Week 2 Challenge

Solution 1

• Various loads to be applied in the abutment of the bridge and clauses as per IRC 6 are mentioned below:

Dead Load – They are the load from the self-weight of the structure and its components. Self-weight of the structure depends on the density of material.

Live Loads – They are the loads from the vehicles or railways or any other mobile, impact loads, Liquid traveling in pipes etc. Loading should be applied as per cl.204 according to the class of vehicles and other parameters as pedestrian loads etc.

Super Imposed Dead Loads – they are the loads from the weight of the pavement, parapet walls, electrical and other utilities etc. Loading should be applied as per cl.203 according to the material provided.

Surcharge – Additional weight acts on the soil or other component such as abutment. Loading is applied to the structure from the load above the soil.

Pressure from soil above foundation – Weight of soil over the footing also needs to be considered.

Lateral Earth pressure – It is the load acts on the structure by the lateral movement of the soil. Loading should be applied as per cl.214 according to the soil condition.

Water current forces – It is the transverse thrust acts on the structure by the movement of the water under it. Loading should be applied as per cl.210 according to the zone of current flow.

Temperature loads – Due to the different weather conditions, structure acts under the effect of expansion and contraction according to the thermal coefficient of its material. Loading should be applied as per cl.215 according to the weather condition of the localized area.

Seismic Loads – Lateral force created by oscillatory vibrations due to the earthquakes acts transverse on the structure. Loading should be applied as per IS 1893 2016 according to the Seismic zone of the location.

Wind Loads - Lateral force created due to the intensity of wind acts transverse on the structure. Loading should be applied as per IS 875 (part 3) according to the basic wind speed of the location.

Buoyancy – Upward thrust offers by the water present in the soil supporting the foundation or the water under the bridge deck over the flooded water bodies. Loading should be applied as per cl.213 according to the material provided.

Vertical Seismic forces – Due to the torsional movement from the earthquake tends to uplift the corner or end supports to move in upward direction.

Vertical Wind Forces – Due to the transmission of wind to the different component of bridge, a circular movement creates to the area beneath the deck slab, which results in the uplifting of bridge or any other structure.

• Given Data:

Safe Bearing Capacity = 200 KN/m²

Eccentricity = 0.3 m

Depth of Foundation = 2.0 m

Height of abutment above ground level = 4.0 m

Coefficient of friction (Φ) = 0.5

Soil density (ρ)= 18.0 KN/m³

#Check against overturning

Coefficient of active earth pressure, Ka = (1-sinΦ)/ (1+sinΦ) = (1-sin50) / (1+sin50) = 0.133

Sr. No.	Description of Load	Magnitude of Load (KN)	Distance of C.G. (m)	Moment about C.G. (KN-m)
1.	Wt. of Stem wall	(0.4+0.65) x 5.35 x 25 = 140.44 KN	1 + [0.42+0.652 – (0.4 x 0.65)] / (0.4+0.65) = 1.307 m	141.747 KN-m
2.	Wt. of Base Slab	3.65 x 0.65 x 25 = 59.313 KN	3.65/2 = 1.825 m	108.25 KN-m
3.	Wt. of Earth fill over heel	[0.5 x (0.65-0.4) + 2] x 5.35 x 18 = 204.64 KN	1.65 + [22+2.152 – (2 x 2.15)] / (2 + 2.15) = 2.692 m	550.9 KN-m
4.	Wt. of Earth fill over toe	1.55 x 1 x 18 = 27.9 KN	1/2 = 0.5 m	13.95 KN-m
		ΣW = 432.293 KN		ΣM = 5772.947 KN-m

Total Horizontal earth pressure over the abutment tending to over turn it = P = ½ x ρ x Ka x H² = 0.5 x 18 x 0.133 x 5.35² = 34.261 KN

Acting at 4/3 from D

Total Overturning Moment, M_o = 34.261 x 4/3 = 45.6813 KN-m

Total Stabilizing moment = 5772.947 KN-m

Factor of Safety (against overturning) = 5772.947 / 45.6813 = 126.374 > 2, Hence Safe

#Check against sliding

Total Horizontal earth pressure over the abutment tending to overturn it = P = ½ x ρ x Ka x H² = 0.5 x 18 x 0.133 x 5.35² = 34.261 KN

Total force opposing sliding = μ x ΣW = 0.5 x 432.293 = 216.15 KN

Factor of Safety (against sliding) = 5772.947 / 34.261 = 168.5 > 1.5, Hence Safe

Solution 2:

The adequate number of bore holes and sufficient depth is critical to gather enough data for the design and construction substructures as foundation, abutments, retaining walls, embankments.

• Borehole numbers should be enough to find out soil strata variations at the particular site or location.
• Borehole should be casted depending upon the area of construction of site, practically on a plane terrain, at least 3 boreholes should be placed at a site with the offset of 30 - 50 m intervals.
• Bore hole should be placed at the heaviest chances of loading, in bridges it means at every position of pier and abutment.
• Quality and depth of boreholes should keep undisturbed so that the detail of each strata at the site can be analysed most precisely.
• In the case of very dense materials or bedrock, the minimum depth of borehole is 6m whereas usually 10m depth hole is dugged out.
• Borehole should drill and carry at least 3m of depth into the bedrock.
• For dense, coarse-grained soil and very stiff clays, extend boring depth up to a range of 5-6m to corroborate that soil layer thickness is sufficient.
• Primarily the depth of borehole must be 1.5 times of the width of the foundation.

Parameters for the site/geotechnical investigation

• Depth of soil strata
• Bulk density (γ-bulk)
• Dry Density (γ_d)
• moisture content
• Gradation of soil (IS Classification)
• silt factor
• Liquid limit
• Plastic Limit
• Plasticity Index
• Specific Gravity
• Swelling Index
• Cohession
• Void ratio & porosity
• Angle of internal friction
• Depth of water table
• SPT (N Value)

Various types of soil are as follows:

1. Sandy soil

It consists of small particles of weathered rock. Soil particles of this soil ranges from 0.075mm to 4.75 mm. This type of soil is very good for the drainage system. Sandy soil is usually by the breakdown or fragmentation of rocks like granite, limestone and quartz.

It has high porosity and cohesion value is 0. Strength is due to interlocking of granular particles. This soil has highly friction property.

2. Silt soil

Silt soil particles are much smaller than the sandy particles and it is made up of rock and other mineral particles, which are smaller than sand and larger than clay. It particles ranges from 0.075 mm to 0.002 mm.

This soil has cohesion both C- Φ value.

3. Clay soil

Clay soil is the smallest particles among the other two types of soil. The particles in this soil are tightly packed together with each other with very little or no airspace. This soil has very good water storage qualities and makes it hard for moisture and air to penetrate into it.

This soil has high cohesion and friction Φ value is 0. And it particle sies ranges less than 0.002 mm.

4. Loamy soil

It is the combination of sand, silt and clay such that the beneficial properties of each are included. For instance, it has the ability to retain moisture and nutrients, Hence, it is more suitable for farming.

5. Rocky soil

Rocky soil is the result of the ground beneath your garden shifting and pushing rocks upwards towards the earth’s surface. Generally, rocks such as limestone, bedrock, and sandstone have substantially high bearing capacities.

Criteria for the selecting the shallow foundation:

• Selection criteria for foundation depend on two factors, i.e. factors related to ground (soil) conditions and factors related to loads from the structure.
• Soil bearing capacity is one of the most crucial criteria that affect the selection of the appropriate type of foundation. Decision can be made to choose shallow or deep foundation based on the soil bearing pressure.
• An allowable bearing pressure of at least 100kN/m^2 i.e 10 ton/m^2 shall be considered. However, higher structures can consider a raft foundation.
• For selecting the founding level of the foundation, the SPT value also plays a major role. The N value should not be less than 10.
• Allowable settlement for the isolated footing as per the Indian standard is upto 50 mm.