Volume Volume after Compaction Factor
Chart 5-2: Correlation between DCP and CBR
DCP Testing 200 100 50 20 10 5.0 2.0 1.0 CBR (per cent) DCP (mm/blow) 1.0 10.0 100.0 4 3 1 2
1. Log10 (CBR) = 2.632 - 1.28 Log10 (mm/blow) 2. Log10 (CBR) = 2.555 - 1.145 Log10 (mm/blow) 3. Log10 (CBR) = 2.503 - 1.15 Log10 (mm/blow)
4. Log10 (CBR) = 2.48 - 1.057 Log10 (mm/blow)
1. Kleyn and Van Heerden (60° cone) 2. Smith and Pratt (30° cone) 3. Van Vuuren (30° cone)
It is however, advisable to do full laboratory CBR tests on selected samples from the test points to verify the results of the DCP tests.
5.4 CONSTRUCTION OF bASE LAyER USING IN-SITU MATERIALS
5.4.1 Subgrade formation to camber
Figure 5-2: Unpaved road camber formation
Unlike the steps involved in constructing the camber on unpaved gravel roads, as shown in Figure 5-2, where excavation is done to a level platform before constructing a triangular “roof-shape” camber, for sealed roads, the subgrade should be constructed in one-go to the required camber slopes. This eliminates the creation of a thin ‘biscuit’ at the edges of the formed camber. The steps for subgrade formation on sealed roads are illustrated below:
i. Select control points over a given section of the road alignment and establish the correct levels that result in a balance of materials within each cross section.
ii. Set the centre line profile level to a height difference (based on the camber slope) from the profile levels on the left and right side shoulders to create an excavated finished subgrade bed with the desired camber over the formation width from the centre line. For low-volume sealed roads, the recommended camber should be in the range of 2.5 – 3.5%.
iii. Ensure that spreading to the required level is achieved by first cutting humps to fill all depressions. and compacting in uniform layers.
iv. Excavate approved material from the sides and place it on each side of the centre line. Increase the width of the cut if more material is required. If the material is dry, make a hole at the top of the heap and add water. Let the water seep in overnight.
v. Carefully add additional water if necessary in order to achieve optimal moisture content (OMC) while spreading and levelling the material to the required camber slope. Once levelled, the surface is compacted to refusal while still maintaining the correct shape of the camber
Figure 5-3: Paved road camber formation
5.4.2 Steps in in-situ base layer construction
If the in-situ soils are found to meet the specifications for base layer construction, the following steps can be followed in constructing the un-stabilized base layer using excavated material from the side drains.
vi. Excavate approved material from side drains and windrow or heap it between the shoulder pegs. Increase the width of the cut if more material is required
vii. If the material is dry, make a hole at top of windrow or heap and add water. Let water seep in overnight. Note: If the material from the side drains is not good (unapproved), it can be stabilized before use. If it cannot
be stabilized, it should be thrown to spoil at a safe location and replaced with approved material imported from elsewhere.
viii. Place shutters in the longitudinal direction, the first set along the centreline and another set along one edge of the base width, with the top of the shutters high enough to allow for the bulking of the base layer to attain the desired compacted thickness layer (say 150mm). It is advisable to construct one half of the width of the base at the time.
ix. Add additional water if necessary and mix at the optimal moisture content. Then spread the material between the shutters, screed off and compact to refusal.
Ensure that the workers do not walk/step on the loose base layer to prevent differential compaction.
x. Remove the centre line shutters, place shutters on the edge of the other half width, and spread material to a height of 50mm above the compacted section (use a 50mm guard rail ). Then compact to refusal which will compress this section to the same level as the first compacted width.
xi. For cladding of the edge of the compacted base layer, place pegs and string line at shoulder breakpoint. xii. Place loose material to a height of 50mm above the compacted layer (using a 50mm guard rail) and compact
to the required slope. 1
Peg
2 Steel shutters placed along edge and centre line
3
50 mm guard rail Edge details as shown in step 4
4 50 mm guard rail
Peg Loose cladding material Compacted Layer
Loose material Compacted Layer
50 mm guard rail Loose cladding material
The photos below further illustrate all the above steps.
Setting out of Steel Shutters to retain Base Layer Use of screed board to level spread Base Layer
Spread and Compacted Base layer Edge Cladding of Based Layer
5.4.3 Constructing a 150mm base course layer
If the pavement is designed with a 150mm thick compacted base course, the layer is too thick to be compacted in a single layer using a 900 kg pedestrian vibratory roller as described above. Still using shutters and bulking rails, the base course should then be constructed in a two-step procedure, as follows:
1) Correct the sub-base/subgrade to within an accuracy of +/-10mm of the design levels.
2) Moisten and scarify using rakes, the surface of the sub-base/subgrade to obtain good bonding with the base course.
3) Mix base course material to OMC, fill loose material and screed off to the top of the 100mm x 100mm steel shutter base section. Compact the material to refusal. This will produce a first layer with a compacted thickness of approximately 67mm.
4) Scarify the surface to obtain good bonding with the next layer.
5) Place a 90mm bulking rail on top of the 100mm x 100mm base section. Mix the second layer of base course material to OMC, place the loose material and screed off to the top of the bulking rail. Once again, compact the material to refusal. This will give a second compacted layer of roughly 83 mm thickness and a total thickness of the two compacted layers at 150mm. Figure 5-4 below illustrates the steps described above.
5.4.4 Constructing 120 mm base course layers
This layer is also too thick to be compacted in one single operation. Instead, follow the steps above, but replace the 90mm bulking rail with a 50mm rail.
5.5 COMPACTION OF LAyERS
5.5.1 Moisture density relationships
When subjected to dynamic compaction, practically all soils exhibit a similar relationship between moisture content and density (dry unit weight). Every soil has an optimum moisture content at which point the soil attains the maximum density under a given compactive effort. This fact as depicted in Chart 5-3 forms the basis for the modern construction process commonly used in the formation of road pavements and embankments, earth dams, levees and similar structures.
In the laboratory, the compaction is simulated by the use of a freely falling weight impinging on a confined soil mass. In road construction, compaction is secured through the use of rollers or vibratory compactors applied to relatively thin layers of soil. In the field an attempt is usually made to maintain the soil at an optimum moisture and bring the soil to maximum density or some specified
percentage thereof. In the laboratory, compaction tests are usually performed using the “standard Proctor” or “standard AASHTO” methods (T99). The optimum moisture and maximum density are usually found in the laboratory by a series of determinations of wet unit weight and the corresponding moisture content. After the moisture content determinations are completed, the dry unit weights may be calculated and plotted. Different soils react differently to compaction at moisture contents that are less than optimum. The moisture content is less critical for heavy clays than for the less plastic sandy and silty soils. Clays may be compacted through a relatively
+/- 134 2.2 2.1 2.0 1.9 1.8 1.7 1.6 5 10 15 20 25 moisture content (%) dry density (g/cm 3) well-graded sand silty sand clayey sand sandy silty clay silty clay clay uniform sand 100% saturation
Chart 5-3: Moisture content – dry