Many professionals involved in raising manhole frames to grade following a repaving operation, or as part of ongoing sewer maintenance, are searching for the best and most economical methods. The last ten years have produced some new methods and tools that should make your choice a little easier.
The following article written by Mr. Manhole explores some old ideas and some newer ones.
Let us look first at some tools currently in use to remove manholes from the road preceding a rebuild. The trusty air hammer ranks up at the top as the most used tool for manhole removal. A number of crews use a large concrete saw to cut a square around the structure before removing the manhole. This still requires an air hammer in most cases. If a saw is used it must have a large diameter blade to fully penetrate the pavement. A typical saw used for this purpose will be in the 65hp range with a 30” blade.
Blade cutters have become more popular in the last few years. They operate on skid steer loaders and have adjustable arms holding fixed blades, allowing various cutting diameters, eliminating the need to purchase different cutting blades. Blade cutters pull the frame and surrounding road surface from the road, reducing labor cost and risk of injury to workers.
Drum cutters are a fairly new tool that use a cutting drum with a fixed diameter. Drum cutters have a more limited cutting depth, and most do not pull the frame and surrounding road surface.
Let’s take a look at a cost comparison for the different available methods. For this estimation, we will use a labor cost of $30 per man hour.
- Air hammer removal will require 2 men at a minimum. The total labor cost will be 10 man-hours for a total cost of $300.
- The concrete saw will require 2 men at 11 man-hours for a cost of $330.
- Blade cutters are the clear winner on cost to operate with a 2 man crew and two man-hour requirement for a total cost of $60.
- Drum cutters require a 2 man crew for 3 man-hours for a total cost of $90.
Unfortunately, there is no perfect manhole frame removal method. Let’s look at the current tools and methods available and compare the pros and cons of each one.
The air hammer–while being the most common removal tool in use today for the purpose of manhole frame removal–is the least effective. Statistics show this method has the highest injury rate to workers. The design of the air hammer bit makes it next to impossible to achieve a smooth, round cut. This method of removal requires the second longest time ranking only slightly above the concrete saw. Because of the time required, employees are exposed to traffic danger for an extended period. This method is not recommended for new construction or resurfacing projects for aesthetic reasons.
The concrete saw method is an interesting concept, since it doesn’t remove any material. The saw is used to make a square cut surrounding the manhole frame. The frame and road material are then removed with an air hammer, tying the two methods together. A saw that will penetrate the road completely will be at least 65hp, and the saw will have a 30” diameter blade. These blades are quite expensive and wear rather quickly. They require water to cool the blade, which can create hazardous waste. Transporting and loading/unloading a saw of this size is a challenge. The saw will leave large over-cuts at each corner of the square surrounding the manhole frame. The workers will still face the challenge of removing the road material with an air hammer. Most engineers object to square repairs.
A blade cutter is able to achieve a perfect round cut through asphalt or concrete and has a variable cutting diameter. They use replaceable carbide teeth, which require no water to cool them. This type of cutter removes the frame and the road over-cut after cutting, greatly reducing labor and risk of worker injury. The blade cutter has a greater cutting depth than the other methods due to the design of the cutting blades. The teeth on the cutting blades can be quickly replaced on the job as needed and are reasonably priced. Blade style cutters weigh less than drum cutters and require minimal maintenance due to their design. Due to the speed of the removal, workers are not exposed to needless traffic hazards. The workers perform minimal physical labor with this method, reducing injury risk.
Drum cutters are similar in some ways to blade-type cutters. They do not provide for multiple cutting diameters without drum replacement. Most drum cutters will not remove the manhole frame and the surrounding road surface from the road, and the design of the cutter causes it to take a while longer to cut through the road than blade style cutters. Drum style cutters have a limited cutting depth and often tend to “skate” or move around as they cut because they don’t use a centering device. The teeth are replaceable in the field and are reasonably priced–labor is almost totally eliminated with the drum cutter, greatly improving worker safety.
Drum cutters have a slow turning speed and cut a very wide groove so they need a lot of horsepower to operate. These machines are quite heavy and require a large machine to operate them.
- A new 65hp concrete saw costs between $15,000 – $22,000
- A new air compressor with 90 lb. air hammer costs between $15,00-20,000
- Drum style cutters start in the $32,000 range
- Blade cutters start at $7,999
In review, air hammers and saws are becoming less popular due to high labor cost and risk of worker injury. Engineers agree that square repairs are far inferior to the round repairs, which are only possible with a blade or drum cutter. When considering a manhole cutter, it is extremely important to achieve a full penetration vertical cut through the pavement surrounding the manhole frame, limiting the effects of frost and pavement creep.
Full penetration round cuts are only possible with blade or drum style cutters. A properly executed repair will be perfectly level with the surrounding road surface and will not let ground water penetrate into the manhole. Such repairs have a long life and will last through many paving cycles.
Historically, many manhole frames shift out of level over time making it necessary to remove and re-level them. Since these new methods are so stable, when it’s time to repave, a high quality lift ring can be installed in the frame, eliminating the need to adjust. It’s also easier to mill to the edge of a concrete repair then simply pave to level. Drum and blade cutters are still the new kid on the block, but they have been in use for over a decade with great success.
If your crews have access to a skid steer loader, consider putting it to work doing manhole frame adjustments. Your crews will thank you for making the job safer and easier.