Question

1. how to select suitable soil as the subgrade material

2.describe the different types of islands used in channelizing at grade intersections indicating the principal function of each type

Answer 1

(1) The selection of suitable soil as the subgrade material can be done by-

The properties of sub grade material must have -

Stability

Incompressibility

Permanency of strength

Minimum changes in volume

Stability under adverse conditions of weather and ground water

Good drainage

Ease of compaction

Sub grade material is an integral part of the road pavement structure because it provides the support to the pavement from beneath.

The sub grade material and its properties are very important in the design of pavement structure.

The function of the sub grade material is to provide adequate support to the pavement and for this the sub grade must possess sufficient stability under adverse climatic and loading conditions.

It is very important to conduct tests for selection of soil as subgrade material. These test includes-

(i)Shear tests

(ii)Bearing tests

(iii)Penetration tests

(i)Shear tests

It is usually carried out on relatively small soil samples in the laboratory.

To find out the strength of soil a number of representative samples from different locations are tested.

Some of the common known shear tests for soil are direct shear test, triaxial compression test and unconfined compression test.

(ii)Bearing tests

It is a loading tests which are carried out on sub grade soils in-situ with a load bearing area.

The results of the bearing tests are affected by variations in the soil properties within the stressed soil mass underneath and hence the overall stability of the part of the soil mass stressed must be studied.

(iii)Penetration test

Penetration test are considered as smallscale bearing tests in which the size of the loaded area is relatively smaller and ratio of the penetration to the size of the loaded area is greater than the ratios in bearing tests.

It can be carried out in the field or in the laboratory.

(2) The different types of islands used in channelizing at grade intersections indicating the principal function of each type are-

An island is an area between traffic lanes for control of vehicle movements.

Within an intersection area a median or an outer separation is considered as an island.

It may range from an area traced by barrier curbs to a pavement area marked by paint.

Types of islands are-

Channelizing Islands

These islands are designed to control and direct traffic movement usually turning.

Divisional Islands

These islands are designed to divide opposing or same direction traffic streams usually through movements.

Refuge islands

The pedestrian islands are provided to serve as safety zones for the aid and protection of persons on foot.

If a divisional island is located in an urban area where pedestrians are present the portions of each island can be considered a refuge island.

The guidelines for selection of island type are-

Since all intersection has a unique geometry, flow values and So it needs special attention as far as the use of channelization devices are concerned.

The factors affecting the selection of the island type are-

Traffic characteristics at the intersection

Cost considerations

Maintenance needs

TYPES ARE-

1 Flush Channelization

The function of flush channelization are -

On high speed rural highways to separate turning lanes.

In constrained locations i.e. the locations where vehicle path definition is desired but space for raised islands not available.

For separating opposing traffic streams of low speed streets.

In the areas where frequent removal of snowfall is required, i.e. in places of high snow fall.

It can also be used as a temporary channelization either during construction or to test traffic operations prior to the actual installation of raised islands.

The disadvantage of flush channelization are-

It is not effective in prohibiting or preventing traffic movements.

It is also not appropriate for islands intended to serve as pedestrian refuge.

2 Raised Islands

The functions of raised islands are-

The main function of the channelizing device is shielding pedestrians or to provide refuge to pedestrians crossing a street.

The primary or secondary function is locating traffic signals or other fixed objects.

The intention is to prevent certain traffic movements.

To separate high volume opposing traffic flows.

The raised islands are also particularly important at intersections with unusual geometry like skewed intersections.

Guidelines for design of Traffic Islands

The main design principles followed for the design of the shape and size and shape of the traffic island are as follows-

1. Shape and size- The islands are generally either narrow and elongated or triangular in shape which are normally situated in the areas of roadway outside the planned vehicle paths, and are shaped and dimensioned as component parts of the intersection layout.

2. Traffic lanes or turning roadways must appear natural and convenient to their intended users.

3. The number of islands should be held to a practical minimum to avoid confusion.

4. The islands s must be large enough to be effective. The small islands do not serve as channelizing devices and pose maintenance problems.

5. It should not be introduced at locations with restricted sight distance or middle of sharp horizontal curves due to sight distance considerations.

Guidelines for design of Median islands

The design principles or guidelines to be followed in the design of median islands are-

1. The approach noses should be offset 0.5 to 1.5 m from through lanes to minimize accidental impacts.

2. The shape should be based on design turning paths and island function.

3. The length of median before the intersection is related to approach speed .It is also affected by available widths, taper designs and local constraints.

4. The width of the medians must serve its primary intended function.

5. The median should be always be provided well past crest vertical curves.