In Malaysian civil engineering practice, dynamic penetrometer tests are widely used for preliminary soil investigations and shallow foundation design. The two most prominent field tests are the Mackintosh Probe and the JKR Probe.
While they look similar to the untrained eye, understanding their technical differences is essential for correct bearing capacity calculation and subsoil profiling.
Technical Specifications Comparison
Both tools consist of a steel rod with a conical tip that is driven into the ground by a sliding hammer drop. However, their physical dimensions differ:
| Specification | Mackintosh Probe | JKR Probe |
|---|---|---|
| Hammer Mass | 4.5 kg | 5.0 kg |
| Drop Height | 280 mm | 280 mm |
| Cone Angle | 60° | 60° |
| Cone Diameter | 25.4 mm (1 inch) | 25.0 mm |
| Rod Diameter | 12.7 mm | 12.7 mm |
The slight increase in JKR hammer mass (5.0 kg vs. 4.5 kg) provides more energy per blow, making it useful in slightly stiffer soils where the Mackintosh probe might encounter premature refusal.
Test Procedure & Data Recording
The procedure is highly standardized:
- The probe is assembled with the cone and first rod.
- The hammer is lifted to its maximum drop height (constrained by the design collar) and allowed to fall freely.
- The number of blows required to drive the probe 300 mm (1 foot) into the soil is recorded.
- Additional rods are connected as penetration progresses, up to a typical maximum depth of 15 meters, or until "refusal" is reached (generally defined as 400 blows for 300 mm penetration).
The recorded blow count ($N$) at each interval is plotted against depth to generate a penetration profile.
Estimating Soil Bearing Capacity
Empirical correlations relate the blow count ($N$) to the allowable bearing capacity ($P_a$) of the soil. A widely used correlation in soft clays is:
$P_a (\text{kPa}) = 1.2 \times N$
For example, a Mackintosh Probe blow count of 10 blows per 300 mm indicates an allowable bearing capacity of roughly 12 kPa. If the count reaches 30 blows, the bearing capacity is approximately 36 kPa.
Note: These empirical formulas are only guidelines and should always be verified with laboratory consolidation and shear strength tests.
When to Choose Which Probe
- Mackintosh Probe: Ideal for very soft cohesive soils, peat, and organic clays. Its lighter hammer provides finer resolution in extremely weak layers.
- JKR Probe: Preferred for slightly firmer clays or silty sand deposits. The 5.0 kg hammer drives the cone more effectively through medium-strength layers, minimizing rod bending risks.
Both methods are limited by skin friction on the rods at depths exceeding 10-15 meters, which can skew blow count results. For deeper subsoil investigations, standard penetration tests (SPT) conducted inside boreholes should be specified.