Week 3 Challenge

1.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

1b. 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)

1.c 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.

Ans: Bridge is structure build over physical obstacles. pathway used for road, river, canal.

Types of bridges are as followed:

· Based on superstructure – Beam bridge, slab bridge, truss bridge, arch bridge, cable stayed bridge.

· Based on material – Steel bridge, RCC Bridge, PCC bridge, PSC Bridge, Timber bridge, composite bridge.

· Based on span length- Culvert ( span less than 6m), Minor bridge ( span in between 6 to 60m), Major bridge ( Span is grater than 60m), Long span ( Span is grater than 120m)

· Based on function- Aquatic, highway, roadway, canal, river, viaduct.

· Based on interrelation span- Simply supported, Cantilever, continuous

· Based on joint- Pinned joint, riveted joint, welded joint.

Point to be considered for design of bridges:

v Topographical survey include detail survey in that area along with alignment. Include each benchmark, school, temple, tress, road level, footpath level etc. mainly level is to consider from nearest railway station because mean sea level is available in railway station. And then we transfer that level further as per our requirement to carry our work.

v Geotechnical investigation it includes soil data collection to determine density and properties of soil using SPT.

v Nearby bridge means any bridge near to proposed bridge to check feasibility.

v Seismology - To determine zone of earthquake accordingly we designed.

v Construction resources- Construction material and machineries is easily available or not. And arrangement necessary requirement accordingly.

v Traffic data- Used to collected traffic data to determine frequency of vehicle and number of vehicle are passed with respect to time.

v Spanning of bridge- Span length is major factor to design bridge as per actual site condition. When obstracel of railway crossing is there in that case we go with special span i.e. increase length of span. This is not tyical span. Generally the spans of a bridge kept similar as per the design consideration but it is not mandatory for keeping all the span length as same. As per physical site requirement the sizes of span may varies.

Foundation is structure used to transfer sub & super structure load into hard strata. Maily divided into two types of foundation. 1) Shallow foundation 2) Deep foundation. Shallow foundation: When depth is less than or equal to width we called as shallow foundation. In this case hard strata is easily available and soil having sufficient bearing stratum.

Ø Based on embedment- When soil data is not available as per IRC 78 we generally go with 2m foundation.

Ø Based on scouring- Scouring is removal of deposit sand, gravel due to fast moving water. When scour failure happen upto 1m from E.G.L. then we go with 3m shallow foundation i.e. 1m scouring + 2m depth as per codel provision.

Ø Based on rock strata- When hard rock available smoothening and chipping for level surface.

Ø Based on SPT- When soil data is not available we do Standard penetration test to determine density and properties of soil.

DATA GIVEN

Silt Factor = 3

Design discharge = 1000 cumecs

Linear waterway = 128 m

SCOUR CALCULATION BY USING LACEY'S EQUATION

Effective linear waterway = 128 m

Assuming a clear span = 16 m

Considering the number of spans at the bridge site = 8 No.

Designed length of proposed bridge 128 m

Designed discharge as per IRC:78-2014

Design Discharge (Q) 1000.00 Cumec

Discharge per meter width (Db) 7.81 Cumec

Silt Factor (Ksf) 3.00

Mean Depth of Scour (dsm) = 1.34 (Db2/Ksf) 1/3 = 3.65 m

FOR ABUTMENT

Maximum Depth of Scour for Abutment( straight) = 1.27 x dsm = 4.641 m

Grip length = 4.671/3 = 1.547 m

FOR PIER

Maximum Depth of Scour for Pier = 2.0 x dsm = 7.309 m

Grip length= 7.309/3 = 2.436 m

1b. 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.

Ans- Scouring is removal of deposit sand, gravel due to fast moving water. Mean Depth of Scour (dsm) = 1.34 (Db2/Ksf) 1/3

Method to prevent scouring- 1)Protection work – using scoring, short creating work, sheet piling.

2) providing flexible and rigid apron

3) providing curtain and cut off level

4) Increased depth of foundation.

1C. What is uplift and how it can affect the foundation.

Uplift pressure, which is also known as hydrostatic uplift, is an upward pressure applied to a structure that has the potential to raise it relative to its surroundings. It is the condition of greater pore water pressure than the overburden pressure of the structure.

The uplift pressure can be problematic if it is greater than the weight of the structure exerted downwards. IRC code, states that the upward pressure of water shall be taken as the full hydrostatic pressure applied over the entire area. The hydrostatic load shall be measured from the underside of construction.

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.

The possibility of occurrence of this upward force on foundation should be check when groundwater table is higher than the bottom of the foundation level. Due to the fact that hydraulic uplift pressure acts in upward direction. Therefore, it reduces the downward weight of the structure and consequently it acts against the stability of structure.

Effects of Uplift Pressure

· It reduces the effective weight of the structure.

· Endanger stability of the structure

Prevention for Uplift pressure

Dewatering procedure may help to control the water away from construction site, but it cannot keep the dewatering the whole life span of the construction as it is not an economical move. There are number of methods by which the influence of uplift pressure is avoided.

· Toeing in the foundation into the surrounding ground

· Increase self weigth of the structure

· Grond anchors

· Various type of piling to with stand the load

height 3,4,5 meters. with water level on top of ramp wall level .

1. Force due to Uplift - (i)u = γwH (for 3 m height)

γw = Unit weight of water = 10 KN/m3

H = Height = in meters (m)

u = 10 x 3 = 30 kN/m2

(ii) u = γwH (for 4 m height)

u = 10 x 4 = 40 kN/m2

(iii) u = γwH (for 5 m height)

u = 10 x 5 = 50 kN/m2