Numerical Methods in Soil Mechanics 27.PDF

Numerical Methods in Soil Mechanics 27.PDF Numerical Methods in Geotechnical Engineering contains the proceedings of the 8th European Conference on Numerical Methods in Geotechnical Engineering (NUMGE 2014, Delft, The Netherlands, 18-20 June 2014). It is the eighth in a series of conferences organised by the European Regional Technical Committee ERTC7 under the auspices of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). The first conference was held in 1986 in Stuttgart, Germany and the series has continued every four years (Santander, Spain 1990; Manchester, United Kingdom 1994; Udine, Italy 1998; Paris, France 2002; Graz, Austria 2006; Trondheim, Norway 2010). Numerical Methods in Geotechnical Engineering presents the latest developments relating to the use of numerical methods in geotechnical engineering, including scientific achievements, innovations and engineering applications related to, or employing, numerical methods. Topics include: constitutive modelling, parameter determination in field and laboratory tests, finite element related numerical methods, other numerical methods, probabilistic methods and neural networks, ground improvement and reinforcement, dams, embankments and slopes, shallow and deep foundations, excavations and retaining walls, tunnels, infrastructure, groundwater flow, thermal and coupled analysis, dynamic applications, offshore applications and cyclic loading models. The book is aimed at academics, researchers and practitioners in geotechnical engineering and geomechanics.

Anderson, Loren Runar et al "RISERS " Structural Mechanics of Buried Pipes Boca Raton: CRC Press LLC,2000 Figure 27-1 Notation for risers, showing radial pressure on the left side and shearing stress (drag-down) on the right side that is caused by soil compression ©2000 CRC Press LLC CHAPTER 27 RISERS Risers are basically pipes that are buried vertically, or nearly so In fact, some risers are inclined They are called risers because they usually "rise" from a buried tank or pipe Risers serve many purposes: access (such as manholes and mine shafts), cleanout, ventilation, collection of gas (methane from sanitary landfills), standpipes (for water pressure control), bins (for feeding underground conveyors), accumulators (to collect entrapped air in water pipes), etc Most risers are cylinders (usually pipes) See Figure 27-1 Basic concerns are ring compression and longitudinal (vertical) thrust The critical location of both is usually at, or near, the bottom of the riser Ring Compression From Chapter 6, ring compression stress is, fc = rs x /t where fc = sx = r = t = circumferential stress in thin-wall riser, external radial pressure against riser, outside radius of curvature of the riser, wall thickness of the riser For design, fc must be less than the yield stress of the riser The safety factor is needed because pressure, sx, is sensitive to soil properties and to soil placement, which never assures uniform pressure Because of soil arching action, sx is neither active soil pressure, nor radial elastic stress These are limits only At the lower limit, if a vertical hole were bored into the ground, and the riser carefully slipped down into it, sx would be zero down to some depth below which the free-standing hole collapses (cavein) under the soil weight Above this collapse depth, the only pressure on the riser is hydrostatic pressure if a water table is above the collapse depth In such a case, stability analysis applies as discussed in Chapter 10 The "vacuum" in the pipe is external hydrostatic pressure The question, of course, is ©2000 CRC Press LLC critical depth of the free-standing bored hole The surest procedure is a test hole At upper limit, if the soil is cohesionless, the riser feels radial active pressure, s = Ks z where sx = K = j = sz = radial pressure on the riser at depth z, (1+sinj)/(1-sinj) from Mohr circle, soil friction angle, equivalent vertical stress caused by compaction of the soil Active pressure is assumed if the soil is loose and slides into place against the riser If the soil is compacted, sz is roughly equivalent to the precompression stress at reversal of curvature of the stress-strain diagram for the compacted soil See Figure 27-2 This analysis is upper limit because arching action (sx) of the soil around the pipe is ignored In fact, arching action is significant The designer can get a feel for the effect of arching action by an elastic analysis Elastic theory provides a conservative stress analysis Radial pressure against the riser at depth, z, is sx Principal stresses on an infinitesimal cube of soil are shown in Figure 27-3 From elastic theory, strains are: Ee z = s z - n(s x + s y) Ee y = s y - n(s x + s z) Ee x = s x - n(s y + s z) where: E = n = e = s = modulus of elasticity, Poisson ratio, strains in the directions indicated, principal stresses on the infinitesimal soil cube in the directions indicated It is reasonable to assume that horizontal strains, e x and e y, are zero because the soil is confined Figure 27-2 Sketch of stress-strain diagrams for soil, showing precompression stresses located where curvature reverses Precompression stresses are approximately equivalent to the effect of compaction (soil density) With no compaction (70% density?) curvature does not reverse Figure 27-3 Infinitesimal soil cube at the riser surface, showing the principal stresses sz is the vertic al soil stress sx is the radial soil pressure on the riser sy is the circumferential stress which develops soil arching action ©2000 CRC Press LLC Figure 27-4 Horizontal stress, s x , is equal to the bearing capacity of the soil that resists horizontal movement of the riser into the soil This is the maximum stress that develops when the riser deflects laterally into the soil horizontally Therefore, sy = s x pressure against the riser, sx = nsz /(1-n) Solving for (27.1) According to elastic theory, pressure on the riser is sensitive to Poisson ratio,