What is geotechnical engineering and why is it important in the UK?

Geotechnical engineering assesses ground conditions to design safe foundations, slopes, and retaining structures. In the UK, variable geology and historical land use make thorough site investigation critical to avoid costly failures and ensure compliance with Eurocode 7.

What standards govern geotechnical work in the United Kingdom?

Geotechnical design in the UK follows Eurocode 7 (BS EN 1997) and its National Annex. Ground investigations comply with BS 5930, and SPT testing adheres to ASTM D1586. These standards ensure consistent, reliable results across the country.

How does your firm approach slope stability analysis?

Our firm uses limit equilibrium and finite element methods to evaluate slope stability, considering soil shear strength, groundwater, and surcharge loads. We apply partial factors per Eurocode 7 to deliver safe, economical designs for natural and engineered slopes.

Infinite Slope FS Calculator — Extensive Slopes

The infinite slope model assumes that the failure surface is a continuous plane parallel to the slope, with a constant thickness of the sliding stratum. This simplification works very well for long slopes with a uniform surface mantle over rock or an underlying rigid stratum, typical of regional colluvial deposits. This calculator provides the factor of safety FS in four scenarios: dry granular soil, granular soil with a water table, dry cohesive soil, and cohesive soil with pseudo-static seismic action. Reference for slope evaluation, linear excavations, and colluvium in mountain ranges.

What is an infinite slope and when to apply it?

It applies when the slope length is much greater than the thickness of the unstable stratum (ratio > 10:1) and the failure surface is planar, parallel to the ground. This is the case for shallow landslides in natural slopes of residual soil or colluvium over weathered rock, mantle-type failures in native forests with organic soil, planar slides over weak stratigraphic contacts, and rock slopes with a parallel joint. It does not apply to circular failures in homogeneous fills (use Bishop), nor to 3D rock wedges (stereographic kinematic analysis).

Applied Formulas

Dry granular soil (c = 0):

FS = tan φ / tan β (β = slope inclination)

Granular soil with water table parallel to slope (depth dw from surface):

FS = (γ − γw·(H−dw)/H) · tan φ / (γ · tan β)

Special case water table at surface (dw = 0): FS = γ'·tan φ / (γsat · tan β) ≈ 0,5·tan φ/tan β

Dry cohesive-frictional soil (c > 0, φ > 0):

FS = c / (γ·H·sin β·cos β) + tan φ / tan β

Pseudo-static with seismic kh:

FS = [c + (γ·H·cos²β − u) · tan φ] / [γ·H·(sin β·cos β + kh·cos²β)]

Acceptance criterion (BS EN 1997-1): FS ≥ 1,5 static; FS ≥ 1,1-1,2 seismic

Calculation example

Input data — colluvium over rock, mountain range area
Parameter	Value
Mantle thickness H	3,0 m
Inclination β	30°
Unit weight γ	19 kN/m³
Saturated unit weight γsat	20,5 kN/m³
Cohesion c	8 kPa (residual soil)
Friction angle φ	30°
Water table	Not detected
kh (Zone 3)	0,24

Case 1 — Dry static: FS = c/(γ·H·sin β·cos β) + tan φ/tan β = 8/(19·3·0,5·0,866) + 0,577/0,577 = 8/24,68 + 1,00 = 0,324 + 1,00 = 1,32. Static FS = 1,32 < 1,5 DOES NOT COMPLY. Case 2 — With heavy rain (water table at surface), without seismic: FS ≈ (γ−γw)/γsat · tan φ/tan β + contribution of c = 10,69/20,5 · 1 + 0,32/1,5 = 0,52 + 0,21 = 0,73. FS < 1 → slope would slide if saturated. Case 3 — Pseudo-static Zone 3, kh = 0,24, dry: numerator = 8 + 19·3·0,75·0,577 = 8 + 24,66 = 32,66; denominator = 19·3·(0,433 + 0,24·0,75) = 19·3·0,613 = 34,94. FS = 32,66/34,94 = 0,94. Seismic FS = 0,94 < 1,1 DOES NOT COMPLY.

Result: Static FS = 1,32 (insufficient) · Saturated FS = 0,73 · Seismic FS = 0,94. Unstable slope.

Interpretation of results

The analysed slope is unstable under marginal static conditions and clearly unstable under heavy rain or seismic action. Solutions: slope flattening (reduce β to 25°), removal of the colluvial mantle and replacement with controlled fill, or stepped retaining walls with back drainage. In central mountain ranges, failures of this type are frequent after winter rains. Evaluation with the infinite slope model is the first filter; if FS < 1,5, further analysis is carried out with Bishop or Janbu.

Reference standards

Duncan, J.M. & Wright, S.G. (2005). Soil Strength and Slope Stability
BS EN 1997-1 (Eurocode 7) — Geotechnical design — Part 1: General rules
BS 8006-2
Skempton, A.W. & DeLory, F.A. (1957). Stability of natural slopes in London Clay
BS EN 1997-1 (Eurocode 7) — Limit states for overall stability

Frequently asked questions

When do I use infinite slope and when Bishop?

Infinite: mantle parallel to the slope with length > 10·thickness, planar shallow failure. Bishop: circular surface, homogeneous fills, excavation cuts, earth dams. If the geometry is not clearly parallel, Bishop or Spencer are more accurate.

How do I consider the water table on a slope?

The most common approach is to assume a water table parallel to the slope at depth dw. If dw = H/2 the FS reduction is ~40 % compared to the dry case. In central mountain ranges, slopes with colluvium > 2 m and intense precipitation reach surface saturation — always evaluate this scenario.

What minimum FS does the BSI require?

BS EN 1997-1: static 1,5 (permanent), 1,3 (temporary); seismic 1,1-1,2. The BS 8006-2 requires 1,5 static and 1,1 seismic for permanent slopes in road works. For critical urban slopes (dense neighbourhood below) this is raised to 1,8 static due to risk.

What if the stratum is colluvium over rock?

Classic infinite slope with c and φ of the colluvium. The rock is the failure surface. The apparent cohesion of the colluvium depends strongly on roots: tree vegetation increases c up to 10-15 kPa; clear-cutting eliminates it. Projects that clear forest on slopes often trigger landslides months later.