{"id":1432,"date":"2026-09-15T12:20:44","date_gmt":"2026-09-15T09:20:44","guid":{"rendered":"https:\/\/setaf2018.com\/?p=1432"},"modified":"2026-09-15T12:20:48","modified_gmt":"2026-09-15T09:20:48","slug":"soil-nail-wall-design","status":"publish","type":"post","link":"https:\/\/setaf2018.com\/en\/post\/soil-nail-wall-design\/","title":{"rendered":"Soil Nail Wall Design: Pullout, Tendon Rupture and Facing"},"content":{"rendered":"\n

A soil nail wall is an excavation support system in which closely spaced passive steel elements are installed into the ground as excavation proceeds downward, reinforcing the soil mass rather than restraining it from beyond. Each nail consists of a steel bar grouted along its full length, connected at the excavation face to a bearing plate and a facing that is usually shotcrete.<\/p>\n\n\n\n

The distinction from a ground anchor is fundamental and determines every subsequent design decision.<\/p>\n\n\n\n

A ground anchor reaches past the problem. A soil nail reinforces through it.<\/strong><\/p>\n\n\n\n

An anchor has an unbonded free length precisely so that it can deliver load beyond the soil that might move. A soil nail is bonded over its entire length, including the portion inside the potentially unstable zone. It does not avoid that zone; it stitches across it, converting a mass of soil and steel into a composite block that behaves as a gravity structure.<\/p>\n\n\n\n

Soil Nail or Ground Anchor?<\/strong><\/h2>\n\n\n\n
<\/th>Soil nail<\/strong><\/th>Ground anchor<\/strong><\/th><\/tr><\/thead>
Bonded length<\/td>Full length<\/td>Bond length only<\/td><\/tr>
Free length<\/td>None<\/td>Present, passes the failure wedge<\/td><\/tr>
Loading<\/td>Passive \u2014 force develops as ground moves<\/td>Active \u2014 prestressed at installation<\/td><\/tr>
Spacing<\/td>Close, typically 1 to 2 m in a grid<\/td>Wide, one row per excavation level<\/td><\/tr>
Element size<\/td>Small diameter bar<\/td>Multi-strand tendon or large bar<\/td><\/tr>
Wall movement<\/td>Accepted; required to mobilise force<\/td>Limited by prestress<\/td><\/tr>
Design concept<\/td>Reinforced soil mass acting as a block<\/td>Discrete restraint forces applied to a wall<\/td><\/tr>
Suits<\/td>Ground that stands unsupported briefly<\/td>Most ground, including where movement must be controlled<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

The practical consequence of passive loading is that soil nail walls move. A nail develops force only when the ground around it deforms, so some displacement is not a defect but the mechanism itself. Where an adjacent structure cannot tolerate that movement, prestressed ground anchors<\/a> are usually the more appropriate system.<\/p>\n\n\n\n

How Does a Soil Nail Wall Work?<\/strong><\/h2>\n\n\n\n

Construction proceeds top-down in lifts. A limited depth is excavated, nails are installed and grouted in the exposed face, facing is applied, and the next lift is excavated.<\/p>\n\n\n\n

This sequence contains the method’s central requirement: the ground must stand unsupported, at full face height, for the time it takes to install nails and apply facing. That is a genuine limitation rather than a construction detail, and it excludes a range of ground conditions regardless of what the capacity calculations say.<\/p>\n\n\n\n

As excavation deepens, the unreinforced soil below and in front of each nail deforms, the nail is stretched, and tensile force develops. Force is transferred into the nail through skin friction along the grout body on the stable side of the eventual failure surface, and out through the facing on the exposed side.<\/p>\n\n\n\n

Excavate a lift \u2192 Install and grout nails \u2192 Apply facing \u2192 Excavate the next lift \u2192 Ground deforms \u2192 Nails mobilise tension<\/p>\n\n\n\n

What Governs Nail Capacity?<\/strong><\/h2>\n\n\n\n

A nail crossing a potential failure surface can fail in two ways: it can pull out of the ground, or the steel can rupture. Its usable capacity is the lesser of the two:<\/p>\n\n\n\n

Rn = min ( Tp \u00b7 x , Rt )<\/strong><\/p>\n\n\n\n

where Tp is the pullout resistance per unit length, x the length of the nail behind the slip surface, and Rt the tendon rupture load.<\/p>\n\n\n\n

The presence of x in that expression is the defining feature of soil nail design.<\/p>\n\n\n\n

A nail’s capacity is not a property of the nail. It depends on where the slip surface cuts it.<\/strong><\/p>\n\n\n\n

The same nail contributes different amounts to different trial surfaces. A surface passing close to the face leaves most of the nail behind it, so the pullout term is large and the tendon governs. A surface passing deep into the slope leaves little embedded length behind it, so the pullout term is small and governs instead. A nail lying entirely in front of the surface contributes nothing at all and is excluded from the analysis.<\/p>\n\n\n\n

This is why nail capacity cannot be established before the stability analysis. The two are solved together.<\/p>\n\n\n\n

How Is Pullout Resistance Determined?<\/strong><\/h2>\n\n\n\n

Pullout resistance per unit length is the skin friction developed over the surface of the grout body:<\/p>\n\n\n\n

Tp = \u03c0 \u00b7 D \u00b7 fs<\/strong><\/p>\n\n\n\n

where D is the diameter of the grouted body and fs the ultimate unit skin friction at the grout\u2013ground interface, expressed as a stress.<\/p>\n\n\n\n

The unit skin friction can be established in several ways, and the reliability of the design depends heavily on which one is used.<\/p>\n\n\n\n

From Effective Stresses<\/h3>\n\n\n\n

For drained conditions:<\/p>\n\n\n\n

fs = K\u2081 \u00b7 \u03c3\u2032v \u00b7 tan \u03c6\u2032<\/strong><\/p>\n\n\n\n

where \u03c3\u2032v is the vertical effective stress at the level of the nail, \u03c6\u2032 the effective friction angle of the soil<\/a>, and K\u2081 a coefficient reflecting the installation method. For elements installed without grouting pressure, K\u2081 typically lies between 1.4 and 2.3, falling to between 1.0 and 1.15 in fine sands and silty soils depending on relative density.<\/p>\n\n\n\n

From Total Stresses<\/h3>\n\n\n\n

For undrained conditions in fine-grained soils, the unit friction is taken as a proportion of the undrained shear strength, with the adhesion factor reducing as strength increases.<\/p>\n\n\n\n

From Empirical Tables<\/h3>\n\n\n\n

Published ranges by soil description and drilling method are widely used and form the basis of FHWA guidance. Because they are derived from case histories rather than site measurements, they are applied with an additional safety coefficient.<\/p>\n\n\n\n

From Pull-out Testing<\/h3>\n\n\n\n

Verification testing on sacrificial nails is standard practice on soil nail projects and is the only route that reflects the actual ground and the actual drilling and grouting method. Where testing is carried out, the measured values supersede the estimates used at design stage.<\/p>\n\n\n\n

Testing occupies a more central role in soil nailing than in anchored systems, because a soil nail wall relies on many small elements whose individual capacities cannot be verified in service, rather than a small number of large elements each of which is proof-loaded at stressing.<\/p>\n\n\n\n

Tendon Rupture<\/strong><\/h2>\n\n\n\n

The second limit is the tensile capacity of the steel:<\/p>\n\n\n\n

Rt = n \u00b7 At \u00b7 fy<\/strong><\/p>\n\n\n\n

where n is the number of bars, At the nominal cross-sectional area of one, and fy the yield strength of the reinforcement.<\/p>\n\n\n\n

In soil nailing this check governs more often than it does in anchor design, because nails are short, closely spaced, and installed in ground where a modest embedded length develops enough friction to reach the capacity of a small-diameter bar.<\/p>\n\n\n\n

How Is the Facing Designed?<\/strong><\/h2>\n\n\n\n

The facing carries the nail force at the face and retains the soil between nails. It is usually shotcrete with welded mesh reinforcement, with a bearing plate, head, and sometimes a waler beam at each nail.<\/p>\n\n\n\n

Three mechanisms are checked:<\/p>\n\n\n\n

Check<\/strong><\/th>Mechanism<\/strong><\/th><\/tr><\/thead>
Flexure<\/td>Bending of the facing spanning between nails, in both directions<\/td><\/tr>
Punching shear<\/td>Local failure of the facing around the bearing plate<\/td><\/tr>
Connection<\/td>Plate, head, welds, and where present the waler beam<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Facing forces are not equal to nail forces. The proportion of nail force carried at the face is smaller than the maximum force in the nail, which occurs some distance behind it, and the facing is designed for that reduced value rather than the full tendon capacity.<\/p>\n\n\n\n

Nail spacing governs facing design as directly as it governs global stability. Wider spacing reduces the number of drilled elements but increases the span of the facing and the concentration at each plate \u2014 a trade-off that is settled by examining both, not by optimising nail count alone.<\/p>\n\n\n\n

Where Does the Slip Surface Come In?<\/strong><\/h2>\n\n\n\n

Because a soil nail wall works as a reinforced block rather than as a restrained wall, its stability is assessed by the same limit equilibrium methods used for slope stability analysis<\/a>.<\/p>\n\n\n\n

Trial failure surfaces are generated through and behind the reinforced block. For each surface, the driving and resisting forces are evaluated, and each nail crossing the surface contributes its capacity Rn as calculated above. The critical surface is the one producing the lowest factor of safety.<\/p>\n\n\n\n

Three families of surface are examined, and they fail in different ways:<\/p>\n\n\n\n

Failure mode<\/strong><\/th>Surface location<\/strong><\/th>Governed by<\/strong><\/th><\/tr><\/thead>
Internal<\/td>Passes through the reinforced block<\/td>Nail pullout and tendon capacity<\/td><\/tr>
Mixed<\/td>Passes partly through and partly behind the block<\/td>Nail lengths near the boundary<\/td><\/tr>
External<\/td>Passes entirely behind and beneath the block<\/td>Block dimensions, sliding and bearing<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Lengthening nails improves internal stability but has diminishing effect on external stability, which depends on the overall geometry of the reinforced block. A wall failing an external check is not fixed by adding steel.<\/p>\n\n\n\n

Nails, like anchors, contribute only their component that resists the driving force. The component acting along the slip surface in the direction of movement is conventionally neglected, which keeps the calculation on the safe side.<\/p>\n\n\n\n

When Is a Soil Nail Wall Not Appropriate?<\/strong><\/h2>\n\n\n\n

The method has clear boundaries, and recognising them early avoids a design that satisfies every equation and cannot be built.<\/p>\n\n\n\n

Ground that will not stand unsupported for the duration of a lift \u2014 clean saturated sands, soft clays, running ground \u2014 excludes the method or requires supplementary measures that erode its cost advantage.<\/p>\n\n\n\n

Groundwater above excavation level is a persistent difficulty. Soil nailing provides no cut-off, seepage at the face undermines the exposed lift, and the facing is not designed to retain water pressure. Where a cut-off is needed, a sheet pile<\/a> or diaphragm wall addresses a problem soil nailing does not.<\/p>\n\n\n\n

Deformation-sensitive surroundings work against a passive system, since the force in every nail is bought with movement.<\/p>\n\n\n\n

And the nails must physically fit. They extend well behind the face, and beneath an adjacent property, road, or utility corridor that may not be available regardless of the engineering.<\/p>\n\n\n\n

Soil nailing works because the excavation stands unsupported for a short time. Where it cannot, the method cannot.<\/strong><\/p>\n\n\n\n

Common Errors in Soil Nail Design<\/strong><\/h2>\n\n\n\n
Error<\/strong><\/th>Consequence<\/strong><\/th><\/tr><\/thead>
Assigning full tendon capacity to every nail<\/td>Nails with short embedment behind the slip surface credited with capacity they cannot develop<\/td><\/tr>
Checking only internal stability<\/td>External failure of the reinforced block not identified; adding nails does not fix it<\/td><\/tr>
Designing the facing for full nail capacity<\/td>Facing over-designed while the true governing force sits behind the face<\/td><\/tr>
Using empirical skin friction without verification testing<\/td>The most uncertain parameter in the design left unmeasured<\/td><\/tr>
Ignoring the unsupported lift height requirement<\/td>A design that cannot be constructed in the ground encountered<\/td><\/tr>
Treating soil nails as prestressed elements<\/td>Wall movement underestimated, because passive elements require deformation to work<\/td><\/tr>
Nails extending beyond the site boundary<\/td>Design not consentable regardless of its engineering adequacy<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Where SETAF2018 Fits<\/strong><\/h2>\n\n\n\n

SETAF2018 handles soil nails within the same wall model as anchors and struts, so a section can combine nail rows with other support types where the design requires it.<\/p>\n\n\n\n

Nails are defined by depth, horizontal spacing, inclination, number and diameter of bars, drilled diameter, and total length, together with the bearing plate, head, and waler beam that form the connection. Rows can be staggered, and the first or last nail in a row omitted where geometry requires it.<\/p>\n\n\n\n

Pullout resistance is established by any of five routes \u2014 direct entry, empirical tables with a safety coefficient, effective stress parameters, total stress parameters, or measured pull-out test results \u2014 with the parameters taken from the layers each nail actually passes through as defined in the soil profile<\/a>. Tendon capacity follows from the bar count, diameter, and steel grade.<\/p>\n\n\n\n

Stability is evaluated by limit equilibrium using Fellenius or simplified Bishop methods, with circular or polygonal slip surfaces and an optimisation routine that searches for the critical surface rather than relying on a trial set. For each surface, each nail contributes the lesser of its pullout and tendon capacity based on the length actually lying behind that surface. A wall section analysed for excavation support<\/a> can be transferred directly into the slope stability model, which is how internal and external stability are checked against the same geometry rather than against two separately built models.<\/p>\n\n\n\n

The facing is modelled as the wall element, with reinforced concrete section checks for combined axial force and bending and for shear, and the connection detail designed at each nail head. Design equations appear in the local reports<\/a> rather than only utilisation ratios, and nail layouts and details are produced as drawings and quantity schedules from the same model.<\/p>\n\n\n\n

Frequently Asked Questions<\/strong><\/h2>\n\n\n\n

What is a soil nail wall?<\/h3>\n\n\n\n

It is an excavation support system built top-down, in which closely spaced steel bars are grouted into the ground along their full length as each lift is excavated, and a facing is applied to the exposed face. The nails reinforce the soil mass so that it behaves as a composite block rather than restraining a wall from beyond the failure zone.<\/p>\n\n\n\n

What is the difference between a soil nail and a ground anchor?<\/h3>\n\n\n\n

A ground anchor has an unbonded free length that carries load past the potentially unstable soil, and is prestressed at installation. A soil nail is bonded along its entire length, is not prestressed, and develops force only as the ground deforms. Nails are smaller, closer together, and reinforce the soil rather than restraining it.<\/p>\n\n\n\n

How is soil nail pullout resistance calculated?<\/h3>\n\n\n\n

From the perimeter of the grouted body multiplied by the ultimate unit skin friction: Tp = \u03c0 \u00b7 D \u00b7 fs. The usable capacity of a nail is that resistance multiplied by the length lying behind the slip surface, unless the tendon rupture load is lower.<\/p>\n\n\n\n

Why does nail capacity depend on the slip surface?<\/h3>\n\n\n\n

Only the portion of a nail behind the failure surface transfers load into stable ground. A surface cutting close to the face leaves a long embedded length and a high pullout capacity; a deeper surface leaves less. The same nail therefore contributes different amounts to different trial surfaces.<\/p>\n\n\n\n

Do soil nail walls move?<\/h3>\n\n\n\n

Yes, and the movement is part of the mechanism. Nails are passive elements that develop force only as the surrounding ground deforms. Where an adjacent structure cannot accommodate that movement, prestressed anchors are usually more appropriate.<\/p>\n\n\n\n

How is the facing of a soil nail wall designed?<\/h3>\n\n\n\n

For flexure spanning between nails, for punching shear around the bearing plate, and for the connection itself. The force at the face is smaller than the maximum force in the nail, which develops some distance behind it, and the facing is designed for the reduced value.<\/p>\n\n\n\n

When should soil nailing not be used?<\/h3>\n\n\n\n

Where the ground will not stand unsupported for the duration of a lift, where groundwater sits above excavation level and a cut-off is required, where adjacent structures cannot tolerate the movement a passive system requires, or where the nails would extend beyond the available site boundary.<\/p>\n\n\n\n

Is limit equilibrium sufficient for soil nail wall design?<\/h3>\n\n\n\n

For most soil nail walls, yes. Limit equilibrium methods evaluate internal, mixed, and external failure surfaces and produce the factors of safety that design standards require. More advanced numerical modelling becomes useful where deformation prediction is critical or where interaction with adjacent structures governs the design.<\/p>\n","protected":false},"excerpt":{"rendered":"

A soil nail wall is an excavation support system in which closely spaced passive steel elements are installed into the ground as excavation proceeds downward, reinforcing the soil mass rather than restraining it from beyond. Each nail consists of a steel bar grouted along its full length, connected at the excavation face to a bearing […]<\/p>\n","protected":false},"author":2,"featured_media":1433,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":"","rank_math_title":"Soil Nail Wall Design: Pullout, Tendon Rupture and Facing","rank_math_description":"How soil nail walls are designed \u2014 pullout resistance, tendon rupture, facing and connection design, the slip surface that governs nail capacity, and how soil nails differ from ground anchors.","rank_math_focus_keyword":"soil nail"},"categories":[22],"tags":[],"class_list":["post-1432","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/setaf2018.com\/en\/wp-json\/wp\/v2\/posts\/1432","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/setaf2018.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/setaf2018.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/setaf2018.com\/en\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/setaf2018.com\/en\/wp-json\/wp\/v2\/comments?post=1432"}],"version-history":[{"count":1,"href":"https:\/\/setaf2018.com\/en\/wp-json\/wp\/v2\/posts\/1432\/revisions"}],"predecessor-version":[{"id":1434,"href":"https:\/\/setaf2018.com\/en\/wp-json\/wp\/v2\/posts\/1432\/revisions\/1434"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/setaf2018.com\/en\/wp-json\/wp\/v2\/media\/1433"}],"wp:attachment":[{"href":"https:\/\/setaf2018.com\/en\/wp-json\/wp\/v2\/media?parent=1432"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/setaf2018.com\/en\/wp-json\/wp\/v2\/categories?post=1432"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/setaf2018.com\/en\/wp-json\/wp\/v2\/tags?post=1432"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}