Soil improvement and ground modification methods chapter 16 soil confinement

Soil improvement and ground modification methods chapter 16 soil confinement Soil improvement and ground modification methods chapter 16 soil confinement Soil improvement and ground modification methods chapter 16 soil confinement Soil improvement and ground modification methods chapter 16 soil confinement Soil improvement and ground modification methods chapter 16 soil confinement Soil improvement and ground modification methods chapter 16 soil confinement Soil improvement and ground modification methods chapter 16 soil confinement Soil improvement and ground modification methods chapter 16 soil confinement

CHAPTER 16 Soil Confinement The practice of confining soil to create high-capacity, load-bearing structures, and to provide erosion control, temporary flood protection, and lateral earth retaining functions, is described in this chapter This general method has existed and been used for many years with very simple designs The advent of geosynthetics and ingenuity in construction methods has expanded the use of soil confinement to many other areas and geotechnical applications Newer materials have allowed rapid and relatively easy construction of temporary and permanent roadway structures, slope stabilization and/or rehabilitation systems, and retaining structures They have also provided a means to contain grout to desired locations and have even provided a method to drain (consolidate) saturated materials such as dredged material or mine spoils 16.1 CONCEPTS AND HISTORY It is well understood from fundamental shear strength theory that the strength and loading capacity of granular (cohesionless) soil is a function of confining stress, and will increase (roughly) proportionately with increased confinement In fact, virtually all soil types will have greater load-bearing capacity and shear resistance if mechanically (or otherwise) confined The use of confinement for constructing various types of earth structures and retaining systems has been demonstrated for many years with the use of timber cribs filled with rocks (rockfill) for support of bridge spans and railroad trestles Rock-filled, wire mesh “cages” called gabions have been widely utilized to buttress slopes and provide slope erosion protection while allowing good drainage of groundwater or rainfall Gabions have also been used as gravity retaining walls with aesthetically pleasing faces, while again providing important drainage capacity Confined soil in the form of conventional sandbags has been utilized for many years for flood control (Figure 16.1), emergency repair of water conveyance structures, or as gravity “bunker” walls Sandbags have even been used for economical home construction in low-income regions, such as parts of South Africa (Figure 16.2) Soil Improvement and Ground Modification Methods © 2015 Elsevier Inc All rights reserved 389 390 Soil improvement and ground modification methods Figure 16.1 Use of sandbags as a temporary “earthen” flood control levee Courtesy of FEMA Figure 16.2 Low-cost, sandbag home construction in South Africa (www architecturalist.com) 16.2 SOLDIER PILES AND LAGGING While soldier piles may be considered a form of structural inclusion, systems of soldier piles integrated with lateral lagging is a methodology that provides excavation support through a combination of lateral earth pressure resistance and confinement of retained soil Piles are typically driven H-piles with wood, steel, or concrete panels inserted between piles to complete the retaining structures (Figure 16.3) Soldier pile and lagging retention Soil confinement 391 Figure 16.3 Soldier piles and lagging installation schematic Courtesy of Hayward Baker structures are most often used for temporary excavation support, and may be further enhanced with anchors or internal bracing, especially for larger wall heights (Figure 16.4) 16.3 CRIBS, GABIONS AND MATTRESSES Cribs have been used, particularly by the railroad industry, for hundreds of years (Figure 16.5) Traditionally constructed by stacked timbers filled with large stone, today’s cribs are often made of concrete or (recycled) plastic elements, providing confinement of the stone for construction of structural piers or retaining walls (Figure 16.6) Another form of historical, timber-retaining structure utilized driven timber piles to confine the rockfill behind it While many of the wooden crib structures lasted for many years, under less 392 Soil improvement and ground modification methods Figure 16.4 Photo of a soldier piles and lagging excavation support Courtesy of Hayward Baker Figure 16.5 Historic use of cribs for railroads Image by Bill Bradley than ideal conditions deterioration of the wooden confining structures has been an issue Timber crib walls continue to be constructed worldwide, although more resilient and durable (albeit heavier) concrete crib walls up to 30 m (100 ft) high have become popular in many regions (Figure 16.7) Gabions are stone-filled rectangular baskets, typically made of (usually PVC-coated) twisted wire mesh Gabions are commonly used as gravity retaining walls for earth retention or as buttresses for slope support (Figures 16.8 and 16.9) They have also been used for scour and/or erosion Soil confinement 393 Figure 16.6 Crib wall under construction Courtesy of Maccaferri, Inc Figure 16.7 High concrete crib wall Courtesy of Maccaferri, Inc protection along channel linings As confined stone they have very high strength, load capacity, and high durability Gabions have a number of advantages over conventional retaining structures in that they are very flexible; they can conform to irregular topography or geometries and can easily tolerate differential settlements without distress Often, they can be used for erosion control for stream or riverbank applications Gabions are typically 394 Soil improvement and ground modification methods Figure 16.8 Gabion (buttress) retaining wall construction schematic Courtesy of Hayward Baker Figure 16.9 Gabion buttress walls for channel bank stabilization/protection Top: Courtesy of Hayward Baker; Bottom: Courtesy of Maccaferri, Inc Soil confinement 395 Figure 16.10 “Green” gabion wall Courtesy of Maccaferri, Inc free-draining and so usually will require no additional drainage construction To ensure their drainage ability, a filter may be used between them and the soil retained behind them Gabions can be filled with stone of various colors or textures to provide a choice of aesthetics The surface layers and/or facings may be lined with a natural fiber (i.e., coconut or coir) and baskets filled with a combination of stone and topsoil materials so that they may be vegetated (Figure 16.10) Gabion mattresses are constructed in a similar manner, but in relatively thin, large-footprint rectangles These mattresses are convenient for use as channel linings, shoreline protection, and other high-energy erosion protection environments (Figure 16.11) As they can be easily placed underwater, they have also been used for offshore and other submerged applications, including foundations for breakwaters, jetties, and groins; pipeline protection; scour protection; and shoreline revetments Where corrosion is a concern or in other harsh environments with salt or acid, gabions, mattresses, and confining structures are now often constructed using copolymer polypropylene geogrids (www.maccaferri-usa.com) (Figure 16.12) Sack gabions are a version of gabions constructed by pouring aggregate into prepared wire (or geosynthetic) “sacks” as a rapid and low-effort means of gabion construction These sacks are mostly used for temporary (and sometimes permanent) erosion/scour protection in high-energy hydraulic environments, and can be placed directly in moving water (Figure 16.13) 396 Soil improvement and ground modification methods Figure 16.11 Gabion mattress for channel lining Top: Courtesy of Maccaferri, Inc.; Bottom: Courtesy of Tensar International Corporation A number of ASTM standards have been devised for the wire mesh, rockfill, and placement of gabion structures These standards are listed at the end of this chapter 16.4 GEOCELLS Geocells are manufactured as 3-D sheets of HDPE membranes (or geogrids) that are shipped as compact, collapsed bundled units When stretched out (typically to 6.6 m (20 ft) lengths), the “sheets” form a series of individual cells into which soil is placed and compacted (Figure 16.14) Presto Products Co., together with the U.S Army Corps of Engineers, developed this technology in the late 1970s and early 1980s The infilled soil is confined by the cells (cellular confinement) such that the combined system can provide Soil confinement 397 Figure 16.12 Geogrid used for confinement of rockfill: (a) gabion mattresses and (b) shoreline confinement structures (Courtesy of Tensar International Corporation) significant load-bearing, lateral load resistance, and erosion resistance (Figure 16.15) A number of advantages have led to the use of geocells in a wide variety of applications Similar to other types of confinement systems such as gabions or mattresses, geocell systems are flexible, easily transported, able to be vegetated, and uncomplicated to install They can also be manufactured in a variety of colors to meet aesthetic requirements, and may be textured or perforated to provide additional frictional resistance If infilled with clean 398 Soil improvement and ground modification methods Figure 16.13 “Sack gabion” being filled and placed in water Courtesy of Maccaferri, Inc granular material and perforated, they will also be free-draining and have very high load capacity (Figure 16.16) Geocells have the additional advantage of providing adequate support for many applications using local on-site soils, rather than select fill material that would otherwise need to be imported Furthermore, the walls constructed with geocells generally provide a significant, relative cost savings compared to other retaining wall types (Figure 16.17) Geocells are often used for retaining wall, free-standing earth berm, or steep slope (embankment) construction They are capable of handling significant bearing loads, by stacking filled horizontal layers of the expanded sheets (Figure 16.18) This technique will typically result in a lighter Soil confinement 399 Figure 16.14 Cellular confinement for rapid construction of high capacity unpaved roads Courtesy of Presto Geosystems structure than other conventional walls, and would apply lower loads to soft, weak and/or compressible foundation soils Geocell wall/embankment construction has been successfully used in locations with poor soil and/or site conditions, including marshlands and rice fields with highly organic soils When placed directly over soft soils, a geotextile is often placed beneath the geocells to provide separation (and some load distribution) In this type of construction the wall and/or slope faces may be very steep, allowing for construction where rights-of-way may be an issue, but are typically always battered to some degree For added stability, walls and slopes may be constructed with intermittent layers of geogrid reinforcement (Figure 16.19) (a) Cellular confinement Wearing surface (b) Figure 16.15 Comparison of distribution and lateral transfer of tire loads with cellular confinement: (a) unconfined; (b) confined Figure 16.16 Very high load capacity gravel-filled geocells Courtesy of Presto Geosystems Soil confinement Wall height (ft) 30 40 20 te n bio in b l tee re ed co c or nf S i Re Cost ($/m2) $40 Ga nc $400 inf ete ncr Co pa tion eten r th r hetic nt osy s/ge ret onc EC MS s/s nel ck e blo $30 l re tee $300 $200 50 s Wall es Slop ms syste $20 Cost ($/ft2) 10 401 a e web Geo $10 $100 12 Wall height (m) 15 Figure 16.17 Relative costs for various earth retention walls Courtesy of Presto Geosystems Figure 16.18 Free-standing geocell wall Another advantage is that the outermost cells may be vegetated by filling with topsoil and/or seeding to provide a natural “green” appearance (Figure 16.20) Geocell confinement systems have been used for rapidly installed load support for emergency and temporary roadways in loose sandy sites, such as desert and beach environments, as well as for permanent support over weak foundations (Figure 16.21) These systems were employed by the military during Desert Storm and in Afghanistan operations to create expedient roadways and other transportation facilities (www.prestogeo com; www.prs-med.com) (Figure 16.22) Several other load-support applications include base stabilization for paved roads, surface stabilization 402 Soil improvement and ground modification methods Geoweb layers (a) Backfill soil Retained soil Foundation soil Geoweb layers Backfill soil Retained soil Geosynthetic (b) Foundation soil Figure 16.19 Geocell walls: (a) gravity wall; (b) geosynthetic reinforced wall Courtesy of Presto Geosystems for unpaved roads, support of railroad ballast, and foundation support for embankments constructed over soft soils The 3-D confinement creates a relatively stiff slab that greatly reduces the rutting and “washboarding” of unpaved roads, and allows for much thinner base layers beneath paved roads, while retaining integrity and reducing necessary maintenance Geocells have also been used very effectively in single sheets as erosion control for protection of slopes and channels, and for protection of geomembrane liners The cells, typically staked down, hold soil securely in place on Figure 16.20 Composite geocell wall before and after vegetation Courtesy of Presto Geosystems Figure 16.21 Geocell reinforcement over weak foundation soils Courtesy of Presto Geosystems 404 Soil improvement and ground modification methods Figure 16.22 Geocells used in rapid road construction for military mobilization in desert environments Courtesy of PRS Mediterranean Ltd slopes, allowing for the establishment of vegetation (Figure 16.23) When used with coarse granular fill, cellular confinement can eliminate the need for riprap or “hard” armor in canals, drainage ditches, storm water swales, and culvert outflows The cells may also be filled with concrete to create flexible, highly resistant concrete mats 16.5 GEOSYNTHETICALLY CONFINED SOIL/GEOSYNTHETIC REINFORCED SOIL Geosynthetically confined soil (GCS®, www.geostabilization.com; geosynthetic reinforced soil (GRS), FHWA) was introduced in Chapter 14 during the discussion of mechanically stabilized earth (MSE) walls GCS/GRS is a version of a traditional MSE wall, but acting more as a composite structure employing close spacing (200 mm or in.) of lighter reinforcement The Soil confinement 405 Figure 16.23 Cellular confinement (geocells) for stabilization of surface soils on steep slopes Courtesy of Presto Geosystems close spacing induces a confining effect in the soil within 100 mm (4 in.) of each reinforcement layer forming a continuously confined soil mass Typical lightweight facing blocks are each held in place only by friction between them and the reinforcement layers (Figure 16.24) A schematic illustrating the differences between the two configurations is shown in Figure 16.25 Figure 16.26 shows a GCS wall supporting a roadway This type of structure has offered a low-cost alternative for new or rehabilitated bridge abutments as well as other earth structures The Federal Highway Administration includes GRS walls as an integral component of an accelerated integrated bridge system (Wu et al., 2006) While they may at first appear very similar to MSE construction, there are a number of distinct differences between these two types of retaining walls The stability of MSE walls relies on the pullout resistance of relatively widely spaced, high-strength reinforcement and to the added shear 406 Soil improvement and ground modification methods Figure 16.24 Geosynthetically reinforced wall with light concrete facing blocks Courtesy of Federal Highway Administration Figure 16.25 Geosynthetic confned soil (GCS) versus mechanically stabilized earth (MSE) wall Courtesy of GeoStabilization International Soil confinement 407 Figure 16.26 Geosynthetically confined soil (GSC/GRS) supporting a roadway Courtesy of GeoStabilization International resistance afforded by the tensile (tear) strength of the reinforcing members Reinforcement in MSE walls is typically physically attached to facing elements, which themselves may be secured to each other GCS walls use close spacing (typically 200 mm or in.) of relatively lightweight geotextiles within a compacted select granular fill The mechanism of support comes from confinement of the fill within 100 mm (4 in) of the reinforcement, which provides an internally stabilized soil mass Research has indicated that GCS structures have bearing capacities of up to 20 times those of traditional MSE walls (www.geostabilization.com) 16.6 FABRIC FORMWORK AND GEOTUBES Another form of confinement involves the use of geotextile “tubes” for containment of grout materials placed around and beneath existing foundations distressed by scour, erosion, or material deterioration This method of controlled confinement is sometimes called fabric formwork, as the geotextile creates a confined form for the placed material This has been shown to be particularly successful when construction or grouting is performed in flowing water environments The flexible geotextile provides a form, which can take irregular shapes, fill voids, or follow undulating topography Geotubes® introduced earlier in Chapter as a means to dewater saturated dredged or spoil materials, have also been used successfully for cost-effective shoreline protection; beach restoration; containment berms; wave barriers (breakwaters); jetties; the creation of wetlands; and for the construction of artificial islands, reclaimed land, or other marine structures (Figure 16.27) (www.infralt.com; www.tencate.com) The tubes are constructed of a high-strength, durable (but flexible) woven fabric If the fabric 408 Soil improvement and ground modification methods Figure 16.27 Construction of sand filled geotubes for a hurricane protection (storm surge) barrier Courtesy of Infrastructure Alternatives, Inc is expected to be exposed for extended periods it may be coated or covered with a UV protective layer In one case study, contaminated dredged materials were reused in place of 450,000 m3 (15.9 million ft3) of expensive imported fill for construction of the largest private containment port in South America that services over two million containers per year (www.tencate.com) It is estimated that reusing the dredged material saved tens of millions of dollars as well as greatly reducing the carbon footprint for that project 16.7 EROSION CONTROL In addition to the confinement methods described earlier, there are a wide variety of erosion control mats designed to confine or hold surface soils in place and resist the forces of surface water flows and wind These mats range from lightweight temporary meshes that are staked to the ground, intended only to stabilize the surface soils until vegetation can be established, and often consisting of “green” biodegradable natural materials, to heavy duty reinforced mats securely anchored to the ground for long-term resilience (Figures 16.28 and 16.29) Confinement of surface soils on slopes and high-energy surfaces provides a means to retain soils subjected to harsh erosional forces Similar confinement schemes have also been used on weathered and/or fractured rock faces (Figure 16.30) Presented at the end of Chapter 15 is a slope stabilization method introduced by Koerner (2005) for relatively shallow, potential slide masses The method works by securing a geosynthetic netting over the slope surface with soil anchors or nails that extend beyond an assumed, potential failure surface, Soil confinement 409 Figure 16.28 High-strength erosion control mat installation Courtesy of Maccaferri, Inc Figure 16.29 Confinement with nailed high capacity steel mesh Courtesy of GeoStabilization International and then post-tensioning the netting and anchor/nail The soil nails act to secure the potential slide mass with the same mechanisms described in Chapter 15, with the addition of confinement of the soil now put in compression by the tensioned netting and nails (Figure 16.31) As described previously, the confined soil mass will have improved strength and stability characteristics Figure 16.30 Confinement of weathered rock face with steel mesh and soil nails Courtesy of GeoStabilization International Netting in tension d) ne fi on (c Failure surface s re mp n sio o nc il i So Anchors/nails in tension Figure 16.31 Slope stabilization incorporating anchored and tensioned netting to provide confinement Top: After Koerner, 2005; Bottom: Courtesy of GeoStabilization International Soil confinement 411 RELEVANT ASTM STANDARDS A974—97(2011) Standard Specification for Welded Wire Fabric Gabions and Gabion Mattresses (Metallic Coated or Polyvinyl Chloride (PVC) Coated), V1.06 A975—11 Standard Specification for Double-Twisted Hexagonal Mesh Gabions and Revet Mattresses (Metallic-Coated Steel Wire or MetallicCoated Steel Wire With Poly(Vinyl Chloride) (PVC) Coating), V1.06 D6711—01(2008) Standard Practice for Specifying Rock to Fill Gabions, Revet Mattresses, and Gabion Mattresses, V4.09 D7014—10 Standard Practice for Assembly and Placement of DoubleTwisted Wire Mesh Gabions and Revet Mattresses, V4.09 REFERENCES Federal Highways Administration, 2011 Geosynthetic reinforced soil integrated bridge system synthesis report Report no FHWA-HRT-11-027, Washington, DC Hausmann, M.R., 1990 Engineering Principles of Ground Modification McGraw-Hill, New York, 632 pp Koerner, R.M., 2005 Designing with Geosynthetics, fifth ed Pearson Education, New Jersey, 796 pp Koerner, R.M., 2012 Designing with Geosynthetics, sixth ed Xlibris Corp, Bloomington, ID, 914 pp Wu, J.T.H., Lee, K.Z.Z., Helwany, S.B., and Ketchart, K 2006 Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing NCHRP Report No 556, Transportation Research Board, Washington, DC http://www.architecturelist.com/2008/04/25/sand-bag-house-in-cape-town/ (accessed 01.04.14) http://www.geostabilization.com/SNL/Design_Build/tools_gsi.html#4 (accessed 08.01.14) http://www.haywardbaker.com (accessed 18.01.14) http://www.infralt.com/content/114/Shoreline-Protection.html (accessed 07.01.14) http://www.maccaferri-usa.com/products (accessed 18.01.14) http://www.moretrench.com (accessed 18.01.14) http://www.prestogeo.com (accessed 25.01.14) http://www.prs-med.com/road-construction/military-roads-applications (accessed 02.04.14) http://www.tenaxus.com (accessed 15.01.14) http://www.tencate.com/amer/geosynthetics/solutions/marine-structures/default.aspx (accessed 07.01.14) http://www.tensarcorp.com (accessed 20.01.14)  ... stabilization 402 Soil improvement and ground modification methods Geoweb layers (a) Backfill soil Retained soil Foundation soil Geoweb layers Backfill soil Retained soil Geosynthetic (b) Foundation soil. ..390 Soil improvement and ground modification methods Figure 16. 1 Use of sandbags as a temporary “earthen” flood control levee Courtesy of FEMA Figure 16. 2 Low-cost, sandbag home construction... securely anchored to the ground for long-term resilience (Figures 16. 28 and 16. 29) Confinement of surface soils on slopes and high-energy surfaces provides a means to retain soils subjected to harsh