A Mackintosh probe is a lightweight portable penetrometer that is designed to be a tool to investigate the soil bearing capacity. The tool consist of high yield steel rods, each of length approximately 120cm that are connected each other by 25mm outer diameter couplings with a 27.9mm diameter of a 30o –apex angle; 12.7 mm diameter solid rods and a 4.5 kg dead weight with standard drop height of 300mm.
The applications of the Mackintosh probe are by assembling the equipment, marked on each 0.3m on the rods, set up the equipment on the ground, pulled up the hammer until maximum height, dropped freely to driven the rod and cone into the soil, continued until the blow reach more than 400 blows per 0.3m penetration or the depth reached 15m.
Under any circumstances of the soil conditions, the rate of driven must be from 15 to 30 blows per minute and the deviation from the vertical of the first extension rod shall not be greater than 2%. The result from the Mackintosh probe test shall be recorded for every blows per 0.3m.
Thus, the value of the bearing capacity will be referred to the standard bearing capacity graph. Mackintosh probe have lots of disadvantages that can lead to misleading test result that are contributed mostly from human error. To overcome this problem, this study will mainly focusing on developing a mechanism that will improve the present Mackintosh probe.
The mechanism is called the Nautical Gauge Mackintosh Probe (NGMP). From the progress test result of this study, it shows the number of blows per 0.3m is exceed the limitation of the standard bearing capacity graph. The recommendation for the problem is to change the location of study to a more adequate location.
Advantages of Mackintosh Probe
Fakher et al. (2005) briefly report the main advantages of the Mackintosh Probe that are :
- i. Speed of operation;
- ii. Use in difficult terrain where access is poor;
- iii. Minimal equipment and personnel are required;
- iv. Equipment is very low cost;
- v. Simplicity of operation and data recording/analysis;
- vi. Use in the interpolation of soil strata and properties between trial pits and boreholes;
- vii. Reduces the number of boreholes required.
Disadvantages of Mackintosh Probe
Despite the many advantages of the Mackintosh Probe, there are also many disadvantages to this tool. The disadvantages are as follows:
- i. Contribute to human error.
a. Wrong counting
b. Non-consistent drop height
c. Not accurately vertical when using of the tool
- ii. This tool can only be used in shallow soil
- iii. Unable to penetrate into medium strength.
- iv. Depending on human strength limitation
- v. Might injured the user.
Mackintosh Probe Test
Mackintosh probe test is widely used to assess soil bearing capacity mainly on soft ground (M-value). In this study, we used one set of 15 mm diameter series mackintosh probe with the length of 1.10 m each. A 25 mm diameter and 60 degree cone screwed onto the lower rod was driven into ground by a 4.5 kg hammer falling freely through a height of 30 mm onto an anvil. The number of blows require for every 300 mm penetration is recorded.
Read Full Mackintosh Probe Test Procedure here.
The relationship between Mackintosh Probe and Safe Pressure is as follows: –
P = (2860 + 550 (R – 40)1/2) x 0.04788 kN/m2 for blows > 40
P = Refer Chart for blows < 40
P = safe pressure (kN/m2)
R = Mackintosh Probe Penetration resistance in blows/0.3m
Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.