Knowing the load bearing capacity of the soil is one of the crucial steps in building a structure. Although it seems simple, it has great importance in the construction industry. To understand how it plays an important role, we first need to understand the soil’s load bearing capacity. In this blog, you will be able to understand the depth of this concept.
The soil’s load bearing capacity is the capacity to support the load applied to it. In simple terms, the amount of weight soil can bear is called the load-bearing capacity of the soil. It is the maximum average contact pressure between the soil and the foundation. It determines the capability of the soil to hold the pressure caused when the foundation is laid. The load bearing capacity is further classified into 2 categories:
Allowable bearing capacity = ultimate bearing capacity/safety factor
It states the soil’s acceptability to handle the weight without failure and with applied safety measures.
In earlier times, builders constructed huge structures using their instincts and experience. The science of understanding soil behaviour and capacity was unknown. They understood that every soil has a different capacity to bear the load. For instance, they realised that dry sand cannot support heavy structures. On the other hand, a rocky surface can easily hold a significant amount of load. With sheer observations and requirements to understand soil behaviour, scientists coined the term ‘load bearing capacity of soil’.
This experience was passed down to generations over the years until, between the 19th and 20th centuries, one of the pioneers in changing civil engineering history came along. Karl von Terzaghi, from Austria, scientifically designed a comprehensive theory of the ultimate bearing capacity of the soil.
His theory defines that if the foundation depth is less than or equal to its width, it is called a shallow foundation. He also developed a method to determine the bearing capacity of soil through general shear failure cases. Furthermore, in 1951, another soil specialist, Meyerhof, added depth to the bearing capacity theory. His theory could also be applied to rough, shallow, and deep foundations. It was mostly the same as Terzaghi’s. However, it included three main factors: shape, depth and inclination.
Soil is a versatile construction material. Every region has a different soil, which has a different texture and load bearing capacity. However, factors that influence the load bearing capacity of soil are constant. Listed below are the factors that influence the load bearing capacity of soil and are required to be taken into consideration while designing and constructing a structure:
Before understanding the formula to derive load bearing capacity, let’s first understand the three main components:
Safe bearing capacity: This is denoted as (qs), the maximum amount of pressure soil can bear with all the safety factors considered.
Net bearing capacity: Net bearing capacity, also represented as (qn), is the difference between ultimate bearing capacity and overburden pressure.
Ultimate bearing capacity can be calculated using Terzghi’s theory and formula, which is as follows:
Where,
Let’s understand this further with the help of an example:
Let’s assume,
Each soil type has a different composition, texture, density, and load bearing capacity. To suit each soil type, several established methods are there, which are as follows:
Standard Penetration Test: The standard penetration test, Also known as SPT, is exceptionally helpful in cohesionless soils. In this test, the bearing capacity is determined by measuring the resistance of the soil.
Cone Penetration Test: The cone penetration test, also known as CPT, is the same as SPT. The only difference between the two is that, unlike SPT, CPT uses cone-tipped equipment to impose weight on the soil.
In certain cases, soil’s bearing capacity is insufficient. Under such circumstances, some factors are considered that can increase the bearing capacity.
Soil drainage: Soil moisture content and load bearing capacity are inversely proportional to each other. The higher the moisture in the soil, the lesser the load bearing capacity. To overcome this challenge, engineers often use the technique of draining the soil moisture, excess water is drained out using pipes.
Soil compaction: It is the most common method to enhance the load bearing capacity of soil. It reduces the gaps and holes between the soil particles, allowing it to become dense and enhancing the load bearing capacity. Soil compaction also removes the chances of soil over-moisturisation.
The bearing capacity of the soil can be remarkably improved using highly efficient geosynthetic materials like geogrids. Here’s how:
Geogrids are highly recommended in areas where seepage or moisture content in soil is excessive. They act as a guard and avoid unwanted growth of moss and other fungal substances which can weaken the foundation.
It also gives excellent stability to the structure during seismic movements below the surface.
Strata Global is a pioneer and global leader in the field of geosynthetic material manufacturing and soil reinforcement technology. We introduced a game-changing soil reinforcement technology, StrataGridTM. This innovative geogrid is a highly efficient and latest technology for reinforcing soil. StrataGridTM. is a cutting-edge product made by interweaving high-tenacity polyester yarn. It has exceptional tensile strength and provides vertical and horizontal soil reinforcement. What stands out in this product is its UV-stabilised coating that ensures longevity and resistance against adverse weather conditions. With these remarkable features, StrataGridTM is a successful soil reinforcement technology and provides exceptional load-bearing capacity to the soil. Let’s learn more about this considering a case study.
Strata Global took on the challenge of elevating a pair of side-by-side railroad tracks through a bustling downtown traffic corridor, collaborating with KS Union Pacific Railroad and Burlington Northern Sante Fe Railroad. Our team of experts found a way to significantly improve the load-bearing capacity of the soil, ensuring the structure could withstand heavy loads. Our experts pitched an idea of installing horizontal layers of StrataGrid™ at 18″ intervals (one layer per wire basket) all the way to the top of the wall. To further bolster the structural stability, the team backfilled the facing of temporary wire baskets with pit sand and wrapped it in a blanket of microgrid face wrap. The project’s impact was substantial, resulting in the creation of new grade separations that connected five new bridges and one rehabbed bridge along a 1.8-mile strip. This seamless integration allowed both trains and vehicles to flow unimpeded through eight major thoroughfares. Our commitment to excellence was evident in the extensive monitoring conducted over several months, which validated our solution. Even under heavy loads, the structure remained rock-solid, moving no more than half-inch in any direction, including vertical settlement.
Strata proposed an innovative and sustainable solution to build a working platform on the edge of a steep slope while minimising the amount of fill material used. In the design calculations, We considered a surcharge of 45 kN/m2 and a required bearing capacity of 200 kPa on the finished surface of the hardstanding equipment for constructing 75 m tall turbines. Our geosynthetics analysis determined that a combination of on-site granular material and StrataGrid™ 35 kN and 55 kN with 600mm spacing could make 10m high, 70-degree walls feasible for the contractor. Our team of experts designed the reinforced soil wall following the guidelines of SD8006:2010 for internal stability and Eurocode EC7 for global stability. By introducing this reinforced earth solution for the steep slope, we significantly reduced the overall height of the structure from approximately 30m to a more practical 10m. This approach not only reduced the overall carbon footprint of the structure but also ensured the platform could support construction traffic and heavy lifting equipment effectively improving the load bearing capacity.
The bearing capacity of soil plays an important role in keeping a structure upstanding and robust. The factors influencing the soil’s load bearing capacity are soil strength, foundation width and depth, spacing between the foundation, and soil moisture. Certain methods, such as plate bearing tests, SPT, CPT, and pressuremeter tests are used to determine bearing capacity of soil.
Considering all these factors, we have introduced a game-changing product called StrataGridTM, which has changed the facet of soil reinforcement technology. It is an exceptional product that enhances the load bearing capacity of the soil.
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