Characterization of Internal erosion susceptibility in Sandstone Dam Foundations

Transcription

1 ICSE6-263 Characterization of Internal erosion susceptibility in Sandstone Dam Foundations Jean-Jacques FRY 1, Gabriel JORGE 2 1 EDF-CIH Le Bourget du Lac Cedex France jean-jacques.fry@edf.fr 2 3Avenue du Maréchal Juin Boulogne-Billancourt France This paper is focused on the susceptibility of sandstone dam foundations to internal erosion. A synthesis of investigations on half a dozen of dam sites, where internal erosion incidents occurred or were suspected, is presented. From field investigations data or from grouting control data, some criteria to assess internal erosion susceptibility are selected and tested. Drilling recorded parameters look very useful to assess the quality of the sandstone foundation. Lugeon tests are interpreted in order to show evidence of internal erosion processes, like wash out, hydraulic fracture, irrecoverable changes of permeability and discharge rate. In conclusion specific parameters and threshold values are proposed and justified from past experience to investigate susceptibility to internal erosion and investigate location of erodible sandstone foundation areas. Key words dam, foundation, piping, internal erosion, investigation, failure. I INTRODUCTION This report deals with the evaluation of the susceptibility to internal erosion of sandstone dam foundations. More than 85 % of the causes of failures or accidents depend essentially on the design of the project and on the methods of construction and operation of dams. It is interesting to note that these elements are not quantified in a precise way by the calculation. For embankment dams, among these elements, the phenomenon of internal erosion is crucial. A list of dam foundation incidents, elaborated in 1994 and illustrated in table 1, gave the feeling that the sandstone foundations are among the most dangerous rock foundations, prone to be eroded. Date Dam Incident Problem in foundation 1791 Puentès Failure Internal erosion of alluvium under masonry dam 1895 Bouzey Failure Sliding of the sandstone foundation with clay lenses 1917 Tigra Failure Sliding and erosion of the sandstone foundation 1923 Gleno Failure Buttress failure on weathered porphyre and sandstone foundation 1928 St-Francis Failure Leakage and old sliding in the left abutment schist 1959 Malpasset Failure Sliding along a fault caused by uplift in joins 1963 Baldwin Failure Internal erosion of silty sand fault filling in foundation 1963 Flagstaff Accident Internal erosion of sandstone foundation from poorly grouted diaclasis 1963 Navarro Accident Leakage 140l/s through the right abutment 1965 Fontenelle Accident Large leakage in sandstone foundation through poorly grouted diaclasis 1979 Sisga Accident Piping and 3m 3 /s leakage in sandstone foundation left bank 1981 Itiyuro Accident Large sinkholes and 1m 3 /s leak in weathered sandstone foundation Table 1: List of major dam foundation incidents. 1577

2 So the question is : On what elements the risk of dam failure caused by internal erosion in weak rock like sandstone foundation may be quantified?. In order to answer that question, this paper is focused on the sandstone foundations with the objective to characterize the susceptibility to internal erosion. To do this, we try to analyze the results of site investigation campaigns and geotechnical studies carried out during feasibility studies or other phase of dam project or during foundation repair works on several dams founded on sandstones. II INVESTIGATIONS FOR INTERNAL EROSION Sandstone is a sedimentary rock consisting in sand cemented by various more or less resistant bonds, as silica, carbium carbonate, iron oxide or clay. According to the quality of the bond, it is frequent to observe sound rock, weathered rock, cracks and diaclasis and lenses of sand or clayey sand. The first objective of the investigation campaign is the localization of the weak zones: weathered sandstone, cracks, diaclasis and sandy lenses. To reach this goal, the drilling recording is one of the most effective methods. Its use is presented here under. The second objective of the investigation campaign is the characterization of the weak zones. One more time the drilling recording is very efficient to do that. In complement a careful interpretation of the Lugeon test is added. II.1 DRILLING RECORDING Figure 1: Example of drilling recording log and soil classification associated. 1578

3 In addition to traditional geological studies by core drilling, records logs during drilling have made a major revolution with the emergence of the first digital recorder EMPASOL, developed in 1981 by Solétanche, then by J.Lutz and others many years later. The many possibilities offered by digital processing have resulted in the highlighting method of drilling parameters as a validated method of foundation investigations. Indeed, the continuous measurement of parameters has significant developments, including correlations with conventional geotechnical tests, but also compound parameters criteria associated with a particular type of soil or with a range of strength. In 1981, the compound parameter criterion used was the Relative Hardness, Dur, which is proportional to the unit energy delivered to the ground, by a drilling machine F320 with a tricone VH1 at constant rate of rotation. Dur is the product of the torque pressure in 100 kpa by the vertical thrust on the tool in 100 kpa divided by the instantaneous drilling rate in m/h. Usual values are between 5 and 10 in estuary sediments, between 150 and 200 in gravelly alluvium, and between 400 and 600 for a soft limestone. Such investigation was carried out after an incident at Itiyuro dam, where large increase of discharge flow and 20 m sinkhole in the dam occurred. The interpretation of recorded drill parameters log led to pinpoint the eroded areas in the sandstone foundation of Itiyuro dam consecutively after the break of the concrete pavement of the gallery of the intake tower (this incident occurred during a flood water release). The Dur values in the eroded sandstone areas were lower than 10 (Figure 2). Figure 2: Use of compound parameters criterion Dur to localize eroded sandstone area in foundation. II.2 LUGEON TESTS The used tests carried out in a rocky dam foundation to measure the permeability are the Lugeon tests in boreholes. The permeability of the sandstone matrix is not the problem. For example, in Las Cuevas site, the values of permeability of the sandstone matrix vary from 0.6 UL (Lugeon units) for a porosity of 10% to 12 UL for a porosity of 19%, while the permeability of joints and cracks can reach 1000 UL. This was also the case for most of the dams in this study. The Lugeon tests have to be located areas prone to be eroded: cracks, joints, sand lenses and weathered sandstone in order to characterize large flows may exist. They have to be interpreted in order to assess their internal erosion strength. On site Lugeon tests in crack or weathered zones are carried out by injecting step by step water under pressure up to 1 MPa between two rubber packers for testing a constant length (3 to 5 meters generally). During the test, the diagram of flow versus pressure is recorded. The recorded curve may be one of the flow patterns shown schematically in Figure

4 If the water absorption followed strictly the laws of the laminar flow in a homogeneous medium, the discharge rate trend versus the pore pressure should be linear (Figure 2. graph. a). In fact, there are often disruptive phenomena of clogging (Figure 2. graph. b) or wash out (Figure 2. graph. c) by increasing pressure. The detailed interpretation of these tests provides several parameters that can identify the internal erosion resistance of the crack filling in the rock medium. a. laminar flow no erosion b. clogging c. wash out Figure 3: Interpretation of Lugeon tests to characterize susceptibility to internal erosion. Ko: initial permeability of the medium (expressed in units LUGEON) where a linear relationship between pressure and flow is observed during the pressure rise. it is characteristic of open joints and cracks unobstructed by deposits or sediments and is obtained by taking the tangent to the curve at its origin. If the pressure is growing faster than the flow rate without hysteresis: it is characteristic of turbulent flow in a homogeneous medium. Kf: final permeability of the medium (expressed in units LUGEON) where a linear relationship between pressure and flow is observed during the pressure drop up to origin. Ier=Ko/Kf: the ratio of final / initial permeabilities of the Lugeon test. The index I, or index of erodibility of the medium, is interesting to describe statistically the sealing capacity of cracks or the erodibility of the medium. This index has its full meaning, where appropriate maximum pressure is compared to the maximum pressure induced by the reservoir. Pc: critical pressure of initial permeability. It is the pressure at which the flow rate - pressure flow relationship is no more linear. It is the beginning of the phenomena of wash out or clogging cracks and joints, possibly associated with water erosion of rock matrix in soft rocks and erodible materials. This parameter, measured across the foundation, compared to the maximum pressure induced by the reservoir, is extremely useful in defining the degree of weathering of the rock and to provide adequate treatment for resisting the future hydrostatic pressure of the reservoir. III OBSERVED CORRELATIONS BETWEEN BASIC PARAMETERS III.1 Unconfined compression strength, time of disintegration and sandstone class The disintegration time in water was studied with a lot of sandstone samples. A sharp difference of behaviour was noticed between the samples staying intact without remoulding of mechanical properties and the samples disintegrating completely. That clear distinction between sand and soft sandstone depends on cohesion. Intact samples without disintegration after immersion get unconfined compression strength higher or equal to 0.5 MPa [Dobereiner, 1986], value close to those proposed by the Geological Society of London (1970) and the International Society of Rock Mechanics (1978). 1580

5 The disintegration time of 5 cm by 5 cm cubes from Itiyuro or Las Cuevas is clearly correlated to the undrained cohesion: disintegration time lower than 1minute or respectively larger to 1 month is associated with cohesion lower than 14 kpa or respectively larger than 65 kpa. The samples with unconfined compression strength between 0,5 and 20 MPa are classified as soft sandstone [Dobereiner, 1986]. III.2 Example of correlation between recorded drill parameters and Lugeon tests Table 2 encompasses parameters from drilling records of the main geological layers of sandstone in the foundation of Itiyuro dam. Sandstone layer Elevation (m) Porosity Permeability (UL Lugeon Unit) Critical Pressure (Pc in kpa) Relative Hardness Dur (10 10 Pa 2 /m/h) Erodibility Ier = Kf/Ko sound weathered+sand Upper Pink Sandstone 585/580 0, > / /1200 0/2/5/20 (sand) >8 Yellow or White Sandstone 577/ / /555 0,24 to 0, /50/70 35/50/70 50 (sand) (sand) 1, /5/10 Pink Sandstone 555/ / / /535 Lower Pink Sandstone Lower Conglomerate 535/530 0,19 to 0,25 0,12 To 0, /20 ~ /75 50/75 50/90 50/70 50/ /120 50/70 5/10 5/10 <530 1 > /1500 Table 2: Example of recorded drill parameters and Lugeon test interpretation. 3,20 1,75 2,50 4,00 III.3 Correlation between critical pressure and other parameters The relation between the critical pressure and the porosity is not clear. Only the matrix porosity higher than 0,20 to 0,22 seems to have a bad influence on the critical pressure at Itiyuro. It corresponds to UL> On the other side, there is a clear correlation between the initial permeability Ko and the critical pressure Pc. Higher is the weathering, weaker is the sandstone cohesion, higher is the permeability and lower is the critical pressure. This correlation is very useful: it gives the final permeability after repair required to stop any internal erosion process. For instance, at Itiyuro site, the maximum required permeability is 8 UL, according to the 550 kpa minimum critical pressure. It is confirmed in another example, at Borde Seco, where the maximum required permeability is between 5 and 10 UL, with the minimum critical pressure between 700 and 900 kpa. Another correlation was observed between the critical pressure and the sandstone cohesion. The extrapolation of the relationship measured in Itiyuro foundation leads to the minimum cohesion of 200 kpa for grouting repair. During the investigations of Las Cuevas foundation, geotechnical classes of sandstone were assessed by the critical pressure, the % RQD and the permeability. From table 3 a clear difference appears between left bank and right bank of the valley. The same previous relationship between initial permeability and the 1581

6 critical pressure observed in Itiyuro and Borde Seco foundations is confirmed in Las Cuevas. In the same time, there is a clear link between the degree of weathering, the critical pressure and the RQD (table 3). % RQD values from boreholes lower than 50% Initial Permeability Ko in U.L critical pressure Pc in 100 KPa min max location min max type of sandstone 30 to 75% 2 6 Bottom+LB 5 7 Hard and well cimented RB to 90% Bottom+LB 2 5 Middle hardness RB 2 5 moyennement fissuré 90 to 99% Bottom+LB 0,5 2 Very weak with diaclasis RB 0,5 2 Very poorly cimented 100% Bottom+LB 0 0,5 Completely weathered RB 0 0,5 Table 3: Characterization of sand stone layers with RQD, Ko and Pc at Las Cuevas site. The mean values of critical pressure of the geological sandstone layer and the mean compound drill parameter Dur in the same layers are correlated in Itiyuro dam foundation (table 4). Type of sandstone layer Dur Pc (kpa) Weathered sandstone like sand 0 à Yellow and White Sandstone 30 à Middle Pink Sandstone 50 à Lower Pink Sandstone 100 à Table 4: Correlation between Dur and Pc at Itiyuro site. IV CRITERIA OF SUSCEPTIBILITY TO INTERNAL EROSION The comparison of the range of values of the basic parameters measured by numerous investigations on erodible dam foundations (Table 5) provides the empirical limit between no erodible and erodible materials. Parameters unit ITIYURO BORDE SECO LAS CUEVAS LA HONDA min max min max min max min max Pc- critical pressure kpa Ko- initial permeability UL Ier=Kf/Ko erodibility > 10 Dur-Relative Hardness C-cohesion kpa n-porosity % % RQD values < 50 % 90% 90% 80% 80% Td-disintegration time minutes <1 2-3 <1 4-5 <1 <1 <1 <2 Table 5: Characterization of susceptibility to internal erosion of sandstone foundations. 1582

7 The internal erosion risk in sandstone foundation can be assessed by the compiled criteria in table 6. On site Investigation Parameters criteria Critical pressure Pc in 100 kpa < 2 or half the pressure induced by the reservoir Relative Hardness Dur < 10 Initial permeability in Lugeon Unit LU >15-50 Ier erodibility Index >3 Table 6: Proposed criteria for assessment sandstone foundations susceptible to be eroded. Criteria for Laboratory tests could be : porosity higher than 0,17, unconfined compression strength lower than 100 kpa and disintegration time lower than 2 minutes. However, they are only complementary to the tests carried out on site. On site tests are crucial: they are very useful to locate the susceptible areas to internal erosion, to define the dimensions of repair work and the nature of repair (grouting pressure, viscosity, number, injected volume). V CONCLUSIONS On site tests based on recorded drill parameters and Lugeon tests interpretation are very useful not only to locate erodible material in weak rock foundation but as well to design the repair. Data investigated from half a dozen of dam sites, where incidents occurred or where suspected, give empirical criteria to define sandstone foundation vulnerable to internal erosion. These criteria were applied to the Vieux Pré Dam. The conclusion is that 95% of the foundation is not erodible. Mainly, 3 quasi vertical diaclasis have sand lenses prone to be eroded. They are protected by drainage and filter system designed for stopping 200 microns sand. After 20 years of operation, the monitoring data are consistent with that diagnosis. VI REFERENCES Dobereiner L. de Freitas M.H. (1986) Geotechnical properties of weak sandstones.-geotechnique 36 n 1 pages