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.
This foundation settlement calculation is actually like a "common language" across various engineering fields. The most directly related field is "Soil Mechanics" itself. In particular, consolidation theory deals with the dissipation of pore water pressure, directly linking it to "Groundwater Engineering" and "Seepage and Permeability Analysis." For example, this calculation is essential for planning "preloading methods," which use drainage works to accelerate consolidation.
Another key area is collaboration with "Structural Mechanics." When a foundation settles, secondary stresses (called "settlement-induced secondary stresses") develop in the steel or reinforced concrete members above it. Conversely, a stiff superstructure can also help suppress differential settlement. Therefore, in the design of buildings and bridges, structural designers and geotechnical engineers constantly exchange information about this settlement amount.
Looking further, it also connects to the world of "Numerical Analysis (FEM)." The Terzaghi formula used in this tool is somewhat empirical, but using FEM allows you to simulate more complex soil profiles and nonlinear soil behavior. Getting a feel for how changing parameters in this tool affects the results will help you develop intuition for interpreting FEM results.
If this tool's calculations pique your interest and you want to learn more, here are some next steps you can take. First, try to "understand the derivation background of the formulas." Terzaghi's bearing capacity formula is derived by assuming shear failure in the soil. Studying "Rankine's earth pressure theory" and "Mohr's stress circle" in textbooks will show you why the bearing capacity factors $N_c, N_q, N_\gamma$ are functions of the internal friction angle φ, transforming the formula from rote memorization into genuine understanding.
Next, explore the "mathematical background of consolidation theory." The differential equation for one-dimensional consolidation, which draws the settlement curve, has exactly the same form as the heat conduction equation. $$ \frac{\partial u}{\partial t} = c_v \frac{\partial^2 u}{\partial z^2} $$ Here, $u$ is the excess pore water pressure and $c_v$ is the coefficient of consolidation. This shows that even when physical phenomena differ, if the governing equations are the same, you can reuse solution methods and concepts. This is a very powerful perspective in engineering.
The final step is to "research other bearing capacity theories and settlement evaluation methods." Following Terzaghi, formulas considering more general conditions, such as those by Meyerhof and Hansen, have been proposed. Also, settlement is evaluated separately as immediate settlement, consolidation settlement, and secondary compression (creep) settlement. For your next topics, learning about differential settlement evaluation and the principles of soft ground improvement techniques (e.g., sand drains, deep mixing method) will help you grasp the entire workflow from design to construction.