This article provides an overview of the engineering concept of bearing capacity and why it is an important consideration for the design of Envirocon structures.
Bearing Capacity
The above video is not an Envirocon produced video.
A key consideration for all structures is the bearing capacity of the underlying ground.
Bearing capacity is the capacity of soil to support the loads that are applied to the ground above. It depends primarily on the type of soil, its shear strength and its density. It also depends on the depth of embedment of the load – the deeper it is founded, the greater the bearing capacity.
It is important to Envirocon structures as, if the bearing capacity is less than the weight of the structure on top it, the structure may fail.
Determining the bearing capacity of the soil is a particular type of engineering called geotechnical engineering. As a standard rule Envirocon does not engage in this aspect of a design as it's a specialised field of engineering and there can be a high degree of variation in ground conditions across a relatively small area.
Customers should engage a geotechnical engineer who will produce a geotech report. This report will then be used by our structural engineers to produce a structure design.
A geotech engineer will prescribe the ‘ultimate bearing capacity’ of the soil, and the ‘allowable bearing capacity’.
The ultimate bearing capacity of soil is the maximum vertical pressure that can be applied to the ground surface, at which point a shear failure mechanism develops in the supporting soil.
In essence, this means the maximum amount of load the soil can take before it fails, or gives way completely. We don’t use this figure on its own in the foundation design process, as it’s also important to consider how soil will settle under pressure, which could affect its ability to support a structure.
The allowable bearing capacity of soil is the amount of load the soil can take without experiencing shear failure or exceeding the allowable amount of settlement. This is the figure that is used in the design of foundations.
The allowable bearing capacity is always lower than the ultimate bearing pressure because it takes into account the settlement of soil, not just the load required to cause shear failure.
- From a practical perspective, bearing capacity is generally expressed in a number from 80 - 300kpa.
- Below 80 is likely to involve remediation work which can make the structure uneconomic.
- Capacity can be higher than 300kpa, however, as this is the definition of ‘good ground’ in New Zealand is the maximum practical number salespeople need to know about.
Where there is insufficient bearing capacity, the ground can be improved. As far as Envirocon structures are concerned this particular approach is generally for retaining and involves removing earth and replacing it with a compacted hardfill or wall embedment as specified by a geotech engineer.
Alternatively the load can be spread over a larger area such that the applied stress to the soil is reduced to an acceptable value less than the bearing capacity. As far as Envirocon structures are concerned this would involve pouring a reinforced concrete foundation which spreads the load over a larger area.
Expansive Soil
Soil class - expansivity of the soil
Expansive soils are present across the wider Auckland region and in many other parts of New Zealand, where residential buildings are increasingly supported on shallow slabs and footings in clay-rich soils subject to shrinking and swelling behaviour.
- Expansive soils most commonly found in Auckland region:
- Class M - Moderately Expansive
- Class H1 - Highly Expansive soils (most problematic)
Practical considerations for expansive soils:
Moderately to Very High Expansive soils will have final design implications. Often geotechnical engineers specify what mitigation action has to be undertaken on-site. In most standard cases, geotechnical embedment depth will be required to achieve “good ground”.
e.g On some sites “good ground”- 300kPa bearing capacity can be found under a thin topsoil layer - this results in a liberal foundation design, often 200mm of the compacted metal base.
When expansive soils are present, the ground falls outside of the “good ground” definition. Geotechnical engineers often request a geotechnical embedment depth of X (often around 400-600mm) to achieve “good ground”. Geotechnical embedment can be achieved by two approaches (example when X=600mm):
- replacing 600mm of soil with engineered fill (GAP65) without block embedment
- achieving 600mm embedment by 200mm compacted base + 400mm embedment of the block
The approach should be always confirmed with the geotechnical engineer as some of them may have specific requirements
Ground Water
Presence of Groundwater has to be considered during retaining wall design. As it may affect the earthworks and further the overall stability of the wall across changes in seasons.
This is something to have in mind while reading the geotechnical report - it will be considered by the engineer and further by the contractor performing site works.
Liquefaction
Soil liquefaction is a process where typically saturated, granular soils develop excess pore water pressures during cyclic (earthquake) loading that exceed the effective stress of the soil.
When initially scanning the geotechnical report, we are trying to find out whether the site is considered to be at risk of Liquefaction. If the report suggests that it may be a risk - the engineer undertaking the design will need to consider the factor in his/her calculations.