Designing Shallow Foundations
In geotechnical engineering, the safe and cost-effective design of shallow foundations depends on accurately determining the soil’s bearing capacity at the intended depth. Without reliable data, there is a risk of over-designing, which increases costs, or under-designing, which compromises structural safety. The Plate Load Test (PLT) is a widely used in-situ method to validate design assumptions for shallow foundations. By directly measuring soil response to applied loads, the test provides valuable information on both bearing capacity and expected settlements, helping engineers reduce uncertainties and design with greater confidence.
What is the Plate Load Test?
The Plate Load Test (PLT), also known as the plate bearing test, is an in-situ geotechnical investigation method used to determine the soil bearing capacity, settlement behaviour, and deformation characteristics under applied loading. A rigid steel plate (typically 300 to 750 mm in diameter, either circular or square) is placed at the proposed foundation level. Load is applied incrementally, and the resulting settlements are measured using dial gauges.
The test generates a load–settlement curve. From this curve, engineers determine the ultimate bearing capacity and calculate the safe bearing capacity using a factor of safety, typically between 2 and 3. The Plate Load Test is especially useful for designing shallow foundations, assessing working platforms, and verifying ground conditions for crane pads and piling rigs. Because the test directly measures the soil response to loading, it provides a reliable basis for reducing uncertainty in design assumptions and improving site-specific foundation design.
Applications of the Plate Load Test
The Plate Load Test (PLT) is widely used in geotechnical engineering to evaluate the bearing capacity and settlement characteristics of soils and granular layers. Its primary applications include:
- Foundation design during site investigations for shallow footings.
- Pavement and infrastructure projects, such as road construction, airport taxiways, and railway trackbeds.
- Temporary works and working platforms, where it provides quality control data for mobile cranes, piling rigs, and other heavy equipment.
- Industrial and energy projects, including tank farms, wind farms, and hardstanding areas, where confirming near-surface ground capacity is critical.
By directly measuring soil response under load, the PLT helps engineers verify design assumptions, ensure safety, and reduce uncertainty in both permanent and temporary works.
Plate Load Test Equipment
The Plate Load Test requires specialised equipment designed to safely apply and measure loads in the field. The primary component is a circular steel plate, with a plate size typically ranging from 300 mm to 750 mm and a minimum thickness of 25 mm. A hydraulic jack with a pump is used to apply the increasing load, while reaction loading is provided by a frame, truss, or platform weighted with dead loads.
Dial gauges with high accuracy are positioned to measure settlement, and a pressure gauge records the applied load. Supporting tools such as tripods, spirit levels, and steel rules help ensure that the test setup is carried out in accordance with recognised test methods. Together, these instruments capture reliable data on soil types, ground surface conditions, and other site-specific characteristics, providing valuable inputs for geotechnical investigations and foundation design.
Procedure for the Plate Load Test
A test pit is excavated to the proposed foundation depth, usually at least five times larger than the plate size. The circular steel plate is set on a level base, and a seating load of about 0.7 T/m² is applied and released to ensure firm contact with the ground.
The load test is then performed by applying incremental loads with a hydraulic jack, while settlement is measured using dial gauges on a stable reference frame. Readings are taken at regular intervals until the rate of settlement reduces. The process continues until the maximum load is reached, generally 1.5 times the expected ultimate bearing capacity.
Calculation of Bearing Capacity
In a Plate Load Test (PLT), the applied load and the corresponding settlement are recorded and plotted as a load–settlement curve. The point at which the soil begins to undergo rapid settlement is taken as the ultimate load on the plate (Qᵤ). The ultimate bearing capacity of the soil beneath the plate (qᵤₗₜ) is then calculated as:
where:
- Qᵤ = ultimate load on the plate (kN)
- Aₚ = area of the test plate (m²)
The safe (allowable) bearing capacity (qₛₐfₑ) is then obtained by applying a factor of safety (FOS), typically ranging from 2.5 to 3, to account for uncertainties in ground behaviour:
Advantages and Limitations of the Plate Load Test
The plate load test provides valuable data on the bearing capacity and settlement characteristics of near-surface soils. It is relatively quick to carry out on site and offers engineers practical information for assessing shallow foundations and temporary works.
Its limitations lie in scale and depth. Because the test plate is small, results may not always represent the behaviour of full-sized foundations or deeper soil layers. Extrapolation requires caution, and the method is generally best suited to conditions close to the ground surface.
While plate load testing is a useful tool, it is often complemented by broader material testing such as soil classification, compaction, aggregate, concrete, and rock testing to ensure a comprehensive understanding of ground behaviour for design and construction.