Soil Mechanics Info

The standard force penetration curve, corresponding to 100 per cent CBR has the following values:

Plunger penetration (mm) 2 4 6 8

Determine the CBR value of the subgrade. Solution

The standard penetration curve is shown drawn in Fig. 11.12.

The test points are plotted and a smooth curve drawn through them. In this case there is no need for the correction procedure as the curve is concave upwards in its initial stages.

From the test curve it is seen that at 2.5 mm the plunger load is 8.3 kN and at 5.0 mm the penetration is 13.5kN.

Concave Graph For

Plunger penetration (mm)

Fig. 11.12 Example 11.3.

Plunger penetration (mm)

Fig. 11.12 Example 11.3.

The CBR value is therefore either

13.24 19.96

whichever is the greater, i.e. 63 per cent or 67 per cent. The CBR value of the subgrade is 67 per cent, say 65 per cent.

Note CBR values are rounded off as follows:

to nearest 1 per cent for CBR values up to 30 per cent; to nearest 5 per cent for CBR values between 30 and 100 per cent; to nearest 10 per cent for CBR values greater than 100 per cent.

11.6.5 Frost susceptibility of subgrades and base materials

Cohesive soils: can be regarded as non-frost-susceptible when Ip is greater than 15 per cent for well drained soils and 20 per cent for poorly drained soils (i.e. water table within 600 mm of formation level).

Non-cohesive soils: except for limestone gravels, may be regarded as non-frost-susceptible if with less than 10 per cent fines.

Limestone gravels are likely to be frost-susceptible if the average saturation moisture content of the limestone aggregate exceeds 2 per cent. Chalks', all crushed chalks are frost-susceptible. Magnitude of heave increases linearly with the saturation moisture content of the chalk aggregate. Limestone: all oolitic and magnesian limestones with an average saturation moisture content of 3 per cent or more must be regarded as frost-susceptible.

All hard limestones with less than 2 per cent of average saturation moisture content within the aggregate and with 10 per cent or less fines can be regarded as non-frost-susceptible.

Granites: crushed granites with less than 10 per cent fines can be regarded as non-frost susceptible.

Burnt colliery shales: very liable to frost heave. No relationship is known, so that tests on representative samples are regarded as essential before the material is used in the top 450 mm of the road structure. Slags: crushed, graded slags are not liable to frost heave if they have less than 10 per cent fines.

Pulverised fuel ash: coarse fuel ashes with less than 40 per cent fines are unlikely to be frost-susceptible.

Fine ashes may be frost-susceptible and tests should be carried out before such materials are used in the top 450 mm.

11.6.6 Traffic assessment

To design the thicknesses of the bituminous layers in a road pavement, a knowledge of the expected traffic flow on the highway is required. This flow is referred to as the cumulative design traffic. Road Note 29 defined traffic in terms of the cumulative number of equivalent standard 80 kN axles that would be carried during the life of the road and the same approach was adopted in the Transport and Road Research Laboratory report LR1132. The Design Manual for Roads and Bridges (DoT, 1991c) however, describes two methods for the estimation of design traffic for road design: the standard method and the full traffic assessment method. The standard method is used for the design of new roads and this section considers only this method. In this method, the flow of commercial vehicles (i.e. those greater than 15kN unladen vehicle weight) is established in order to determine the cumulative design traffic. Commercial vehicles are placed into categories depending on the type of the vehicle (e.g. coach or lorry), the number of axles and the vehicle rigidity. Buses and coaches are placed in the public service vehicle (PSV) category, and lorries are placed into one of two other goods vehicles categories (OGV1 and OGV2) depending on their size. Traffic flow data should be determined from traffic studies and, from these, the percentage of OGV2 vehicles in the total flow is determined. To establish the cumulative design traffic, use is made of design charts. The charts give the cumulative design traffic (in millions of standard axles) depending on the forecast traffic flow at opening together with the percentage of OGV2 vehicles in the flow.

11.6.7 Design life of a road

An important part in the design of a road is the decision as to the number of years of life for which the road can be economically built. The DMRB suggests a design life of 20 years for bituminous roads, because their life may be extended by a strengthening overlay. With a concrete road a major extension of life involves considerable problems and the DMRB therefore recommends a design life of 40 years.

11.6.8 The moisture condition value, MCV

The selection of a satisfactory soil for earthworks in road construction involves the visual identification of unsuitable soils, the classification tests described in Chapter 1 together with the use of at least one of the three compaction tests described in this chapter. The compaction test chosen is the one that uses a compactive effort nearest to the expected construction compactive effort and is used to determine the optimum moisture content value, i.e. the upper value of water content beyond which the soil becomes unworkable. The system can give good results, particularly with experienced engineers, but there are occasions when the assessment of a particular soil's suitability for earthworks is still difficult.

The moisture condition test is an attempt to remove some of the selection difficulties and was proposed by Parsons in 1976. It is essentially a strength test in which the compactive effort necessary to achieve near full compaction of the test sample is determined. The moisture condition value, MCV, is a measure of this compactive effort and is correlated with the undrained shear strength, cu, or the CBR value, that the soil will attain when subjected to the same level of compaction (Parsons and Boden, 1979).

Details of the apparatus are shown in Fig. 11.13. Basically the test consists of placing a 1.5 kg sample of soil that has passed through a 20 mm sieve into a cylindrical mould of internal diameter 100 mm. The sample is then compacted to maximum bulk density with blows from a 7 kg rammer, 97 mm in diameter and falling 250 mm. After a selected number of blows (see Example 11.4), the

Upper cross member

Upper cross member

Plunger Penetration Apparatus
Fig. 11.13 The moisture content apparatus (MCA) (reproduced from Parsons and Boden, 1979).

penetration of the rammer into the mould is measured by a vernier attachment and noted. The test is terminated when no further significant penetration is noted or as soon as water is seen to extrude from the base of the mould. The latter requirement is essential if the water content of the sample is not to change.

Required calculations

The difference in penetration for a given number of blows, B, and a further three times as many blows (i.e. 4B blows) is calculated and is plotted against the logarithm of B. The calculation is repeated for all relevant B values so that a plot similar to Fig. 11.14 of Example 11.4 is obtained. The MCV is taken to be lOlogioB where B = the number of blows at which the change of penetration = 5 mm. As is seen from Fig. 11.14 the chart can be prepared so that the value of 101ogi0B, to the nearest 0.1, can be read directly from the plot. The value of 5 mm for the change in penetration value was arbitrarily selected as the point at which no further significant increase in density can occur. This avoids having to extrapolate the point at which zero penetration change occurs.

The MCV for a soil at its natural moisture content can be obtained with one test, the wet soil being first passed through a 20 mm sieve and a 1.5 kg sample collected.

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