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Together with the assessment of the traffic loading,

the support provided by the subgrade is the most

important factor in determining the required pavement

thickness.

There are a number of methods of measuring the

subgrade strength, ideally each method being

appropriate to the design philosophy being adopted.

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The principal methodsare as follows:

(A) CBR (California Bearing Ratio) – the

most commonly measured parameter

for assessing the strength of the

subgrade.

The principal methodsare as follows:

(B) MR (Resilient Modulus) – this provides a

measure of the elastic property of the

subgrade soil and is determined in

accordance with the method defined in

AASHTO Test MethodT294.

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The principal methodsare as follows:

(C) K (Modulus of Subgrade Reaction) – this effectively

provides a measure of subgrade support in terms of a

bearing capacity and is used in the design of rigid

pavements. The k-value can be determined by field tests

or by correlation with other tests. There is no direct

laboratory procedure for determining k-value.

CBR is adopted as the principal

method of determining subgrade

strength.

NOTE:

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The assessment of the subgrade is

dependent on

Soil Type DensityMoisture Content

WHAT IS A WATER TABLE?

The level below which the ground is saturated with water.

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In the tropics, including the Philippines, subgrade moisture content under

surfaced (asphalt or PCC) pavements can be classified under 2 main categories:

1) Category 1

Subgrade where the water table is sufficiently close to the ground

surface to control subgrade moisture content. The type of subgrade soil

governs the depth at which a water table becomes the main influence on the

subgrade moisture content. Typical values of depth related to soil type are as

follows:

(i) Non-plastic soils, depth of water table below ground surface 1m or less.

(ii) Sandy Clays (PI < 20%), depth of water table below ground surface 3m or less.

(iii) Heavy Clays (PI > 40%), depth of water table below ground surface 7m or less.

In the tropics, including the Philippines, subgrade moisture content under

surfaced (asphalt or PCC) pavements can be classified under 2 main categories:

2) Category 2

Subgrades with deep water tables and where rainfall is

sufficient to produce significant changes in moisture

conditions under the road.

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Methods of estimating the subgrade design moisture content are discussed

below:

a) Category 1

The easiest method of estimating the subgrade design

moisture content is to measure the moisture below existing

pavements in similar situations, at the time of year when the

water table is at its highest level. Existing pavements should be

at least 2 years old and greater than 3m wide. The sample

should preferably be taken from under the carriageway about

0.5 m from the edge.

Methods of estimating the subgrade design moisture content are discussed

below:

b) Category 2

When the water table is not near the surface, the

subgrade moisture condition under an impermeable pavement

will depend on the balance between water entering the

subgrade through the shoulders and at the edges of the

pavement during wet weather and the moisture leaving the

ground by evapotranspiration during dry periods.

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Having determined the design moisture content for the

subgrade, it is then necessary to define the density at which the

CBR is to be measured.

When an existing pavement is to be upgraded and existing

pavement layers which od not exhibit defects due to subgrade

failure are to be retained, the density to be used will be the

insitu density of the subgrade as measuredin the field.

In case of a new pavement, subgrade compaction will be in

accordance with the specification which will define the

required minimumdensity.

Having determined the design moisture content and density of

the subgrade soil, samples of the subgrade soil at the design

subgrade moisture content can then be compacted in CBR moulds to

the specified density (or insitu density where appropriate) and

tested to determine the CBR values.

If saturated subgrade conditions are anticipated, the compacted

samples for the CBR test should be saturated by immersion in water

for four days before being tested. In all other cases when CBR is

determined by direct measurement, the CBR samples should not be

immersed since this results in over design.

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ADOPTION OF TYPICAL CBR VALUES

Typical values of CBR may be used for feasibility study purposes and it is particularly

and are given in the table below. For all other purposes, laboratory testing must be carried

out.

Estimated design subgrade strength class under sealed roads in the presence of a

water table.*DEPTH OF WATER

TABLE FROM

SUBGRADE LEVEL

(METRES)

CBR VALUES

NON-PLASTIC SAND SANDY CLAY

PI = 10

SANDY CLAY

PI = 20

SILTY CLAY

PI = 30

HEAVY CLAY

PI > 40

0.5 8 -14 8 - 14 3 - 4 3 - 4 2

1 15- 29 8 - 14 5 - 7 3 - 4 2

2 15 – 29 15 - 29 8 - 14 5 - 7 3 - 4

3 30 15 - 29 8 -14 5 - 7 3 - 4

The highest seasonal level attained by the water table should be used.

The design philosophy is that all inputs to the

design process should be mean values. It is

sufficient to take the mean of all CBR values for

design purposes.