Soil improvement and ground modification methods chapter 15 in situ reinforcement ,

Soil improvement and ground modification methods chapter 15 in situ reinforcement , Soil improvement and ground modification methods chapter 15 in situ reinforcement , Soil improvement and ground modification methods chapter 15 in situ reinforcement , Soil improvement and ground modification methods chapter 15 in situ reinforcement , Soil improvement and ground modification methods chapter 15 in situ reinforcement , Soil improvement and ground modification methods chapter 15 in situ reinforcement , Soil improvement and ground modification methods chapter 15 in situ reinforcement ,

ered inChapter 6 In this chapter, an overview of the installation of struc-tural members for reinforcement will be covered For many construction applications, the earth pressure loads that are otherwise directly applied to conventional sheet piles or soldier piles and lagging, are transferred into the ground (or rock) beyond the potential failure surface by an anchoring system In other situations, structural members may provide compressive (bearing) reinforcement Common in situ reinforcement/anchoring schemes include ground anchors, soil nails, micropiles, helical piles, and bolts Some smaller bearing reinforcement inclusions (not including deep pile foundations) are also described

15.1 TYPES, INSTALLATIONS, APPLICATIONS

As suggested, a number of different types of in situ reinforcement systems may be utilized depending on the particular application, loads, permanency, and so forth Some of these variations of reinforcement schemes are very similar in many respects, and sometimes the name refers as much to the func-tion of the inclusion, such as “anchors” for pullout resistance, or piles/piers for bearing support A number of in situ reinforcement types will be described here Conventional deep foundation piles will not be covered 15.1.1 Ground Anchors

Ground anchors are defined as structural units (typically grouted tendons) that transmit loads to stable soil or rock through tensile reinforcement Grouted ground anchors are also sometimes called tiebacks, or tiedowns when subjected

to uplift forces (Federal Highway Administration, 1999) These types of reinforcing systems are used to support temporary or permanent new wall construction, as well as for rehabilitation or reinforcement of critical and

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potentially hazardous slopes, steep cuts, or fractured and/or weathered rock faces and tunnels When used for slope stabilization or to remediate land-slides, ground anchors are often used in conjunction with other structural inclusions (e.g., beams, blocks, stub walls)

The most commonly used anchored wall systems in the United States are soldier beam and lagging walls (Figure 15.1) These are nongravity cantilev-ered wall systems consisting of discrete vertical wall elements (usually driven wide-flange “H-piles” or double channel), spanned by timber lagging, rein-forced shotcrete, or cast-in-place panels In addition to providing support for lateral and downward forces as generated by excavations, cuts, and slopes, there are a number of applications where this type of tensile reinforcing system provides resistance to uplift forces These may include hydraulic structures subjected to high internal water pressures, other structural slabs with high hydrostatic uplift pressure, buoyant underwater structures, foundations of tall, slender structures (such as transmission towers and wind turbines) subject to high overturning (i.e., wind) loads, support cables for utility poles, and so on Anchors are generally installed by inserting sleeved structural tendons into predrilled holes or trenched excavations, and grouted (or epoxied) into place over a length of the tip or deepest section of the tendon Most commonly, anchors are placed at an inclined angle of between 15 and 30 below the horizontal (although some anchors have been installed between 0and 45) For most simple applications, the deeper portion of the hole is first grouted and the anchor tendon is then inserted into the uncured grout The pullout

Figure 15.1 Ground anchor supported soldier beam and lagging excavation wall Courtesy of Hayward Baker.

by meeting test specifications, the unbounded length may later be grouted to form a fully grouted anchor Grout holes may be gravity fed or pressure-grouted, the latter more applicable to granular material and fissured rock Some anchors may be postgrouted, often in stages, allowing the development of an enlarged grout bulb over the anchored length for increased pullout resistance in softer material Some other anchoring systems may employ underreaming

or installation of an anchoring plate at the anchor tip to provide additional sup-port (Figure 15.3) Anchor tendons may be comprised of a single steel bar (available in various diameters from 26 to 64 mm) in lengths of up to 18 m (60 ft), or as multistrand tendons (typically groups of seven 15-mm diameter strands), which can be manufactured in any length (Federal Highway Admin-istration, 1999) Another version of anchor, known as a “helical anchor,” will

be described in a later section of this chapter Ground anchors may have typical capacities of up to 1000 kN (112 U.S tons) in soil, and in excess of 10,000 kN (1120 U.S tons) if grouted into rock (Hausmann, 1990)

Wall

plate

Trumpet

Sheath

Anchor diameter

Bo nded anch

or length

Unb onded len gth

tendon grout head

Figure 15.2 Components of a grouted soil anchor After FHWA (1999)

Permanent anchors have proven to be an integral part of an economical construction and permanent wall support system, where anchored walls are installed, tested, and finished in sequence with advancing excavation depths Anchors are also used in conjunction with continuous walls such as sheet pile, secant pile, slurry, or soil-mixed walls These types of structures are more often used for temporary excavation support (Federal Highway Administration, 1999)

15.1.2 Soil Nailing

Soil nailing consists of driving, screwing, drilling, or “shooting” a series of steel or fiberglass bars into the ground, most commonly for excavation sup-port or stabilization of steep slopes or cuts Nails have also been used for rock slope stabilization (see Section 15.1.3) The “nails” are typically fully grouted in place to secure the inclusions and provide additional pullout resis-tance, although some versions of nails have barbed ends (Figure 15.4) and are driven into the ground without grouting There are a number of principle differences between soil nails and anchors Soil nails are usually much shorter than tieback anchors, and generally have no structural (vertical) wall member (e.g., soldier pile or structural wall) for reaction They utilize both passive

Excavation Plate anchor

Figure 15.3 Various types of ground anchors: (a) plate anchor, (b) helix (screw) anchor, (c) underreamed anchor, (d) grouted anchor After Hausmann (1990).

and active forces mobilized on either side of a slip surface, rather than the active anchored wall systems They are generally not tensioned like an anchor system at a distance (depth) below/behind the ground surface Instead, nails reinforce a groundmass through resistance along their entire length Soil nail walls may be “finished” with reinforced shotcrete, precast panels, heavy steel mesh, or vegetated “cells.” Nail tendons typically have lower tendon strength and must be spaced closer together due to their lower capacity

With their closer spacing relative to anchors, they form a coherent, rein-forced soil mass capable of providing support for excavations or slope stabi-lization For shallower-depth slides, both shear and pullout resistance of the nails reinforces potential sliding mass Soil nails have also been used for reha-bilitation of historic stone retaining walls (Figures 15.5)

Design of soil nailing systems consists of proper dimensioning of nail spacing, length, and inclination to assure that the ground mass is stabilized When used to secure a rotational-type slide, the nails must be secured well beyond any potential failure surface Monitoring of any movements of the stabilized ground mass is vital to ensuring that any problems are detected early In extremely corrosive environments, fiberglass nails may be used

to resist deterioration

Dynamically “launched” soil nails are placed into the ground by a com-pressed air cannon at speeds of up to 400 kph (250 mph), usually without a need for grouting The use of launched soil nails for slope stabilization has many benefits over the construction of more conventional retaining systems Costs may be on the order of one-tenth of a traditional system, and launched soil nails can be installed in a fraction of the time without nearly as much dis-ruption to the environment or to ongoing serviceability of the structure under

Figure 15.4 Detail of a launched soil nail with “barbed” end.

repair (www.geostabilization.com) In addition, as the nails are launched into the ground, they generate a shock wave that causes the earth materials to elas-tically deform without the full static resistance that might normally have been expected After insertion, the earth materials collapse onto the bar in a relatively undisturbed state with increased normal stresses In addition, the dis-placement of the ground surrounding the nail is densified The combination

of these effects results in pullout capacities up to 10 times that of driven piles (www.geostabilization.com) without the need for grouting A version of the launched soil nail employs a perforated hollow rod that can be injected with grout after installation, resulting in further densification/stabilization of the surrounding soil, as well as providing increased pullout resistance Another

Figure 15.5 Rehabilitation of historic rock retaining walls with soil nails Courtesy of GeoStabilization International.

modification employs insertion of a solid rod inside the grout-filled, perfo-rated, hollow launched nail to provide added strength These versions have been termed SuperNails®(www.geostabilization.com)

Some of the advantages to using launched soil nails is the ability to mobi-lize rapidly for emergency repairs, the rapid rate of installation, and reduced need for design planning (Figure 15.6) Traditional soil nailing includes a lengthy delay while cement grout hardens Launched soil nails can provide effective stabilization almost immediately after installation Several private and government-sponsored research projects have verified and supported the effectiveness and economy of utilizing soil nails rather than more con-ventional soldier piles and lagging for slope and retaining structure repairs Case studies have shown that the use of launched soil nails in place of tra-ditional temporary shoring or slope stabilization methods has realized cost savings of around 50-80% (in some cases, millions of dollars), with installa-tions in days as opposed to several weeks (www.geostabilization.com) This type of solution can be critical, especially when disruption of service of road-ways, rail lines, or utilities is a serious concern For highly corrosive envi-ronments (e.g., coastal bluffs, highly acidic soil, etc.) fiberglass nails have been employed (Figure 15.7)

15.1.3 Rock Bolts

Rock bolts are a type of drilled soil nail or anchor used when the ground to

be stabilized consists mostly of rock materials These types of inclusions are typically grouted in place and posttensioned, similar to soil anchors

Figure 15.6 Launched soil nails for emergency slope stabilization Courtesy of GeoStabilization International.

(Figure 15.8) The bolts hold potentially unstable jointed or fractured rock masses together in compression with bearing faceplates providing passive resistance, forming a more stable structural entity Rock bolts have been used for many years as temporary roof support in the mining industry and for tunneling, and are now used routinely for stabilizing roadcuts, rock cliffs, steep slopes, bridge abutments, and reinforced concrete dams Two other versions of rock bolts sometimes used are those that have expansive shells (like a drywall anchor) rather than grout for resistance, and untensioned steel rods grouted into boreholes referred to as dowels (Hausmann, 1990) 15.1.4 Micropiles

Micropiles (a.k.a minipiles, pin piles, root piles) are essentially small-diameter piles (often steel bars or pipes) grouted into predrilled holes to form short friction piles with high capacity and a generally lower amount of settlement

Figure 15.7 Stabilization of coastal bluff with fiberglass launched soil nails: Before and after stained shotcrete finish Courtesy of GeoStabilization International.

compared to much more expensive driven piles (Figure 15.9) They can be installed in almost any type of soil, and even rock Micropiles are most com-monly used for structural foundation support, underpinning, wall support, and slope stabilization One of the significant advantages of using micropiles

is the lack of required overhead or lateral site constraints that would prohibit installations requiring much larger equipment As opposed to the array of structural reinforcement methods described previously, micropiles can pro-vide significant compressional capacity as well as tensile restraint The indus-try reports that micropiles can have working capacities up to 2200 kN (250 tons) (www.rembco.com) Traditional micropiles are installed in concrete-filled predrilled holes They are often used in groups to transfer bearing loads to subsurface soils in place of expensive deep foundations (Figure 15.10) For higher capacity, micropiles are pressure-grouted in

Figure 15.8 Rockfall mitigation with nails/rock bolts Top: Courtesy of Layne Christensen; Bottom: Courtesy of GeoStabilization International.

place, which increases lateral pressures and densifies surrounding soil (if compressible), greatly increasing side resistance

Launched micropiles, installed with the same type of equipment used for launched soil nails, can be rapidly installed and used for soil reinforcement, as

is required for shallow excavation support, for support of retaining structures and embankments, or even for scour protection around bridge piers or cul-vert discharge channels (Figure 15.11)

Figure 15.9 Components of a micropile.

Figure 15.10 Minipiles (vertical soil anchors) for roadway support Courtesy of GeoStabilization International.

15.2 DESIGN BASICS

15.2.1 Capacity Estimates

For the most part, the final design of anchors, bolts, and nails is empirical and may rely on actual field testing of some selected, installed test anchors Some empirical guidelines are available for initial feasibility estimates based upon soil/rock type and condition, identified by field borings and standard soil/

Launched scour micropiles

Launched scour micropiles

Bridge abutment/Culvert scour protection Figure 15.11 Launched micropiles for scour protection Courtesy of GeoStabilization International.

evaluation of the shaft resistance of a deep foundation pile or shaft For driven or “launched” inclusions, the stresses normal to the elements may

be increased by displacement When designing for slope stabilization, the required resisting forces may be significantly greater than for walls and should be estimated based on limit equilibrium analyses

15.2.2 Spacing

Both vertical and horizontal spacing of in situ reinforcing inclusions must

be designed to cover a load proportionate to the area attributable to each inclusion In addition, closer spacing may be needed to attain a stiffer com-posite wall if smaller deformations are required The Federal Highway Administration (FHWA) (1999) recommends that horizontal spacing should be no<1.2 m to avoid group effects between adjacent inclusions, while maximum spacing must consider the flexural capacity and tolerances

of the wall A minimum depth should also be observed for the uppermost grout application to prevent heave and provide sufficient confining stress

to contain grout pressures (if used) and provide sufficient pullout resistance capacity

15.2.3 Other Considerations

Designs should also consider other possible failure modes, including bond strength between the tendon and grout, tendon strength, stiffness/bending potential of the wall material(s), and depth of most critical ground failure (which could be significantly deeper than a Rankine active failure wedge) This last parameter is essentially addressed by evaluating maximum lateral earth pressures as well as deeper possible ground failure (limit equilibrium rotational stability) to ensure adequate embedment depth of anchors Some general guidelines are provided byFHWA (1999) Where appropriate, seis-mic ground forces should be added to capacity requirements

Protection of metallic elements from corrosion is also an important con-sideration for long-term durability and performance of permanent inclu-sions This may be accomplished by providing one or more physical barriers, including corrosion-inhibiting compounds, sheaths, epoxy coat-ings, and grouts (FHWA, 1999)