Civil Engineering Subgrade Design

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Questions and Answers

What is the significance of the Linear Shrinkage (LS) in the CBR calculation?

It influences the calculated CBR value. (correct)

It is a direct measure of moisture content.

It affects the plasticity index results.

It is not related to the CBR calculation.

Which of the following is NOT a factor considered in selecting CBR for moisture conditions?

Design moisture content

Thickness of pavement layers (correct)

Equilibrium conditions

In-service moisture regime

What is meant by 'in-service' moisture conditions?

Moisture is consistent and remains unchanged.

Moisture conditions are evaluated at the center of the pavement.

Moisture is influenced by seasonal variations. (correct)

Moisture conditions are tested solely in dry seasons.

What approach is recommended for CBR testing during adverse climate conditions?

Testing should be undertaken under worst climate conditions. Signup and view all the answers

Which equation represents the first estimate for calculating CBR from grading?

log CBR = 1.688 - 0.00506P + 0.00186P - LS + 0.000385P Signup and view all the answers

What does the repeat load triaxial testing provide?

Modulus values across varying stress conditions. Signup and view all the answers

What criteria should be considered when selecting the design CBR?

The worst-case scenarios under worst climate conditions. Signup and view all the answers

Which sieve size is relevant for determining the fines content in graded materials for CBR calculations?

2.36 mm Signup and view all the answers

What is the primary purpose of pavement drainage?

To control the water that may affect the pavement structure Signup and view all the answers

Which type of asphalt is specifically designed to allow for very high binder content?

Stone Mastic Asphalt Signup and view all the answers

What do table drains primarily do in the context of rural roads?

Drain water from the pavement away from the structure Signup and view all the answers

What is the significance of California Bearing Ratio (CBR) in pavement design?

It serves as a measure of shear resistance for subgrade strength. Signup and view all the answers

What role do cut-off drains serve in pavement engineering?

To control slope scour and manage erosion Signup and view all the answers

Why is subsoil drainage important when groundwater levels are high?

It minimizes water flow into the pavement structure. Signup and view all the answers

Which material can be used in a spray seal application?

Hot bitumen or bitumen emulsion Signup and view all the answers

What should be considered when selecting materials for batters in pavement design?

Materials that resist erosion Signup and view all the answers

For effective pavement drainage, catch pits are used to divert rainwater into what system?

Pipe systems Signup and view all the answers

What is an additional benefit of an excellent culvert beyond passing water?

Allowing for aquatic life passage Signup and view all the answers

What is the maximum allowed CBR value for a stabilised subgrade?

20% Signup and view all the answers

In the example of a subgrade with three layers, what is the thickness of the first layer?

0.2m Signup and view all the answers

How is the CBR value of the second layer calculated when a subgrade is stabilised to a depth of 0.4m?

It is an average of the original CBR and the stabilised CBR. Signup and view all the answers

What must be true about the layers when using the Japan method for CBR calculations?

Layers must decrease in CBR value. Signup and view all the answers

For a layered subgrade with CBR values, how do you calculate the average CBR?

Multiply each CBR percentage by its layer thickness and divide by the total thickness. Signup and view all the answers

What is the depth range for the original CBR to be assigned to the remaining material after the stabilised layer?

0.4m to 1m Signup and view all the answers

If a three-layered subgrade has layer thicknesses of 0.2m, 0.5m, and 0.3m, what should the total thickness of layers equal?

1.0m Signup and view all the answers

What happens to the CBR of the lower 0.2m of a stabilised subgrade?

It is averaged with the stabilised CBR value. Signup and view all the answers

What should be done if anomalies cannot be explained in pavement analysis?

Take the lowest value Signup and view all the answers

When designing heavy duty pavements, what value is primarily used for the design CBR?

Lowest reasonably confirmed CBR Signup and view all the answers

What is the maximum allowable CBR value typically recommended for sands?

10 Signup and view all the answers

Which of the following is NOT a method for treating soft subgrades?

Increasing CBR to above 5 Signup and view all the answers

What is one result of lime stabilisation applied to subgrades?

Improves stiffness Signup and view all the answers

Which method assigns the CBR for the lower 200 mm of a stabilised layer?

Average of the stabilised CBR and existing CBR Signup and view all the answers

What is the purpose of geogrids in pavement construction?

Tensile reinforcement only Signup and view all the answers

For good drainage conditions, what is the typical CBR value for sandy clay?

5-6 Signup and view all the answers

Which statement is TRUE regarding composites in pavement construction?

Combine functions of geofabrics and geogrids Signup and view all the answers

What is the typical effective depth for road pavements below the top of the subgrade?

1 m Signup and view all the answers

Which test is used to measure the California Bearing Ratio (CBR) for surface levels only?

Clegg Hammer Signup and view all the answers

What is one of the properties that should be tested for subgrade classification?

Atterberg Limits Signup and view all the answers

What is the purpose of soaking a sample for 4 days during a CBR test?

To simulate field conditions Signup and view all the answers

How is the CBR determined from load and penetration?

By plotting and correcting initial loads Signup and view all the answers

Which method is NOT an insitu test for determining subgrade CBR?

Undisturbed CBR test Signup and view all the answers

What corrective action is needed when the CBR curve is initially concave up?

Adjust for shape Signup and view all the answers

Which of the following measurements helps find the soil type using the Electric Friction Cone Penetrometer?

Tip Resistance Signup and view all the answers

What is an essential factor to be considered during the CBR test that may affect the accuracy of results?

Water Table Level Signup and view all the answers

What is a key characteristic of industrial pavements in terms of effective depth?

Can be 2 to 3m Signup and view all the answers

Which factor primarily influences the classification of subgrades?

Natural Soil Composition Signup and view all the answers

Which type of CBR test focuses on fully saturated conditions?

Soaked CBR Signup and view all the answers

What effect do polymer materials have before CBR testing?

They require a drying cycle Signup and view all the answers

Which formula approximates the CBR from the Clegg Impact Value (CIV)?

CBR ≈ 0.06CIV² + 0.52CIV + 1 Signup and view all the answers

Study Notes

CBR and Pavement Stabilisation

CBRmax ≤ 20 for cement-stabilised subgrades.
Subgrade layers must decrease in CBR and be thicker than 0.2m, totaling 1.0m for stability.
Example involves 3 layers with CBR values: Layer 1 (0.2m, 8%), Layer 2 (0.5m, 4%), Layer 3 (0.3m, 2%) resulting in an average CBR of 3.8%.

Japan Method for CBR Calculation

Utilises weighted average of layer CBR and thickness to determine the effective CBR.
Involves calculating CBR based on multiple layers, assessing only top two layers when used for certain depths.
In case of stabilised subgrades, the original and stabilised CBR must be averaged for the lower 0.2m layer.

Heavy Duty vs. Road Pavements

Road pavements are typically designed with an effective depth of 1m, while industrial pavements may extend to 2-3m due to heavier loads.
Variances in behavior and requirements for heavy duty due to unregulated axle loads.

Subgrade Characterisation

Subgrades vary significantly based on natural soil properties, requiring geological mapping and systematic sampling.
Essential properties like Particle Size Distribution, Atterberg Limits, and California Bearing Ratio (CBR) must be tested.

CBR Testing Methods

Insitu tests include the CBR Test, Dynamic Cone Penetrometer, and others.
Laboratory tests include Undisturbed CBR and Triaxial tests, highlighting that variability across labs is common.

CBR Testing Procedure

Tests performed in a compacted sample, soaked if required, with load applied via a piston to determine resistance.
Corrected load values at specified penetration depths are used for accurate CBR determination.

Equipment and Measurements

Static/Dynamic Cone Penetrometers and Clegg Hammers provide quick field estimates of CBR by measuring penetration resistance.
Falling Weight Deflectometers assess pavement response to dynamic loads, estimating subgrade modulus.

Drainage in Pavement Design

Effective drainage systems are critical to manage surface water and prevent pavement failure.
Proposed methods include table drains and culverts, ensuring both aquatic life passage and road integrity.

Subgrade Support and Strength

CBR values influence pavement thickness design; maximum CBR values are suggested (e.g., sands CBRmax = 10).
Soft subgrades with CBR < 5 may require special construction techniques and groundwork stabilization.

Moisture Balance and CBR Selection

Moisture conditions (equilibrium, in-service, and design) impact subgrade behavior; field tests should reflect worst-case scenarios.
Statistical methods facilitate CBR estimates based on measured results, with adjustments for soil variability.

Treatment of Soft Subgrades

Strategies for soft subgrades include drainage, replacement of material, stabilisation, and geotextile applications to enhance performance.

Lime Stabilisation and Geotextiles

Lime stabilisation addresses soil stiffness, permeability, and water sensitivity, influenced by lime demand testing.
Geotextiles provide separation and reinforcement, while geogrids enhance structural integrity when laid under pavement.

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Description

This quiz focuses on the principles and calculations involved in civil engineering subgrade design using the Japan Method. It includes understanding CBR values, layer thickness requirements, and the overall design framework for layered soil structures. Test your knowledge and application of these essential concepts.