Analysis and Design of a Deep Subsurface Dam

Transcription

1 International Journal of Civil & Environental Engineering IJCEE-IJENS Vol:10 No: Analysis and Design of a Deep Subsurface Da Ahed T. Khairy, Abdullah S Al-Ghadi and Saud A Gutub Abstract A 3-D finite eleent (FE) nuerical odel was used to analyze structurally a proposed subsurface concrete da to serve as a strategic water supply storage for the Holy city of Makkah, Saudi Arabia. The work was done ainly to show the behavior of the subsurface da under the subjecting loads. The plastic concrete was chosen for the construction of the cut-off wall required for the reservoir. Three-diensional finite eleent analyses were ade for three thicknesses of the cut-off walls: 0.6 to 0.8, 0.8 to 1.0 and 1.0 to 1.2 for the alluviu depths of 30, 50 and 70, respectively. The change in soil rigidity with depth was taken into account. The vertical and horizontal boundary conditions were designed to siulate the proper behavior of the structure It was found that increasing the thickness of the wall iproves the distribution of the obilized passive pressure opposite to the water pressure acting on the upstrea face of the cut-off wall, while, the vertical and horizontal stresses developed in the wall due to the applied loads increased. In addition, stresses in the wall increase increentally with the increase of wall height. The axiu horizontal and vertical stresses developed in proportion to the 1/6 botto height of the wall. In the lower part of the wall, the ix with rich ceent content ust be used. It is recoended also to use vertical reinforceent ibedded by the anchorage length in the lock of the wall inside the bedrock and extended 1/20 the wall height. Results showed that the two vertical boundaries in the FE esh ust be placed at iniu distance equaling twice the height of the alluviu deposits fro the centerline of the cut-off wall.. Index Ters subsurface da analysis, cut-off wall analysis, underground da analysis, structural analysis of das. I. Introduction Using concrete cut-off walls as subsurface das to intercept or obstruct the natural flow of groundwater and provide storage for water underground is a coon practice in any parts of the world, notably in India, Africa, Brazil, Japan and southwest of Saudi Arabia. This type of da has any advantages, for instance; a) it stores water underground with less daage to the environent, thus, it does not suberge Ahed T. Khairy, Khatib & Alai Consolidated Engineering Copany, United Arab Eirates, P.O. Box 688, King Faisal Street, Qasieh, Sharjah, UAE. Abdullah S Al-Ghadi**Civil Engineering Departent, King Abdulaziz University, Jeddah, Saudi Arabia, P.O. Box 80204, Jeddah 21589, Saudi Arabia corresponding author (e-ail: alghadi@kau.edu.sa). Saud A Gutub Civil Engineering Departent, King Abdulaziz University, Jeddah, Saudi Arabia, P.O. Box 80204, Jeddah 21589, Saudi Arabia land area, b) it does not cause socioeconoically probles such as the forced igration of the local people or relocation of roads, c) eliinates the loss of water due to evaporation, d) the groundwater reserved behind the da aintains its quality and is less subject to pollution, e) storing water underground eliinates epideic water borne diseases spread by osquitoes and other insects, f) the da is usually, stable fro the viewpoint of dynaics, and thus it does not need aintenance, and g) if da failure occurs, for any reason, the daage or loss of lives in the downstrea area will be liited or negligible because the breakage occurs underground. The ain types of vertical cutoff walls are sheet pile walls, geoebrane walls, and Slurry/hardening slurry (bentoniteceent ix) trench cut-off walls (Daniel and Koerner, 1993). The Holy City of Makkah, the capital city of Musli world, has peranent residents of 1.4 illion inhabitants, but the population of the city exceeds 3 illion during the pilgriage season. At present, the city gets 90 % of its water supply fro the sea water desalination plants located on the Red Sea coast soe 110 k west of Makkah for any reasons. The water supply fro the desalination plants ay be interrupted due to planned and/or accidental shutdowns for a short or a long period of tie. If the shutdown requires a considerable aount of tie to rectify the proble, the city faces a severe water shortage, as there is no adequate strategic water reserve syste to supply the city with water in case of eergencies (Al-Ghadi, 2009). The situation ay be exaggerated if the situation arises just before the pilgriage season and before filling the short ter reserve tanks. To avoid such vulnerable situation, previous studies call for the developent of water resources in adjacent Wadis to for a natural reserve of water capable of supplying the city with water in eergencies. A subsurface da is proposed in Wadi Naan which is located east of the city to provide a natural underground storage reservoir for eergency use (Al-Ghadi, 2009). In this contribution, a study using 3-D FE technique for the structural analysis of the da cut-off wall was conducted. II. Site Investigations Wadi Naan, which is located about 10 k east of Makkah and less than 3k fro the holy shrine of Arafat, as shown in Figure 1, is considered to be the best location for a strategic water reserve for the city of Makkah. Previous study suggested the construction of an underground (sub surface) da across the Wadi in the location shown in Figure 1 to serve as a barrier wall to intercept the groundwater flow and hence storing the water in the alluviu behind the da for future use (Al-Ghadi, 2009). The alluviu thickness at the location ranges fro 20 near the sides to a 70 in the iddle of the section. Figure 2 presents the centerline of the proposed da which has a total length of

2 International Journal of Civil & Environental Engineering IJCEE-IJENS Vol:10 No: Araf Pro pos Figure. 1: Location of the Proposed Subsurface Da in Wadi Naan (After Space Iage Atlas for KSA 2007). The boreholes logs indicate that the soil type is in general a poorly graded coarse to ediu grained sandy soil ranging fro loose near the surface to very dense in the deeper layers. Soe interediate layers ixed sand and gravel with occasional interediate layers of boulders with cobbles 4-5 thick. The results of the soil investigations reveal that the depth to the solid bedrock ranges fro to over The bedrock consists of slightly to very highly fractured strong granodiorite rock with low to coplete weathering and RQD values ranges fro 0-90 % and TCR values range fro 35%-94%. The unconfined copressive strength values range fro to kg/c 2. Cheical analysis of the soil indicates that sulphates (as SO4 -- ) is in the range of 90 to 130 pp, while Chloride (as Cl - ) is range fro 160 to 240 pp and the ph is in the range of 8.60 to The total dissolved solids (TDS) values in the soil saples are range fro 343 to 433 pp. Conservatively, SM sandy soil with friction angle Ф = 30 o was considered. The static properties of the soil can be calculated using U.S. Navy (1972) charts and weight-volue relationships. Table 1: Values of Soil Properties. Angle of Internal Frictio n, (Ø o ) 30 o Specific Gravity G s Dry Unit Weight, w d (kn/ 3 ) Saturated Unit Weight, w s (kn/ 3 ) Void Ratio, e o Relativ e Density, Dr (%) 24.86% Poisson's Ratio, ν The increase in the soil rigidity with depth was accounted by using Hardin and Drnevich (1972) Correlation. The variations of the calculated initial odulus of elasticity (Eo) with depth for the dry and saturated soil are shown in Figure. 3. The nonlinearity of the soil was ipleented approxiately by considering: E 0.65 Eo Modulus ofelasticity kn/2 Figure. 2: Da Centerline The da alignent was selected to avoid, as uch as possible, the geological faults in the Wadi as deterined by geologists and to iniize the construction cost by avoiding the intersection with highway bridges in the area and iniizing the da length. Depth () Saturated Soil Dry Soil Three boreholes were drilled along the proposed axis of the da and the following field and laboratory analysis of the soil were conducted: Visual Exaination, Standard Penetration Test (SPT), Rock Quality Designation (RQD), Total Core Recovery (TCR), Grain Size Distribution, Moisture Content, Unconfined Copression Strength, Bulk Density, and Cheical Analysis. Figure 3: Variation of Modulus of Elasticity with Depth for Dry and Saturated Soil. III. Proble Forulation For the proposed subsurface da, it was recoended to use plastic concrete cut-off walls. Plastic concrete was adopted

3 International Journal of Civil & Environental Engineering IJCEE-IJENS Vol:10 No: due to its high deforability (plasticity) without cracks thus reaining ipervious as it was built and low-cost copared to ordinary concrete Diaphrag walls are the ideal for the construction of these walls (FS, 2010). Carey et. al (1997) illustrated the success of installing a cut-off slurry barrier wall in a containent project through area with large voids. Grouting was used in sealing the voids and containing the slurry during construction. Figure 4 illustrates the forces acting on the cut-off wall. Three diensional linear analyses were perfored for three heights (H) of soil deposits above the bedrock: 30, 50 and 70 using Finite eleent technique. Staad Pro progra (2004) was used in these analyses. The change in soil rigidity with depth was taken into account. The horizontal and vertical boundary conditions were carefully studied to siulate the infinity of soil edia behind the finite eleent esh and the roughness between the soil deposits and bedrock at the interface between the respectively. Results were presented for three selected thicknesses of the cut-off wall. Upstrea Saturated Soil Water pressure t Ground Surface Downstrea Cut-off Wall Dry Soil Bedrock Surface H = 30, 50 and 70 Figure 4: Proble Description for the Full Critical Loading. Mahboubi and Ajorloo (2005) conducted an extensive experiental paraetric study of the echanical responses of various types of plastic concrete in unconfined and triaxial copression tests. Table 2 shows the ixing proportions and consistency of the fresh concrete ixes for the selected ix (4Z) used in this study. Mahboubi and Ajorloo [8] observed that the behavior of the plastic concrete was ore and ore ductile for increasing confining pressure. It was shown, also, that any increase in confining pressure increases the copressive strength as well as the elastic odulus and the deforability of the specien. It is shown that an increase in ceent factor increases the shear strength as well as the elastic odulus. It was obtained that the increase of bentonite content, decreases the copressive strength as well as the elastic odulus. Increasing the age of the speciens causes an increase of the copressive strength as well as the elastic odulus. In addition the shear strength paraeters are affected. It has also been observed that increase in confining pressure and ceent factor reduces pereability. Table 2: Mixing proportions and consistency of the selected fresh concrete ix (Mahboubi and Ajorloo, 2005). Mix Ceent factor (kg/3) Water ceent ratio, W/C Bentonite content B/C Slup (c) Sand (kg/3) Gravel (kg/3) Z C: Ceent, W: Water, B: Bentonite. Although at the sae strain, the stress ratio (σ1/σ3) greatly decreases when the confining pressure increases, the ultiate value of σ1 increases with the increase of σ3. Fro figures provided by Mahboubi and Ajorloo [8], the calculated ultiate values of σ1 are 3.5, 4.17 and 5.1 MPa when σ3 equals 0.3, 0.5 and 0.8 MPa respectively. The shear paraeters given by Mahboubi and Ajorloo (2005), are: c = 0.73 MPa and φ = 30.5 o. The shear capacity of the wall section at its base can be calculated approxiately using the forula: V = Ø [c*t + (t*h* w wall ) * tan φ] Conservatively, a strength reduction factor (Ø = 0.75) as given in ACI (2005) for concrete was considered. Accordingly, the calculated values of the shear capacity at the base of the diaphrag walls are given in Table 3. Table 3: Shear capacity at the base of cut-off walls. Wall Height Wall Thick. Shear Capacity at Wall Base (kn) t = 0.6 H =30 H =50 H =70 t = 0.8 t = 0.8 t = 1.0 t = 1.0 t = Based on Mahboubi and Ajorloo s (2005) results, the axiu elastic odulus (E) doesn t exceed 1/20 of the elastic odulus of ordinary concrete. Since the stresses developed in the cut-off wall increase with the value of E, the value considered for E is 2200 MPa (approxiately 1/20 the elastic odulus of ordinary concrete) to be on the safe side. To show the effect of E, one additional case (H = 50 ) was studied where E equals ½ the odulus of elasticity of ordinary concrete (E = MPa). Structural Model and Finite Eleent Mesh Three diensional linear Finite Eleent analyses were used in this work. Eight nodded solid eleents (with different properties) were used to represent soil and wall edia. For the current proble, Finite Eleent analysis using solid eleents provides a powerful tool [9]. The two interfaces (upstrea and

4 International Journal of Civil & Environental Engineering IJCEE-IJENS Vol:10 No: downstrea) between the wall and the soil continuu were represented by short links connecting the corresponding (adjacent and at the sae level) nodes in the two continuus. This was done to avoid the error resulting fro transferring excessive vertical self weights fro the soil to the wall, due to the relative high rigidity of the wall. This transferring is not real because the soil was already settled before constructing the wall. Since the proble is syetrical along the wall direction (longitudinal, z axis), three are no displaceent in this direction (plane strain case). Therefore, displaceents in z direction are restrained. Figure 5 shows the Finite Eleent eshes for one selected esh (H = 30 ). Since the eshes are syetric about the vertical axis of the wall, only the upstrea and a portion of the downstrea esh are shown. U/S D/S interediate depth) W was taken to be equal to H, 2H and 3H. Investigation was done for the following scenarios: 1- The copressive stresses (induced in the soil edia resulting fro the lateral water pressure acting on the wall) at the vertical boundary of the downstrea is not significant copared to the horizontal pressure in the sae place induced fro the self weights of the soils. 2- The values of the tensile stresses (induced in the soil edia, due to the lateral water pressure acting on the wall) at the vertical boundary of the upstrea are not significant copared to the absolute values of the corresponding stresses (copression) resulting fro the self weights of the soils. 3- There are no significant tensile stresses developed in soil edia when the two priary cases of loadings (self weights and lateral water pressure) are cobined. For H = 50 and t = 0.8, Figures. 6 and 7 show the horizontal stresses developed in the soil edia due to effective vertical weights and lateral water pressure acting on the wall for W = H, 2H and 3H respectively. Fro these figures, it is shown that the proper value of W can be started fro 2H. Cut-off Wall Figure 5: Finite Eleent Mesh H = 30. Boundary Conditions The vertical and horizontal boundary conditions were designed to siulate the proper behavior of the structure as possible. a) Horizontal Boundary The nodes at the horizontal boundary (the surface of the bedrock) were constrained fro horizontal and vertical oveents (having pin joints). For all cases the horizontal reactions at these supports were checked against sliding. In case the value of the horizontal reactions is bigger than the shear capacity at its node, the horizontal restrain was released. The values of the shear capacities of the wall joints are given in Table 3. While the values of the shear capacity at the soil continuu nodes were calculated by ultiplying the effective weight of the soil colun served by the node by tan φ. b) Vertical Boundaries Both the vertical boundaries at the upstrea and downstrea were placed at equal distances (W) fro the face of the wall. The nodes at the vertical boundaries were restrained fro horizontal oveent but allowed to ove vertically. The distance (W) was checked by trials. For H =50 (the Figure 6: Horizontal Stresses due to Effective Weights for H = 50 and W = 50, 100 and 150. Figure 7: Horizontal Stresses due to Lateral Water Pressure Acting on Wall for H = 50 and W = 50, 100 and 150.

5 International Journal of Civil & Environental Engineering IJCEE-IJENS Vol:10 No: IV. Results and Analysis The ain results obtained fro the analyses will be displayed as follows: 1) horizontal stresses exerted in the soil and wall edia due to critical full loading, 2) vertical stresses exerted in the soil and wall edia due to critical full loading, and 3) stability of the cut-off walls. 1) Horizontal Stresses: Figure 8 shows the horizontal stresses exerted in the soil and wall edias resulting fro the critical case of full loading for a selected case (H = 30 and t = 0.8 ). For all studied cases, focus is done on the area having the axiu exerted stresses (Figures 9 to 10). Results showed that this area lie at about the 1/6 the height of the wall fro the base and around the wall. These figures show that for all cases of H, increasing the wall thickness leads to reduce the values of the horizontal stresses exerted in the downstrea (resisting) side. The stresses increase with the increase of the wall height. All the axiu values of the stresses decrease the copressive strength of the plastic concrete with big a arginal of safety. a) t = 0.6 a. Upstrea Part b. b) t = 0.8 c. Downstrea Part Figure 9: Horizontal Stresses due t o the Full Critical Loading on the Lower Part of the Wall (H = 30). Figure 8: Horizontal Stresses due to the Full Critical Loading for H = 30 and t = 0.8.

6 International Journal of Civil & Environental Engineering IJCEE-IJENS Vol:10 No: a) t = 0.8 a) t = 1.0 b) t = 1.0 Figure 10: Horizontal Stresses due to the Full Critical Loading on the Lower Part of the Wall (H = 50). b) t = 1.2 Figure 11: Horizontal Stresses due to the Full Critical Loading on the Lower Part of the Wall (H = 70).

7 International Journal of Civil & Environental Engineering IJCEE-IJENS Vol:10 No: ) Vertical Stresses Figures 12, 13 and 14 show the vertical stresses exerted in the soil and wall edia resulting fro the critical case of full loading for heights of 30, 50 and 70 respectively. The areas (about the 1/6 the height of the wall) having the axiu exerted stresses are shown only in these figures. These figures show that the stresses in the cut-off wall increase with the increase of the wall height. All the axiu values of these stresses lie in the safe zone (Figure. 5-a). a) t = 0.8 a) t = 0.6 c) t = 1.0 b) t = 0.8 Figure 13: Vertical Stresses due to the Cobination of Vertical Weights and Lateral Water Pressure on the Lower Part of the Wall (H = 50). Figure 12: Vertical Stresses due to the Cobination of Vertical Weights and Lateral Water Pressure on the Lower part of the Wall (H = 30).

8 International Journal of Civil & Environental Engineering IJCEE-IJENS Vol:10 No: Pressure kn/ Active Pressure - t=0.6 Upstrea 5 Downstrea Water Pressure Full Passive Pressure 10 Mobilized Passive P - t=0.8 Mobilized Passive P - t= H = a) t = 1.0 Depth () Base Shear = kn/ t = kn/ t = 0.8 Figure 15: Horizontal Pressures of Soil and Water Acting on the Cut-off Wall (H = 30). b) t = 1.2 Figure 14: Vertical Stresses due to the Cobination of Vertical Weights and Lateral Water Pressure on the Lower Part of the Wall (H = 70). Pressure kn/ Upstrea Downstrea Active Pressure 5 Water Pressure 10 Full Passive Pressure 15 Mobilized Passive P t= Mobilized Passive P t= H = ) Stability of the Wall Figures 15 to 17 show the pressures acting on the horizontal pressures of soil and water acting on cut-off wall for the heights of 30, 50 and 70 eters. The base shears for all cases are given in these figures. Fro these figures, the iniu values for the factor of safety (full passive pressure/obilized passive pressure) are given in Table 4. Fro these figures and this table the followings can be concluded: 1- All the values of base shear are less than half of the base shear capacities which eans good factor of safety against base sliding. 2- For all heights, increasing the thickness of the cut-off wall gives a slightly better distribution of the obilized passive pressure. This is due to the increase in the relative rigidity between the wall and the surrounding soil. In addition the stability factor of safety showed sall iproveent. This indicates that the selected wall thicknesses are in the suitable range. Deterining the wall thickness is affected also by seepage analysis. Depth () Base Shear = kn/ t = 0.8 = kn/ t = 1.0 Figure 16: Horizontal Pressures of Soil and Water Acting on the Cut-off Wall (H = 50).

9 International Journal of Civil & Environental Engineering IJCEE-IJENS Vol:10 No: Depth () Pressure kn/ Active Pressure Upstrea Figure 17: Horizontal Pressures of Soil and Water Acting on the Cut-off Wall (H = 70). Table 4: Least values of the FOS (Full passive pressure/obilized passive pressure). H () t () FOS Downstrea Water Pressure Full Passive Pressure Mobilized Passive P - t=1.2 Mobilized Passive P - t=1.0 Base Shear = kn/ t= 1.0 = kn/ t = H = V. Suary and Conclusions The work undertaken in this study served to ainly describe and analyze the behavior of the cut-off walls of the proposed ground reservoir under the subjecting loads. The ai of this proposed reservoir is to contribute in the water supply of the Holy city of Makkah. The plastic concrete was chosen for the construction of the cut-off walls required for the reservoir. Three diensional Finite Eleent analyses were ade for three cases of wall heights: 30, 50 and 70 with various thicknesses. Conducted analyses were based on the following considerations: 1- Linear properties for all plastic concrete, soils and the interface between the. 2- The nonlinearity of the soil behavior was ipleented approxiately by taken elastic odulus of elasticity equals to 0.65 the initial elastic odulus of elasticity. 3- The increase of soil rigidity (odulus of elasticity) with depth was taken into account. 4- The properties of soils and plastic concrete were taken fro published correlations. 5- Critical case of loading: The ground water table is at the surface of the ground at the upstrea, while the soil is dry at the downstrea. 6- The soil and wall continuus were represented by 3D solid eleents. 7- The interface between the wall and the soils at the upstrea and downstrea were represented by nontension eleents. 8- The nodes at the horizontal (botto) boundary were restrained fro horizontal and vertical oveents (pin supports). Check was done to assure that the horizontal reactions fro the underlay bedrock don t exceed the shear capacity at the interface surface between the soil strata and the bedrock. 9- The nodes at the vertical boundaries were restrained fro horizontal oveent but they have the liberty to ove vertically. Trials were ade to assure the proper distances between the centerline of the cut-off wall and the vertical boundaries. Based on the work done here, the following conclusions were ade: 1- The choice of a plastic-concrete aterial in constructing the cut-off walls is certainly coendable. 2- Structurally, the ranges of thickness of the cut-off walls: 0.6 to 0.8, 0.8 to 1.0 and 1.0 to 1.2 for the deposit heights up to 30, 50 and 70 eters, respectively, are suitable. 3- Increasing the thickness of the wall (which has good effect in preventing water seepage) leads to iproving the distribution of the obilized passive pressure opposite to the water pressure acting on the upstrea face of the cutoff wall, while, the vertical and horizontal stresses developed in the wall due to the applied loads increase. 4- Stresses that develop in the wall increase with the increase in wall height. 5- The axiu horizontal and vertical stresses developed in about the 1/6 the botto height of the wall. 6- In the lower part of the wall, the ix with rich ceent content ust be used. It is recoended also to use vertical reinforceent ibedded by the anchorage length in the lock of the wall inside the bedrock and extended 1/20 of the wall height. 7- The vertical boundaries ust be placed at iniu distances equaling twice the height of the deposits. It is optial for the to be placed at three ties this height. VI. References 1. Al-Ghadi, A. S., (2009), "Developent of strategic water reserve for the Holy City of Makkah, Saudi Arabia", Journal of Water Science and Technology, 9.5, IWA publishing, pp ACI / ACI 318R-05, (2005), Building Code Requireents for Structural Concrete and Coentary, Aerican Concrete Institute. 3. Carey, M. J, Fisher, M. J. and Day, S. R. (1997), Case Study Installation of Soil-Bentonite Cutoff

10 International Journal of Civil & Environental Engineering IJCEE-IJENS Vol:10 No: Wall through an Abandoned Coal Mine, International Containent Technology Conference and Exhibit, Florida. 4. Daniel, D. E. and Koerner, R. M. (1993), Quality Assurance and Quality Control for Waste Containent Facilities, Risk Reduction Engineering Laboratory Office of Research and Developent U.S. Environental Protection Agency, Cincinnati, Ohio 45268, Das, B.M. (2002). Principles of Geotechnical Engineering, 5th Edition, Thoson Brooks/Cole. 6. Hardin, B. O. and Drnevich, V. P. (1972). Shear odulus and daping in soils: Measureent and paraeter effects, Journal of the Soil Mechanics and Foundations Division, ASCE, Vol. 98, No. SM6, pp FS Foundation Specialists, inc. (2010), Web Site: 8. Mahboubi, A. and Ajorloo, A. (2005). Experiental study of the echanical behavior of plastic concrete in triaxial copression, Ceent and Concrete Research, Vol. 35, Issue 2, Feb., pp STAAD PRO User Manual, Release (2004), Research Engineers Intl. 10. U.S. Departent of the Navy (1971). Soil Mechanics, NAVFAC DM7.1, U.S. Governent Printing Office, Washington.