Week 2 Challenge
The loads to be considered for abutment design according to IRC 6 (Indian Road Congress):
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Dead Load (DL):
- Self-weight of the abutment components, including the stem, heel, toe, and any other permanent parts.
- Permanent loads due to utilities, railings, etc.
2. Live Load (LL):
- IRC classifies vehicles into various classes, such as Class AA, Class A, Class B, etc., each having different axle loads and configurations.
- Consider the live load as per the applicable vehicle class and load combinations specified by IRC 6.
3. Earth Pressure Load:
- Lateral earth pressure due to the soil behind the abutment is an essential consideration.
- IRC 6 recommends using Rankine's theory for determining earth pressure distribution.
4. Impact Load:
- Consider an impact factor on live loads as per IRC 6, which depends on the type of vehicle class.
5. Thermal Load:
- Account for the temperature variation and its effect on the bridge components.
To check the overturning and sliding of the abutment, we'll perform stability checks for both conditions.
Overturning Check:
Given data:
- Abutment height (h) = 4 m
- Embedment depth (d) = 2 m
- Soil Bearing Capacity (SBC) = 200 kN/m²
- Eccentricity (e) = 0.3 m
- Coefficient of friction (μ) = 0.5
- Soil density (γ) = 18 kN/m³
1. Overturning due to Vertical Load: The overturning moment (M) due to the vertical load can be calculated as follows:
M = Weight of the abutment x Eccentricity
Weight of the abutment = Volume of the abutment x Density of the abutment
The volume of the abutment = Area of the abutment x Height of the abutment
Assumptions: let's consider the abutment as a rectangular block.
Area of the abutment (A) = Width x Depth Width can be taken as the minimum dimension (assuming it is facing the direction of loading).
Let's assume Width (b) = 2 m.
Volume of the abutment = 2 m x 2 m x 4 m = 16 m³ Weight of the abutment = 16 m³ x 18 kN/m³ = 288 kN
M = 288 kN x 0.3 m = 86.4 kN·m
2. Overturning due to Soil Resistance: The resisting moment (MR) due to soil resistance can be calculated as follows:
MR = SBC x Area of the base x (h/2)
Area of the base = Width x Depth Area of the base = 2 m x 2 m = 4 m²
MR = 200 kN/m² x 4 m² x (4 m / 2) = 800 kN·m
Stability Check for Overturning: To check stability, we need to ensure that the resisting moment (MR) is greater than or equal to the overturning moment (M).
MR >= M 800 kN·m >= 86.4 kN·m
Since MR is greater than M, the overturning is stable.
Sliding Check: let's check the sliding stability of the abutment.
The sliding force (Fs) can be calculated as follows:
Fs = SBC x Area of the base
Fs = 200 kN/m² x 4 m² = 800 kN
The resisting force (Fr) due to friction can be calculated as follows:
Fr = μ x Weight of the abutment
Fr = 0.5 x 288 kN = 144 kN
Stability Check for Sliding: To check stability, we need to ensure that the resisting force (Fr) is greater than or equal to the sliding force (Fs).
Fr >= Fs 144 kN >= 800 kN
Since Fr is less than Fs, the abutment will slide. This is not acceptable.
Design of Sections and Detailing of Steel: To improve the sliding stability, we can provide a shear key at the base of the abutment. This will increase the resistance to sliding.
Deciding Borehole Location and Depth of Exploration for Shallow Foundation:
The location and depth of boreholes for geotechnical investigation depend on several factors, including the size and type of the structure, local soil conditions, and potential variability of the subsurface strata. Here are some general guidelines to decide borehole locations and depths:
Borehole Locations:
- Boreholes should be placed at critical points where the foundation loads are concentrated, such as column locations, wall ends, and corners.
- If the foundation is spread over a large area, additional boreholes should be located in between to capture any variations in soil properties.
- Boreholes should be placed outside the influence zone of any nearby existing foundations or construction activities.
- Consider the accessibility and safety aspects while selecting borehole locations.
Borehole Depth:
- The depth of exploration should extend below the planned foundation level to assess the soil conditions at the depth where the foundation loads will be transferred.
- Typically, the borehole depth should extend beyond the foundation depth by at least 1.5 to 2 times to investigate the underlying strata adequately.
- In areas with significant depth variations in strata, deeper boreholes may be required to assess the deeper soil layers' properties.
3. Parameters from Geotechnical Investigation for Foundation Design:
A geotechnical investigation provides crucial information for the foundation design. The following parameters need to be obtained from the site investigation:
- Soil Type: The type of soil, its properties, and its stratification.
- Soil Bearing Capacity: To determine the safe bearing capacity of the foundation soil.
- Groundwater Table: To assess potential issues related to water table fluctuation.
- Slope Stability: If the site has slopes, their stability should be assessed.
- Presence of Rock: The depth and type of rock (if present) can affect foundation design.
- Shear Strength Parameters: Parameters like cohesion and angle of internal friction are essential for design calculations.
- Compressibility and Settlement: Information about soil compressibility and expected settlement under foundation loads.
- Seismic Properties: If the site is in a seismic zone, seismic parameters are critical for foundation design.
- Frost Depth: In cold climates, the depth of frost penetration can influence foundation design.
4. Different Types of Soils/Rock (Minimum 5): Here are five common types of soils/rock encountered in geotechnical investigations:
- Clay: Fine-grained soil with low permeability and high compressibility.
- Silt: Fine-grained soil with moderate permeability and moderate compressibility.
- Sand: Coarse-grained soil with good drainage and moderate compressibility.
- Gravel: Coarse-grained soil with high drainage and low compressibility.
- Rock (Granite, Limestone, etc.): Hard and dense geological formations.
5. Criteria for Shallow Foundation Selection:
Criteria for selecting the foundation type and level for shallow foundations depend on various factors such as soil characteristics, expected loads, and site conditions. Some general guidelines include:
- Bearing Capacity: Ensure the soil has adequate bearing capacity to support the loads from the structure.
- Settlement: Avoid excessive settlement by choosing the appropriate foundation depth and type.
- Groundwater: Consider the groundwater table to prevent issues related to buoyancy and water infiltration.
- Stability: Ensure the foundation is stable against sliding, tilting, and overturning.
- Frost Depth: Consider frost depth to prevent frost heave issues in cold regions.
- Seismic Considerations: In seismic-prone areas, design foundations to resist earthquake forces.
Typical Borehole Drawing:
A borehole drawing typically includes:
- Borehole identification number and location on the site plan.
- Depths of soil layers and rock encountered.
- Soil types, descriptions, and properties (e.g., colour, texture, moisture content).
- Groundwater table level.
- Rock type, description, and properties .
- Any encountered bedrock and its depth.
- Any anomalies or inclusions encountered during drilling.
- Any additional notes or observations made during the investigation.