Challenging conditions affective servicing
Chapter 6: Challenging Conditions Affecting Servicing
This chapter presents design guidelines, suggestions and ideas which may assist in the application and design of alternate technologies for underground servicing of areas that are affected by challenging conditions. For more detailed information on this subject the designer should refer to the ASCE/CSCE (1996), Cold Regions Utilities Monograph, (formerly Cold Climate Utilities Deliver Design Manual, Environment Canada) available from the ASCE bookstore website.
6.1 General
Challenging conditions may be a result of: climate, geology, hydrogeology, area location (remoteness), topography or any combination of these factors. Challenging conditions are often associated with northern communities, but can also occur in urban areas where above-ground piping is necessary (i.e., bridge crossings or over permafrost) and/or shallow buried obstructions such as culverts cause pipe to be placed in the frost zone. Generally biological treatment processes operate most effectively at sewage temperatures above 10°C (41°F) and critical design conditions need to be assessed at low operating temperatures to ensure year round effluent objectives can be met. This low temperature evaluation is particularly important for nitrifying plants that have low ammonia effluent objectives.
6.2 Climatic Factors
The main climatic elements that can affect low ground temperatures are cold air temperatures and the amount of snow cover. With below freezing temperatures, the designer should determine whether the conditions are such that sewage in the proposed municipal sewer pipe would freeze or glaciate and the pipe affected by frost heave may hinder gravity flow by forming pockets. Historical information on Ontario climate is available from the Environment Canada website. Design temperature data are also provided in the Supplementary Standard SB-1 of the Building Code (O. Reg. 350/06) made under the Building Code Act, 1992. Other references of note for additional information include:
- Smith, DW and Hrudey, SE (1981), Design of Water and Wastewater Services for Cold Climate Communities; and
- Air Force (1987), Technical Manual - Arctic and Subarctic Construction Utilities, AFR 88-19, Volume 5.
The main indicator utilized to determine the relative “air coldness” of an area is the “Freezing Index”. The “Freezing Index” is defined as the number of degree days [above and below 0°C (32°F)] between the highest point in the autumn and the lowest point the next spring on the cumulative degree-day time curve for one freezing season. It is recommended that the designer consider the coldest month.
The climatic factor most seriously impacting the design, cost and operation of sewer pipes is frost. The depth to which it penetrates depends upon the Freezing Index as well as the frost susceptibility and the thermal conductivity of the soil. The designer should refer to the Environment Canada, Cold Climate Utilities Delivery Design Manual (Environmental Protection Service Report No. EPS 3-WP-79-2).
Another factor to be considered is frost heave. As the water in the pores of the soil freezes, there is an associated increase in the volume of the soil of up to 5%. If ice lenses form in the soil, much greater volume increases may occur. Any sewage works that is to be constructed within the frost zone should be designed with consideration given to the rise of the ground surface due to frost action.
For more information regarding frost and freezing, the designer should refer to the National Research Council of Canada, Institute for Research in Construction and the Ontario Ministry of Transportation.
6.3 Geological Factors
The predominant geological factor which can have an effect on the design of sewage works is the presence of rock and its proximity to the surface. This phenomenon is common in many areas of the Province and predominant in Northern Ontario where the main geological feature is the Precambrian formation of the Canadian Shield.
Other factors concerning the geology of the northern parts of the Province that affect the design of sewer systems are:
- The presence of muskeg which can be found in depths varying from less than 0.3 m (1 ft) to in excess of 3.0 m (10 ft);
- Soil classification and frost susceptibility;
- Soil thermal conductivity;
- Soil chemistry (i.e., acidic and alkali soils); and
- The presence of a high water table.
6.4 Remote Location
In certain regions of the Province, the mere location of the community to be serviced may be a factor in the design. Access to the site may be difficult, limited and/or expensive due to the lack of adequate road or rail transportation.
Access problems can affect for example the supply of materials, construction equipment, replacement parts and servicing. The designer should ensure that the servicing methods are adapted to suit local conditions. If special fittings and accessories are required that may be difficult to obtain, replace and service, this should be considered at the design stage and spares purchased during construction.
6.5 Permafrost
Permafrost is defined as soil, bedrock or other material that has remained below 0°C (32°F) for two or more years. Continuous permafrost occurs in areas that are underlaid by permafrost with no thawed areas. Discontinuous permafrost occurs in an area underlain mostly by permafrost but containing small areas of unfrozen ground.
In Ontario, a state of discontinuous permafrost exists north of the line drawn from the southern tip of Hudson Bay, westerly to the point where the 53°N parallel intercepts Ontario’s western boundary, to the 55°N parallel. More information on the distribution of permafrost in Ontario is available from the Natural Resources Canada atlas.
Passive construction is usually used in permafrost conditions. This maintains the state of frozen permafrost by constructing insulated municipal sewer pipes. Permafrost conditions will not likely be met in any but the most remote northern areas of the Province.
6.6 Difficulties Associated With Conventional Practices
Conventional design practice of installing sanitary sewers is to provide gravity flow from the basement of the building being serviced to a gravity collector system. At the high point in the system the collector sewer is generally in the order of 2.5 metres (8.2 ft) below grade. This depth is, in most instances, adequate to prevent freezing/glaciating of the sewage and to prevent any pockets from forming in the gravity sewers due to frost heave.
In areas where little or no overburden exists (i.e., rock), it is the practice to blast the required trench and remove the rock. The fragmented rock is often returned to the trench after a “protective” layer of cover material is placed over the pipe. The large voids present in rock backfill are conducive to greater frost penetration. Replacement of the rock backfill with granular backfill is recommended. In either case, the high thermal conductivity of the surrounding rock may be the governing design factor and should not be forgotten. This situation often necessitates installation of the sewers below the depth normally required for gravity flow from the basements of the buildings being serviced. In areas with a high water table, infiltration into the system through pipe and manhole joints and private services can be a problem in many installations.
In some areas, the development density of the lots may be so low as to make sanitary sewer servicing infeasible due to the high installation costs per lot. In such areas, the use of septic tanks has been practiced. However, in an area where the presence of rock associated with very little overburden (e.g. less than 600 mm (24 in)) is the predominant feature, the use of septic tanks is not practical.
6.7 Retrofitting of Existing Sewers
Frost can exert considerable load on a buried conduit and frost action can separate the sections of a precast manhole if the sections are not strapped.
There would appear to be little that can be done to an existing sewer system to protect it from the increased loads due to frost, although some studies suggest that slab insulation above the pipe will distribute the load. This may be considered in situations where the cover over an existing sewer is reduced due to the reconstruction of the road or re-grading.
Where existing manholes have separated and are permitting extraneous flows, the manholes should be grouted and/or reset and straps installed on the inside of the manhole to prevent any further movement.
In areas where the freezing index is greater than 500 freezing degree-days Celsius (932 degree days Fahrenheit), precast manholes/chambers should have three (3) steel straps extending vertically from top to bottom and held by bolts in the top and bottom sections.
The three steel straps should be located at points equidistant around the circumference of the manhole/ chamber.
When the design freezing index equals or exceeds 1,800 freezing degree-days Celsius (3272 degree-days Fahrenheit), an additional granular water draining layer at least 0.3 m (1 ft) thick should surround the manhole.
There are less significant problems with freezing of gravity sewers, service connections or forcemains in comparison to watermains and water services.
6.8 Alternative Design Practices
In general, the costs of installing sanitary sewer services increases as the depth of burial increases. In areas that are subject to the effects of adverse conditions (such as the presence of rock, extreme frost or a high water table) the costs would be much greater as the depth to which these services need to be installed increases. The designer is referred to Chapter 12 in the ministry Design Guidelines for Drinking-Water Systems for information on water services in challenging conditions.
6.8.1 Thermal Considerations
When dealing with services and/or forcemains that are located in the active frost zone, it is possible to reduce heat loss and increase time before freezing by using pre-insulated piping with or without electric heat tracing.
6.8.2 Shallow Buried Pre-Insulated Servicing Systems
“Shallow buried” means a system that is partially or totally within the frost zone (i.e., cover only for physical protection) and “insulated” means reducing the heat loss from the pipe by applying various amounts of insulation to the buried pipe with or without heat tracing.
The specification for the work should indicate that “factory fabricated, preinsulated, flexible piping system” is required.
6.9 Sanitary Sewer Systems
The fundamental concepts of sanitary sewer system design, including such items as layout and appurtenances, should follow the recommendations contained in Chapter 5 - Design of Sewers.
In the laying out of gravity sanitary sewers in areas where frost depth penetrations are great and/or other adverse conditions exist, the designer should consider alternate routing of the sewers (i.e., off the traveled/plowed portion of the road).
It is recommended that all gravity sewer system designs include the provision of frost straps on all manholes and that all pipe designs reflect the increased loading that will be experienced through either improved bedding or increased pipe strength, due to the increase in weight on the pipe.
Other manhole design considerations include:
- Use of a plastic film around the outside of the manhole to prevent bonding of the soil to the structure and damage from frost heaving;
- Use of manhole insulation, generally 75 mm (3 in) of polystyrene or urethane; and
- An insulated manhole cover to further reduce heat loss.
Attention to service connections should include consideration for insulation, heat tracing and flexible connections where damage due to freezing or frost heave is a concern.
Forcemains need to maintain an adequate scouring velocity and be able to drain between pumping cycles in cold climates. This can be accomplished by an electrically operated ball valve in the line to allow drainage back into the wet well between pump cycles.
The soil conditions such as rock or high groundwater can significantly affect the cost of the sewers and, in some instances, contribute to extraneous flow problems. It may be advisable in such circumstances to design and construct the system such that gravity drainage is only provided for the first floor and up. In assessing such an alternative, it is essential that the following factors be considered:
- The presence or absence of basements in the existing dwellings;
- The extent of “finishing” in an existing basement;
- The presence or absence of fixtures in the basement; and
- The need for a solids handling sewage pump in the basement, should the basement contain fixtures.
6.9.1 Alternate Sewage Collection Systems
In the recent past, several alternate methods of communal servicing have been introduced in Ontario and other jurisdictions with success. The designer should refer to Section 5.15 - Alternative Sanitary Sewer Systems for more details. If a previously untried sewage collection system is being proposed, the designer should refer to Section 3.9 Technology Development for guidance.