Unfavorable or Difficult Soil Conditions for Soil NailingArticles > Unfavorable or Difficult Soil Conditions for Soil Nailing
Examples of unfavorable soil types and ground conditions are provided below:
- Dry, poorly graded cohesionless soils. When poorly graded cohesionless soils are completely dry, contain no fines, or do not exhibit any natural cementation, apparent cohesion is not available. Therefore, the required vertical or nearly vertical cuts are difficult to achieve.
- Soils with high groundwater. Perched groundwater occurring behind the proposed soil nail wall will require significant drainage, which is necessary to stabilize the mass of soil in this location. Additionally, large amounts of groundwater can cause drill holes (particularly in loose granular soils) to collapse easily, thus requiring a costly soil nail installation. Excessive groundwater seeping out to the excavation face may cause significant difficulties for shotcrete application.
- Soils with cobbles and boulders. A large proportion of cobbles and boulders present in the soil may cause excessive difficulties for drilling and may lead to significant construction costs and delays. When only a few boulder sand cobbles are present, modifying the drilling orientation from place to place may minimize or eliminate most of the difficult drilling. However, this approach has practical limitations when too many boulders are present.
- Soft to very soft fine-grained soils. These soils typically have SPT N-values less than 4 and are unfavorable for soil nailing because they develop relatively low bond strengths at the nail-grout-soil interface, thereby requiring unreasonably long nail lengths to develop the required resistance. Long-term deformations (creep) of the soils may be a concern for highly plastic clays. Concerns for creep deformations are generally less critical for temporary applications. As with any retaining system constructed in a top-down manner, the potential for instability at the bottom of the excavation is high in soft fine-grained soils. Additionally, high-plasticity soils may be expansive and may induce additional localized pressure on the facing due to swelling.
- Organic soils. Some organic soils such as organic silts, organic clays and peat typically exhibit very low shear strengths and thereby low bond strengths, which causes uneconomical nail lengths. While some organic soils can exhibit acceptable shear strengths, other organic soils like fibrous peat may be highly heterogeneous and highly anisotropic. In this case, while the soil shear strength can be reasonable along some orientations, it may be significantly low along other orientations. These unfavorable orientations may have a detrimental impact on the wall stability and very long soil nails will be required. In addition, organic soils tend to be more corrosive than inorganic soils.
- Highly corrosive soil (cinder, slag) or groundwater. These conditions may lead to the need of providing expensive corrosion protection. These conditions are obviously more disadvantageous for permanent applications of soil nail walls.
- Weathered rock with unfavorable weakness planes and karst. Weathered rock with prevalent unfavorable weakness planes such as joints, fractures, shears, faults, bedding, schistosity, or cleavage may affect the drill hole stability and make grouting difficult. In addition, the presence of these discontinuities may cause the formation of potentially unstable blocks in the retained mass behind the wall during excavation. The marginal stability of blocks may rapidly deteriorate due to various factors, such as gouge in the joints, uplift and lateral hydrostatic pressures, and seepage forces. The stabilization of individual blocks may be necessary and can make this solution uneconomical when compared to conventional soil nails. In addition, grouting in rock with very large open joints or voids will be very difficult and/or expensive due to excessive grout loss. Grouting in karstic formations is not appropriate.
- Loess. When it is dry, loess may exhibit acceptable strengths that would allow economical installation of soil nails. However, when sizable amounts of water ingress behind the proposed soil nail wall, the structure of the loess may collapse and a significant loss of soil strength may take place. Therefore, the collapse potential upon wetting of these soils must be evaluated. Appropriate measures to avoid excess water migration to the soil nail area must be provided in loess exhibiting significant collapse potential. Additionally, considerably low soil shear strengths may arise for the wetted condition. In these cases, unusually long soil nail lengths may result in using conventional methods of nail installation. Regrouting (an atypical and more costly step) has been used to increase bond strengths in loess.
In addition to the difficulties described above, other aspects related to soil conditions must be considered when assessing the feasibility of soil nail walls:
- The prolonged exposure to ambient freezing temperatures may cause frost action in saturated, granular soils and silt; as a result, increased pressures will be applied to the temporary and permanent facings.
- Repeated freeze-and-thaw cycles taking place in the soil retained by the soil nail wall may reduce the bond strength at the soil nail grout-ground interface and the adhesion between the shotcrete and the soil. To minimize these detrimental effects, a suitable protection against frost penetration and an appropriate shotcrete mix must be provided.
- Granular soils that are very loose (N ≤4) and loose (4 < N ≤10) may undergo excessive settlement due to vibrations caused by construction.
- Loose and very loose saturated granular soil can be susceptible to liquefaction in seismically exposed regions. Several ground modification techniques (typically with significant associated costs) may be utilized to densify granular soils and thereby minimize these damaging effects. equipment and traffic.
Despite the difficulties associated with unfavorable soil conditions described above, soil nail walls may still be built. It should be recognized that these wall systems would typically be more expensive to design and construct when compared to conventional walls in a more suitable soil. It is likely that significant extra effort and cost is needed in the design and construction of soil nail walls in these marginal conditions and that more strict long-term performance requirements might be necessary to allow soil nailing in such challenging conditions.
Share: Follow our official Facebook page (@civilengineeringbible) and Twitter page (@CivilEngBible) and do not miss the best civil engineering tools and articles!