Visual inspection of a CMP culvert

How to Determine the Level of Deterioration of Your Culvert

The successful rehabilitation of existing culvert pipe structures through the process of sliplining is an essential undertaking for municipalities.

A significant concern in this process is ensuring that the liner pipe will seamlessly fit inside the host pipe. Various issues, including misaligned joints, out-of-roundness, protruding laterals, long radius curves, elbows, or pipeline breaks and deflections, can influence the fit of the liner pipe, and can often lead to unexpected complications. These complications can keep designers, contractors, and suppliers awake at night, fearing that on-site contractors will be faced with pipes that do not fit. However, with a meticulous focus on planning, particularly in the condition assessment phase, these situations can be prevented.

// ** Advertisement ** //

The key aspects of determining the level of deterioration of your culvert include a discussion of three primary inspection methods used to assess pipe conditions: visual inspection, closed-circuit television video (CCTV), and laser profiling. Each method serves as a valuable tool in ensuring the success of sliplining projects and optimizing cost-efficiency while enhancing safety.

CCTV of an existing 48-in. RCP revealed material spalling and misaligned joints.
CCTV of an existing 48-in. RCP revealed material spalling and misaligned joints.

The Importance of Early Planning

Before delving into the inspection methods, it’s crucial to emphasize the significance of early planning through the condition assessment phase. The old adage “an ounce of prevention is worth a pound of cure” applies aptly here. The nightmares that can arise from encountering pipe-fitting issues can be avoided through comprehensive planning. While Contech does not generally perform condition assessments on existing pipelines beyond collaborative CCTV review or culvert condition review, we often receive inquiries about the essential information required to determine whether our solutions will fit. The choice of inspection method largely depends on factors such as the purpose of the pipe, its diameter, pipeline layout, existing conditions, and budget.

// ** Advertisement ** //

Now, let’s delve into the three primary inspection methods for assessing culvert pipe conditions:

1. Visual Inspection

Visual inspections represent the most cost-effective method for assessing the condition of a pipe. To make visual inspection a viable option, the pipe should be 48 in. or larger, straight, and relatively short. This method is primarily used for open-ended culverts and is often conducted by municipal maintenance crews during the project’s design phase. Smaller culverts can be visually inspected from each end, but the pipe will need to be is straight and shorter for potential problems to be effectively identified. With skilled measurement-taking, it is possible to determine vertical and horizontal angles within the pipe.

// ** Advertisement ** //

Nonetheless, for culverts with multiple curves, angles, or closed ends, an alternative inspection method may be more suitable.

Contractors may also verify the measurements of the culvert prior to ordering material. During visual inspection, measurements are taken at predetermined intervals and at locations where specific problems are identified.

// ** Advertisement ** //

The measurements obtained from visual inspection determine the minimum inside dimension of the host pipe. The maximum outside diameter of the liner pipe is provided by the liner material supplier. The maximum outside diameter of the liner pipe is subtracted from the minimum inside diameter of the host pipe to determine the spacing between the two pipes. The larger the space between the two pipes, the easier the liner pipe installation will be.

2. Closed Circuit Television Video (CCTV)

CCTV is the next level of pipe inspection and involves the use of a small cart equipped with cameras that can be operated remotely. This method is typically carried out by specialized contractors, although some municipalities may have their own CCTV equipment and crews. Due to the specialized equipment and crews required, CCTV is generally more expensive than visual inspections. However, it is highly effective for small and large diameter host pipes alike along with enclosed systems, such as storm sewers and sanitary sewers.

// ** Advertisement ** //

CCTV provides the capability to determine the locations of existing conditions, including laterals, misaligned joints, pipeline deflections, and alignment curves. Nonetheless, determining the degree or severity of these issues can still be challenging with this method.

Laser profile of an existing structural plate culvert shown in red. The proposed liner pipe, shown in green, was added to the plot to check for fit
Laser profile of an existing structural plate culvert shown in red. The proposed liner pipe, shown in green, was added to the plot to check for fit

3. Laser Profiling

Laser profiling of existing culvert pipes is considered the “no-doubt” fit assessment. Similar to CCTV, a small cart is used to traverse the host pipe, but, in this case, it is equipped with laser emitters that locate and map all deflections, protrusions, elbows, and other obstructions. This specialized work is conducted by expert contractors and is more expensive than CCTV. The benefit of laser profiling is substantial, as it allows the plotting of all known points to create an exact model of the existing pipe. This model, in turn, permits the precise modeling of the liner pipe to ensure it fits inside the host pipe long before liner material is purchased.

// ** Advertisement ** //

However, it is important to note that the primary drawback of laser profiling is that the laser cannot penetrate water, which can make locating invert information tricky. To overcome this challenge, laser profiling should either be (1) conducted in low flow conditions, or (2) supplemented with SONAR readings for point plotting pipe conditions below the water surface. Although laser profiling may be considered overkill for culverts, it is an ideal choice for pipes with multiple elbows, deflections, and other pinch points.

Potential Culvert Deterioration Indicators

What we are looking for during a culvert inspection depends upon the general type of pipe; rigid pipe or flexible pipe.

// ** Advertisement ** //

Rigid pipes are generally designed to carry all live and dead loads in the pipe itself. An example of a rigid pipe is RCP. As such, we are looking for signs that the pipe is no longer capable of carrying the designed loads. Symptoms such as cracking, mis-aligned joints, spalling, and/or visible rebar point to the need for rehabilitation. References such as Post Installation Evaluation and Repair of Installed RCP from the ACPA and ASTM C76.15 are good references to help determine allowable limits for various conditions.

Mis-aligned Joint (left), Cracking & Spalling (center), Exposed Rebar (right)
Mis-aligned Joint (left), Cracking & Spalling (center), Exposed Rebar (right)

Flexible pipes are designed so all dead loads and live loads are carried by the pipe/soil composite structure. Examples are CMP and plastic pipe. Symptoms that the pipe needs rehabilitation include local deformation, out of roundness, invert heaving, invert material loss, and/or soil intrusion. Available references for review are Design Data Sheet 19 – Load Rating and Structural Evaluation on In-Service Corrugated Steel Structures from the NCSPA and the Plastic Pipe Institute Handbook Chapter 10.

PVC Local Deflection (left),  HDPE Out of Roundness (center),, Invert Loss (right)
PVC Local Deflection (left), HDPE Out of Roundness (center),, Invert Loss (right)

Safety First

Before concluding, it is essential to underscore the paramount importance of safety during pipe inspections. Whether using visual inspection, CCTV, or laser profiling, strict adherence to all safety precautions and compliance with OSHA guidelines is non-negotiable. Ensuring the safety of all crew members involved in the inspection process is of utmost priority and should never be compromised.

In the realm of culvert rehabilitation and sliplining, determining the level of deterioration of your culvert is a pivotal step towards a successful project. Early planning and the selection of the appropriate inspection method are crucial in preventing unforeseen complications. Each inspection method has its advantages and limitations, and the choice ultimately depends on various project-specific factors. By investing in a meticulous assessment of your culvert’s condition, you can save time, resources, and potential headaches in the long run. Safety remains paramount, and all necessary precautions should be taken to ensure the well-being of the inspection crew. Ultimately, the more information you have at your disposal, the better positioned you are to achieve a seamless and efficient sliplining process, contributing to the long-term integrity and functionality of your culvert infrastructure.

Rian A. McCaslin is area pipe manager in the West for Contech Engineered Solutions.

// ** Advertisement ** //