Week 3 Challenge

1(a):Hydrology and geotechnical studies:-

                                               Both preliminary and detailed, along with the consideration of the zone of influence, are vital in determining the span of a bridge as per IRC standards and the type of bridge. Let's delve into how each of these studies contributes to the process:

1. Preliminary Hydrology and Geotechnical Studies:

Preliminary studies are conducted in the early stages of bridge planning and design to gather essential data and information. Here's how these studies are utilized: -

1.1 Site Selection:

Hydrology studies help identify potential bridge locations where water bodies (rivers, streams, etc.) may need to be crossed. Geotechnical studies assist in understanding the underlying soil conditions at each potential site, evaluating their suitability for bridge construction.

1.2 Rough Span Estimate:

 Based on the preliminary hydrological analysis, engineers can estimate the approximate span required to provide adequate waterway opening and account for potential flood levels. Geotechnical studies help identify any potential soil challenges that may impact bridge foundation options, which can further influence the span length.

 2. Detailed Hydrology Studies:(IRC-78-2000, Page No-18, Cl-703.1.1)

Detailed hydrology studies involve comprehensive analysis and data collection, providing more accurate information for bridge design. Here's how they contribute to fixing the bridge span:

2.1 Design Flood Determination: -

Detailed hydrological analysis allows engineers to identify the design flood, which represents the maximum flood event expected during the bridge's design life. This information helps determine the required waterway opening and influences the span length and number of spans.

2.2 Water Flow Modelling: -

Hydrological modeling is used to simulate the flow of water during different flood scenarios. This modeling assists in understanding potential scour and debris accumulation, which can impact bridge design, including the span length and foundation protection measures.

3.Detailed Geotechnical Studies:

Detailed geotechnical studies involve extensive soil investigation and laboratory testing to obtain accurate soil parameters. Here's how they affect the bridge span:

3.1. Foundation Design:

 Detailed geotechnical studies provide critical information about the soil's bearing capacity, settlement characteristics, and potential for scour or erosion. This information helps in selecting the appropriate foundation type (shallow or deep) and designing the foundations to support the bridge's span length and structural loads.

 3.2 Soil Improvement Techniques:

In cases where poor soil conditions are found, detailed geotechnical studies recommend soil improvement techniques. These techniques, such as soil stabilization or ground improvement, can enhance the soil's bearing capacity and influence the span length.

4. Zone of Influence:

The zone of influence refers to the area surrounding the bridge site that may be impacted by the bridge's construction, operation, and maintenance. Hydrology and geotechnical studies assess this zone to consider potential environmental impacts and land use considerations. The zone of influence can affect the bridge alignment, approach, and overall span length.

In conclusion, hydrology and geotechnical studies, both preliminary and detailed, are used in fixing the span of a bridge as per IRC standards and the type of bridge. These studies help in determining the waterway opening, foundation design, and potential environmental impacts, all of which influence the final span length and overall bridge design. The combination of these studies ensures that the bridge is safe, sustainable, and well-suited for its specific location and function.

 

Determine the Scour depth at the pier and Abutment location:-

                                                                                   To calculate the maximum depth of scour from the highest flood level for the design of piers and abutments with foundations without any protection works, you can use the guidelines provided by engineering standards and codes. The process typically involves empirical formulas or numerical methods based on the characteristics of the river or stream where the structure is located. One commonly used method is the "Einstein-Brown" equation for estimating scour depth, which is as follows:-

                                                                      

Where,

d_sm = Maximum depth of scour in meters.

K_sf = Silt-Factor (depends on the type of bridge, shape of piers/abutments, and other factors)

D_b = Design-discharge

 

^{Solution: -}

^{Given Data}

^{Silt Factor k}_sf                              ^{=             3 ( IRC:78-2014, Cl.No.703.2.2)}

^{Design-discharge, Db                    =             1000m3/s}

^{Linear Waterway                           =             128 m}

^{Height of Water level Upto HFL       =             5.5 m        (Assumed)}

^{Cross-sectional Area (m²)              =             Width (m) * Depth (m)}

^{Cross-sectional Area (m²)              =             128 m * 5.5 m = 704 m²}

^{Now, we can find the velocity:}

^{(Velocity (m/s) = Discharge (cumecs) / Cross-sectional Area (m²)}

^{Velocity (m/s)                               =             1000 cumecs / 704 m² = 1.421 m/s}

^{Determine the Maximum depth of Scour for Design of Foundation: -(As per IRC:78-2014, Cl.No.703.3)}

^{Under this IRC:78-2014, Cl.No.703.3.3.1:-}

^{We get the following criteria as below without any protection work:-}

^{Maximum Scour Depth (for Flood without Seismic Combination): - (IRC:78-2014, Cl.No.703.3.3.1.1)}

• ^{Piers          = 2.0 x Mean depth of scour = 2x1.174m=2.348 m for all round scours.}
• ^{Abutments = 1.27 x Mean depth of scour = 1.27x1.174=1.49 m for one side retained}
• ^{Abutments = 2.0 x Mean depth of scour=2x1.174=2.348 m for all round scour}

^{Maximum Scour Depth (for Flood with Seismic Combination): - (IRC:78-2014, Cl.No.703.3.3.1.2)}

• ^{Piers          = 2.0 x0.9x Mean depth of scour = 2x1.174x0.9m=2.113 m for all round scours.}
• ^{Abutments = 1.27 x 0.9xMean depth of scour = 1.27x1.174x0.9=1.341 m for one side retained}
• ^{Abutments = 2.0 x0.9x Mean depth of scour = 2x1.174x0.9m=2.113 m for all round.}

 ^{Maximum Scour Depth (for Low water level without Flood condition with Seismic Combination): - (IRC:78-2014, Cl.No.703.3.3.1.3)}

• ^{Piers          = 2.0 x0.8x Mean depth of scour = 2x1.174x0.8m=1.878 m for all round scours.}
• ^{Abutments = 1.27 x 0.8xMean depth of scour = 1.27x1.174x0.8=1.193 m for one side retained}
• ^{Abutments = 2.0 x0.8x Mean depth of scour = 2x1.174x0.8m=1.878 m for all round.}

^{(***** In respect of Viaduct and as well ROB having no possibility of Scour)}

 

^{(1b): Solution:-}

^{To avoid scour at bridges with shallow foundations, various measures can be implemented to protect the bridge piers and abutments from erosion caused by flowing water.  Here are some common scour prevention measures for shallow foundations:}

^{Riprap or Gravel Protection:}

^{Placing a layer of riprap (large stones) or gravel around the bridge piers and abutments can help dissipate the energy of the flowing water, reducing its erosive force and preventing scour. The riprap or gravel acts as a protective armor to stabilize the riverbed and prevent erosion.}

^{Gabion Baskets:}

 ^{Gabion baskets are wire mesh containers filled with stones or rocks. They can be strategically placed around bridge foundations to provide scour protection. The gabion baskets help control the flow of water and prevent excessive erosion.}

^{Concrete Collars or Aprons:}

^{Constructing a concrete collar or apron around the base of bridge piers and abutments can protect against scour. The collar extends horizontally into the riverbed and provides additional resistance to the erosive forces of the water.}

^{Sheet Piling:}

                              ^{Sheet piling is a method of installing vertical barriers around the foundation to prevent scour. The sheet piles can be made of steel, vinyl, or other materials and act as a barrier against the flow of water.}

^{Submerged Vanes:}

^{Submerged vanes are structures placed in the waterway that redirect the flow away from the bridge piers. By diverting the water around the foundation, they reduce the risk of scour.}

^{Guide Banks:}

^{Guide banks are constructed parallel to the waterway to guide the flow and prevent lateral movement of the river, reducing the potential for scour around the bridge.}

^{Armor Units:}

 ^{Armor units, such as concrete blocks or articulated concrete blocks, can be placed around bridge piers to provide additional protection against scour.}

^{Soil Stabilization:}

^{Proper soil stabilization techniques can be employed to reinforce the riverbed around the bridge foundation and reduce the susceptibility to scour.}

^{Bridge Elevation:}

^{Raising the bridge deck or elevating the piers can minimize the exposure of the foundation to high water flows during floods, reducing the risk of scour.}

 

Some system are adopted to prevent scouring & its diagram also:-

There are:-

• Cut-off walls/Curtain walls:

 

• Providing adequate depth of foundation:-

 

 

 

 

 

• Protection work: (near by Abutment location lonly)

 

 

(1c) Solution:-

 

 

In geotechnical engineering, uplift refers to the vertical force that acts on a foundation or structure, pushing it upwards. This force occurs when the upward pressure from the soil below the foundation exceeds the weight of the structure, causing the foundation to lift or lose contact with the ground.

Uplift can be caused by various factors, such as: -( Part -1)

1.Hydrostatic Pressure:

When the water table is high or there are significant water pressures in the soil, it can exert an upward force on the foundation

2.Expansive Soils:

 Some types of soils, such as clay, can expand when they absorb water, leading to uplift forces on the foundation.

3.Frost Heave:

In cold climates, freezing of moisture in the soil can cause the ground to expand, resulting in uplift on the foundation.

4.Buoyancy:

In cases where structures are built in or near bodies of water, the buoyant force of water can cause uplift on the foundation.

 

The effect of uplift on a foundation can be detrimental and lead to "loss of contact" between the foundation and the ground. When uplift forces are greater than the downward forces (e.g., the weight of the structure), the foundation may start to lift or become partially or fully disengaged from the supporting soil. This can result in several issues, including:

1.Settlement and Stability Problems:

Uplift can cause differential settlement, where parts of the foundation lift while other areas remain in place, leading to structural instability and potential damage to the building.

2.Structural Damage:

The movement and shifting of the foundation due to uplift can result in cracks in walls, floors, and other parts of the structure.

3.Impaired Load Bearing Capacity:

When the foundation loses contact with the ground, it loses its ability to effectively transfer the loads of the structure to the supporting soil, which can lead to structural failures.

4.Misalignment:

Uplift can cause the building to become misaligned, impacting its overall integrity.

To mitigate the effects of uplift and prevent foundation "loss of contact," engineers employ various measures, such as:

5.Adequate Foundation Design:

 Properly designed foundations, considering site-specific soil conditions and potential uplift forces, can minimize uplift-related issues.

6.Foundation Anchoring:

Installing anchors, piles, or other systems that provide downward resistance against uplift forces can help stabilize the foundation.

7.Drainage Systems:

Proper drainage to manage water levels around the foundation can prevent excessive uplift due to hydrostatic pressure.

8.Geotechnical Investigations:

Conducting thorough soil investigations to understand soil properties and potential uplift risks is essential for designing appropriate foundation solutions.

By taking these factors into account during the design and construction phases, engineers can help ensure the stability and longevity of the structure, even in the presence of uplift forces

 Calculate the uplift force or buoyancy in a submerged structure or member:-( Part -2)

Case-1 for 3 m height of the water level Upto ramp wall level:

 

To calculate the lifting force (uplift) on a section with a height of 3 meters, we need to consider the hydrostatic pressure of the water above the section. The uplift force is the result of the pressure exerted by the water on the section due to its weight.

The formula to calculate the uplift force (U) is:

U = γ * h * A

 

where:

γ = Unit weight of water (approximately 9.81 kN/m³) h = Depth of water above the section (in meters) A = Area of the section (in square meters)

 

To calculate the uplift force for a section with a height of 3 meters:

 

Step 1:

Determine the depth of water (h) above the section. Since the section height is 3 meters, the depth of water above it will also be the following height as below-

 

Overall Height, h= (100+580+3000)mm=3680mm=3.68m

 

Step 2:

Calculate the area of the section (A). The area of the section depends on its shape. If the section has a uniform width (b) and the water level covers the entire width, then the area (A) is simply the product of the width and the height of the section.

Given data:-

width of the structure  Upto raft top,w1=8000+410+600+1200=10210mm

width of the structure bottom  of 2500m ,w2=(410+300)/2+(300+600)2+9200=355+450+9200=10005mm

width of the structure upto ramp wall level ,w3=300+9200+300=9800mm

Average width of the section,=w1+w2+w3=(10210+10005+9800)/3=10005mm

                                                    A = w * h

So, Submerdge area of the section,A= Height x width=10.005x3.68=36.82m²

Step 3:

 Calculate the uplift force (U_3m). U_3m = 9.81 kN/m³ * 3.68m * 36.82m²

                                                            =1329.23kN(Upwoard force)

Similarly, 4 & 5 m height, we can calculate the uplift force

 

Calculate the uplift force (U_4m). U_4m = 9.81 kN/m³ *4.68m * 36.82m²

                                                            =1690.40kN(Upwoard force)

And

Calculate the uplift force (U_5m). U_5m = 9.81 kN/m³ *5.68m * 36.82m²

                                                            =2051.64kN(Upwoard force)

After that, total resultant uplift force ,U= U_3m + U_4m + U_5m =1329.23+1690.40+2051.64kN=5071.27kN

 

(1c) Part 3:-

We should be taken some common measure to avoid uplift or buoyancy issues in structures, especially when dealing with water or hydrostatic pressure, several measures can be taken to ensure stability and prevent uplift.

Here are some common measures:

Increase Weight:

One straightforward approach is to increase the weight of the structure itself or its foundation. By increasing the mass, the structure becomes more resistant to buoyant forces.

 

Use Counterweights:

 Adding counterweights can help to offset the uplift force. The counterweights should be designed to provide enough downward force to balance the buoyant force.

 

Increase Foundation Depth:

A deeper foundation can anchor the structure more securely to the ground, reducing the potential for uplift.

 

Use Piles or Piers:

 Piles or piers can be driven deep into the ground to anchor the structure and prevent uplift. They provide more resistance against buoyancy forces.

 

Grouting:

 Injecting grout or cement into the soil beneath the foundation can help improve soil stability and resist uplift.

Use Tie-Downs or Anchors:

 Installing tie-downs or anchors that extend deep into the ground can help secure the structure against uplift.

Sloping Design:

Designing the structure with a sloped or tapered base can help reduce the potential for uplift forces.

Incorporate Suction Piles:

In underwater or marine structures, suction piles can be used to increase the foundation's resistance to uplift forces.

Adequate Drainage:

 Proper drainage systems can help prevent the buildup of water pressure around and beneath the structure.

Watertight Seals:

Ensuring that the structure has watertight seals and joints can minimize water infiltration and reduce the possibility of buoyancy.

Design for Uplift Forces:

Engineers should consider uplift forces during the design phase and implement appropriate measures to counteract them.

Conduct Geotechnical Investigations:

 Thorough geotechnical investigations can provide valuable information about the soil conditions, allowing engineers to design foundations that resist uplift effectively.

Consideration of Wind Uplift:

In certain cases, wind uplift can also be a concern. Properly designed roof systems, anchoring, and bracing can mitigate the effects of wind uplift.

Conclusion :-

After that, each structure and site present unique challenges, so the measures taken to avoid uplift should be tailored to the specific conditions and requirements of the project. Professional engineering expertise is essential to ensure that the appropriate measures are implemented effectively.