The tallest building in the world, Burj Khalifa, in Dubai, UAE, stands at 828 meters. The twisting Shanghai Tower in China reaches 632 meters. While the One World Trade Center in the USA stands at 541 meters. How do these towering structures remain stable and safe? The answer lies in the bearing capacity of soil testing.
The foundation is the main thing as every great structure relies on it. But the foundation is solid only if the soil is strong. Imagine building a house on soft sand vs solid rock. The house would not be safe. Without proper testing, even the most well-designed buildings can face consequences.
This is why it is important to do testing on the bearing capacity of soil. In this article, let’s delve deeper into the bearing capacity test and the methods.
Understanding Bearing Capacity of Soil Testing
The bearing capacity of soil is important in construction because when a weight is placed on it, the ground must have the capacity to support it without failure. Before initiating the project, if the test is not done, it will have negative consequences such as collapsing or damage.
Soil bearing capacity means the maximum pressure or load the soil can withstand without any issues. The process of the bearing capacity test assesses the ability of soil to support the loads applied to it. This is measured in pounds per square foot (psf) or kilopascals (kPa).
Accurate testing helps engineers make informed decisions when constructing foundations, roads, bridges, and other infrastructure projects.
Methods of testing the bearing capacity of soil and foundation
Plate load test
This is a method that directly tests the bearing capacity of soil. A rigid plate is placed on the ground, and a load is applied to observe the settlement of the plate. This measures the settlement of the plate and the bearing capacity. They apply to all kinds of soil, soft rock, and weathered rock bodies.
- Plate Material and Size: the plates are circular or square steel with at least 25 mm thickness. The size ranges from 300 mm to 750 mm. The smaller plates are used for dense or stiff soils and larger ones are used for loose or soft soils.
- Test Area: A pit is dug to a depth 5 times the width of the plate, and the plate is placed in the center of the pit at the proposed foundation depth.
- Water Table: If the water table is above the test level, water is pumped out before placing the plate.
- Applying Loads: A load of 70 g/cm² is applied first and then released. Additional loads are added in increments of one-fifth of the estimated safe load until the plate either fails or settles by 25 mm.
- Measuring Settlements: Settlements are measured at specific time intervals: 1, 2.25, 4, 6.25, 9, 16, and 25 minutes, and then once every hour. A load-settlement curve is plotted based on the data.
Since the plate load test is short-term, it only measures immediate settlement and cannot measure consolidation in clays. Therefore, it’s not recommended for testing in clayey soils.
Spiral plate load test
Instead of the typical plate, a spiral-shaped plate is used for more reliable results. It has a continuous surface that distributes the load applied uniformly across the soil beneath it. It’s especially used in varying soil including soft soil, general cohesive soil, pulverized soil, and sandy soil.
The procedure is similar to the regular plate load test and after the measurements, the load-settlement curve is plotted.
The spiral plate distributes the load more evenly across the soil, making the test useful in areas with uneven soil conditions. It allows for a more continuous and consistent testing process.
Standard penetration test
This is one of the most commonly used methods for testing the bearing capacity of soil. This drives a split-barrel sampler into the soil to measure the number of blows needed to reach a specific depth. It is based on the principle that the denser the soil, the more difficult it is to drive the sampler into it. It is suitable for general cohesive soils, pulverized soils, and sandy soils.
- A split spoon sampler is driven into the soil using a 65 kg weight that falls from a height of 75 cm.
- The borehole is drilled to the desired depth to determine the N-value (N-value is the number of blows per foot of penetration).
- The sampler penetrates 150 mm first, but the number of blows for this is not recorded. Then, the sampler is allowed to penetrate another 300 mm, and the number of blows to reach this depth is recorded as the SPT N-value.
- The next test is conducted 750 mm below the previous one and for deeper boreholes, tests are spaced 2 to 2.5 meters apart or at changes in soil layers.
Static force touching
This is a cost-effective test that can test tension as well as compression. This measures the load and movement at the top of the pile to find the worst-case stresses it will experience when supporting the structure.
The load is applied either by pressing against a grillage and Kentledge system (Kentledge Test) or a reaction beam secured by anchorage piles (Reaction Test).
Load and movement data are recorded using load cells and displacement gauges, with results showing load vs. movement and load & movement vs. time. Piles can also be tested in tension and laterally.
- The tests can run through the night and provide quick results.
- The test is controlled and monitored remotely.
- Removes any potential human error.
This test can be done in soft soil, cohesive soil, pulverized soil, sandy soil, and soil layers containing a small amount of gravel.
Rock body straight shear test
This measures the shear strength of rock materials that are hard to assess the stability of the foundations and slopes. It’s suitable for measuring rock bodies with soft and weak structural surfaces and soft rock.
The test measures the shear strength by recording the amount of force required to cause the rock to fail or slide along the shear plane. This is done by gradually increasing the applied load while measuring displacement or deformation until failure occurs. The results provide valuable data on the rock’s resistance to shear stress, which is essential for evaluating its suitability for construction projects such as tunnels, dams, or foundations.
The Rock Body Straight Shear Test helps engineers determine the rock’s internal friction, cohesion, and overall shear strength, which are vital factors in the design and safety of structures built on or within rock formations.
Pre-Drilled Side Pressure Test
This test measures cohesive soil, silt, loess, sandy soil, soft rock, and weathered rock. It evaluates the shear strength and pressure resistance of soil or rock.
- A hole is drilled into the ground to a specific depth.
- A pressure is applied using a pressure chamber or specialized equipment that exerts force.
This test measures how much pressure the surrounding material can withstand before failure. It is especially useful for determining the characteristics of weak or loose soil layers, and it can provide insights into the material’s capacity to support foundations or structures.
By analyzing, engineers can calculate critical parameters such as the soil’s cohesion and friction angle, which are essential for ensuring the stability of the planned structure.
Cross-plate shear test
The Cross-Plate Shear Test is a laboratory method used to measure the shear strength of soil, particularly for granular soils. This applies a shear force to the soil sample between two plates.
The test measures the amount of force required to cause failure or shear along this plane, and the data collected can be used to determine the soil’s shear strength parameters, such as cohesion and the angle of internal friction. These parameters are important for understanding the soil’s resistance to sliding or shifting under loads, which is critical in foundation design, slope stability, and geotechnical engineering.
The Cross-Plate Shear Test is often used when other testing methods (like direct shear tests) may not be feasible or when a more controlled environment is required for testing granular soils under specific conditions. Undrained shear strength and sensitivity of saturated soft cohesive soils can be measured with this soil-bearing capacity test.
Conclusion
The bearing capacity of soil is important to design the foundation. How much the soil is capable of supporting the load applied by the structure? There are various testing methods like Plate Load Test, Standard Penetration Test, and Shear Test to measure the soil bearing capacity. But each of them should be done depending on the type of soil and other requirements.
Engineers can design foundations for safety with the bearing capacity of soil. However, it is important to choose the most appropriate testing method for accurate results.
Visit Langry and browse through the site to find the necessary tools.
