher pressures during grouting and you also touched on the benefits of using computers and sensors to monitor the grouting process itself and I just like to essentially chip in that we shouldn't let the precision of the computers and the trend toward higher pressures distract us from the fact that when we are grouting in the upper portions of the circuit grouting the consolidation grouting and the first stage of the curtain grouting that we very closely watch those pressures and to keep them under control because of the low overburden pressures that exist Well your point again has been borne out by experience in numerous cases there has been hydro fracturing or hydrojacking of rock particularly along the abutments or in a shallow foundation of stratified material in the dam bottom it is surprising in Massey Brock a normal let's say a granite with a normal amount of jointing the very high pressure that you can use with surface consolidation grouting and not see any lifting or any damage whatsoever but a very good periculation of the grout throughout so I'm in favor of using fairly high pressure a lot higher than we normally do in the deeper rock let's say below 15 20 feet but again we have to be very careful in zones of questionable rock in zones along the abutments where the stress condition is not real good for resisting a high grouting pressure I could name two or three instances where we have jacked up beginning parts of dam construction with the concrete already poured and set by adjacent grouting which got on a little too high however Frank the computer grouting can also if you're watching that closely the computer plots and I like live time plots of the pressure versus time and of the volume rate of volume versus time and just you see a little block where all of a sudden the pressure drops and the volume goes up you've hydrofracked and then you can reduce the pressure if you try to keep grouting that same pressure it often goes along the same and then another hydrofrack but as you say you ought to start off with pressures that are not too high to induce that I think the point is certainly well taken Dave you had a point I think that you were discussing earlier about maintenance yes during your comments you indicated how essential drainage is to the performance of the foundation that the Bureau of Reclamation has used drainage as a defensive measure for the dams and leaching from the upstream grout curtain and from the parent rock tends to reduce the efficiency of the drains and I believe the drainage really needs to be maintained in order to continue to verify the performance of the foundation and of the grouting deferred maintenance their delay of maintenance to keep these drains clean will have a tendency to make cleaning very expensive and quite often I'll be delayed to the extent that the drains actually have to be re-drilled but I liked your point about the drainage being essential in the part of the dam foundation well I think you've touched a very critical point in looking at existing dam foundations particularly in old dams one of the things that we often see are plug drains their drainage system has lost its efficiency over time and the maintenance has not always been done and then our recommendation has to be to clean the drains and almost automatically to add a certain number of new drains because the cleaning doesn't always get you back to the zero condition but drainage is essential and I'm very glad you brought that point up I would like to add that in a project where earth and dam fairly high some 400 feet high was founded on a sandstone foundation of a fair amount of weathering along the joints that made the whole sandstone very friable and so we had to be particularly careful that no seepage water exited downstream in an unfiltered condition because we knew it was going to bring some eroded fine sand a fairly good job was done was decided to cancel the grout curtain and to go in with a plan and to go in with a cutoff so a cutoff wall was done a panel wall in the abutment tying in with drainage curtain under the bottom of the river however they forgot to treat the area adjacent to the spillway and the spillway was not part of the dam it took off from a low area some distance away and while visiting the site some 2 or 3 months after the dam was in the filling stage they took us to see a wet spot where they had controlled it by literally pounding in about a 6 inch diameter white I remember that very well there was a certain PVC pipe and that pipe was flowing half full of water the most beautiful clear water you ever saw and I said are you sure you're not getting any leak or any erosion and piping of fines I said well how about all of those little slide scars taken here and there well it's rainfall well it wasn't rainfall the seepage coming in to this downstream slope and the seeps had actually little deltas of sand that they were carrying and in the pipe that was carrying this crystal clear water I took off my hard hat put it under the pipe brought it out poured the water off and the bottom was covered with nice beautiful clean sand and it's amazing when you have a good flow going and what seems to be clear water the amount of sediment it can bring one reason in your drainage curtain in dams to put little collector pipes down underneath or collector containers let the drainage water go into those and then to flow out through a pipe to another tank and that way you can make a certain number of the drains as monitor wells to check on the plaguing or the erosion of vines well I think that we've covered this topic fairly well and our next topic is going to deal with foundation assessment for existing dams I now would like to talk about foundation assessment of existing dams while the previous discussions were more oriented toward dams in design or under construction, many of the same principles apply to the assessment of existing dams for example, the weak geologic features that require treatment for a new dam probably also occur to some extent at most existing dams if those features occurred and were identified during construction they likely were treated the two key questions are were all of the weak features discovered and were the recognized features analyzed and treated sufficiently the difficulty in providing answers to these two questions is precisely the difficulty in assessing the foundations of existing dams the fact that the dam is still in operation and is functioning is a welcome observation but it does not necessarily provide the assurance that all is well for future operations for the maximum design flood has not occurred as yet second, detail inspection indicate that the dam's foundations have not performed flawlessly as indicated perhaps by cracks in concrete seepage and wet spots in the dam in abutments and drains in a plugged state third, time dependent processes that may lead to detrimental behavior down the road are still ongoing seepage and piping, drain hole plugging, increasing piezometric levels locally or slow creep displacements the background study for an existing dam differs in several ways from that for a new dam they both would include studies of regional and local geology but the site inspections would be different and that the reservoir would obscure much of the upstream geology but new exposures may be available in the downstream area along new road cuts and other excavations of particular value in the foundation assessment are records of the original design exploration in the form of boring logs, rock cores geologic maps and geologic profiles together with the foundation design report the plan and specifications must also be made available for the study for some very old dams these documents may not all be available which creates a hardship in dam assessment of equal value are the records of construction these would include construction photographs excavation records grouting records design and construction memos and change orders and the as-built drawings again, if not all of these are available the foundation assessment will suffer the history of performance of a dam may provide vital clues the records of visual inspections and instrumentation records may be the keys to the historical performance of the dam and its foundation time plots of the data are helpful in recognizing trends and correlations with temperature reservoir level or rainfall of importance are dam displacements drainage discharge from the drain holes drainage from the galleries or springs in the abutment in piezometric levels attention must be focused on anomalous behavior and any increasing trends after the completion of the aforementioned study the assessment team will probably have some preliminary thoughts about the dam foundations an important item is whether new investigations are needed and of what types commonly several new borings will be drilled targeting the anticipated problem areas the borings may be made from the top of the dam or angled as necessary from galleries in the dam or the abutments careful drilling with the most appropriate equipment and pressure blowout devices may be needed particularly if you're drilling from galleries that are well below the reservoir level undisturbed cores from weak zones may be desired for modulus or direct shear laboratory testing and Lujon permeability in borehole photography or TV scanning down the borehole may be desirable the installation of permanent piezometers is frequently done including the sophisticated multi-port piezometer in certain cases infrequently a drainage gallery will be driven or an existing gallery extended to allow observations to be made borings to be drilled or monitoring devices to be installed at this stage of the assessment the background information and the results of the new exploration are analyzed to see if any particular foundation problem exists and if so what are the remedial measures that should be considered by the assessment team the degree of urgency must be assessed as well as the need for an immediate lowering of the reservoir perhaps only long-term monitoring devices will be required together with a partial restriction on reservoir height frequently no additional work is required the next discussions will deal with remedial measures that might be used in case the assessment so indicates it is important that the design assessment team have access to an experience construction advice in conducting the alternative studies of the remedial measures in my own experience it seems that the most common remedial measure either singly or in combination with other methods is an enhancement of the drainage system to bring about a lowering of piezometric levels or to reduce piping and sloughing enhancement methods have included the following cleaning out of the pumping sump and providing standby pump and emergency power sources cleaning out drain holes and adding new ones excavating a drainage gallery or extending an existing one drilling long drainage holes from the surface into the abutments and covering seepage areas with an inverted filter only occasionally has additional grouting been done in some cases where drain holes have had increasing flows and have indicated sediment transport the holes have been grouted and new drains drilled in others the grout curtain was reinforced with new grout holes where it was suspected that the grout had deteriorated or the clay had been washed out in limestone particularly where the clay has been partially eroded by the leakage water passing through solution cavities that has been carried out both to reduce seepage and to prevent further enlargement of the cavities by erosion of the infield soil that could threaten the dam by the occurrence of a sinkhole where sliding resistance or overturning resistance is judged to be insufficient under maximum design flood either along a weak sub-horizonal zone in the foundation rock or at one of the construction lift joints of a concrete gravity dam high capacity grouted steel tendons have been installed and post-tensioned to increase the vertical force on the dam the steel tensioned anchors are often needed for dams where the spillway capacity has been found to be inadequate and the dam could conceivably be overtopped a number of concrete gravity dams have undergone this treatment as well as having the spillway capacity enlarged where leakage or piping has been observed in abutments concrete cutoff walls have been chosen in some cases as a treatment to be carried out rather than additional grouting or drainage while the time and cost of installation of a cutoff wall may be greater the security given by the wall should be superior to just grouting different construction techniques available as noted previously and the choice of the type of concrete wall to be installed depends on several factors including the reservoir level at the time of the remedial construction work the availability of the specialty contractors and the preference of the designer and the construction team stability berms of compacted earth gravel or rock fill have been used numerous times to increase the stability of either the downstream or upstream slope or both of an earth embankment weak foundation material is most often associated with the slope failure in some cases the berm is needed when the height of the dam is to be raised a few concrete dams both arch and gravity have been buttressed by concrete or rock fill following cracking and leakage during their first filling I would ask the panel members to comment on their experiences with respect to investigations assessment or remedial construction for existing dam Don, when you've got holes drilled in a dam foundation to collect samples there's often a tendency to put too much instrumentation and without a thought to just exactly what that instrumentation is going to tell you I think the instrumentation that you do put in has to be specifically designed to answer a question and so you need to know what those questions are first then pick your instrument to give you the answers to those questions Duane, I again agree wholeheartedly I think a number of dams have been over instrumented to collect data and many times the data goes into a file system and is not really used because there's so much of it that may not be pertinent so they think well we just have it for the record and as you say it is much better to identify what the potential problem areas are and to get the instrumentation in that will give some answers to questions that you might have Don, you know, we've talked about explorations for new dams and for existing dams here and it's been in my experience that in the existing dams the tendency kind of is to go with the data that you have in order to try to make the assessment and make the decisions and I guess I would like to emphasize that when you're when the public safety is at issue and when you're trying to assess what a problem is and determine what's going on that a lot of times the exploration has to be done regardless of the effort it requires and the cost that it requires in order to do it Again, I think the point is extremely well taken and I've known several instances and where it was judged to be necessary to lower the reservoir level and if this is a power hydro project this is a very serious monetary consideration and it even could get into public relations problems and regulatory problems but on several instances it has been deemed to be necessary and it had to be done as you say you must make your case and stick with it and convince the owners that these things have to be done Don, I think one of the things you brought attention to was the need to pay attention to anomalies in the dam's performance over the long term I believe that we can become complacent with the performance of a structure the monitoring that is going on and independent review or new eyes can help pick up changes in that performance and I believe are critical to the long term assurance of the performance of the structure I think that programmatic emphasis needs to be placed on the value of independent reviews or that new look Yes, well, I think those are good comments I believe that a number of the projects I'm involved in where there is a board of consultants the fact that they can come to the project maybe every six months during construction and again during the first filling gives a little bit of the same independent look at things that a parent or not as a parent to the people who are working on a full time but during the operation period which I think is the one you are referring to primarily why it's also a very good period if we all can think back on projects where the dam and the reservoir were just going along functioning perfectly and five years, ten years down the road suddenly you go into the drainage gallery and you find that you have 20 cubic yards of mud on the floor you can hardly get into the drain that piped and so this means this is a time dependent process it keeps going on and then you must have the monitoring and the surveillance as an ongoing thing for the life of the project no doubt of it I appreciate that comment Dave I'd like to reiterate and sort of reinforce what Dave said with respect to assessment and UIs sometimes when we routinely take measurements on instrumentation we don't really stop to think about what we're taking and interpret what we're looking at and we look at the overall perspective of what the trend of the data is and sometimes if we just look at a short period that trend may be that there is nothing being changed but if we look more closely at what that particular instrument is supposed to be telling us and look at all the data that is in the file and not just a segment of it we find that there's a different story so the perspective with respect to new eyes the new eyes the fresh eyes would then undoubtedly look at the entire record and it's good periodically to have somebody different look at this not only from the perspective of an independent evaluation but also with respect to inspections that are done routinely so this is really an important feature and I know we have one situation where this happened and it wasn't recognized that something was going on because the narrow time was being looked at and once somebody recognized this the new eyes an analysis and evaluation was done and it was determined that the dam was actually moving and lots of money had to be spent to remediate that situation I think that's a good example of new eyes looking at the data and looking at the data over a period that is sufficiently long to allow you to judge things that are happening on a yearly basis and not necessarily on a month by month Jim, you had a comment Yeah, I'd like to add a little bit to the use of a board of consultants this is something that TVA has been committed to for over 35 years our board meets once every six months and reviews at that point all the projects that are at whatever stage be it design, construction, maintenance operation anything that relates to public safety and where issues are presently being undertaken and we found that it serves two purposes one is to keep us sharp because we have to present to the board every six months the details of things looking at the whole forest and not just the trees and secondly it brings an independent judgemental review that we feel is based on the highest level of experience to the projects and preserves the public interest in terms of safety and everything else in the public domain and I feel very strongly that it's enhanced the integrity and quality of our projects through the years the independent look is just extremely important As an independent consultant I couldn't agree with you more Gus, I think you were going to tell us a little about some cut off experience that you've had This really plays a role into the proper type of remediation once it's identified that there is a problem and particularly with a seepage problem what is the correct remediation measure to implement for that particular situation and we had one situation where there was a difference of opinion but what ended up happening was a partial slurry wall was put in to address a seepage condition that was causing some potential piping in the one end of the embankment and so it was attempted and it turned out that it didn't work and then a different a different set of eyes again as we mentioned before came in evaluated everything that had been done and disbanded with a cut off but then addressed it by an elaborate drainage system to control and monitor the seepage and this worked very well and it's been in operation now for about two to three years and there are no problems so it's just a situation where the proper assessment needs to be made and as you mentioned earlier with respect to all the various disciplines to bring all those people together to discuss the necessary remediation Well thank you for all of these comments on this particular topic I think that they've added to the value of this session very much In completing my presentation I wish to thank the Interagency Committee on Dam Safety for inviting me to make this video and special thanks are given to the panel members whose comments have enhanced the breadth and the depth of my presentation on Dam Foundations and finally to the production crew that have helped in making this a most pleasant experience