Unit 03b : Advanced Hydrogeology Engineering Implications
Groundwater flow systems have important implications for certain engineering activities and can be significantly modified by others. –Large reservoir impoundments –Induced seismicity –Excavation inflows and stability –Landslides and slope stability
Reservoir Filling Groundwater discharge to a flooded valley is usually inhibited as a reservoir fills. Recharge continues unaffected by flooding. Flooded Natural Dams are generally constructed in groundwater discharge areas (because aquifers predominantly discharge to river valleys). Reservoir heads are generally greater than the aquifer heads.
Transient Readjustment The flow regime adjusts by filling groundwater storage until a new steady state is established. Where the water table was near the surface, new discharge zones become established. (e.g. Flathead Reservoir, Mt.) Flow direction reversals in the subsurface are likely to occur. (e.g. Lake Diefenbaker, Sk.) Flooded Natural Reversal New Springs
Valley Bottom Stability Reservoir impoundments can also lead to stability problems. Beneath the reservoir, increased pore-pressures are partially compensated by the total stress increase due to the water loading. Downstream of the impoundment, pore-pressures are increased to similar levels with no total stress compensation. Post-Reservoir Head Pre-Reservoir Head Zone of potential uplift and slope failure Aquitard Aquifer
Valley Wall Stability Pore-pressures can reactivate bedrock shears, faults and gouge (mylonitic) zones Increased uplift pressure can cause heave of the valley floor. Bedrock slide blocks and landslides can be reactivated or initiated by large changes in pore-pressures in valley walls. Regional Aquifer Slide Debris Slide Block Water Table Piezometric Surface River
Induced Seismicity Many human activities are known to induce or increase seismic activity (Simpson,1986): –fluid injection for various purposes: waste disposal, solution mining, geothermal power generation, and secondary oil recovery; –deep underground mining; –removal of large volumes of rock during quarrying; –fluid extraction in petroleum production; and –impoundment of large reservoirs behind high dams (Simpson, 1986).
Seismic Risk Of all causes, reservoir impoundment has produced the largest earthquakes. There is evidence linking earth tremors and reservoir operation for more than 70 dams. Reservoirs are believed to have induced five out of the nine earthquakes on the Indian peninsula in the 1980s which were strong enough to cause damage. Reservoir induced seismicity (RIS) is well documented but relatively poorly understood.
RIS The mechanisms of RIS are not sufficiently well understood to predict accurately which dams will induce earthquakes or how strong the tremors are likely to be. Most of the strongest cases of RIS have been observed for dams over 100 metres high - but dams just half this height are also believed to have induced quakes. Reservoirs filling can –increase the frequency of earthquakes in areas of already high seismic activity and –cause earthquakes to happen in areas previously thought to be seismically inactive.
nn Mechanism I When the pressure of the water (u) in the rocks increases, it acts to reduce the normal load ( n ) on fault planes thereby reducing the frictional resistance mobilized and increasing the tendency to shear. = c + ( n – u) tan( ) where is the shearing resistance cis the cohesion and is the friction angle Note that the principal stresses remain in the same orientation. The most widely accepted explanation of how dams cause earthquakes is related to the extra water pressure created as the reservoir fills.
nn Mechanism II In this case the normal load ( n ) on fault plane has increased but the deviatoric stress has changed more to induce failure = c + n tan( ) where is the shearing resistance cis the cohesion and is the friction angle Note that in this case the orientation of the principal stresses are changed and that this mechanism will only trigger normal faults where 1 is vertical. It is also possible to cause failure by increasing the vertical principal stress as a result of the weight of impounded water.
Evidence For most well-studied cases of RIS, the intensity of seismic activity increased within around 25 km of the reservoir as it was filled. The strongest shocks normally occurred relatively soon - often within days but sometimes several years - after the reservoir reached its operating level. After the initial filling of the reservoir, RIS events normally continued as the water level rose and fell but usually with lower frequency and magnitude than the initial events. The pattern of RIS is, however, unique for every reservoir.
Interpretation of Evidence The evidence is consistent with a mechanism involving stress-relief. The early events release the initial stresses more quickly the nearer they are to the critical level for slip. Later changes in stress trigger less violent releases as the fault plane weakens (c, approaches residual ) with each successive event. nn nn
Another Perspective Seismologists have published a list of about 100 cases of RIS. These cases show that after the completion of a dam, the reservoir area experienced earthquakes of micro- level magnitude - 2.0 or 3.0 on the Richter scale. Dense seismic networks have increased the detection potential and increased the number of cases cited as instances of RIS. The earthquakes that the Indian peninsula has so far experienced may not be attributable to dams. Construction of dams should be done in such a way as to withstand anticipated seismic activity and minor stress adjustments are inevitable.
Indian Earthquakes India is unique as far as earthquakes are concerned. The northern part of India, the Himalayan frontal arc, is one of the seismically most active regions in the world. Four great earthquakes (>8.0M) have occurred in a the period 1897-1950 – the largest subsequent earthquake occurred in Gujarat in 2001. A catalogue of Indian earthquakes from the earliest times has been compiled. The 1967 earthquake at Koyna M6.3 in Western India confirmed that peninsular India, believed until then to be aseismic, is vulnerable to earthquakes. The more recent 1993 Killari earthquake M6.4 in the Deccan Traps was unexpected and devastating.
Seismicity of India The map shows the location of the Koyna and Killari earthquakes in the largely aseismic Indian penninsula. The recent M7.9 Gujarat quake is also shown. Gujarat Killari Koyna
Koyna Dam Earthquake The area between the Koyna and the Warna dams, in the vicinity of the Shivaji Sagar and Vasant Sagar reservoirs, is unique for its ongoing, high level of seismic activity. Seismicity at Koyna has close correlation with the filling cycles of the Koyna reservoir. The 1967 Koyna event, in the watershed of the Krishna River in Maharashtra state, is a classic example of earthquake activity triggered by reservoir. The world's worst confirmed reservoir-induced earthquake was triggered by the Koyna Dam. Nearly 200 were killed in the magnitude 6.3 tremor.
Koyna Dam Background The height of the Koyna-Dam is 103 m, reservoir volume is 2.78×10 9 m 3. Seasonal fluctuations of the lake level are typically 30 to 35 m and are dominated by monsoon rainfalls. The site is now highly instrumented and the subject of active research Since its first impoundment in 1962, more than 150 earthquakes of magnitude 4.0 have been recorded. Events are mostly restricted to an area 40 × 25 km 2 south of the Koyna-Dam. This marks the area as probably the best in the world to study the phenomenon of reservoir induced/triggered seismicity (RIS).
Killari Event The most puzzling event in Peninsular India is the Killari earthquake. The devastating magnitude 6.4 earthquake struck Killari, Maharashtra in 1993, killing 10,000 people. The event was totally unexpected as it was located in the Deccan Trap-covered stable Indian Shield. There was no record of any historical earthquake in the region. The Killari earthquake is considered the most devastating SCR (Stable Continental Region) event in the world. Some seismologists believe that the Killari event was triggered by a nearby (Tirna) reservoir.
Tirna Reservoir The Killari earthquake was about 10 km from the Lower Tirna Reservoir. The maximum water depth is about 20m, which is at the low end of the range of depths of reservoirs where induced seismicity has been documented. The reservoir level was low at the time of the main shock, which is consistent with the expected negative effect of the loading by the reservoir on an underlying thrust fault. Several other recent earthquakes in peninsular India appear to be located close to reservoirs. Whether the Killari earthquake was triggered by the Lower Tirna reservoir is not known, but it cannot be ruled out at this time.
Narmada Valley Indian seismologists have noted an increase in seismic activity in the Narmada Valley over the past 20 years, which may be linked to reservoir impoundment. In the Narmada Valley, a series of tremors were felt soon after the completion of the Sukta Dam. A strong earthquake hit the Narmada Valley on May 22, 1997, killing around 50 people and injuring 1,000 in the city of Jabalpur in the state of Madhya Pradesh. The epicentre of this magnitude 6.0 earthquake is believed to have been about 20-40 kilometers from Bargi Dam, which completed filling in 1990. The recent earthquake has focused attention on the seismic risks faced by the large dams planned for the Narmada Valley, and on the risk of reservoir-induced earthquakes.
Seismic Hazard Assessment Seismic hazard assessments are an integral part of site investigation for large dams and reservoirs. In order to interpret the recorded seismicity of a region, a thorough review of the available previous seismicity and seismotectonic studies is performed. The analysis is further deepened through the integration of three-dimensional velocity structures and inversion studies beneath this area. The compilation of all these data makes it possible to define and gain considerable insight concerning the major seismic sources active in the region.
Talembote Case History The assessment of seismic hazard within the Talembote area, Morocco, is a study of a dam located within the actively deforming intermountain belt of the Rif region, considered the most active zone in Morocco. The historical seismic data available on Morocco extend to about 11 centuries back in history. Of more importance is the 20th century seismicity data, which reveals the occurrence in 1909 of a M6.4 event about 50 km away from the dam.
Talembote Seismic Setting Of particular importance are shallow surface features; mostly normal and strike-slip faults, which are identified as local faults that are running right next to the dam-site. However, most of the seismic activity seems to be related to reverse faults along Rif-nappes connected to a detachment surface at about 20 km-depth. This detachment runs right underneath the dam-site. The detachment zone may coincide with a low strength layer that decouples the overlying sediments from the basement of the African Plate. As a result, there is a high level of small magnitude earthquakes.
Talembote Seismic Analysis The analysis of seismic hazard of the site of the Talembote dam has shown that the Maximum Credible Earthquake (MCE) is in the order M6.8, risking to produce a maximum acceleration of 0.5g. This event could possibly be generated once every ten thousand years by one of the faults passing in the immediate proximity of the dam. When considering the much shorter design life for the dam-structure, it is normal to use an earthquake return period 7 or 8 times the 75-year design life. An acceleration of 0.085 g, corresponds to a return period of 550 years. This acceleration is rounded to predict an operational basic earthquake of 0.1 g.
Induced Seismicity Many human activities are known to induce or increase seismic activity (Simpson, 1986): –fluid injection for various purposes: waste disposal, solution mining, geothermal power generation, and secondary oil recovery; –deep underground mining; –removal of large volumes of rock during quarrying; –fluid extraction in petroleum production; and –impoundment of large reservoirs behind high dams.
nn nn Fluid Injection and Extraction It is easy to see that increases in pore pressure due to fluid injection may reduce the effective strength of faults below the critical shear stress causing failure and earthquakes. It is also apparent that a reduction in pore pressure due to fluid extraction will have the opposite effect, effectively strengthening the fault.
Fluid Extraction It is more complex to explain how fluid extraction can induce seismicity. In fact, earthquakes are not induced in reservoirs undergoing extraction. The failures occur in the surrounding low permeability rocks where high pore pressure gradients are generated. The analysis of Grasso (1995) and Segall (1992) shows that seismicity occurs near the reservoir magins, both above and below the depressurized formation. The analysis is an example of the application of the theory of poroelasticity (Segall et al., 1994).
Fluid Injection For fluid injection to induce seismicity several conditions must be satisfied: –Differential stresses must be high –Absolute stress level may be low –Pore pressure build up must occur so low permeability reservoirs are more susceptible nn nn
Excavations Ibrahim and Brutsaert (1965) provide a simple analytical solution for predicting inflows and water-level decline in the vicinity of excavations. For the simple 2D case with an initial head H, in an isotropic homogeneous region, length L, the solution, h(x,t) depends on h/H and x/L. H H h(x,t) L x Excavated face Seepage face t=0 t=t
Transient Excavation Heads The transient response depends on the dimensionless Fourier number N F = t / T* where T* = KH/S y L 2 The dimensionless discharge is given by: = T*q’ / HL where q’ is the inflow per unit length of excavation. H H h(x,t) L x t=0 t=t NFNF 0 4 0.01 0.1 1 10 1 1 0 h / H x / L 0.1 0.5 1.0 5.0
Transient Excavation Heads For example, let S y = 0.26, L = 100 m, K = 10 -4 m/s, H = 5 m, so T*= 60 days. After 30 days N F = 0.5. At the seepage face (x=0) the predicted head is 0.2H and at the boundary (x=L), 0.7H The dimensionless discharge is about 0.35 so the inflow per unit length is about 350 m 3 /d NFNF 0 4 0.01 0.1 1 10 1 1 0 h / H x / L 0.1 0.5 1.0 5.0
Trench Problems One of the most common problems in construction is bottom heaving or boiling in trenches. Heave occurs because the material removed was providing a normal load preventing upward displacement. Boiling results from removal of granular particles by high seepage exit velocities
Heaving Before trenching the total stress at A is: s h 1 After trenching the total stress is: s h 2 The maximum uplift pressure at A is thus: s (h 1 – h 2 ) Uplift pressures can result in heaving at the base of trenches, particularly in plastic cohesive soils. ss h2h2 h1h1 A
Boiling Before trenching the effective stress at A is: s (h 1 - h 2 ) - w h w After trenching before any pore pressure dissipation the effective stress is: s h 2 – w h w If the effective stress at A is zero the soil is fluid and boils when: h 2 < ( w / s )h w ww ss h2h2 hwhw h1h1 For boiling to occur the materials must be both cohesionless and have a relatively low hydraulic conductivity (high gradient). Fine sands and silts are most prone to boiling. A
Flow Net Boiling is entirely a groundwater seepage phenomenon. Consider a The flow net around the base of a trench as a result of seepage. The head distribution is such that the flow per unit cross sectional area is the same for all net elements. The velocities are a maximum at the seepage face, where the head gradient is highest.
Piping Now consider a single flow tube. All the flux through this tubes exits through a small segment at the base of the trench. For a soil particle on the base of the trench, the exit velocity may be sufficient to move the particle. Particle removal results in a shorter flow path and a higher exit velocity so the next particle is more easily removed. Erosion backwards along a flow tube is called piping. Piping occurs mainly in fine sands and silts (cohesionless soils with low K)
Landslides Terzaghi (1950) provided the classic treatment of landslide development and attributed rapid movements to: 1.External changes (surcharge loading of crest, toe erosion, undercutting, etc) 2.Earthquake shocks (through horizontal loads due to increased g-forces) 3.Lubrication by water (dismissed as wet soils tend to have more friction mobilized than dry soils) 4.Groundwater level rises (increases in pore pressure in the Mohr-Coulomb equation)
Mohr-Coulomb Equation = c + ( n – u)tan where is the shearing resistance (FL -2 ) c is cohesion (FL -2 ) n is the normal stress (FL -2 ) u is the pore pressure (FL -2 ) is the friction angle. As pore pressure increases the frictional resistance (second term on RHS) is reduced For a cohesionless soil, c=0 so all shearing resistance is lost when u = n For a cohesive soil, failure will occur when pore pressure overcomes both the cohesion and friction components. Notice that suction increases shearing resistance Some believe that “cohesion” does not exist in soils and that what is observed is a suction phenomenon induced by dilation.
Periodicity in Slope Movements From the Mohr-Coulomb equation it is clear that pore pressure is a critical factor in slope stability. Pore pressure varies seasonally as precipitation and evapotranspiration rates change Slopes thus become more susceptible to failure in the wetter seasons (no suction) and failures show periodic movements. Controlling and preventing slope movements are thus largely groundwater control problems. Drainage and dewatering are the first line of defence for both prevention and remediation of slope stability problems.
Soil Slope Remediation Infiltration ControlCrest Drain Well ControlToe Drain
Rock Slopes Rock slopes present some different problems because of the presence of discrete fractures or joints. Water pressures can build much more quickly because of the much lower storage characteristics of fractured rock masses ( 50%). Joint orientation and persistence are significant factors in stability. Inclined “daylighting” joints that are not free-draining (plugged) are often most troublesome.
General Case For Rock Wedge Consider the forces on a wedge bounded by a dipping joint and a vertical tension crack. W U V The weight of the block, W The water pressure due to the tension crack, V The water pressure due to the joint, U The forces resisting sliding are: cA + (W cos – U – Vsin tan The forces tending to induce sliding are: Wsin + Vcos
Factor of Safety The factor of safety against sliding is: F = cA + (W cos – U – Vsin tan Wsin + Vcos W U V where A = (H – z)/sin U = ½ w z w A V = ½ w z w 2 W = ½ r H 2 [(1-(z/H) 2 cot – cot ] and r and w are the specific weights of the rock and water respectively Some “cohesion” may be provided by interlocking joint asperities. H z zwzw A
Factor of Safety The factor of safety against sliding (ignoring cohesion) is: F = (W cos – U – Vsin tan Wsin + Vcos W U V where U = ½ w z w A V = ½ w z w 2 W = ½ r H 2 [(1-(z/H) 2 cot – cot ] and r and w are the specific weights of the rock and water respectively Prudent conservative design assumes zero cohesion. H z zwzw A
Water in Rock Masses Hoek and Bray (1974) list the following concerns with groundwater around open pits: 1.Water pressure reduces slope stability. 2.High moisture content results in increased specific weight and moisture content changes in shales leads to accelerated weathering. 3.Freezing of water in fractures can result in ice wedging. 4.Erosion of particulates can lead to plugging of fracture drainage. 5.Liquifaction of saturated overburden or waste tips can occur as a result of excess pore pressures. 6.Discharge of water into pits requires pumping to avoid difficulties with mining equipment in wet conditions. The first item is by far the most important consideration in pit wall design
Rock Slope Remediation Infiltration ControlCrest Drain Well ControlWeep Hole Drains