By Vince Hunt — April 5, 20165 as previously printed in the April edition of the Compact Equipment magazine
Soil compaction has been an accepted practice since Roman Empire roadbuilders determined that their cut stone road surfaces were only as good as the foundation in which they rested. Obviously, the equipment used for soil compaction has vastly changed and improved over the years, but one thing remains true: Soil compaction is an integral part of the construction process, and it is vital that any civil engineering project including structures, roadways, walkways and earth-retaining systems have a plan for compaction. Properly done, soil compaction adds many years to the useful life of structures by increasing the foundational strength and stability.
To compact soil in confined areas, vibratory rammers and vibratory plates are often used. Vibratory rammers are the most universal of all confined area compactors. They work in most soil types and are ideal for very confined spaces such as narrow trenches, foundations and walls. Rammers really excel in cohesive (clay) soils because the high shoe stroke, ideal speed and subsequent high energy combine to produce a shearing effect in the soil which kneads the soil particles together, squeezing air pockets to the surface. This high shoe lift allows the machine to advance forward even in sticky, highly cohesive soils. A high percussion rate (impact speed) is generally considered greater than 650 blows per minute (bpm).
There are a number of different types of rammers offered in the market. When choosing a rammer, contractors take into consideration weight (for ease of transport), shoe size, stroke height and a variety of engine options: two-cycle, four-cycle and diesel. In the late 1990s, because of stringent environmental regulations, most rammer manufacturers were required to switch from two-cycle to four-cycle engines which, at that time, had lower emissions. The exception was Wacker Neuson that designed and manufactured its own two-cycle engine, which passes all EPA regulations and has lower emissions than rammers powered with four-cycle engines. Initially, four-cycle rammers did not perform as well as rammers powered with two-cycle engines. They were also not as dependable on jobsites.
The four main considerations when selecting any type of construction machine are performance, durability, ergonomics and reliability/support. Production is the factor that helps the contractor make money. A rammer must be capable of producing compaction densities that are required on the jobsite or are needed to meet specifications. On large jobs, contractors are often required to meet predetermined Proctor density values and are usually required to have a third party confirm the results. When attempting to compare performance between manufacturers, buyers need to be aware that rammer force specifications are not all created equally.
In 2004, several rammer manufacturers discontinued publishing impact force because the industry could not determine an accurate and repeatable standard testing method for measuring impact force. There is currently no “apples to apples” standardized method for manufacturers to measure force output. Unlike many other specifications, force cannot be calculated theoretically using equations because of the spring-mounted design. As a result, force calculations vary greatly on different soil types, soil densities and shoe sizes as well as being heavily influenced by the downward force of the user during operation. Therefore, rammer impact force is a moving target and can be misleading when used for comparative purposes and/or fitness-for-use analysis. The best way to compare compaction performance of rammers is by doing a side-by-side comparison and observing the performance differences.
Durability is important because rammers actually strike the soil rapidly and violently at about 10 to 12 blows per second. As a result, rammers create extremely high accelerations approaching 10 to 12 times the force of gravity. When you consider that a jet aircraft gets exposed to 3 to 4 Gs during accelerations and turns, rammers are quite amazing. They are actually self-destructive tools. Therefore, durability becomes very important because contractors need uptime and rental companies require a high utilization rate for their ROI.
Ergonomics is always important, especially to the operator. With the high speed and acceleration of rammers, minimizing vibration felt by the operator’s hands can be a design challenge. Be sure to choose a rammer with low hand arm vibrations (HAV). This is often a design requirement for manufacturers and includes analysis of handle geometry and specially designed shock mounts properly positioned to reduce HAV vibration. Other ergonomic factors include low weight, low center of gravity and overall weight distribution which combine to make rammers well balanced, easy to handle and self-advancing, which reduces operator fatigue. These all contribute to better efficiency as operators are able to work for longer periods of time.
Lastly, support is always an important consideration when choosing a rammer brand or any piece of construction equipment.
Contractors generally prefer local support, ease of doing business, quick spare parts availability and jobsite consultation for all phases of the construction process. The effects of inadequate soil preparation may take weeks or even years to become apparent. However, once the results begin to show, the costs of repairs far outweigh the time and rental costs of proper soil compaction the first time around.
VINCE HUNT IS A PRODUCT APPLICATION AND TRAINING SPECIALIST FOR WACKER NEUSON.