Introduction:
The content of specifications (Nigeria General Specifications for Roads and Bridges – Volume II (revised 1997)) and the discretion of the Engineer or Engineers Representative (ER) guides road construction projects. However, the decision of the Engineer holds the upper hand in conflicting situations. The reason could be related to a peculiarity in site conditions which may not be fully captured by what is laid down in the specifications. Hence, the Engineer needs adequate training and knowledge in order to make the right and informed decisions at every stage of the project. While the decision of the well-trained Engineer holds the upper hand as noted earlier, I hereby lay down specification guidelines for road geometry and earthworks in road construction projects in Nigeria. My decision to mention these two was born out of the fact that they took the most time in road construction projects and how they are managed and properly done would conduce to a safe and durable road.
Earthworks are usually done over the road geometry that comprises the horizontal and vertical alignments, super elevations, and cambers of the road (see Figures 1 and 2) and over the road width/formation width of the road between the drainages that comprises the carriageway, the hard shoulder and the soft shoulder (Clause 6005). They shall be constructed in accordance with the drawings or as directed by the Engineer in the following sequence of operations:
> Dumping of materials
>> Spreading by the grader
>>> Watering and mixing and finally,
>>>> Compacting by approved means
In the process, there shall be regular field trials, supplemented by any necessary laboratory investigations to determine types of compaction equipment, suitable layer thickness, and moisture contents all within the requirements of the specification for placing and compaction of various materials to be used in the earthworks.
What Constitutes Unsuitable Materials in Road Works
Clause 6122 of the specifications identified unsuitable materials as materials from swamps, marshes and bogs, peat, logs, stumps, roots, and other perishable or combustible materials. Topsoil and highly organic clay and silt are also in this class. In addition, clay having a liquid limit exceeding 80% and plasticity index exceeding 55 and highly micaceous materials fall into this class. These materials shall be removed wherever they are found. Topsoil is soil that is seen to have been broken down by agricultural cultivation and/or is seen to be capable of supporting vegetable growth. This includes turf.
Topsoil shall not be removed from any area until excavation or filling operations are about to start in that area. Topsoils on the sites of cuttings and embankments shall be excavated to the average depth directed by the ER or as specified in the specifications.
Earth Fill Materials
Earth fill can be seen as all soil materials that are not in the class of unsuitable materials and may even contain boulders not exceeding 3m3 in size. Tests for the selection of fill material include plasticity tests, grading tests, density/moisture compaction tests, and laboratory CBR tests. Fill materials shall be placed, spread, and levelled layer of loose depth not exceeding 230 mm of general fill. Other fills like rock fill have their own specifications but are beyond scope of this article. Isolated boulders or isolated large fragments of solid rock forming part of the general fill shall be evenly distributed throughout each layer but no stone exceeding 150 mm in size shall be placed within 300 mm of the formation level.
Work in compaction of fill materials shall be carried out only when the moisture content is within the range shown by the field or by laboratory tests whichever is appropriate for that material be suitable for the attainment of the density specified below;
- For material that is within 600 mm formation level or in the filling adjacent to structures (including backfilling), not less than 100% of MDD for the material obtainable by BS standard compaction.
- For other materials, not less than 95% of MDD for the material obtainable in BS standard compaction.
Subgrade
Tests for the classification of compacted subgrade and filling shall be carried out for each 2 km of subgrade or each fill. The number below can be used for tender purposes while the actual number would be determined by the Engineer.
Classification tests
-2 no liquid limit tests
-2 no plastic limit tests
-2 no sieve analysis tests
-4 no BS compaction tests
-2 no laboratory CBR tests at specified density and moisture content
Control tests
-20 no in situ dry density tests
In places where there is the certainty of deposits of clay and peat, these should be subjected to consolidation tests.
According to clause 6005, XIII A/B, the subgrade of the road shall be shaped to the dimensions, formations, and levels, as shown on the drawings or as directed by the ER, and the top 150 mm subgrade, shall be watered and compacted to 100% BS Standard compaction at the optimum moisture content (OMC). Where fill materials are used for the subgrade, each compacted layer shall not be more than 150 mm.
Sub-base Course
This is also basically on the carriageway but may be continuous over the hard shoulder. It shall be constructed of approved material having a finished compacted thickness shown on the drawings or as directed by the Engineer which depends on the CBR value of the sub-base concerned.
For subbase type I, a minimum strength of 30% CBR shall be expected after 24 hours of soaking. For subbase type II, a minimum strength of 20% CBR shall be expected after 24 hours of soaking for light traffic.
Base Course
The base course is basically within the carriageway but can be continued over the hard shoulder or extended a minimum of 300 mm at either side of the surface width. It shall be constructed of approved material having a finished compacted thickness of not less than 150 mm. The base course extended to the hard shoulder may be constructed of different material from that of the carriageway but must also have a finished compacted thickness of not less than 150 mm.
The minimum strength of the base course material shall not be less than 80% CBR value being determined at maximum dry density (MDD) and OMC unsoaked with either West African Standard or Modified ASHTO compaction. Where there is a risk of perched water table etc, a CBR value of 80% is obtained after 24 hours of soaking.
N.B.:
a. the compaction of the subbase course and the base course shall be carried out to a dry density not less than 100% of that achieved in the West African Standard compacted test at OMC. But the compaction of the base course may be carried out to a dry density of not less than 100% of that achieved in the modified AASHTO compaction test at specified OMC.
b. exposed surface of each lift of fill or subbase course or base course after compaction shall be protected by a cambered surface from excessive moisture content changes due to rainstorms until the next lift is placed. If the materials are prone to excessive drying, the surface should be moistened from time to time till the next lift (layer) is placed.
c. if there are patches of unsound ground, excessive sand, or clay, the areas should be rectified by admixture of suitable materials or by cutting out and replacement to ensure uniformly compacted formation.
d. in all cases, except where any particular area fell below the minimum required in the compaction test, double compaction should be avoided as it has the capacity to increase the percentage passing 75μm sieve to more than 30%, thus, reducing the strength of the material. Every effort and care therefore should be taken to achieve the required degree of compaction in the first compaction.
Materials for Hard Shoulder
Surface-dressed hard shoulders should be constructed on both sides of the roadway as shown on the drawings or as directed by the Engineers representative (ER).
Material for Soft Shoulder
The material selected for the upper 75 mm of the soft shoulder shall or preferably be well-graded lateritic soil with a plasticity index of not less than 10 and not more than 20. But where topsoil is used instead, suitable grass used for sowing or planting on the verges, the slopes of embankments, and cuttings shall be used and these shall preferably be daub grass (cynodont dactylion) or Bahama grass.
Compaction Required for Backfilling Operations
West African Standard (WAS) compaction is the specified compaction required for materials used for backfilling operations and for compaction of verges, medians, and hard shoulder for subgrades.
Sandy Soils
Where sandy soils are encountered in the borrow pit, they should preferably be placed on the top layer of the fill. Where these soils are specifically required for fill, whether as hydraulic fill or otherwise, the sand-fill material shall be such that no more than 15% passes the 75μm sieve and in no case will isolated pockets or layers of silt, clay, or other organic impurities be accepted.
The relative density of hydraulics sand fill placed in embankments shall be as follows;
Top 600 mm – 90%
Next 600 mm – 80%
Below the top 1200 mm – 70%
Sand fill placed in any other method or than the hydraulic method shall be compacted in layers to a density of not less than 100% of the MDD obtained in the BS compaction test. A vibrative compaction system is recommended for the purpose.
For hydraulic sand fill, they shall be deposited in layers but in any case, the thickness of such layers shall not exceed the limit stipulated as follows;
First layer – not exceeding 450 mm in thickness
Second layer – not exceeding 750 mm in thickness
All subsequent layers – not exceeding 900 mm in thickness
Bituminous Surfacing
In addition, the surfacing of the carriageway shall be two coats of bituminous surface dressing or asphaltic concrete pavement as shown on the drawings or as directed by the Engineer.
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