PANDA® DCP dynamic cone resistance (qd) – Glossary & FAQ
|For soil investigation, the bearing capacity (qu) is calculated from the dynamic cone resistance (qd) as follows:
Lateral friction is avoided with ‘lost’ cones of area 4 or 10 cm².The maximum resistance you can test with the PANDA® is about 30 MPa. Note that 1 Mpa = 1000 kpa = 10 bars = 0,1 KN/cm²
We want to use the PANDA® to test crane pad locations and we have calculated that the ground must withstand 100 Tonnes / Square Metre. How does this relate to the PANDA® results?
100 t/m² = 100000 kg/m² = 1 MPa = 10 bars : that is the pressure the crane gives to the soil under the crane foundation (pads)(insulated foundations).
So the soil must be able to “retain” 1 MPa under the foundation. This must be the minimum bearing capacity qu of the soil under the crane foundation.
And with the following correlation qu (in bar) = qd (panda in MPa) x 10 / (12 to 15) – let’s take 15 for safety and use only the 4 cm² PANDA® lost cone in order to avoid the lateral friction around the rods.
In this case the minimum qd panda® tip resistance would be : qd (in MPa) = qu x 15 /10 = 1 MPa x 15/10 = 1.5 MPa
So you should need a qd PANDA® cone resistance (qd) minimum of 1.5 MPa for an enough depth of 2 times the width of the foundation (insulated foundations). For example, with a foundation of 1m x 1m, you should need 1.5 MPa with the PANDA® for 2 meters under the foundation.
Test Hole Spacing – at the university which makes the calibration of the PANDA®, they consider that you can make tests with the PANDA® with 15 cm between each test hole. 20 cm is a conservative value.
Does the PANDA® account for any dynamic effects in the test results?
The dynamic effects have not been considered for the PANDA®. Nevertheless, these effects are reduced to a minimum as the speed of deformation is very low. The advantage of the PANDA® sounding (compared to pile driving) is that you can reduce the hammering speed (and force) when you perform a sounding on a “weak” soil.
Is a continuous measurement of force and penetration is made?
Yes, the force and penetration is measured with each blow. This continuous measurement is what the user sees on the Dialogue Terminal and on the software: it is the penetrogram. The user can also check the data table on the dialog terminal or through the software. They will see, for every impact, the driving depth and the cone resistance per blow.
What is the relationship between qc and qd?
qc is cone resistance measured with the Cone Penetration Test (CPT) where a cone is pushed into the ground at a constant speed (e.g. 2cm/second). It’s also know as the static cone penetration test. qd is cone resistance but measured dynamically and is known as the Dynamic Cone Penetrometer (DCP). To do a comparative test, the geology should be the same so test probes should be done approx 30-40cm apart. A minimum of three comparative tests would be required. When comparing qc and qd, care needs to be taken with sandy soils because of lateral friction / skin friction on the rods.
What is the Maximum Depth that the PANDA® can reach?
It depends of the resistance of soils, but the PANDA® can be made to reach 5 to 7 m in classicals soils (<10 MPa). However, in certain cases, it will be only a few centimeters (e.g. rocks). The softer the soil, the deeper you can go. In exceptional circumstances, we have known tests done to 17 meters.
When doing soil investigation using the PANDA® (with the 4 cm² lost cone), the main reason to stop a PANDA® test is because of lateral friction on the rods. If you cannot turn the rod with your hand, it means that the lateral friction becomes important and then the value qd in MPa is not only the resistance under the cone (which is the principle of the DCP), but the resistance under the cone added with the resistance along the rods. The qd value will be higher than it should be and we are not able to quantify this error. The operator has to take this into account.
The lateral friction will appear normally more rapidly with sandy soils than with silty soils or clayed soils. Sandy soils will collapse more easily into the hole (and generate lateral friction on the rods) than clayed soils. This can be overcome with a 20mm PVC plastic pipe sleeve following the sacrificial cone (larger diameter than the rod) as it is driven into the ground.
PANDA® Variable Energy Light Weight Dynamic Cone Penetrometer is also known by a number of different names that include:
- PANDA® 2 Ultra Light Dynamic Cone Penetrometer
- PANDA® 2
- PANDA® 2
- PANDA® Probe
- Dynamic Probe
- Cone Penetrometer
- Ultra Light Dynamic Cone Penetrometer
- Dynamic Cone Penetrometer
- Variable Energy DCP
- PANDA® 2 Variable Energy Ultra-Light Weight Dynamic Cone Penetrometer