Engineering Survey

Engineering survey represents the survey which is done before, during and after engineering works. Engineering survey has three distinct stages; conceptual stage, construction stage and post-construction stage.

  • Conceptual stage: This usually happens before an engineering works. Examples include topographical survey and cadastral surveying works.
  • Construction stage: Major activities involved at this stage include setting out of works as well as extensions of control to the engineering work.
  • Post-construction stage: this stage involves the monitoring of engineering works. In addition, there is verification of positions of elements as well as design points.

Setting Out

This is a survey process where information on designs of works is transferred to the ground for purposes of construction of engineering works.

Elements of Setting Out

  1. Provision of horizontal controls
  2. Provision of vertical control.
  3. Vertical alignment for buildings
  4. Accurate positioning techniques

Aims of Setting Out

  1. To ensure the engineering works are set out in their correct relative and absolute positions.
  2. To make sure the work continues smoothly with minimal cost.

Stages in Setting Out

There are two main stages:

  • Planning of the setting out where individuals acquire the design plans and the necessary labor and technical staff.
  • The establishment of control which include locating design points and elements of construction as well as setting out levels and excavations.

Four Basic Control Points Used in Engineering Survey

  1. Primary control points – these represents the high order control points for triangulation, trilateration or GPS positioning.
  2. Secondary control points – densification of primary points using traversing and GNSS positioning techniques.
  3. Detail points- marks the location of features on the project construction works (Marks and stakes).
  4. Temporary Benchmarks – Provided near construction works for setting out levels.

Ways of Setting Out Detail Point

  1. Offset method.
  2. Intersection.
  3. Traverse.
  4. Ties.

Example: Given two primary control points A (1000mE, 2000mN), b (986.72mE, 1897.46mN), calculate the data for setting out coordinates of a point S (1025.00mE, 1950.00mN).

Ways of Referencing and Setting Out Design Points

  1. Baseline established between control points.
  2. Use of reference grids.
    1. Survey grid  – grid values from survey plan which has coordinates from national geodetic systems.
    1. Side grid – done along perimeter of the construction site. Provide closer control points (Secondary control points).
    1. Structural grid – used for referencing and also for positioning. It bring control closer to the building (tertiary control points).
    1. Secondary grid – it is used to place the design points inside the building.
  3. Offset pegs – these are the extra pegs which are located on the lines of buildings but at an offset distance away from a corner position. The offsets distance should be enough to avoid disturbance of the pegs during excavations. They are used together with the profile boards.

Vertical Controls

Vertical controls are used in geomatic engineering works to plan position, determine height and elevation of points, to assess the depth of excavation as well as for leveling works.

Examples:

Consider a proposed sewer line which is 60m long to be constructed from manhole A to B. The invert level for manhole A is 30.02m. The gradient from A to B is 1:100 falling from A to B. The ground level at A is 32.90m. If the height of the traveler is set at 3.75, determine:

  1. Invert level of manhole B
  2. Reduced level of sight rail at A and B.

Example 2.

A section of a sewer is to be laid between A and B 100 m apart. Sight rails have been established as shown below. If the sewer is to be constructed using a travelling rod of 3.25m, determine the invert level at the manholes and the gradient of the sewer lines as designed.

BS IS FS Distance Remarks
1.480     50 BM RL 48.290m
  0.855   75 Sight rail at MHA
  2.105   40 Sight rail at MHB
    1.480 50 BM RL 48.290

Determine the invert level of A and B.

Example 3.

A sewer is to be constructed between 2 points P and Q 80m apart. Levels were taken to establish the longitudinal profile as provided in the table below. The invert level at P is to be 120.750m above datum and the sewer is to fall towards Q at a gradient of 1:120. The length of the traveler is 3 meters.

BS IS FS Distance Remarks
0.633        
  2.925   0 BM RL 124.82m
  2.697   20 Ground level at P
  2.560   40  
  2.915   60  
  3.020   80 Ground level at Q
    0.633   BM

At the setting out stage, the level was set up close to its previous position and a back sight of 0.587 was recorded on a staff held on the benchmark. Determine:

  1. The staff reading required for fixing the sight rails at P and Q.
  2. The heights of those rails above ground level at P and Q.

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