Input foundation width, embedment depth, applied load, N-value, and soil type to compute Terzaghi bearing capacity, safety factor, and consolidation settlement. Visualize the soil profile with stress bulb.
The core of the analysis is Terzaghi's Bearing Capacity Formula, which calculates the ultimate pressure the soil can withstand before shear failure.
$$q_u = cN_c + qN_q + 0.5 \gamma B N_{\gamma}$$Where:
$q_u$ = Ultimate bearing capacity (kPa)
$c$ = Soil cohesion (kPa)
$q$ = Effective overburden pressure at foundation base = $\gamma D_f$ (kPa)
$\gamma$ = Soil unit weight (kN/m³)
$B$ = Foundation width (m)
$N_c, N_q, N_{\gamma}$ = Bearing capacity factors, dependent on soil friction angle $\phi$
The settlement is calculated using the consolidation theory for clays or elastic methods for sands. A common form for primary consolidation settlement is:
$$S_c = \frac{C_c}{1 + e_0}H \log_{10}\left(\frac{\sigma'_0 + \Delta \sigma}{\sigma'_0}\right)$$Where:
$S_c$ = Consolidation settlement (m)
$C_c$ = Compression index (from soil tests)
$e_0$ = Initial void ratio of the soil
$H$ = Thickness of the compressible soil layer (m)
$\sigma'_0$ = Initial effective stress in the soil layer (kPa)
$\Delta \sigma$ = Stress increase due to the foundation load (kPa)
Residential Building Design: Before constructing a house, engineers use this exact calculation to size the concrete footings. For instance, on soft clay, they might recommend wider footings or a deep foundation system to limit settlement and prevent cracked drywall and jammed doors.
Industrial Storage Tanks: Large tanks for oil or water exert massive loads on the ground. Settlement analysis ensures the tank settles uniformly. Differential settlement could cause a rupture, so engineers often use a ring beam foundation designed based on these principles.
Bridge Abutment Design: The supports (abutments) at the ends of a bridge transfer huge loads from the structure into the soil. Calculating bearing capacity and settlement is critical to prevent the bridge approach from sinking relative to the deck, creating a dangerous "bump."
Wind Turbine Foundations: A modern wind turbine mast presents a massive, tall structure with high overturning moments. The foundation must have ample bearing capacity and minimal tilt. These calculations are the first step in designing the large reinforced concrete mats or piles used.
When you start using this tool, there are a few points you should be careful about. First, there's the common misunderstanding that "a larger factor of safety is always better." While it certainly increases safety, it's a trade-off with economy. For instance, setting the factor of safety to 5.0 or 10.0 leads to designing an unnecessarily large foundation, causing costs to skyrocket. In practice, considering the accuracy of soil investigations and the importance of the structure, you should aim for an "appropriate" range, typically between 2.5 and 3.0.
Next, pay close attention to the "units" for parameter input. This is crucial! The tool uses [kN/m²] and [kN/m³], but field data often comes in [tf] or [g/cm³]. For example, if you mistakenly input the unit weight γ as 1.8 [tf/m³] (the correct value is 18 [kN/m³]), your calculation result will be off by a factor of 1/10, leading to a major error. Always double-check unit conversions before inputting values.
Finally, remember that this calculation assumes a "homogeneous soil" and "central loading." Real-world sites are more complex. When the soil is layered or the foundation is subjected to eccentric loading (e.g., placing machinery at the edge of a building), the formulas become much more complicated. Treat this tool's results as a "first approximation," and keep in mind that for complex conditions, specialized software or detailed analysis is necessary.
For a shallow square foundation on clay: B=1.5m, Df=1.0m, φ=28°, c=35kPa, γ=18kN/m³, applied q=150kPa. Terzaghi analysis yields N_c=30.14, N_q=17.69, N_γ=15.70, giving q_u=892kPa and safety factor F_s=5.95. Immediate settlement S_i=4.2mm from elastic theory (E=15MPa, μ=0.35). Primary consolidation S_c=18.7mm over 2-year period (C_c=0.28, e₀=0.82, Δσ'=145kPa). Total settlement approximately 23mm.