Week 3 Challenge

AIM OF THE EXPERIMENT:-

How hydrology and geotechnical studies (preliminary and detail studies, zone of influence) are used in fixing span as per IRC standards and type of bridge. Mention the clauses applicable for the same.

Determine the scour depth at the pier and abutment location having silt factor =3

Design discharge = 1000 cumecs

Linear waterway = 128 m

Sol.

Bridge:-

• A bridge is a structure bridging gap over an obstacle without closing the way beneath
• The pathway may be for a road, railway, pedestrans a canal or a pipeline
• It can be used to cross a road , railway or a valley

Type of Bridge:-

• Based on the type of Superstructure

1. Slab Bridge
2. Beam Bridge
3. Truss Bridge
4. Arch Bridge
5. Cable stayed/ Suspension Bridge

• Based on the Materials:-

1. Timber Bridge
2. Stone Bridge
3. Steel Bridge
4. Concrete Bridge
5. RCC Bridge
6. PCC Bridge
7. Composite Bridge
8. Aluminium Bridge

• Based on Relative Position between bridge floor and superstructure:-

1. Deck trough Bridge
2. Suspension Bridge

• Based on the Length:-

1. Culvert ( span < 6m )
2. Minor Bridge ( 6m<span< 60m )
3. Major Bridge ( span > 60m )
4. Long Bridge ( span > 120m )

• Based n the Fuctions:-

1. Aquaduct
2. Viaduct
3. Pedestan Bridge
4. Highway Bridge
5. Railway Bridge

How hydrology and geotechnical studies (preliminary and detail studies, zone of influence) are used in fixing span as per IRC standards and type of bridge. Mention the clauses applicable for the same.

Hydrological studies:

Hydrological studies are done for the following reasons:

• To calculate design discharge which helps in deciding total length of the bridge, depth of the foundation and river training works if required.
• Calculation of discharge is done by calculating the catchment area and then, discharge formulas can be used to calculate the velocity and discharge of the river.

Geotechnical studies: Geotechnical Investigation helps us to understand the properties of the soil in the bridge location. It helps us to determine soil properties and ground water conditions & Ground water table, Selection of foundation type and depth, suitability of the site for the proposed project. Seismic activity in the area, to know the depth of types of soil below the foundation, to know SBC, also to improve the site condition.

Scouring & Methods to avoid scouring: Scouring is defined as the lowering or loss of top bed soil (erosion) around the bridge piers, abutments, spurs etc. due to the water flow currents. Scouring is always a potential threat to bridge stability eventually it can lead to foundation failure.

IRC CLAUSES:-

As per IRC:5-2015 Clause 106

Design discharge for which the waterway of the bridge to be provided, shall be based on maximum flood discharge of return period of 100 years. The design discharge shall be the maximum estimated discharge determined by consideration of the following:

From the rainfall and other characteristics of the catchment:

• By the use of an empirical formula, applicable to that region, provided that empirical formula can predict the maximum discharge for given return period.
• By a recognized method, provided it is possible to evaluate, for the region concerned, the various factors employed in that method.
• By the area velocity method with the help of hydraulic characteristics of the channel.
• By unit hydrograph method.

As per IRC:5-2015 Clause 106.5

After determining the design discharge, one can find out the linear water way for the bridge.

For natural channels in alluvial beds but with well-defined banks and for all natural channels in beds with rigid in-erodible boundaries, the linear waterway shall be distance between banks at HFL, at which the designed maximum discharge determined can be passed.

For natural channels in alluvial beds and having undefined banks, the linear waterway shall be determined from the design discharge using some accepted rational condition is

W = C√ (Q)

• W = regime width in meters (equal to effective linear waterway under regime condition)
• Q = the maximum design discharge in m3/sec;
• C = a constant usually taken as 4.8 for regime channels but it may vary from 4.5 to 6.3 according to local conditions.

As per IRC:5-2015 Clause 106.6

If the waterway is restricted, it will cause afflux at the bridge and design HFL will accordingly be raised.

The afflux shall be calculated by using any suitable method based on conservation of energy or momentum. In absence of any rigorous method, the afflux may be calculated by using Molesworth formula as given below :

h = (V2/17.88 + 0.015) x ((A/a)2 – 1)

Where:

• h = afflux (in meters)
• V = average velocity of the water in the river prior to constriction (in m/sec)
• A = unobstructed sectional area of the river at proposed site (in sq. m)
• a = constricted area of the river at proposed site (in sq. m)

As per IRC:5-2015 Clause 106.9.3

The following theoretical method may be adopted when dealing with the natural channels flowing over scour able beds for the estimation of mean depth of scour ‘dsm’ in meters. Mean depth of scour is given as,

Where,

Db = The discharge in m^3/s per meter width. The value of “Db” shall be the maximum of the following:

Ksf = the silt factor

The maximum scour depth below the highest flood level (HFL), which is to be considered for the purpose of design of foundations, protection works, and river training works.

Maximum depth of scour for foundation design for flood without seismic condition

• For piers = 2dsm
• For abutments = 1.27dsm with approach retained or LWL (whichever is deeper)

= 2.00 dsm with scour all around

Determine the scour depth at the pier and abutment location having silt factor =3

Design discharge = 1000 cumecs

Linear waterway = 128 m

Sol.

Given Data:

Silt factor = 3

Linear waterway = 128m

Design discharge Q= 1000 cumecs

As per IRC:5-2015

For the margin of safety, Q x 1.3 = 1000*1.3 =1300 m3/s

Mean scour depth

dsm = 1.34 * ((((1300/128)^2)/3)^(1/3)

dsm = 4.36m

Maximum depth of scour for piers = 2 dsm = 2*4.36 = 8.72m

Maximum depth of scour for abutment = 1.27 dsm = 1.27*4.36 = 5.537m

What measures can be taken to avoid scour at bridge for shallow foundation. Also draw the section of measures on the downstream and the upstream side.

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

Measure to avoid the scouring at bridge for shallow are as follows:

Protection works – The rigid flooring should be provided under the bridge and it should extend for a distance at least 3m on upstream and 5 m on downstream side of the bridge. However, in case the splayed wing walls of the structure are likely to be longer, the flooring should extend up to the line connecting the end of wing walls on either side of the bridge.

Flooring should consist of 150mm thick flat stone/brick on edge in cement mortar 1:3 laid over 300 mm thick cement concrete M15 grade laid over a layer of 150 mm thick cement concrete M10 grade. Joint at suitable spacing may be provided.

Curtain walls - The rigid flooring should be enclosed by curtain walls (tied to the wing walls) with a minimum depth below floor level of 2 m on upstream side and 2.5 m on downstream side. The curtain wall should be in cement concrete M15 grade or brick/stone masonry in cement mortar 1:3. The rigid flooring should be continued over the top width of curtain walls.

Flexible apron- Flexible apron 1 m thick comprising of loose stone boulders (weighing not less than 40 kg) should be provided beyond the curtain walls for a minimum distance of 3 m on upstream side and 6 m on downstream side. Where required size stones are not economically available, cement concrete blocks or stones in wire crates may be used in place of isolated stones

What is uplift and how it can affect the foundation. (loss of contact)

Calculate the uplift in U trough of below section with height 3,4,5 meters. with water level on top of ramp wall level .

What are different measures that can be taken to avoid uplift.

Uplift can be defined as the upward pressure provide to the structure against the direction of gravity in such a way that has the potential to raise the structure from its intial position. It can be occurred due to increase of the pore water pressure within the soil resulting summation of those pressure lead to uplifts the structure.

IRC code, states that the upward pressure of water shall be taken as the full hydrostatic pressure applied over the entire area.

The structure must be designed properly to provide greater resistance against uplift forces in order to prevent risks due to water pressure. So, the uplift pressure must be taken into consideration during design stage since rehabilitation and maintenance of foundation cannot be carried out easily in the future.

Due to the uplift force structure tends to lose its stability and endangers to collapse. Due to the uplift force, the structure loses its anchorage with soil, which it maintained by the soil above the foundation, and tends differential positioning of foundation datum.

Prevention against uplift pressure

• Dewatering of the site before construction plays important role in it.
• Increasing the self-weight of structure.
• Provision of anchor bolts.
• Increase the depth of footing in order to increase the weight of soil above foundation
• Provision of deep foundation in order to achieve hard strata.

1. Force due to Uplift –

(i)Fu = γw x H (for 3 m height)

γw = Unit weight of water = 10 KN/m3

H = Height = in meters (m)

Fu = 10 x 3 = 30 kN/m²

(ii) Fu = γw x H (for 4 m height)

Fu = 10 x 4 = 40 kN/m²

(iii) Fu = γw x H (for 5 m height)

Fu = 10 x 5 = 50 kN/m²

Measure to prevent uplift are mentioned below as follows:

Toeing in the Foundation into the Surrounding Ground

When a substructure is built inside an open excavation or temporary cofferdam, the base slab or raft foundation can be extended outside perimeter to increase resistance to uplift.

The weight of backfill materialls on the tied-in foundation increase the weight of the structure against uplift pressure. This method is not feasible where a diaphragm or secant pile wall is used as a permanent retaining structure.

Increase Self-weight of the Structure

The self-weight of the building can be increased by increasing the size of structural members. sometime, the dead weight of the low-rise podium in a high-rise complex is increased by incorporating a rooftop garden with a thick soil fill.

Ground Anchors

Pre-stressed anchors can be used as a temporary measure to counteract uplift forces. Permanent application of ground anchors is limited by concerns about their long-term performance with respect to corrosion.

Piling Method

The piling method is the most effective method to resist Uplift pressure. Various types of piles are available to be used for countering uplift pressure based on soil condition and their necessity on site.