Jumping Jack Compactor: Rammer Uses, Lifts and Rental Tips

A jumping jack compactor is the tall, noisy machine you see hopping along the bottom of a utility trench. Its proper name is a rammer. A small engine drives a piston and springs that pound a flat shoe into the ground, while the forward lean of the machine makes it creep ahead with each blow. That high-impact action is what clay and other cohesive soils need, and the narrow shoe fits where plates and rollers cannot. This guide explains how rammers work, which soils they suit, how thick to place each lift and how to avoid the classic trench backfill mistakes.

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Quick facts

Also called: rammer, trench rammer, jumping jack tamper.
Best for: cohesive and semi-cohesive soils, trenches, around pipes and foundations.
Impact rate: about 500 to 750 blows per minute, per Multiquip's compaction handbook.
Typical weight: roughly 140 to 170 lb for common walk-behind units.
Shoe size example: 11 by 13 inches on the MBW R442.

How a rammer works

The engine turns a crank that drives a piston inside a spring-loaded housing. Each stroke throws the whole machine up a little and then drives the shoe down hard. Multiquip describes rammers as high-impact, high-amplitude machines that combine impact, vibration and kneading. In clay, that blow shears the bonds between particles and pushes out air, which a low-amplitude plate cannot do. The machine is built tilted forward, so each hop moves it ahead; the operator mostly steers.

Jumping jack vs plate compactor

Feature	Jumping jack (rammer)	Vibratory plate
Action	High impact, high amplitude	Low amplitude, high frequency vibration
Best soils	Clay, silt, mixed soils	Sand, gravel, crushed stone, asphalt patches
Footprint	Narrow shoe for trenches	Wider plate for open areas
Finish	Dimpled surface	Smooth surface

For granular base, see our guide to lift thickness for a plate compactor. A rammer will pound gravel too, but a plate does it faster and leaves a smoother top.

Lift thickness and passes

Thin lifts are the rule in trenches. The MBW R442 is rated to compact up to 18 inches in clean sand at optimum moisture, yet MBW still recommends lifts of no more than 12 inches. On clay, many specifications ask for less. New Zealand's forest road manual suggests no more than about 200 mm (8 inches) per layer for cohesive soils with a power rammer. The U.S. Unified Facilities Criteria note that some inorganic clays can be compacted adequately in confined zones with rammers of at least 100 lb, provided lift thickness and water content are closely controlled. Follow the job spec, and check density rather than counting passes.

Running a jumping jack well

Moisture first. Squeeze a handful of the backfill. It should hold together without smearing water onto your hand.
Place a thin lift and level it roughly with a shovel.
Work in a pattern. Run the length of the trench, overlap each pass, and pay attention to the edges and around the pipe.
Keep the shoe flat. Hold the throttle wide open so the machine hits with full force, and let it travel at its own pace.
Protect the pipe. Hand-tamp or use bedding stone near the pipe until there is enough cover; see pipe bedding stone.
Check, then repeat with the next lift.

Why trench patches sink

Most sunken utility patches come from backfill dumped in thick lifts and tamped only at the top. The bottom settles over the next year and the pavement follows it down. Our guides on sinking utility cut patches and trench paving cover the repair side. Where compaction is hard to guarantee, some agencies use flowable fill instead of soil.

Safety

Never enter an unprotected trench deeper than your agency or OSHA rules allow; shoring or a trench box comes first.
Rammers produce exhaust. In a deep, narrow trench, fumes can collect.
Wear hearing protection, safety glasses and steel-toed boots.
Shut it off before refueling or moving it in and out of the trench.

For wider trenches or long runs, a remote-controlled trench roller can do more work while the operator stays out of the cut. Plan backfill quantities with the gravel calculator, the crushed stone calculator or the sand calculator.

Questions people ask

What is a jumping jack compactor?

It is a walk-behind rammer: a small engine drives a piston and spring stack that slams a foot into the ground hundreds of times a minute. The machine hops forward as it works, which is where the nickname comes from.

When should I use a jumping jack instead of a plate compactor?

Use the rammer on clay, silt and mixed soils, and in narrow trenches or tight spots around pipes, posts and foundations. Use a vibratory plate on sand, gravel and crushed stone, which compact best from vibration rather than impact.

How deep can a jumping jack compact?

It depends on the machine and the soil. One manufacturer lists up to 18 inches for its 150 lb rammer in clean sand at optimum moisture, yet still recommends lifts no thicker than 12 inches. On clay, thinner lifts are safer, and the job spec rules.

How much does a jumping jack weigh?

Common walk-behind rammers weigh roughly 140 to 170 lb. The MBW R442, for example, is listed at about 145 to 154 lb depending on the engine.

How many blows per minute does a rammer deliver?

Multiquip's soil compaction handbook gives a range of 500 to 750 blows per minute for rammers. Individual models list their own rate; the MBW R442 is rated up to 720.

Is a jumping jack hard to use?

It takes some practice. Let the machine lead, keep it tilted forward so it travels, and do not lean on the handle. Wear hearing protection and steel-toed boots, and keep feet clear of the shoe.

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