Chapter 19 Bridge Works : Substructure

1 Unified Standard Specifications For Works & Materials Chapter 19 : Bridge Works : Substructure Page 707 : Chapter 19 Chapter 19 Bridge Works : Substructure GENERAL Scope The Specifications given in this Chapter deal with items pertaining to all types of foundations for bridges and Bridge superstructure viz., Piers, abutments, wing walls, bed blocks and ballast walls / dirt walls. A section is included at end to cover water supply wells and tube wells. Types of Foundations Bridge foundations can be of i) Open or Shallow Foundations including footings, blocks and raft foundations. ii) Pile Foundations iii) Well Foundations Types of sub-structures:- Abutment : i) Wall Type abutment ii) Spill through abutment Pier i) Wall type ii) Columns - Solid or hollow iii) Trestle type Wing Walls i) Splayed Wing walls ii) Box Type Wing walls Design and inspection aspects of bridges Design and inspection aspects of bridges are not covered in these Specifications.

2 User may refer to the Codes and Specifications listed above and Indian Railways Bridge Manual and departmental instructions for same. Provisions made in other chapters of the specifications are applicable to the extent they are relevant. SETTING OUT FOR FOUNDATIONS Setting out for Minor Bridges and Culverts Shall be carried out by a competent / qualified engineer employed by the Contractor and checked by the Engineer s representative for all bridges and culverts. Contractor shall provide necessary instruments, linear tapes, pegs etc. The setting out for foundations and sub-structure shall be carried out with a theodelite and steel tapes / Invar tapes in case of Works not involving deep foundations or standing water. All levels will be measured using a precise levelling instrument. Errors in location of piers / abutments and fixing levels shall be within following limits.

3 Linear Measurements + 5 mm Levels + 3 mm Setting out for Major Bridges a) Locations of piers and abutments along with the centre line of the Bridge should be accurately laid out by establishing one or more base lines as directed and a system of pegs and posts. Also sufficient reference pegs and pillars should be established for checking the positions with ease during progress of work. Reference Bench Marks for levelling should be established nearby on a permanent structure or on a pillar to be built up in vicinity. b) The principal reference lines and level pegs should be established at easily accessible locations. They include i) Longitudinal Centre line ii) Transverse Centre lines of abutments and piers iii) Tangent points of the curve at either end, if alignment is on a curve. c) For Bridge Works involving deep excavations, pile driving or well sinking and / or where there is standing water, use of base line is obligatory.

4 They should be preferably at right angle to centre line of Bridge , with one on either end on high bank in case of long bridges or on one side bank of Bridge for shorter ones. d) In case of bridges of length exceeding 1000 Metres, base lines and reference towers will have to be established. Provision of all assistance in form of measuring instruments, linear tapes as may be required by the surveyor, technical and skilled staff and labour required to assist them, fixing pegs, pillars and towers including all building materials, and maintaining and guarding them including supply of all materials, tools and plant shall be done by the Contractor at his cost. Unified Standard Specifications For Works & Materials Chapter 19 : Bridge Works : Substructure Page 708 : Chapter 19 Nothing extra will be payable to them on this account.

5 Important points to be observed in this activity are: 1. Linear Measurement shall be carried out with invar tape or electronic distance measuring instruments 2. Spring balances shall be used for giving specified tension to the tape. Tape readings shall be corrected for tension, temperature and slope. 3. Concrete pillars with steel plates fixed over them shall be located at intermediate points (at tape lengths) and ends. 4. Reference pillars at pier and abutment position along centre lines and reference pillars on base lines shall be to standards to be prescribed by the Engineer. During construction, since centre line pillars at abutment / pier locations will be disturbed, reference pillars and lines shall be fixed around each structure by the Contractor under Site Engineer s supervision.

6 Reference diagrams at Annexures 4/1 and 4/2 and Clause 401 of IRBM shall be referred to for more details. SOIL EXPLORATION Shallow Foundations .1 Shallow Foundations are provided where sufficiently hard soil is available at shallow depths. They shall be taken down at least below bed level or anticipated scour level except in case of pipe and Box culverts. The bearing capacity of soil and nature of soil should be ascertained by making trial pits at location of culvert/ Bridge ..2 The depths of such pits shall extend for sufficient depth below the proposed founding level. If the soil in the pit by observation is found hard and stable plate load test shall be conducted at a level to be decided by the Engineer but not less than below proposed foundation level for determining safe bearing capacity of soil and the characteristics of the strata in accordance with provisions of IS: 1888.

7 (Plate Load Test) .3 In addition, at the discretion of the Engineer, exploratory bores or sounding shall be done for depth of about times the proposed width of foundation below proposed foundting level. In case of exploratory bores, sample shall be collected and laboratory tests conducted as detailed in Para .4 The results of laboratory tests and of plate load tests and penetration tests shall be compared for conformity. It shall be ensured that no layer of soft soil is met with upto times depth below the founding level. Soil Exploration for Deep Foundations .1 At least one exploratory bore shall be drilled at the location of each pier. The bore shall be taken down to a minimum depth of times the proposed width of foundation below design founding level. If rock is met with in the bore, the bore should be taken down at least 5m into the rock.

8 2 In case the rock layer is suspected to be sloping or dipping or fissures are noticed, more than one bore per foundation will be required to be done. Additional bores should be done as instructed by the Engineer. See also Para .3 Only rotary drills shall be used for drilling bore holes and collection of samples. Use of percussion or wash boring equipment may be permitted by the Engineer in certain types of soils, based on his judgement..4 SPT tests and collection of undistributed samples shall be done at intervals of m depth unless otherwise specified in contract or directed by the Engineer, and in accordance with IS:1892. Samples collected shall be properly protected, packed and sent for tests. Tests shall be conducted as per provisions in relevant part of IS: 2219.

9 Tests:- In- situ and laboratory tests listed below shall be carried out according to soil type and site requirement. Cohesionless soils a) Classification tests, density etc. Standard Penetration Test or Dynamic Penetration Test (as per IS 2131) when considered appropriate. b) Laboratory Tests: Triaxial or shear or box shear test on collected samples and saturated samples where sub soil water level is likely to reach that level. Cohesive soils a) Classification tests, density etc. b) Field Tests i) Plate Load Tests ii) Unconfined compression Test (IS 2720-Part IX) iii) Vane Shear Test (IS 4434) Unified Standard Specifications For Works & Materials Chapter 19 : Bridge Works : Substructure Page 709 : Chapter 19 iv) SPT (IS 4968 Part III) c) Laboratory Tests i) Shearing strength Test Triaxial Tests (IS:2720-Part XI) ii) Consolidation Test (IS 2720-Part XV) iii) Special Tests a) Permeability tests should be conducted on samples in case need for dewatering is expected for / during excavation / or well sinking.

10 B) Chemical tests of soils and sub-soil water where considered necessary c) Look out should be maintained for presence of any gas, in case peaty soils are met with or otherwise suspected, specially when well foundations are proposed. Coverage a) The exploration shall cover the entire length of the Bridge and also extend at either side for a distance of about twice the depth below bed of the lowest main foundations. b) The depth of exploration should be at least 1 times the minimum width of foundation below the proposed foundation level. Where such investigation ends in any unsuitable or questionable foundation material the exploration shall be extended to a sufficient depth into firm and stable soils or rock but not less than four times the minimum depth of foundation below the earlier contemplated foundation level.