Soil nailing
Soil nailing is a remedial construction measure to treat unstable natural soil slopes or unstable man-made (fill) slopes as a construction technique that allows the safe over-steepening of new or existing soil slopes. The technique involves the insertion of relatively slender reinforcing elements into the slope – often general purpose reinforcing bars (rebar) although proprietary solid or hollow-system bars are also available. Solid bars are usually installed into pre-drilled holes and then grouted into place using a separate grout line, whereas hollow bars may be drilled and grouted simultaneously by the use of a sacrificial drill bit and by pumping grout down the hollow bar as drilling progresses. Kinetic methods of firing relatively short bars into soil slopes have also been developed.
Bars installed using drilling techniques are usually fully grouted and installed at a slight downward inclination with bars installed at regularly spaced points across the slope face. A rigid facing (often pneumatically applied concrete, otherwise known as shotcrete) or isolated soil nail head plates may be used at the surface.[1] Alternatively, a flexible reinforcing mesh may be held against the soil face beneath the head plates. Rabbit proof wire mesh and environmental erosion control fabrics and may be used in conjunction with flexible mesh facing where environmental conditions dictate.
Soil nail components may also be used to stabilize
Since its first application using modern techniques in
Preliminary Analysis
Four main points should be considered in determining if soil nailing would be an effective retention technique. First, the existing ground conditions should be examined. Next, the advantages and disadvantages for a soil nail wall should be assessed for the particular application being considered. Then, other systems should be considered for the particular application. Finally, cost of the soil nail wall should be considered.
A list of unfavorable or difficult soil conditions for soil nailing can include dry, poorly graded cohesion-less soils, soils with a high groundwater table, soils with cobbles and boulders, soft to very soft fine-grained soils, highly corrosive soils, weathered rock with unfavorable weakness planes, and loess.[4]: 15–16 Other difficult conditions include prolonged exposure to freezing temperatures, a climate that has a repeated freeze-and-thaw cycle, and granular soils that are very loose.[4]: 16
Origins
Soil nailing evolved from the
The first application of soil nailing was implemented in 1972 for a railroad widening project near
Design
After a preliminary analysis of the site, initial designs of the soil nail wall can be begin. This process starts with a selection of limit states and design approaches. The two most common limit states used in soil nail wall design is strength limit and service limit states.[3]: 77 The strength limit state is the limit state that addresses potential failure mechanisms or collapse states of the soil nail wall system.[3]: 77 The service limit state is the limit state that addresses loss of service function resulting from excessive wall deformation and is defined by restrictions in stress, deformation and facing crack width under regular service conditions.[3]: 77 The two most common design approaches for soil nail walls are limit state design and service load design.[3]: 77
Initial design considerations include wall layout (wall height and length), soil nail vertical and horizontal spacing, soil nail pattern on wall face, soil nail inclination, soil nail length and distribution, soil nail material and relevant ground properties.[4]: 123 With all these variables in the mind of the design engineer the next step is to use simplified charts to preliminarily evaluate nail length and maximum nail force. Nail length, diameter and spacing typically control external and internal stability of the wall. These parameters can be adjusted during design until all external and internal stability requirements are met.[4]: 130 After the initial design is completed, final design progresses where the soil nail wall has to be tested for external and internal failure modes, seismic considerations and aesthetic qualities.[4]: 144 Drainage, frost penetration and external loads such as wind and hydrostatic forces also have to be determined and included in the final examination of the design.[4]: 144 Soil nail walls are not ideal in locations with highly plastic clay soils. Soils with high plasticity, a high liquid limit and low undrained shear strengths are at risk of long-term deformation (creep).[4]: 144
Construction
With the design complete, construction is the next step. Most soil nail wall construction follows a specific procedure. First, a cut is excavated and temporary bracing is put in place if necessary. This is done with conventional earth moving equipment and hydraulic
In terms of construction, soil nail walls have a decisive advantage over other alternatives. Soil nail walls require a smaller
Cost comparison
One great advantage of soil nail walls is their cost-effectiveness over other alternatives. When conventional soil nailing construction procedures are used, soil nail walls are much more economical than concrete gravity walls and similarly or more cost effective than ground anchor walls.[4]: 18
Inspection and performance monitoring
Inspection activities play a vital role in the production of high-quality soil nail walls because conformance to project plans and specifications should result in a soil nail wall that will perform its intended duty for its designed duration. Inspections usually involve evaluation of the following: conformance of system components to material specification, conformance of construction methods to execution specifications, conformance to short-term performance specifications, and long-term monitoring.[4]: 156 Short-term performance specifications are checked with loads tests, which use hydraulic jacks and pumps to perform several load applications. Three common load tests for short-term performance are verification or ultimate load tests, proof tests and creep tests. Verification or ultimate load tests are conducted to verify the compliance of the soil nails with pullout capacity and strengths resulting from the contractor's installation method.[4]: 163 Proof tests are intended to verify that the contractor's construction procedure has been consistent and that the nails have not been drilled and grouted in a soil zone not tested in the verification stage.[4]: 163 Creep tests are performed to ensure that the nail design loads can be safely carried throughout the structure's service life.[4]: 163
Long-term performance monitoring is used to collect data to ensure adequate performance and refine future design practices. Parameters to be measured include vertical and horizontal movement of the wall face, local movements or deterioration of facing elements, drainage to the ground, loads, load distribution and load changes in the nails, temperature and rainfall.[4]: 170 These parameters are measured using several specific tools including inclinometers, load cells and strain gauges.
See also
References
- ^ Goldstein, Natalie (Sep–Oct 2001). "Soil Nailing". Erosion Control. 8 (6). Forester Media. Archived from the original on 2011-07-10.
- ^ "Construction d'un mur de soutènement entre Versailles-Chantiers et Versailles-Matelots", S. Rabejac and P. Toudic, General review of the railways, 93rd edition, pp 232-237.
- ^ a b c d e f g h i j Manual for Design and Construction Monitoring of Soil Nail Walls (Report). Washington, D.C.: U.S. Federal Highway Administration (FHWA). October 1998. FHWA-SA-96-069.
- ^ a b c d e f g h i j k l m n o p q r s t u v w Geotechnical Engineering Circular No. 7: Soil Nail Walls (PDF) (Report). FHWA. 2003. FHWA-IF-03-017.