Hydromechanical analysis ofPlacing and protecting fill(坝体填料的填筑保护及基础流体力学行为分析).doc

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1、 Hydromechanical analysis of Placing and protecting fillflow behavior in dam Frist Placing and protecting fillFill shall be placed so that mo part of the final foundation surface remains exposed for more than 72 hours.Fill shall be placed in such methods as will prevent segregation of the material.W

2、here the Contract requires the placing of different types of fill in separate zones,the Contractor shall carry out the work so as to prebent mixing of different types of fill.Shoud there be ,in the opinion of the Engineer, any excessive mixing of different types of fill , such mixed materials shall

3、be removed to a spoil tip and replaced with fresh fill.Any undesirable material accumulated on the fill surface shall be removed before placeing the next layer of fill.No fill material shall be placed on a previous layer of that has dried out,become saturated or in any way deteriorated by exposure o

4、r by spilling of other material or disterbance by mechanical transport or by deposition of wind blown particles or by any other means. Before fresh fill material is placed aoo such deteriorated fill or foreign material shall be removed to a depth at which material of an acceptable standard is expose

5、d. The surface of each layer is to be approved by the Engineer before the next layer is placed. Any fill shall be placed in uniform layers not greater than the approved thickness as specified hereafter and in an orderly sequence approximately horizontal along the centreline of the embankments.Except

6、 where specified of directed otherwise, no portion of any embankment shall be stepped more than 3 feet higher than any immediately adjacent portion except where permitted by the Engineer and the slope formed by such steps shall not exceed 1V:3H and not less than 1V:4H from one level to another.Excep

7、t as shown on the Drawings or as otherwise directed, all fill placement surfaces shall be sloped at right angles to the centerline of the embankment in both the upstream and downstream direction from the downstream edge of the core so as to allow run-off and prevent the accumulation of water. The dr

8、ainage slope on the temporary surface of anny zone shall not exceed 1 on 30 and the highest point shall be de downstream edge of the core.Where,due to the specified geometry of the excavation into the top of the existing embankments, the surface slope is towards the downstream edge of the core, the

9、Contractor shall take such measures an necessary to prevent erosion of fine material being washed into the filter zones downstream of the core. Any surface layer of filter material contaminated by such drainage or other cause shall be removed and replaced with fresh filter material before placing th

10、e next layer above. Construction of any one embankment shall be carried out over the maximum possible length,mo less than 1500 feet,of that embankment in such a manner that mo temporary construction slope crosses the axis of the embankment except as approved by the Engineer. Where a temporary constr

11、ction slope crossing the axis of the embankment is permitted by the Engineer it shall be formed at a gradient of 1V：5H. When subsequently placeing material against this slope it shall be cut back in steps equal to the layer thickness to avoid feather edges. The Contractor shall complete each layer o

12、f fill fully up to the abutment contacts and structures and against sloping foundations and ensure that the fill is compacted an specified throughout. The Contractor shall not allow the fill in those areas to lag behind or to get ahead of the normal fill placing operations and form feather edges, ex

13、cept where fill has been placed in advance to cover grouted surface.Where the Contractor is allowed to use either grvel fill and /or sandstone no intermixing of the two materials in a layer shall be allowed. The Constractor may place either of the materials in adjacent layers or sections of the emba

14、nkment.The Constractor shall be responsible for protecting temporary fill surfaces against damage of erosion. At the end of each working day,or if it start to rain ,the surface of the fill shall be made smooth and compacted with a smooth drum roller with a drainage slope to induce runoff from the fi

15、lled areas and leave no areas that can retain water. Where necessary, grips,drainage ditches and the like shall be formed to assist drainage and to prevent runoff from damaging placed material.Runoff from heavy rain shall be controlled to prevent gully erosion of the placed fill. Any gully erosion s

16、hall be repaired with material compacted in accordance with the Specification, and eroded surfaces shall be restored and graded to ensure a proper bond with new fill placed on them.Any eroded material other than gravel and any contaminated material shall be removed from the embankment and placed in

17、designated spoil tips. In particular the Contractor shall ensure that no material is washed into filter or drain material.Where placing of the filter material of drain material is not continuous ,the Constractor shall protect such filter or drain materials by a 2 foot thick layer of course filter ma

18、terial or in such other manner approved by the Engineer,and the Contractor shall maintain the protective layer.The Contractor shall keep the work free from standing water to prevent damage to the fill material. When working below the surrounding level, the Contractor shall ensure that material from

19、adjacent areas does not contaminate the fill material,and that runoff does not flow onto the fill.The Contractor shall arrange the timing and rate of placing fill material in sucn way that no part of the workes is over stressed,weakened or endangered. Any part of the fill that be comes saturated or

20、attains excessive moisture content or that is rendered unsuitable due to poor surface drainage, uncontrolled traffic,or for any other reason, shall be excavated and removed to a spoil tip and replaced by fresh fill .If permitted by the Engineer, such fill may be scarified and re-compacted.Unless oth

21、erwise approved by the Engineer unrestrained edges of fill, whether for temporary or permanent slops, shall be overbuilt as necessary to allow full compaction to be achieved within defined limits of the fill. The excess material shall be trimmed and removed to leave a regular compacted surface.Slope

22、 exposed to view,including riprap and downstream protection slopes, shall be dressed to neatly appearing final surfaces matching the existing slopes.Temporary access ramps shall be removed when work in that area is completed. Any ramps or other areas within the limits of an embankment which, in the

23、opinion of the Engineer have been over-compacted or damaged by the concentrated use by construction equipment,shall be reworked and re-compacted or,if the Engineer requires,shall be excavated, removed to spoil tip and replaced by the fresh fill.When necessary the surface of the layer of fine grained

24、 fill material(rolled clay,rolled silt Type A and B ,Rolled Sandstone Type A and B) shall be sprayed with water to prevent drying out and to maintain the correct uniform moisture content prior to placing the next layer.The Contractor shall ensure that a good bond is achieved between layers of fillan

25、d unless otherwise directed, previously compacted layers of fine grain materials shall be harrowed, scarified or otherwise roughened to depth of at least 3 inches and made suitable for covering with future layer of fill.Two Hydromechanical analysis of flow behavior in dam Evaluating the safety of co

26、ncrete gravity dams against sliding requires an understanding that rock foundations and the structure above them are an interactive system whose behavior is controlled by the mechanical and hydraulic properties of concrete materials and rock foundations. About a century ago, the failure of Boozy Dam

27、 prompted dam engineers to start considering the effect of uplift pressures generated by seepage within the damfoundation system and to explore ways to minimize its effect. Today, with modern computational resources and much more precedent, it is still most challenging to determine the pore-pressure

28、 distribution along foundation discontinuities to assess pertinent stresses and evaluate factors of safety. It is our opinion that observing and monitoring the behavior of large dams on well mapped and adequately instrumented foundations can bring important insights for a better understanding of fac

29、tors controlling joint opening, crack propagation, and pore-pressure development in foundations of concrete gravity dams.Fig.1.Hydromechanical behavior of natural joints :(a) mechanical;(b)hydraulic.This paper presents behavior representative of cycles of reservoir operation in the last 20 years col

30、lected from monitored data of Albigna Dam, Switzerland, and also describes the results of a series of numerical analyses carried out to assess the hydromechanical behavior of its foundations. Comparisons are made between results of numerical modeling and the actual behavior monitored in the field. B

31、ased on these comparisons, a series of conclusions are drawn regarding basic pore-pressure buildup mechanisms in jointed rock masses with implications that may be considered in other engineering projects, where the hydromechanical behavior of jointed rock should be considered. Such projects include

32、pressure tunnels, hazardous waste disposal, and other situations dependent on geologic containment controlled by flow behavior along rock discontinuities.Hydromechanical behavior of natural joints A brief summary of the state-of-the-art of mechanical and hydraulic behavior of individual rock joints

33、is presented here. A more detailed description of rock joint Hydromechanical behavior can be found in Alvarez(1997)and Alvarez et al.(1995)and in investigations in laboratory and numerical model simulations carried out by Raven and Gale (1985), Gentier (1987),Esaki et al.(1992),and others.The mechan

34、ical behavior of the joint can be represented by a nonlinear hyperbolic relationship between the applied effective normal stress,， and joint closure, During loading, significant joint closure takes place at low effective normal stresses. The magnitude of the closure per unit of stress decreases rapi

35、dly, however, as the stress level increases. The hyperbola is defined by the initial tangent stiffness, and the asymptote maximum joint closure, . This relationship is also nonlinear and hysteretic for the unloading condition until effective normal stresses become zero (Fig.1a).The values of and are

36、 estimated by regression analysis on experimental data. For natural and induced fractures in granite, these parameters are interrelated and range between the following limits Alvarez et al. (1995):Where is in M pa/m and is in mRough joints exhibit the largest joint maximum closure and the lowest ini

37、tial joint stiffness, whereas smooth joints have the lowest and the largest The hydraulic behavior of the rock joint is characterized by the linear relationship between hydraulic aperture, which controls the magnitude of flow, and mechanical joint closure, , which depends on stress levels. Hydraulic

38、 apertures are plotted versus their corresponding joint closure (Fig.1b)to obtain the line intercept, ,initial hydraulic aperture, and the coupled slope coefficient, ,which characterizes the hydromechanical behavior of the joint ,i. e., the relationship between changes in hydraulic aperture due to c

39、hanges in mechanical aperture, given byWhere is the residual hydraulic aperture.For a given rock joint, there is a relationship between roughness and the coupled coefficient, because f depends on the distribution and tortuosity of flow channels along the joint surface. For ideal parallel plates, wit

40、h a single flow channel along the entire joint surface, f=1.0.For concentrated flow channels meandering across the joint surface, f1.0.Hence, the classic cubic law expresses flow rate through a rock joint:Where Q is the flow rate; is the unit weight of the water; is the head drop along the rock join

41、t; is the dynamic viscosity of the water(1.005Pas ); Is the joint hydraulic aperture; and G is the shape factor, which depends on the geometry of flow. For straight flow, G=W/L (where W and L are the width and length, respectively, of the joint); and for divergent radial flow, G=2/ln (re/), where an

42、d re are the borehole and external cylindrical surface radiuses, respectively.Jointed rock mass permeability change with depthAlternatively, the rock mass equivalent permeability, km, can be expressed in the same form as the modified cubic law, or in terms of hydraulic aperture, to account for spaci

43、ng of the joints, S:Changes in jointed rock mass permeability due to overburden and confining stresses were calculated using eqs. 1 3.The results of a theoretical relationship of rock mass permeability, k, for depths up to 1000 m, using eq. 5 are presented in Fig.2.The reduction of hydraulic apertur

44、es with increasing overburden stresses results in a rock mass permeability that decreases with an increase in depth from cm/s near the surface to cm/s at depths of 600 1000 m.The rock mass permeability estimates were obtained assuming f=1.0, = and =10, which are representative of the values obtained

45、 in laboratory tests carried out in granitic formations(Alvarez et al.1995)similar to those of the Brazilian test location described in this section. Overburden stresses were estimated using a unit weight of 26.0 kN/m3.In this case it was assumed that horizontal and vertical stresses are about the s

46、ame (coefficient of earth pressure at rest Ko=1.0), which are also considered to be representative of the igneous formations at the Brazil test location, but other values of in situ stresses could be estimated, e.g., for Ko1.0, vertical joints would have larger permeabilities.Field permeability meas

47、urements obtained in Packer tests at a deep open-pit mining project in granitic rock in Brazil are also plotted in Fig.2 for comparison with the theoretical relationship. Values of joint spacing observed from borehole cores are in the range of a few meters, and thus a constant spacing of 5m was assu

48、med in the computations. Values of aho in the range of 3001000m were used to determine the theoretical relationships of km=f (z), where z is the depth, and compare with field measurements.Measured permeability values in the two boreholes are relatively high at depths between 100 and 200m, probably d

49、enoting the presence of a sheared zone or a zone of more jointed rock. The measured rock permeabilities decrease steadily with an increase in depth, however, and their values correspond well with the theoretical trend of rock mass permeability estimated with the model. Typical hydraulic apertures of 400500m and joint stiffness following a hyperbolic relationship with =10V mc and = seem to agree well with observed field behavior for these crystalline rock mas