Week 2 Challenge

Aim and Introduction:

• To explain about loads, that is need to be applied in the abutment of the bridge as per IRC 6.
• And to check for overturning and sliding of the abutment.
• To design the section of stem, heel and toe. and to present the detailing of the steel in the foundation and wall.
• To explain about a selection of borehole location and its depth of exploration if it is the shallow foundation.
• To explain the parameters taken from a geotechnical investigation that is needed for the design of the foundation.
• To List five different types of soils or rocks and criteria that should be followed for selecting the foundation level of shallow foundation.
• Overturning occurs when the turning moment due to lateral forces exceeds than to the self-weight of the wall.
• Overturning failure is a result of excessive lateral earth pressures with relation to retaining wall resistance thereby causing the retaining wall system to topple or rotate.
• Sliding failure is the change in the position of the retaining wall far away from the backfill due to the forces from the backfill on the wall.
• The sliding failure will displace the base of the wall, which is mainly due to the shearing failure.

Elaboration:

• 
• the above image shows the dotted line, which is the original location of the retaining wall and the wall with the black represents the final location of the retaining wall due to the effect of the sliding of the wall and the pressure created on the wall from the backfill.
  
• 
• The above image shows the overturning failure.

Abutment and loads applied on it :

• Abutments are constructed at the ends of bridges to retain the earthfill and to carry the vertical and horizontal loads from the superstructure to the foundation.
• The abutments designed must be stable against overturning and sliding.
• An abutment is subjected to both horizontal and vertical loads from the superstructure and also resists loads from the backfill material and any water that may be present.

Loads acting on abutment

• Dead Load: The dead load carried by a member shall consist of the portion of the weight of the superstructure which is supported wholly or in part by the girder or member including its own weight.
• Live Load: The design live load shall consist of standard wheeled or tracked vehicles or trains of vehicles
• Vehicular braking force from live load: Tractive effort caused through the acceleration of the driving wheels, Braking force is due to the application of the brakes to braked wheels
• Wind load: The wind pressure acting on a bridge depends on the geographical location, the terrain of the surrounding area, the fetch of terrain upwind of the site location, the local topography, the height of the bridge above the ground, horizontal dimensions and cross-section of bridge or its element under consideration.
• Earth pressure: The pressure exerted by filled material over the abutment.
• Water load: Any part of a road bridge that may be submerged in running water shall be subjected to water load.
• Live load surcharge.
• Temperature and shrinkage: Daily and seasonal fluctuations in shade air temperature, solar radiation, cause the Temperature load.
• Earth surcharge.

Check for overturning and sliding:(Gravity retaining wall)

• The wall can slide along the bottom due to the lateral soil pressure. The factor of safety can be calculated using the formula.
  
  The factor of safety against sliding is defined as forces preventing sliding along the bottom divided by the forces that will cause sliding along the bottom surface.
  
  The factor of safety should not be less than 1.5 for sliding conditions.
  
  
  

• The moment about toe is induced from the lateral soil pressure causing it to overturn. The weight of the wall will have a moment resisting the overturn. The factor of Safety against Overturning is expressed using the formula.
• FS = FR/ \PH
  
  The Factor of Safety is defined as the sum of moments of forces preventing the rotation about (A) and (B) divided by the sum of the moment causing the rotation about the toe.
• Sum of moments that resist overturning _ MR
  Sum of overturning moments M0
  
  The Factor of Safety against Overturning should not be greater than 2.0.

• Pa = Ka γsoil h2/2
• Gravel and coarse sandy backfill soil φ 33-36
• Height of wall = 4m
• Pa =1/2 - x 18.5 x (4*4)x 1/3
• pa=48.53Kn
• Ph = Pacos β =48.3 cos 15' = 45.5 kN / m
• Ph = v = Pasinβ = 48.3 cos 15'= 12 kN / m

overturning :

The overturning moments are calculated by taking moments about point O, the whole section is divided into parts. Let these forces be represented by w1, w2 and the corresponding lever arms asl1, l2, ... l5

• assume the weight of concrete yc = 24 kN/m3
• MR — W1l1 + w2 + ... w5/5

Section No.	Area (m2)	Unit weight kN/m	Weight kN/m	Lever arm(m)	Moment kN-m
1	1.20	18.5	22.2	3.75	83.25
2	18.75	18.5	346.9	3.25	1127.4
3	3.56	24	85.4	2.38	203.25
4	3.13	24	75.1	1.50	112
5	0.78	24	18.7	1.17	230.85
			vertical = 596.9		Mr = 1779.3

• M0= 45.5x2.6 =118.3
• fs=1779.3/118.3 is greater than 2.0 hence safe.

sliding:

To find FR and PH (FD)

Assume h = 4m, y= 19 kN/m3, c = 60 kN/m , total vertical force Iv = 596.9 kN

Pp = 1/2 x 19 x 4 x 4 x 2.46 + 4 x 60 x 2 x (2.46)^1/2 =840kn/m

FR = 60 x 4.75 + 596.9 tan 25° + 840 =1363.76

PH = 181.2

fos = 1363.76/181.2 is greater than 1.5 hence safe.

A borehole and Geotechnical investigation of the site:

• Borehole investigations are done to get the properties of materials underneath soil layers, The type of foundations is given based on the condition of the soil layers.
• To perform a borehole investigation, we need to decide the number of boreholes to be drilled,

The borehole depends on factors

1. Plot coverage of the building.
2. The scale of the project.
3. Structural loading and load distribution arrangments.
4. Condition of the ground.
5. Importance of the structure.

A borehole is a deep vertical hole with a small diameter drilled into the ground to obtain soil samples for soil investigation required for the construction of the suitable foundation for the planned structure.

The depth of Exploration carried out to a depth up to which the increase in pressure due to structural loading is likely to cause perceptible settlement or shear failure of the foundation, which depends upon the following factors,

• Type of structure
• Load on structure
• Size and shape of the foundation
• Position of loaded areas
• Soil profile and its properties

The depth of exploration should be one and half times to two times the estimated width of the footing, single or combined, from the base level of the foundation.

Site Investigation:

- Soil Exploration (Boring Holes)
- Collection of Soil Samples
- Conducting the in-situ tests
- Study of Groundwater conditions and Collection of water
sample for chemical analysis
- Preparation of Drawings, Charts, etc
- Analysis of Data Collected
- Preparation of Repor

TYPES OF SOIL :

• Black cotton soil: It is a residual soil containing a high percentage of the clay mineral – montmorillonite. It has a very low bearing capacity and high swelling and shrinkage properties.
• Clay: It consists of microscopic and sub-microscopic particles derived from the chemical decomposition of rocks. It contains a large number of clay minerals. It can be made of plastic by adjusting the water content. It exhibits considerable strength when dry. Clay is a fine-grained soil. It is cohesive soil. The particle size is less than 0.002mm.
• Gravel: It is a type of coarse-grained soil. The particle size ranges from 4.75mm to 80mm. It is a cohesion-less material.
• Sand: It is coarse-grained soil, having a particle size between 0.075 mm to 4.75 mm. The particles are visible to the eye. The soil is cohesionless and pervious.
• Peat: It is a type of organic soil having fibrous aggregates of macroscopic and microscopic particles. It is formed from vegetal matter under conditions of excess moisture, such as in swamps. It is highly compressible and not suitable for foundation
• Cobbles: Cobbles are large size particles in the range of 80mm to 300mm.

criteria Shallow foundation level :

Soil layer: The topsoil is organic soil or waste, which should be completely removed and the foundation is placed below the topsoil. If the topsoil extends to a larger depth, it may be replaced with a suitable backfill material and the foundation may be placed at the minimum depth.

Soil strength: The foundation should be taken to a level where soil with reasonably high bearing capacity and less compressibility is available. This is the usual rule unless the depth to the hard stratum is very large. This will mainly ensure that the size of the foundation is not excessively large and hence economical.

Groundwater: The foundation should be located above the groundwater table (GWT) to avoid undesirable effects of the groundwater on the foundation as well as the foundation soil. However, the GWT level changes with seasons.

Scour Depth: all bridges, the foundations should be located below the maximum scour depth plus the minimum grip length below the highest flood level.

RESULT :

• The recommended borehole depth is equal to one to three times the width of the loaded area.
• Hence the parameters taken for geotechnical investigation have been explained and the condition for sliding, overturning conditions were explained.
• The soil investigation report is the final document of the sub-soil investigation which contains important information’s for the designer. The report must be prepared in such a manner that the reader is able to get a complete picture of the subsurface condition of the site.
  A good soil report should include the following:
  1. Introduction
  2. Borehole log
  3. Method of investigation
  4. Laboratory test results
  5. Analysis of results
  6. Recommendations.