Setting Out Base Lines and Pier Positions

Setting Out Base Lines and Pier Positions

• The position of piers/abutments is determined by the intersection of three sight lines: alignment sighted from stations located on either end, base line on the down stream side, and base line on the upstream side.
• Ideally, these three lines should intersect at one point, but a triangle of error may form, and the correct center is fixed by judgment within this triangle.

Setting Out Base Lines

• Linear measurements should be carried out with invar tape or Electronic Distance Measuring (EDM) equipment.
• Concrete pillars with steel plates should be located at tape lengths for accurate measurements.
• Spring balances should be used to give specified tension to the tape.
• Tape readings should be corrected for tension, temperature, and slope.

Pipe and Box Culverts with Open Foundation

• Pipe and box culverts can be constructed by removing top soil, replacing it with a layer of lean concrete, and leveling and consolidating the bed.
• They should not be used where the bed is likely to be scoured.

Other Bridges with Open Foundation

• Open foundations must rest on a stratum with adequate bearing capacity.
• The base can be widened by providing footings to reduce bearing pressure.
• Footings will rest on a lean concrete bed of adequate thickness.
• The foundation should be taken to a depth not less than 1.75 meters below the lowest anticipated scoured bed level in ordinary soil.

Soil Exploration for Design and Construction of Piles

• Detailed soil exploration should be carried out to a depth generally not less than 10 meters below the anticipated level of pile tip.
• The following particulars should be collected:
  • Groundwater table and its tidal and seasonal fluctuations
  • Soil profile and borehole log
  • In-situ bulk and dry density
  • Index properties of soil
  • Shear properties of soil (if required, Standard Penetration Test (SPT) may be done)
  • Consolidation properties (in case of clays)
  • Chemical analysis of soil and groundwater to identify sulfate and chloride content or any other deleterious chemical content
  • Additional data such as high flood level, maximum scour depth, normal water level during working season, etc.

Classification of Pile Foundations

• Based on the manner of transfer of load:
  • Friction piles: transfer load primarily by skin friction developed along their surface
  • Bearing piles: transfer load primarily by bearing resistance developed at the pile tip or base, without taking into account frictional resistance
  • Bearing-cum-friction piles: transfer load by both skin friction and bearing resistance
• Based on construction methods:
  • Driven pre-cast piles
  • Driven cast-in-situ piles
  • Bored cast-in-situ piles
• Large diameter bored piles (more than 1 meter diameter) are normally used for Railway bridge construction.

Selection of Type of Piles

• Factors to be considered while selecting the type of piles:
  • Availability of space and headroom
  • Type of soil and its properties
  • Load-carrying capacity required
  • Depth of water
  • Velocity of water
  • Depth of pile
  • Construction difficulties and costs

Sinking of Wells in Water

• Sinking of well is done by removing soil with grabs or chiselling and drawing out the soil.
• Sinking of steining should not be done within 48 hours of casting.
• In stagnant water and in water with velocity up to about 2.0 m/s and of depth up to about 5 meters, construction of island may be resorted to.
• In greater depths, the use of steel caissons would become unavoidable.

Construction of Island

• The island should provide sufficient working space of about 2 to 3 meters all round the well.
• A minimum freeboard of 0.6 meter should always be provided.
• For depths of water up to 1 meter, an island may be constructed by laying a few rings of sandbags enclosing the area of the island and filling with sand.

Tilt and Shift of the Well

• Limits of tilt and shift:
  • A tilt of up to 1 in 100 (1%) and a shift of D/40 subject to a maximum of 150 mm can be permitted.
  • Excessive tilt and shift, which cannot be corrected, should be taken into account for rechecking the design of the well and the resulting foundation pressure.

Founding the Well

• Settling the well by blasting:
  • As far as possible, the well shall be evenly seated on sound rock devoid of fissures, cavities, etc.
  • One or more cylindrical holes may be made in the base to give good anchorage to the well.
  • Alternatively, anchorage can be provided by the rock itself, which is allowed to remain projecting at the center.

Bottom Plugging of the Well

• Sand hearting:
  • The joints should be made as air-tight as possible to prevent escape of air.
  • Waterproof cement plastering can be resorted to on the inside face of steining.

Construction of the Top Plug

• Setting out of the piers on top of the well:
  • The top plug should be constructed with a good bond between the base rock and the bottom plug.

Determination of Pier/Abutment Position

• The position of piers/abutments is determined by the intersection of three sight lines: one along the alignment, one from a station on the base line on the downstream side, and one from a point on the base line on the upstream side.
• Theoretically, all three lines should intersect at one point, but practically, a triangle of error forms, and the correct center is fixed by judgment within this triangle.

Setting Out Base Lines

• Linear measurements should be carried out with invar tape or Electronic Distance Measuring (EDM) equipment.
• Concrete pillars with steel plates fixed over them should be located at tape lengths for accurate measurements.
• Spring balances should be used to give specified tension to the tape.
• Tape readings should be corrected for tension, temperature, and slope.

Pipe and Box Culverts

• Pipe and box culverts can be constructed after removing the top soil in the bed to the required depth and replacing it with a layer of lean concrete after the bed is levelled and well consolidated.
• Pipe and box culverts should not be provided where the bed is likely to be scoured.

Open Foundations

• Open foundations must rest on a stratum with adequate bearing capacity.
• The base can be sufficiently widened by providing footings to reduce the bearing pressure.
• The footings will rest on a lean concrete bed of adequate thickness.
• The foundation should be taken to a depth not less than 1.75 meters below the lowest anticipated scoured bed level in ordinary soil.

Spacing of Piles

• The spacing of piles is determined based on the type of soil and empirical approach.
• The spacing will be governed by the competency of the end bearing stratum when piles are found on a very hard stratum and derive their capacity mainly from end bearing.
• The minimum spacing in such cases shall be 2.5 times the diameter of the pile shaft.
• Piles deriving their bearing capacity mainly from friction shall be sufficiently apart to ensure that the zones of soil from which the piles derive support do not overlap to such an extent that their bearing values are reduced.

Load Carrying Capacity of a Pile/Group of Piles

• The load carrying capacity of a single pile is affected by the type of soil and the method of installation.

Defective Piles

• Defective piles should be removed or left in place, whichever is convenient, without affecting the performance of the adjacent piles or the group as a whole.
• Additional piles should be provided to replace them as necessary.

Tremie Concreting

• The concrete should be coherent, rich in cement (not less than 370 kg/m3), and of slump between 150 and 180 mm.
• When concreting is carried out under water, a temporary casing should be installed to the full depth of the bore hole or 2m into a non-collapsible stratum.
• The hopper and the tremie should be a closed system embedded in the placed concrete, through which water cannot pass.

Sinking of Wells

• Caissons can be used for sinking wells in deeper channels and swift rivers.
• The caisson is assembled on a temporary platform, and after testing for leakages, it is lifted from the platform and lowered into position.
• The process of open sinking of wells involves using timber shear leg, derrick, or timber scotch for smaller wells.

Tilt Correction

• Tilt correction methods include eccentric dredging, eccentric loading, applying pull to the well, and applying push to the well.
• The method of tilt correction depends on the site conditions.

Use of Divers

• Divers can be used for sinking wells at greater depths.
• Divers should follow the relevant British standard specifications for durations of ascends and stoppages at various depths and stay under water at various pressures.

Pneumatic Sinking of Wells

• Pneumatic sinking is used when other methods are not found feasible, particularly when the wells have to pass through considerable depths of intervening layers of rock or when the bed is full of large boulders or interlocked small boulders.

Learn how to determine the position of piers and abutments in engineering surveying, including the use of alignment sight lines and base lines to fix the center point.