The Mackintosh Probe Test: A Practical Guide to Shallow Soil Investigation

In Malaysian civil engineering practice, light dynamic penetrometer tests are widely used for preliminary soil investigation and shallow foundation design. The Mackintosh probe is one of the most common of these field tests, especially on soft ground where a quick, low-cost profile of subsoil strength is needed before committing to a full borehole programme.

What the Mackintosh Probe Is

The Mackintosh probe is a hand-operated dynamic penetrometer. It consists of a string of steel rods fitted with a conical tip, driven into the ground by a hammer that is repeatedly lifted and dropped through a fixed height. Because the hammer mass and drop height are fixed, each blow delivers a consistent amount of energy, so the number of blows needed to advance the cone is a direct indication of how much the soil resists penetration.

The equipment is light enough to be carried by a small field crew, which makes it well suited to sites with limited access, such as residential lots, drainage alignments, and soft riverine or peat areas where heavy drilling rigs cannot easily reach.

Test Procedure & Data Recording

1. The probe is assembled with the cone and the first rod, and set vertically at the test point.
2. The hammer is lifted to its full drop height and released to fall freely onto the anvil.
3. The number of blows required to drive the probe each 300 mm into the soil is recorded.
4. Additional rods are connected as penetration progresses. The test continues to the target depth, or until the refusal criterion specified by the engineer for the project is reached.

The recorded blow counts are plotted against depth to produce a penetration profile, which shows where soft layers, firmer layers, and potential bearing strata lie beneath the test point.

Interpreting the Results

Blow counts are interpreted using empirical correlations that relate penetration resistance to the allowable bearing capacity of the soil. These correlations were developed for particular soil types and conditions, so the results should always be interpreted by a qualified engineer familiar with the local geology, rather than read off a single formula.

Note: Mackintosh probe results are a guide for preliminary design. Where foundation loads are significant or ground conditions are variable, they should be confirmed with laboratory classification, consolidation, and shear strength tests on recovered samples.

When the Mackintosh Probe Is the Right Choice

• Soft cohesive soils, peat, and organic clays, which are common across low-lying parts of Sarawak, where fine resolution in weak layers is more useful than raw driving power.
• Preliminary investigation to map how the depth of firm ground varies across a site before specifying boreholes at the most critical locations.
• Light structures and shallow foundations, such as houses, drains, culverts, and small retaining walls.
• Restricted-access sites where drilling rigs cannot be mobilised economically.

Limitations

• Depth: skin friction along the rods increases with depth and can inflate blow counts, so results become less reliable in deeper probing.
• Dense or gravelly layers: hard layers, boulders, or buried debris can cause early refusal that is mistaken for a competent bearing stratum.
• No samples: the probe does not recover soil, so it cannot identify soil type on its own.

For deeper subsoil investigations, or where soil samples and groundwater information are needed, boreholes with standard penetration tests (SPT) should be specified.