Comparative Evaluation of Shallow and Deep Foundation Design in Variable Cohesionless-Cohesive Soil Strata: A Structural-Geotechnical Perspective

Every building, from a modest home to a towering skyscraper, relies on a hidden partner to stand upright: the foundation. This subterranean structure acts as a critical bridge, transferring the immense weight of the walls, floors, and roof down into the earth below. However, the ground is not a uniform, solid block; it is a complex, living medium of soil and rock that behaves differently under pressure. Some soils are dense and gritty, like dry sand, offering firm resistance. Others are soft and sticky, like wet clay, which can squish and settle over time. If a builder places a heavy structure on soft ground without adjusting the design, the soil may compress unevenly. This uneven sinking, known as differential settlement, can cause walls to crack, doors to jam, and in severe cases, lead to structural collapse. The central challenge for engineers is to match the type of foundation to the specific nature of the soil, ensuring the building remains safe and level for decades.

In a recent study, researchers from the Structural and Geotechnical Engineering Research Group at D Y Patil Technical Campus in Talsande, India, tackled this exact problem. They set out to compare how different foundation designs perform when subjected to the same heavy load across three very different types of soil. The team focused on a standard five-story building frame, applying a consistent downward force of 1200 kilonewtons to see how various foundation strategies would hold up. They examined four distinct approaches: the isolated pad footing, which is a single concrete block under each column; the combined footing, which links two columns; the raft or mat foundation, a large continuous slab supporting the whole building; and the deep pile foundation, where long concrete columns are drilled deep into the ground to bypass weak layers. Their goal was to determine which method offered the best balance of safety, cost, and stability in soils ranging from dense sand to soft, compressible marine clay.

The study began by modeling the behavior of three specific soil layers. The first was a dense, cohesionless sand, representing strong ground where the soil particles lock together tightly. The second was a medium-stiff soil that possessed both some stickiness and some friction, a common mix in many construction sites. The third was a highly problematic layer of soft, saturated marine clay, a material known for being weak and prone to significant sinking under load. Using established theories of soil mechanics, the researchers calculated how much each foundation type would sink and whether the soil could support the weight without failing. They looked at two main criteria: whether the soil would shear or break under the pressure, and whether the building would settle too much or unevenly, which could damage the structure above.

The results revealed a clear and dramatic divide based on soil type. In the dense sand, the simplest solution proved to be the most effective. An isolated pad footing, a single concrete block measuring 1.8 meters by 1.8 meters, easily supported the load with a safe capacity of 385 kilopascals. The building settled by only 11.4 millimeters, a tiny amount that poses no risk to the structure. This confirmed that for strong, granular soils, individual footings are highly efficient and economical. However, as the soil quality degraded, the performance of these simple footings deteriorated rapidly. In the medium-stiff soil, the required footing size had to increase to 2.7 meters by 2.7 meters to prevent failure, and the settlement rose to 28.6 millimeters.

The situation became critical in the soft marine clay. Here, the researchers found that an isolated pad footing would need to be enormous, covering 4.8 meters by 4.8 meters, to even approach a safe load limit. Even with this massive size, the predicted settlement was a dangerous 118.5 millimeters. The study highlighted a crucial geometric problem: when individual footings become this large, their zones of influence overlap significantly. This overlap creates a cumulative pressure deep underground that accelerates uneven sinking and can induce secondary bending moments in the building's columns and beams, threatening the integrity of the entire frame. The researchers noted that when the total area of isolated footings exceeds half the building's footprint, the design is no longer viable.

To solve the problem in soft clay, the team tested alternative strategies. They found that switching to a raft foundation, a large slab covering the entire building footprint, helped by spreading the load more evenly. This reduced the settlement to 64.2 millimeters, a significant improvement, but the pressure on the soil remained high, and the settlement was still substantial. The most effective solution for the soft clay was the deep pile foundation. By driving bored cast-in-situ concrete piles 14 meters deep, the structure bypassed the soft marine clay layer via shaft adhesion and developed high end-bearing resistance in the underlying dense gravelly sand. This approach reduced the total settlement to just 8.5 millimeters, a reduction of 86.7 percent compared to the isolated footing. This level of stability brought the settlement well within the safe limits required to prevent structural damage.

The researchers concluded that bearing capacity alone is not enough to design a safe foundation; the ability of the soil to settle without damaging the building is often the deciding factor. In strong soils, simple, isolated footings are the best choice. In intermediate soils, combined footings or stepped designs may be necessary. But in weak, soft clays, the study explicitly ruled out isolated footings for anything beyond very low-rise structures. Instead, the authors recommend that medium-to-high-rise buildings in such conditions must utilize deep pile systems or a combination of piles and rafts. The findings provide a clear decision framework for engineers: as soil strength decreases, the foundation must transition from simple, shallow blocks to deep, interconnected systems that reach competent ground, ensuring that the building above remains level and secure.