Site and civil drawings

Before a building can rise, its site must be surveyed, shaped and serviced. Site and civil drawings describe that work: where the property lines are, how the ground is shaped now and how it will be shaped, where the building, roads, parking and walkways go, how rain and snowmelt will drain, and how water, sewer and other services reach the building. They are drawn by land surveyors and civil engineers, usually at small scales, and use their own conventions: contour lines, spot elevations, slopes, pipe inverts, stations and profiles. This lesson explains how to read surveys, site plans, grading plans, servicing plans, plan-and-profile sheets and erosion and sediment control plans. It focuses on reading the drawings; the earthwork and construction methods behind them belong to CON-110.

Surveys: the starting point

Two kinds of survey usually come before design. A legal or boundary survey establishes the property lines, easements, rights-of-way and encroachments. A topographic survey records the existing ground surface, buildings, trees, pavements, utilities and other features, with elevations. Together they form the base on which the civil engineer and architect design the site.

In Canada, boundary surveys are performed by provincially licensed land surveyors, such as Ontario Land Surveyors or Alberta Land Surveyors, and in federal lands and the territories by Canada Lands Surveyors. A plan of survey or a surveyor's real property report shows boundaries, buildings and encroachments, and may be registered in the provincial land registry. Topographic information is often added by the same surveyor. Before excavation, buried utilities must be located through the provincial one-call service, such as Ontario One Call or the Click Before You Dig services in other provinces.

Survey drawings show the property boundary as a heavy line with long dashes and short dashes, labelled with bearings and distances, such as "N 45°12'30" E 62.48 m". Corners are marked with symbols for found or set survey monuments. Easements, setbacks and rights-of-way are shown with their widths and purposes. Existing features are drawn with standard symbols: trees with their trunk diameters, hydrants, manholes, catch basins, light standards and utility poles.

Contour lines

The shape of the ground is shown with contour lines, lines joining points of equal elevation, like the lines on a topographic map. The vertical distance between adjacent contours is the contour interval, typically 0.25 m, 0.5 m or 1 m on metric site plans and 1 ft or 2 ft on US plans. Some contours, often every fifth one, are drawn heavier and labelled as index contours. Reading contours takes practice, but a few rules help:

• Contours never cross, except at overhangs, which are rare.
• Closely spaced contours mean steep slopes; widely spaced contours mean gentle slopes.
• Water flows downhill perpendicular to the contours.
• Contours bend upstream (in a V pointing uphill) where they cross a valley or swale, and downstream where they cross a ridge.
• Closed loops indicate a hill or a depression, depending on whether elevations rise or fall towards the centre.

Grading plans show existing contours as thin dashed lines and proposed contours as solid lines. Where they differ, the ground will be cut or filled. Spot elevations, marked with a small cross or dot and a number, give the exact proposed elevation at key points, such as building corners, entrances, high points, low points, catch basins and the tops and bottoms of curbs and walls.

Slopes and grades

Slopes on site drawings are expressed in several ways:

• As a percentage: the rise or fall per 100 units of horizontal distance. A 2 % slope falls 2 m in 100 m, or 20 mm per metre (about 1/4 in per foot).
• As a ratio of horizontal to vertical, especially for embankments: 3:1 (or 3H:1V) means 3 units horizontally for each unit vertically.
• As a ratio of rise to run for ramps: 1:12 means one unit up for every 12 units along.
• As an angle, used less often on civil drawings.

Typical design values include at least about 1 % to 2 % for paved surfaces to drain, about 2 % cross slope on sidewalks, 5 % or more for landscaped ground falling away from a building for the first few metres, and no steeper than 3:1 for mowed grass embankments. Accessible routes and ramps have strict maximum slopes set by building codes and accessibility standards.

Interpolating between contours

You often need the elevation of a point that lies between two contours. If the ground is assumed to slope uniformly between them, the elevation can be found by linear interpolation: measure the distance from the lower contour to the point and the total distance between the contours along the steepest line, and add the same proportion of the contour interval to the lower elevation. For example, a point 6 m from the 100.0 contour along a 10 m line to the 100.5 contour is at about 100.0 + 0.6 × 0.5 = 100.30. The difference between existing and proposed elevations at a point gives the depth of cut or fill there, which estimators use to calculate earthwork volumes.

The site plan

The site plan, often prepared by the architect with civil and landscape input, shows the building in relation to its site. It typically includes:

• Property lines, easements and the required setbacks from each line, with dimensions locating the building.
• The building footprint, entrances and finished floor elevation.
• Roads, driveways, parking stalls and aisles, loading areas, fire routes and curbs.
• Sidewalks, accessible routes, ramps and barrier-free parking.
• Fences, retaining walls, signs, lighting and site furniture.
• Zoning data: lot area, building area, coverage, height, parking counts and other figures required by the municipality.
• A north arrow, scale and key to adjacent streets.

Setbacks and building location are critical. Building in the wrong place by even a small amount can violate zoning, encroach on a neighbour or an easement, or fail to meet fire separation distances, and the consequences can be severe. That is why the building is set out from survey control by a qualified surveyor and checked by survey once the foundations are in.

Site servicing drawings

Servicing drawings show the underground pipes that bring water to the building and carry wastewater and stormwater away. The main systems are:

• Water: a service connection from the municipal main, with valves, a meter (often inside the building), fire hydrants and a fire service line for sprinklers, often separate from the domestic line.
• Sanitary sewer: a gravity pipe carrying wastewater from the building to the municipal sanitary sewer, with cleanouts and manholes.
• Storm sewer: catch basins, area drains and roof drain connections feeding pipes to the municipal storm sewer, a ditch or a stormwater management facility.
• Stormwater management: ponds, underground storage tanks or chambers, oil and grit separators and orifice controls that limit the rate of discharge to the municipal system.
• Other utilities: gas, electrical, telecommunications and district energy, often shown on the electrical or composite utility drawings.

Pipes are shown as lines labelled with their size, material and slope, such as "200 mm PVC SAN @ 1.0 %" or "8" PVC SAN @ 1.00 %". Manholes and catch basins are numbered and listed with their rim (top) elevations and the invert elevations of each pipe entering and leaving. The invert is the elevation of the inside bottom of the pipe, the level at which water flows.

Plan and profile drawings

For linear work such as sewers, water mains and roads, civil drawings use plan-and-profile sheets: the plan view in the upper half of the sheet and a profile, a vertical section along the centre line, in the lower half. The profile shows the existing and proposed ground surface, the pipe with its slope and inverts, the manholes, and crossings with other utilities. Profiles use an exaggerated vertical scale, often ten times the horizontal scale, so that slopes and clearances are visible.

Positions along the centre line are given as stations. A station is the distance from a starting point, written with a plus sign.

Canadian civil drawings in metric give stations, often called chainage, in metres from the start of the alignment, written as kilometres plus metres: 0+120.000 means 120 m from the start, and 1+045.500 means 1,045.5 m.

Erosion and sediment control plans

Construction exposes soil, and rain carries sediment off site into storm sewers, streams and wetlands. Erosion and sediment control (ESC) plans show the measures that prevent this: silt fences along the downhill edges of the site, catch basin inlet protection, stabilized construction entrances of clean stone, sediment ponds or traps, check dams in swales, slope protection with mulch or blankets, and the sequence for installing, maintaining and removing them. ESC drawings typically include inspection and maintenance notes, and the measures must be in place before stripping and grading begin.

In Canada, erosion and sediment control is regulated mainly by provinces, municipalities and conservation authorities (in Ontario), through site alteration by-laws, permits and guidelines, along with the federal Fisheries Act, which prohibits depositing harmful substances such as sediment into waters frequented by fish. Many municipalities require an ESC plan as a condition of site plan approval or a grading permit.

Paving, curbs and site details

Site drawings also show paving types, with sections giving the thickness of asphalt or concrete and of granular base and subbase layers; curb types and transitions, including depressed curbs at driveways and curb ramps at crossings; pavement markings and signs; site lighting; fences and gates; retaining walls; and landscape elements. Most of these are covered by standard details, often taken from the municipality or the provincial or state transportation department's standard drawings, which the civil sheets reference by number.

Reading site drawings together

Site work involves several disciplines on separate sheets: survey, civil grading and servicing, landscape, architectural site plan, electrical site lighting and utilities, and sometimes geotechnical recommendations. Elevations on the civil drawings must match the finished floor and entrance elevations on the architectural drawings; roof drain locations on the plumbing drawings must match storm connections on the civil drawings; and site lighting bases must avoid underground pipes. Checking these interfaces before construction prevents expensive surprises in the ground.