Raft Foundation Design

Use raft foundation for very low bearing soil, like soft clay, to prevent excessive and differential settlement; with basements or if isolated footings exceed 70% of the area, choose raft.

Learn when to use isolated footing instead of raft foundation, comparing bulb areas, soft soil layers, and expansive soil effects to prevent excessive settlement and uplift.

Explore three raft foundation types: flat slab raft, a beam-area with field-area configuration for uneven loads, and the siller type for very weak soils and heavy loads.

Explore how lab-derived net bearing capacity differs from site gross allowable bearing pressure, and how overburden pressure and borehole data from geotechnical reports govern foundation design.

Calculate column loads for an eight-floor building with a basement, then compare gross versus net bearing capacity to decide if direct soil support suffices or piles are needed.

Explore how asymmetrical load distributions affect the center of load on raft foundations, showing that bearing capacity and stresses require life load variations and induced moments beyond simple calculations.

Analyze raft foundation stresses under eccentric loads, determine the center of load, eccentricity, and bending moments using a cross bearing capacity of 125 kPa.

Identify the moment direction and locate maximum compressive stresses on raft foundations by analyzing the normal force and X and Y directions.

Explore how small eccentricity in raft foundation loading can markedly raise total stresses, including moments and shear, with a fourteen percent increase, emphasizing not ignoring load distribution in design.

Illustrate the conventional rigid method by dividing the earth into strips and calculating stresses and restraining actions on each strip to achieve equilibrium.

Identify the three assumptions of the conventional rigid method—infinite rigidity, uniform stress below the raft, and load center aligning with the soil pressure centroid.

Explore the approximate finite element method for raft foundation analysis, dividing soil into elements modeled as elastic springs. Understand how the coefficient of subgrade reaction governs stiffness and settlement.

Examine how foundation geometry and depth affect subgrade reaction for raft foundations, showing how length-to-width ratio and deeper levels modify the coefficient and its estimation from field tests.

Explore the plate load test, a field method to determine soil bearing capacity by loading a steel plate, recording settlement, and calculating ultimate capacity for use in approximate flexible models.

Calculate the coefficient of subgrade reaction for different soil types, using plate load test data, and select K values from soil tables to adjust for square and rectangular foundations.

Calculate the coefficient of subgrade reaction for raft foundations using the approximate flexible method, considering soil type and footing dimensions, and compare manual and finite element approaches.

Learn how to design construction joints in raft foundations by matching joint quantity to daily pour capacity and placing joints at the middle third or zero-shear zones to minimize shear.

Learn to evaluate differential settlement of mats by calculating raft inertia from foundation, framing, and shear walls per unit area, using the rigidity factor kr to assess risk.

Wrap up the raft foundation design course by reviewing fundamentals and analysis methods for raft foundations, including manual and finite element approaches, with real examples and common design mistakes.