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 tures, and to provide erosion control, temporary flood protection, and lateralearth retaining functions, is described in this chapter This general methodhas existed and been used for many years with very simple designs Theadvent of geosynthetics and ingenuity in construction methods hasexpanded the use of soil confinement to many other areas and geotechnicalapplications Newer materials have allowed rapid and relatively easy con-struction of temporary and permanent roadway structures, slope stabilizationand/or rehabilitation systems, and retaining structures They have also pro-vided a means to contain grout to desired locations and have even provided amethod to drain (consolidate) saturated materials such as dredged material ormine spoils

struc-16.1 CONCEPTS AND HISTORY

It is well understood from fundamental shear strength theory that thestrength and loading capacity of granular (cohesionless) soil is a function

of confining stress, and will increase (roughly) proportionately withincreased confinement In fact, virtually all soil types will have greaterload-bearing capacity and shear resistance if mechanically (or otherwise)confined The use of confinement for constructing various types of earthstructures and retaining systems has been demonstrated for many years withthe use of timber cribs filled with rocks (rockfill) for support of bridge spansand railroad trestles Rock-filled, wire mesh “cages” called gabions have beenwidely utilized to buttress slopes and provide slope erosion protection whileallowing good drainage of groundwater or rainfall Gabions have also beenused as gravity retaining walls with aesthetically pleasing faces, while againproviding important drainage capacity

Confined soil in the form of conventional sandbags has been utilized formany years for flood control (Figure 16.1), emergency repair of water con-veyance structures, or as gravity “bunker” walls Sandbags have even beenused for economical home construction in low-income regions, such as parts

of South Africa (Figure 16.2)

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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 providesexcavation support through a combination of lateral earth pressure resistanceand confinement of retained soil Piles are typically driven H-piles withwood, steel, or concrete panels inserted between piles to complete theretaining structures (Figure 16.3) Soldier pile and lagging retention

Figure 16.1 Use of sandbags as a temporary “earthen” flood control levee Courtesy

structures are most often used for temporary excavation support, and may befurther enhanced with anchors or internal bracing, especially for larger wallheights (Figure 16.4).

16.3 CRIBS, GABIONS AND MATTRESSES

Cribs have been used, particularly by the railroad industry, for hundreds ofyears (Figure 16.5) Traditionally constructed by stacked timbers filled withlarge stone, today’s cribs are often made of concrete or (recycled) plastic ele-ments, providing confinement of the stone for construction of structural piers

or retaining walls (Figure 16.6) Another form of historical, timber-retainingstructure utilized driven timber piles to confine the rockfill behind it.While many of the wooden crib structures lasted for many years, under less

Figure 16.3 Soldier piles and lagging installation schematic Courtesy of Hayward Baker.

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than ideal conditions deterioration of the wooden confining structures hasbeen 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 (usuallyPVC-coated) twisted wire mesh Gabions are commonly used as gravityretaining walls for earth retention or as buttresses for slope support(Figures 16.8and16.9) They have also been used for scour and/or erosion

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.

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protection along channel linings As confined stone they have very highstrength, load capacity, and high durability Gabions have a number ofadvantages over conventional retaining structures in that they are very flex-ible; they can conform to irregular topography or geometries and can easilytolerate differential settlements without distress Often, they can be used forerosion control for stream or riverbank applications Gabions are typically

Figure 16.6 Crib wall under construction Courtesy of Maccaferri, Inc.

Figure 16.7 High concrete crib wall Courtesy of Maccaferri, Inc.

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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.

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free-draining and so usually will require no additional drainage construction.

To ensure their drainage ability, a filter may be used between them and thesoil 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 ings may be lined with a natural fiber (i.e., coconut or coir) and baskets filledwith a combination of stone and topsoil materials so that they may be veg-etated (Figure 16.10)

fac-Gabion mattresses are constructed in a similar manner, but in relativelythin, large-footprint rectangles These mattresses are convenient for use aschannel linings, shoreline protection, and other high-energy erosion protec-tion 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 protec-tion; scour protection; and shoreline revetments Where corrosion is a con-cern or in other harsh environments with salt or acid, gabions, mattresses,and confining structures are now often constructed using copolymer poly-propylene geogrids (www.maccaferri-usa.com) (Figure 16.12)

Sack gabions are a version of gabions constructed by pouring aggregateinto prepared wire (or geosynthetic) “sacks” as a rapid and low-effort means

of gabion construction These sacks are mostly used for temporary (andsometimes permanent) erosion/scour protection in high-energy hydraulicenvironments, and can be placed directly in moving water (Figure 16.13)

Figure 16.10 “Green” gabion wall Courtesy of Maccaferri, Inc.

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A number of ASTM standards have been devised for the wire mesh,rockfill, and placement of gabion structures These standards are listed atthe 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 individualcells into which soil is placed and compacted (Figure 16.14) Presto ProductsCo., together with the U.S Army Corps of Engineers, developed this tech-nology in the late 1970s and early 1980s The infilled soil is confined by thecells (cellular confinement) such that the combined system can provide

Figure 16.11 Gabion mattress for channel lining Top: Courtesy of Maccaferri, Inc.; Bottom: Courtesy of Tensar International Corporation.

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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 etated, and uncomplicated to install They can also be manufactured in avariety of colors to meet aesthetic requirements, and may be textured or per-forated to provide additional frictional resistance If infilled with clean

veg-Figure 16.12 Geogrid used for confinement of rockfill: (a) gabion mattresses and (b) shoreline confinement structures (Courtesy of Tensar International Corporation)

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granular material and perforated, they will also be free-draining and havevery high load capacity (Figure 16.16) Geocells have the additional advan-tage of providing adequate support for many applications using local on-sitesoils, rather than select fill material that would otherwise need to beimported Furthermore, the walls constructed with geocells generally pro-vide 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, orsteep slope (embankment) construction They are capable of handling sig-nificant bearing loads, by stacking filled horizontal layers of the expandedsheets (Figure 16.18) This technique will typically result in a lighter

Figure 16.13 “Sack gabion” being filled and placed in water Courtesy of Maccaferri, Inc.

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structure than other conventional walls, and would apply lower loads to soft,weak and/or compressible foundation soils Geocell wall/embankment con-struction has been successfully used in locations with poor soil and/or siteconditions, including marshlands and rice fields with highly organic soils.When placed directly over soft soils, a geotextile is often placed beneaththe geocells to provide separation (and some load distribution) In this type

of construction the wall and/or slope faces may be very steep, allowing forconstruction where rights-of-way may be an issue, but are typically alwaysbattered to some degree For added stability, walls and slopes may be con-structed with intermittent layers of geogrid reinforcement (Figure 16.19)

Figure 16.14 Cellular confinement for rapid construction of high capacity unpaved roads Courtesy of Presto Geosystems.

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(b)

Wearing surface Cellular confinement

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.

Another advantage is that the outermost cells may be vegetated by fillingwith topsoil and/or seeding to provide a natural “green” appearance(Figure 16.20).

Geocell confinement systems have been used for rapidly installed loadsupport for emergency and temporary roadways in loose sandy sites, such

as desert and beach environments, as well as for permanent support overweak foundations (Figure 16.21) These systems were employed by themilitary during Desert Storm and in Afghanistan operations to createexpedient roadways and other transportation facilities (www.prestogeo.com; www.prs-med.com) (Figure 16.22) Several other load-supportapplications include base stabilization for paved roads, surface stabilization

Figure 16.18 Free-standing geocell wall.

Steel bin

Reinforced concrete

MSE Concrete bloc

ks/geosynthetic

Concrete panels/steel reinf

.

9 Wall height (m)

for unpaved roads, support of railroad ballast, and foundation support forembankments constructed over soft soils The 3-D confinement creates arelatively stiff slab that greatly reduces the rutting and “washboarding”

of unpaved roads, and allows for much thinner base layers beneathpaved roads, while retaining integrity and reducing necessary maintenance.Geocells have also been used very effectively in single sheets as erosioncontrol for protection of slopes and channels, and for protection of geomem-brane liners The cells, typically staked down, hold soil securely in place on

Geoweb layers

Backfill soil

Backfill soil

Geosynthetic

Retained soil

Retained soil

Figure 16.19 Geocell walls: (a) gravity wall; (b) geosynthetic reinforced wall Courtesy of Presto Geosystems.

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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.

slopes, allowing for the establishment of vegetation (Figure 16.23) Whenused with coarse granular fill, cellular confinement can eliminate the needfor riprap or “hard” armor in canals, drainage ditches, storm water swales,and culvert outflows The cells may also be filled with concrete to createflexible, highly resistant concrete mats.

16.5 GEOSYNTHETICALLY CONFINED

SOIL/GEOSYNTHETIC REINFORCED SOIL

Geosynthetically confined soil (GCS®,www.geostabilization.com; thetic reinforced soil (GRS), FHWA) was introduced inChapter 14duringthe discussion of mechanically stabilized earth (MSE) walls GCS/GRS is aversion of a traditional MSE wall, but acting more as a composite structureemploying close spacing (200 mm or 8 in.) of lighter reinforcement The

geosyn-Figure 16.22 Geocells used in rapid road construction for military mobilization in desert environments Courtesy of PRS Mediterranean Ltd.

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close spacing induces a confining effect in the soil within100 mm (4 in.) ofeach reinforcement layer forming a continuously confined soil mass Typicallightweight facing blocks are each held in place only by friction betweenthem and the reinforcement layers (Figure 16.24) A schematic illustratingthe differences between the two configurations is shown inFigure 16.25.Figure 16.26shows a GCS wall supporting a roadway This type of structurehas 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 integralcomponent of an accelerated integrated bridge system (Wu et al., 2006).While they may at first appear very similar to MSE construction, thereare a number of distinct differences between these two types of retainingwalls The stability of MSE walls relies on the pullout resistance of relativelywidely spaced, high-strength reinforcement and to the added shear

Figure 16.23 Cellular confinement (geocells) for stabilization of surface soils on steep slopes Courtesy of Presto Geosystems.

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Figure 16.25 Geosynthetic confned soil (GCS) versus mechanically stabilized earth (MSE) wall Courtesy of GeoStabilization International.

Figure 16.24 Geosynthetically reinforced wall with light concrete facing blocks Courtesy of Federal Highway Administration.

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resistance afforded by the tensile (tear) strength of the reinforcing members.Reinforcement in MSE walls is typically physically attached to facing ele-ments, which themselves may be secured to each other GCS walls use closespacing (typically 200 mm or 8 in.) of relatively lightweight geotextileswithin a compacted select granular fill The mechanism of support comesfrom confinement of the fill within100 mm (4 in) of the reinforcement,which provides an internally stabilized soil mass Research has indicated thatGCS structures have bearing capacities of up to 20 times those of traditionalMSE walls (www.geostabilization.com).

16.6 FABRIC FORMWORK AND GEOTUBES

Another form of confinement involves the use of geotextile “tubes” for tainment of grout materials placed around and beneath existing foundationsdistressed by scour, erosion, or material deterioration This method of con-trolled confinement is sometimes called fabric formwork, as the geotextile cre-ates a confined form for the placed material This has been shown to beparticularly successful when construction or grouting is performed in flow-ing water environments The flexible geotextile provides a form, which cantake irregular shapes, fill voids, or follow undulating topography

con-Geotubes®introduced earlier inChapter 8as a means to dewater saturateddredged or spoil materials, have also been used successfully for cost-effectiveshoreline 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 con-structed of a high-strength, durable (but flexible) woven fabric If the fabric

Figure 16.26 Geosynthetically confined soil (GSC/GRS) supporting a roadway Courtesy

of GeoStabilization International.

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