Slope and substrate

SRS NX models the slope as an overburden (the potentially unstable surface layer) resting on a substrate (soil or rock) into which the nails are anchored. This page explains the parameters of the Slope and Substrate sections and how SRS uses them to compute the pre-intervention safety factor FS₀.

Geometric scheme

The scheme below summarises the input geometry: slope inclined at α, surface layer of thickness S (measured perpendicular to the slope), water table within the layer (S′ = m·S), anchor dipping β below the horizontal with total length L_a = L_nc + L_b, where L_nc = S / sin(α+β) is the non-collaborating length in the surface layer and L_b the bonded (bulb) length in the substrate.

Typical section: surface layer, substrate, water table and anchor.

Slope (overburden)

Symbol	Parameter	Unit
I.1 α	Slope inclination	°
I.2 S	Overburden thickness	m
I.3 γ_col	Unit weight	kN/m³
I.4 φ_col	Friction angle	°
I.5 c'_col	Drained cohesion	kPa
I.6 m	Dimensionless seepage-flow thickness	—

m represents the position of the water table within the overburden: m = 0 means no water table, m = 1 water table at ground level, intermediate values an intermediate depth. It affects the uplift force and reduces the available shear resistance.

Substrate

Choose Soil or Rock with the two radio buttons: the card shows the fields relevant to your choice.

Soil

Symbol	Parameter	Unit
I.8 ad_soil	Mortar-soil bond stress	MPa
I.9 α_iniez	Injection-mode coefficient	—

The dropdown next to ad_soil suggests typical values by lithology (basalt, limestone, sandstone, dolomite, schist, weathered schist, gypsum, slate, or loose soils such as clays, sands and gravels), to be used as a reference and adjusted based on site data. The α_iniez coefficient distinguishes repeated injection (higher values) from simple injection, for the same range of loose lithologies.

Rock

Symbol	Parameter	Unit
I.10 ad_rock	Mortar-rock bond stress	MPa

Same lithology-preset principle as the soil bond stress.

Indicative values, not a substitute for site investigation

The lithology presets are typical reference values from technical literature. The mortar-substrate bond stress used in design must always be confirmed by pull-out tests on site or by the mortar/anchor manufacturer's data.

How SRS computes FS₀

Pressing Calculate FS₀ (card Design parameters, step 1), SRS determines the equilibrium of the overburden wedge tributary to a single anchor, of plan area equal to the grid spacing i_x × i_y:

1. Acting volume V = i_x · i_y · S
2. Weight W = V · γ_col
3. Uplift force U = 10 · m · V · cos(α)
4. Horizontal seismic force F_h = W · K_h (0 if K_h = 0)
5. Vertical seismic force F_v = F_h / 2
6. Resisting shear force — from cohesion and friction on the sliding surface, net of the seismic components
7. Acting shear force — the component of weight and seismic forces along the slope
8. FS₀ = resisting force / acting force

If FS₀ comes out negative, the slope is already unstable even before any intervention: check the values you entered.

Update FS₀ after every slope change

FS₀ is computed once and stays stored until you recalculate it. If you change the overburden's inclination, thickness or geotechnical parameters after computing it, press Calculate FS₀ again before running the full check.

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