Measuring Bearing Capacity with the Light Weight Deflectometer

Clients are?always?looking?for?ways to get a better understanding of the ground conditions?at a specific point in time,?and how?the bearing capacity of a site has?altered due to changing conditions, like rain.??

Let’s take a crane lift as an example.?The?insitu ground conditions?need to be identified in order for the?temporary works platform?to be designed.?However,?immediately prior to the crane lift the insitu conditions?are required to be verified. This is to check that the site conditions remain as per the design assumptions – and thus being able to provide real time results?from tests that directly model the proposed loading scenario are important.??

For this article, we describe the Zorn Light Weight Deflectometer (LWD) as an example of equipment suitable of achieving these verification outcomes for the ‘as-encountered’ site conditions. Safety factors are not considered.

Method 1 – Varying the Applied Load

The LWD used in the standard, calibrated configuration – 10 kg weight, full height?drop?and 300 mm plate – will provide a single stress vs. deflection datapoint (i.e. deflection magnitude under 100 kPa stress). However, varying the weight mass, plate diameter (and potentially drop height) facilitates the construction of an extended stress vs. deflection dataset, such that the extent of a linear (elastic) response can be determined and the allowable bearing capacity directly identified.?10kg and 15kg drop weights and 300mm and 150mm diameter plates are available.

For example, the following LWD configurations would yield varying test stress for which the deflection would be measured:?

• 15 kg drop weight, 300 mm?diameter?plate – Approx.?150?kPa stress?
• 10 kg drop weight, 150 mm diameter plate – Approx.?400?kPa stress?
• 10 kg drop weight, 300 mm diameter plate – Approx.?100?kPa stress?

After factoring for the varying plate diameters, the measured deflections would be graphed as deflection vs. test pressure. The allowable bearing capacity is represented by the point of deviation from the initial linear response. If the test stress is of sufficient magnitude, the?ultimate bearing capacity?may also be able to be?identified.?

This technique is shown on the below graph – thanks?to?our friends at?FSG Geotechnics?+ Foundations?– which also identifies the demonstrated allowable bearing pressure of the site.?

To provide?additional?data points,?the user could also?change the drop height?within any specific weight / plate diameter configuration of the LWD.?For example, the?drop height?could be split?into thirds?– one?being?the calibrated position (full drop height),?and the other two are positions?temporarily marked upon?the drop shaft.?Although the Zorn LWD doesn’t have a load cell, you?can?calculate the approximate applied pressure for each drop height?based on the proportionate heights.?Complete the LWD test as per the normal test technique and the?deflection?magnitude?will be recorded.?These can then be added as (approximate) supplementary stress vs. deflection datapoints to those taken under the calibrated test configurations.?

Note the measured deflections are obtained from the LWD –?a dynamic, surface-based test – represent the composite arrangement within the limited zone of influence of the test (penetration depth). Assessment and interpretation would still be required in confirming the applicability of the LWD test results to the proposed outrigger dimensions. Similarly, the use of the LWD to directly demonstrate allowable bearing capacity onsite, is a step-change from the Safety Factor method used to estimate the same parameter from other geotechnical test results.?

Method 2 – Determining if permanent deformation has occurred

This is the easiest method to determine if an allowable bearing capacity threshold has been reached. In this example, we are using a 10 kg drop weight with a 300 mm diameter plate. Dropping the drop weight causes an stress applied (bearing pressure) under the plate of approximately 100 kPa.

Looking at the graph below, we can see the no permanent (plastic) deformation has been caused (the plate has returned to its original level) which means we are operating in the elastic zone. Only elastic deformation of the ground is demonstrated. Hence, we can determine that the elastic response of the material is greater than e.g. 100 kPa. If our allowable bearing capacity is 100 kPa, this threshold has not been reached.

Remember the zone of influence (depth) is limited by the diameter of the base plate, as is the case with the Plate Load Test.

As a test is done and results captured within 3 minutes, a large number of tests can be carried out very quickly by a single operator and all results are GPS located.

In conclusion, the?Light Weight Deflectometer?is a useful tool to help?directly demonstrate the?allowable and ultimate bearing capacity?of a site, and monitoring any variation of these values due to changing site conditions throughout the timeframe of project delivery.?