Week 2 Challenge

Question 1 -Define all loads to be applied in the abutment of the bridge as per IRC 6 and mention the clauses. and check overturning and sliding of 4 m height of Abutment (from ground level) , embedded 2 m in ground.

SBC = 200 kn/m2 ( Eccentricity from bearing and abutment center is 0.3 m )

Use coefficient of friction =0.5

Soil density =18 kn/m3

Design the section of stem, heel and toe. Also show the detailing of the steel in the foundation and wall.

(Note: Draw all the drawings to the scale and neatly).

Answer 1 -

As far as the design is concerned we have to assume various data for the design :

Assumed Data-

1. The Length of the Span of the Bridge is 10.6 m (Overall Length) & 10.0 m (Effective Length).
2. The Depth of the Girder and Deck Slab Combined is 1.066 m.
3. All The Levels Such as FRL, HFL, Cap Top, Cap Bottom, Sill Level & Foundation Level is Assumed Accordingly.
4. Height of the Shaft is 4.0 m, Width of the Shaft is 12.9m, which is carrying a Carriageway of 10.5 m.
5. Safe Bearing Capacity of the Soil is 200 KN/M2 is given.

Level Details-

1. F.R.L. - 100 m.
2. Soffit Level - 98.859 m. (100 - 0.075 (Wearing Coat) - 1.066 (Depth of Girder).
3. Cap Top Level - 98.559 m. (Soffit Level - Bearing Depth 90mm and Pedestal Depth 210 mm).
4. Cap Bottom - 98.259 m.
5. Bed Level - 94.759 m.
6. Sill Level - 94.259 m.
7. Foundation Level - 92.759 m.

Structure Details-

1. Superstructure Grade of Concrete - M30
2. Substructure Grade of Concrete - M30
3. Foundation Grade of Concrete - M30
4. Heel Length - 3.0 m.
5. Toe Length - 2.0 m.
6. Foundation Straight Part Depth - 0.5 m.
7. Foundation Sloped Part Depth - 1.0 m. (1/3rd of Heel Portion)
8. Abutment Cap Width - 12.9 m.
9. Abutment Cap Length - 1.0 m.
10. Shaft Length at Top - 1.0 m
11. Shaft Height at Bottom - 1.2 m.

Question 2 -How to decide the boreholes location and its depth of exploration in case of shallow foundation? What all the parameters we need to get from the site/geotechnical investigation for design of the foundation.

List different type of soils/rock (minimum 5) and what criteria should be followed for the shallow foundations for selecting the foundation level, their characteristics and collect a picture of each. Draw a typical borehole showing all the data.

Answer 2 -

Bore Hole Locations are basically decided where the Substructures are likely to be placed, if the hydrology data states that the bridge is required of 2 span having 2 abutments and 1 pier then bore hole locations decided in this location will be 3, Similarly in case of 3 span where 2 piers are present and 2 abutments then 4 bore hole locations will be decided. In Short, Bore Hole Locations = Span + 1.

As far as Shallow Foundations are concerned, These foundations are provided when the soil is having a good Safe bearing Capacity as well as strata contains weathered or soft rock and not in greater depths. If these ideal conditions are available at site only then we consider to provide the Shallow Foundation if the Safe bearing Capacity of the Soil is not good at the top so we have to excavate deeper to gain strength, in such conditions if we place our shallow Foundation in to greater depths the cost of Excavation is likely to be more also the substructres and foundations concrete as well as steel requires huge Cost which is not economical, and we opt for Deep Foundations in those cases.

There are various Parameters which we take from soil Exploration data

Suppose there is the Bridge of 7 Span and the Bore hole locations are 8 below are the images attached in which the details of these bore locations are presented.

Sub Soil Profile (Bore Log Data)

Individual Field Test Data at A1

Individual Field Test Data at P1

As we can see in the Field test data of P1 there is various details present Such as Description of Strata, Soil SPT N Value, Core Drilling Etc.

If we have to decide the foundation depth for the above case we can see the strata is silty sand which is also having very less SPT N Value this means this soil has poor safe bearing Capacity, So to rest our founding level we will place our foundation upto the depth of hard rock and as IRC Suggest in case of weathered rock the embeddment of the foundation will be 1.5m and in case of Hard Rock we embedd our foundation upto the depth of 0.6m. therefore, the depth of foundation in P1 will be 10.6m deep from the bed level.

Also the the rock we considering as Weathered rock or soft rock is totally dependent on the crushing strength of the rock if the rock is having crushing strength of more than 10 MPa then it is considered as Hard Rock else it is a Weathered Rock.

There are Different Types of Soil such as :

1. Clayey Soil.

2. Sandy Soil.

3. Silty Soil

4. Peaty Soil

5. Chalky Soil

6. Loamy Soil