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Unsuitable Material for Earthwork in Road Construction

posted Sep 6, 2021, 8:13 AM by jeffery jim
Understanding unsuitable material is important as it is the prerequisite for works involving soil bearing or strength. I am fond about this issue as one of my friends who stuck in IT department for Pan Borneo project finally got a job as a Pan Borneo project engineer because he answered this accurately during his interview. Hence, it is a very important topic when it comes to road construction.

2. In most constructions, the criteria is rarely obliged or ignorantly overseen and thus, lead to poor subgrade formation in most road projects. Most civil engineers know how to differentiate good soil and unsuitable to a certain degree. Most understood that soil should be free from filth, peat, log, roots and other perishable matters as well as dead logs and stumps which is quite visible to the eye during the formation of subgrade and typically embankments.

3. Those who are in QAQC understand the essential values that soil must have through laboratory testing where soil properties are determined through Sieve Analysis for constituent grading, and Atterberg Limit when it comes to liquid and plasticity limits and index. These two test will highlight the classification of the soil name and type and refined based on the A-line based on Plasticity Chart and Soil Standard Classification.

4. The other test would be the laboratory CBR test for determine soil strength based on penetration value for design purposes of the pavement. Besides that, many have not take prerogative action to further check the volumetric change which not supposed to swell more than three percent.

5. Two other tests which rarely taken place in a lot of road projects are the test to check for organic matter content through loss of weight by ignition as well as the clay content as sieving is only done until wet sieve with aperture size of 63 micron which partially grade silt. Here, assumptions are made for silt and clay which can be very menacing for quality control.

6. Another way to determine unsuitable material before tests are conducted would be the blows required to penetrate one foot of stratum using the Macintosh Probe. A typical unsuitable material will take less than 40 blows for a foot of penetration. For embankment fills, the criteria changed based on height of the proposed fill level and may be a minimum of 150 blows, depending on the geotechnical consulting engineer's instruction.

7. There are more to be specific when it comes to unsuitable material and here is where many have overlooked. Soil should not be mixed or from worn down argillaceous rocks. The strategy here is to understand what is defined as argillaceous or part of argillites or worst, melange where the matrix have no indication of good bonding among constituents.

8. There are some bases which a civil engineer or QAQC engineer can use to determine melange or argillites at site. Melange is visible based on the profile of a section of open cut or a trial pit as shown in the annexed photos. This kind of soil is not suitable for earth filling and should be carted away and disposed unless exemption is given as counterbalance or counterweight or filler for less critical undulating toes.

9. The reason why melange or argillites is not suitable as filling material is due to its behavior which easily dissolved under constant water exposure or saturation. There are Garinono, Ayer and Wario Formation in parts of Sabah, this kind of argillaceous material have given JKR Sabah a lot of problems during and post construction. In some other formations like this Crocker Formation, melange can be spotted in localized clusters. This area have interbedding and intertwining geological features of older Oligocene to early Miocene and Quartenary Alluvium.

10. As a civil engineer, one should be good enough to establish unsuitable material at site and order the removal from site from any use primarily for earth fill. Quantity surveyor should also understand that this material is not suitable and reflect it clearly in the bills of quantities.

May be an image of outdoors

May be an image of outdoors