How Strong is the Ground? A Guide to Determining Soil Bearing Capacity
Before any structure can rise, its builders must first look down. The single most important question in foundation design is: "Can the ground support the load?"
The answer lies in determining the soil bearing capacity, a term that defines the maximum pressure a soil can withstand before it fails or settles excessively.
Failure to get this number right is not an option. An overestimated bearing capacity can lead to catastrophic structural failure, while an underestimated one leads to over-designed, unnecessarily expensive foundations. The geotechnical investigation is how we find the correct, safe value.
The process boils down to two key concepts:
Ultimate Bearing Capacity (q_u): This is the theoretical pressure at which the soil will fail through shearing—think of it as the ground punching through.
Allowable Bearing Capacity (q_a): This is the practical, safe number used for design. It is the ultimate capacity divided by a Factor of Safety (typically 2.5 to 3) to account for uncertainties and to limit settlement.
So, how do we find this critical value? We use a combination of field tests, lab tests, and proven engineering formulas.
Method 1: In-Situ (Field) Testing
Field tests are the most common and direct way to assess soil properties as they exist on-site.
Standard Penetration Test (SPT): This is the workhorse of geotechnical investigations. A "split-spoon" sampler is hammered into the soil at the bottom of a borehole. The number of blows required to drive the sampler a specific distance (one foot) is recorded as the "N-value." This N-value is a powerful indicator of soil density and strength. Engineers use established empirical formulas (like Meyerhof's) to directly correlate the corrected N-value to the allowable bearing capacity.
Cone Penetration Test (CPT): A more advanced method, the CPT involves pushing an instrumented cone into the soil at a constant rate. It continuously measures cone resistance (q_c) and sleeve friction (f_s). This provides a detailed profile of the soil layers and their properties. These (q_c) values can then be used in formulas to calculate the ultimate bearing capacity.
Plate Load Test: This is the most direct method. A large steel plate is placed on the soil at the proposed foundation depth, and a hydraulic jack is used to apply load in increments. The settlement is measured at each step. This test provides a real-world load-settlement curve, from which engineers can directly determine the ultimate bearing capacity of the soil at that specific location.
Method 2: Laboratory Tests & Analytical Formulas
Field tests are often supplemented by laboratory tests on soil samples (both disturbed from the SPT and undisturbed "Shelby" tubes).
Soil Classification: First, the soil is classified (e.g., clay, sand, silt) to determine its behavior.
Shear Strength Tests: Tests like the Direct Shear Test or Triaxial Shear Test are performed to find the soil's two critical strength parameters:
Cohesion (c): The "stickiness" of the soil, crucial in clays.
Angle of Internal Friction (\phi): The resistance to sliding between soil grains, crucial in sands.
The Terzaghi Equation: These (c) and (\phi) values are then plugged into the foundational formula of soil mechanics: Terzaghi's Bearing Capacity Equation.
This formula calculates the ultimate bearing capacity (q_u) by combining the soil's cohesion, the surcharge (weight of the soil above the foundation), and the weight of the soil itself.
The equation uses "bearing capacity factors" (N_c, N_q, N_{\gamma}), which are standard values derived from the soil's friction angle (\phi).
Ultimately, a geotechnical engineer never relies on a single test.
They synthesize all the data—SPT N-values, CPT readings, lab-tested strength parameters, and the position of the water table—to build a comprehensive model of the ground.
From this model, they determine a final, safe allowable bearing capacity that ensures the structure will stand for generations.