Planning and Design Gravity Sewers

Jerry D. Morrow, P.E.

Course Outline

Gravity sewer design is an area of civil engineering that is often taken for granted. The result can be a 50-year storm or sanitary system that could allow years of inflow and infiltration, deposits and blockages, invert erosion, structural failure and systems that are hydraulically over-loaded, causing flooding and backups.

This course starts with the basics and addresses planning, expected flows, hydraulics, structural pipe design and appurtenances. Tables and Charts and sample problems are provided, so that that student has a practical guide to the successful design of a gravity system.

The course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the reader will:

• Understand the advantages and disadvantages of a gravity drainage system vs. a pressure system;
• Be able to establish design criteria for a gravity system;
• Learn the relationship between pipe size, velocity, erosion and system cleansing;
• Correctly identify the average design flows for various types of development;
• ccess minimum slopes for each pipe size to achieve cleansing velocity;
• Be able to establish correct design periods, drainage areas, and the resulting ultimate and design flows;
• Be able to establish design flows for undeveloped areas by land use;
• Be able to develop hydraulic profiles, including Hydraulic gradient and Energy gradient;
• Learn how often appurtenances are required by regulation and by design;
• Be able to use Manning's Equation for Q and V;
• Be able to calculate various velocities at various depths of flow in pipe;
• Be able to accurately determine the load on a pipe in a trench;
• Understand the effect of a load on a pipe and its relationship to the primary and secondary trench plains;
• Use the Marston Equation to determine earth loads on a pipe and the D-Loads that can be taken from manufactures literature on pipe;
• Be able to calculate the impact factor on pipe from such items as surface traffic;
• Be able to calculate where loads on a pipe change by calculating the transition width of the installation trench;
• Learn the change in pipe load due to bedding conditions;
• Be able to select the proper appurtenances for each pipe size and condition, including drop manholes and junction boxes;
• Learn what factors determine the depth of bury of a sewer;
• Be able to design the type of connections to manholes and appurtenances and assure a water-tight joint and a structurally sound result;
• Be knowledgeable of the factors that can cause odors, explosions and corrosion in a gravity system and learn to design against them;
• Use population projections to establish the ultimate development of a watershed and convert population to per capita peak flow;
• Be able to recognize and address peak flows in large watersheds through the use of "time of concentration" concepts and Peak Flow Factor;
• Be able to address the types flow monitoring equipment used in sewer system;
• Be able to work through an actual sample sewer design problem, including establishing peak flows and eventually calculate flows draining from each manhole in the system;
• Be able to establish reasonable hydraulic design criteria prior to beginning hydraulic design;
• Learn what factors to consider when plotting a sewer profile as a design tool;
• Learn how to protect the structural integrity of an existing pipe with a change load condition, such as additional surface load, by changing the class of bedding used;
• Be able to determine how much of the sanitary sewer design can be used in storm water design;
• Understand the kind of items that are customarily presented on sewer plans; and
• Review a set of guidelines established by an agency to guide plan preparation that are very typical of those used by a number of other agencies.

Intended Audience

This course is intended for civil engineers and technical civil designers.

Benefit to the Audience

Attendees of this course will get a review of the factors that must be considered to plan and design a sanitary sewer system. The class further addresses hydraulic design to establish pipe size, structural analysis to select the proper pipe strength and bedding conditions that support the pipe and discusses appurtenances such as junction boxes and precast manholes.

Course Introduction

Sanitary sewer systems must be properly planned and sound criteria established prior to beginning hard design. The larger the watershed, the more important the planning portion of the exercise. The course establishes design criteria and the engineering necessary to provide a sound design product.

• Criteria
• Planning
• Hydraulic Design
• Structural considerations

Sample design problems are presented to allow the attendee to work through a design of the sanitary sewer system for a watershed with nine sub-watersheds. The calculation charts, graphs and design curves are presented to assure the attendee has the tools to undertake the design task as a new designer or provides a good review with a collection of design tools.

The course content can be printed out in MS Word, with embedded charts, drawings and design curves that will provide a future reference document.

Course Content

The course content is in a PDF file Planning and Design Gravity Sewers .

Please click on the above underlined hypertexts to view, download or print the documents for your study. Because of the large file size, we recommend that you first save the file to your computer by right clicking the mouse and choosing "Save Target As ...", and then open the file in Adobe Acrobat Reader.

Course Summary

Often sanitary sewer design is undertaken without evaluation of design periods, downstream receiving systems, and future uses within the watershed. Some basic planning prior to beginning design can greatly improve the longevity of the product.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.