Stabilising Steep Slopes with Soil Nails
- deepexcavation
- Jul 14
- 4 min read
Revisiting a TRL Case Study
In infrastructure schemes where land take is restricted, steepened slopes are often necessary to optimise available space. A classic example of this is found in the TRL case study reported by Johnson et al. (2002), where two cuttings were widened by forming 60° slopes and stabilised using soil nails. One of these cases is revisited using the software SnailPLUS to better understand the implications of reinforcement design and the effect of a toe excavation on global stability. Figure 1 (adapted from Johnson et al., 2002) illustrates the slope geometry, stratigraphy, and nail layout.
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Figure 1 – Scheme of the geometry and stratigraphy of the site, adapted from Johnson et al. (2002). |
Case Summary and Original Design Approach
In the original scheme, the steepened slopes ranged from 0.5 m to 1.5 m in height and were reinforced with up to two rows of nails (20 mm steel bar, 13 m and 8 m length), typically installed at horizontal spacings of 1.8 m (Site 1) and 1.5 m (Site 2), with a vertical spacing of 1.0 m. The nails were fixed to a steel cage, which was filled with topsoil and vegetated with a grass mix to achieve a natural finish. The ground profile comprised glacial deposits overlying the Reading Beds, which in turn overlie the Upper Chalk.
An important aspect of their methodology was the use of pull-out tests to confirm the nail design assumptions — a critical step to ensure field performance matches analytical expectations.
Present Study Reappraisal and Modelling Approach
The case is revisited by modelling the slope using SnailPLUS, employing the same stratigraphy and soil parameters from the original report. However, opting for a more conventional shotcrete facing in the design to reflect current practice in certain transport infrastructure projects. The nails are installed with a vertical spacing of 1 m and horizontal spacing of 1.8 m (according to site 1), bar diameter of 20 mm, the top bar has a 13 length and the bottom, 8 m.
The stability assessment is performed using the Morgenstern-Price method, a rigorous limit equilibrium approach capable of accounting for both moment and force equilibrium across non-circular failure surfaces, with automatic search.
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Figure 2 – Representation of the model geometry and stratigraphy in SnailPLUS. |
The analysis produced a factor of safety (FS) = 1.507, practically equal to the minimum recommended value of 1.5 for serviceability conditions. The presence of reinforcement clearly contributes to the stability, yet the toe excavation induces a deeper critical slip surface—a reminder that even in reinforced systems, geometry changes at the toe can trigger unfavourable failure mechanisms. Figure 3 illustrates the critical failure mechanism obtained in the model, for the most critical stage.
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Figure 3 – Result of the stability analysis, failure mechanism (stage 2 of 4, SnailPLUS). |
The nails proposed in the original study demonstrated good performance considering the site's geotechnical conditions. Figure 4 presents the results of the critical stability check, with a summary provided in Table 1 for verification purposes. The original authors also reported pull-out test results, which serve as a benchmark for comparison with the axial forces obtained in the numerical analysis. This allows for a direct assessment of how well the modelled nail loads align with the measured field performance.
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Figure 4 – Representation of the structural check of the nails (stage 3 of 4, SnailPLUS). |
Table 1 – Soil nail critical check for FS= 1.864 (last stage, SnailPLUS).
Nail/Units | F (kN) | Fmax (kN) | CAP STR (kN) | CAP GEO (kN) | TC1 STR (kN) |
0: N0 | 133.9 | 203.8 | 526.6 | 211.3 | 263.3 |
1: N1 | 81.2 | 99.8 | 526.6 | 103.1 | 263.3 |
F = Soil nail axial tension force for critical failure surface (may not be the greatest)
Fmax = Maximum soil nail tension from all analysed critical failure surfaces
CAP STR = Tensile structural design capacity for soil nail
CAP GEO = Tensile geotechnical pull-out resistance for soil nail
TC1 = Structural soil nail shear resistance
Maximum pull-out capacity
The original study reports a measured pull-out force of Pmes = 200 kN for a test conducted on a 13-metre-long soil nail. The soil nail adhesion (qs) was calculated as a function of the friction and the effective overburden pressure acting over the effective length of the nail. In the present study a bond resistance was assumed, according to each soil type (glacial deposit qs = 91.7 kPa, clay 1 qs = 22.0 kPa, and clay 2 qs = 137.9 kPa). In comparison, the present study generates a slightly higher maximum pull-out force of Pmax = 211.3 kN, indicating close agreement between the two results. The ratio Pmes/Pmax = 0.95 corresponds to a 95% match between the experimental and analytical values. This strong correlation supports the reliability and validity of the methodology employed in the present study.
Conclusions
This case reaffirms the importance of integrating robust slope reinforcement with careful attention to construction sequencing, particularly when geometry is modified prior to the installation of reinforcement. Using soil nails remains a viable and effective strategy for reshaped slopes, on the condition that the design is supported by field testing and detailed analysis.
The study here also demonstrates SnailPLUS’ capabilities as a reliable tool for assessing the stability of slopes stabilised by nails. Although the present approach is slightly differed from the original—particularly in terms of the facing system—the stabilisation solution remained as faithful as possible to the source case. It reaffirms the effectiveness of soil nailing in the stabilisation of steep cut slopes under constrained site conditions.
Reference:Johnson, M., Card, G. B., & Dixon, N. (2002). Soil Nails for Slopes. TRL Report TRL537, Transport Research Laboratory, UK.
Morgenstern, N. R., & Price, V. E. (1965). The Analysis of the Stability of General Slip Surfaces.
