Dredging Grabs

DREDGING TECHNIQUES

Dredging is a critical industry to keep waterways navigable.  There are several reasons waterways need to be dredged.   It could be a new waterway which needs to be deepened or a contaminated waterway that is need of remediation.  Over time waterways will fill with silt and debris which needs maintenance to keep them to certain depths.  There are several dredging techniques which dredgers use to accomplish this dredging.  Cutterhead, suction, and mechanical dredging are several of the techniques which can be utilized; each has its place and its purpose.  This document will focus on mechanical dredging.

MECHANICAL DREDGING

Mechanical dredging is dredging by use of buckets to remove rocks, sediment, and debris from the surface of a waterway.  Mechanical dredging typical require three main pieces:

1. A crane, excavator or material handler
2. A bucket for the actual dredging
3. Some method for transporting the material away from the site, such as a dump truck or barge.

The two main options when using mechanical dredging are cable vs. hydraulic and each has their pros and cons. 

1. CABLE OPERATED BUCKETS

Cable cranes typically will have a much larger operating capacity allowing for much larger buckets to be used.  Cable operated buckets are most commonly used on two rope cable cranes and can be over 60 cubic yards weighing over 100,000 pounds.   The equipment for cable operations is very expensive.  These buckets rely on the weight of the bucket and number of sheaves for its digging power.

PRO’S

• Larger buckets can be used for great capacity
• More material is moved per cycle
• Greater depth of dredging
• Higher boom reach if needing to go over walls, etc.

CON’S

• Slower cycle times
• Digging power is based on the weight of the bucket and number of sheaves
• More sheaves mean more cable required which slows cycle times

2. HYDRAULICALLY OPERATED BUCKETS

Hydraulic cylinders power hydraulically operated clamshell buckets.  Hydraulic machines, such as a material handler will have much less operating capacity than a cable crane, meaning smaller buckets will have to be used. These buckets are typically in the 3 to 6 cubic yard range.  Hydraulic machines have a faster cycle time than a cable crane, and the operator is also able to push the bucket into the material.   These machines typically max out at around 50ft digging depth with extensions.  The limiting factor on the size of these buckets is the operating capacity of the machine.  An Anvil model HC5 3 yard bucket weighs about 6,000 pounds.  Dredge material typically weighs about 120 pounds per cubic foot.  This means the total weight of the bucket and material is 14,748 pounds.  The most common size material handler has an operating capacity of between 10,000 and 14,000 pounds when fully extended down and then out.

PRO’S

• Fast cycle times
• Operator can push bucket into dense materials
• Less expensive

Cons

• Lower operating capacity, smaller cubic yards per cycle
• Depth of digging constrained by boom length
• Lower boom height

TONS PER CYCLE COMPARISON

This will illustrate the difference in tons per cycle between a 4-yard hydraulic bucket and a 12-yard cable bucket.  This is estimating 120 pounds per cubic weight of the material.

               ANVIL HC5-400 4 YARD HYDRAULIC BUCKET

• HC5-400 WEIGHT: 6,600 POUNDS
• MATERIAL WEIGHT: 120 POUNDS PER CUBIC FOOT
• 4 YARDS OF MATERIAL (EST 90% FILL): 11,664 POUNDS HC5-400 AND MATERIAL WEIGHT: 18,264 POUNDS

               ANVIL RN-1200 12 YARD 2 ROPE CABLE BUCKET

• RN-1200 WEIGHT: 38,000 POUNDS
• MATERIAL WEIGHT: 120 POUNDS PER CUBIC
• 12 YARDS OF MATERIAL (EST 90% FILL): 34,992 POUNDS
• RN-1200 AND MATERIAL WEIGHT: 72,992 POUNDS

CABLE OPERATED BUCKET STYLES

There are several options when selecting a cable clamshell bucket for dredging.  The two most popular types are a Round Nose style bucket and a General Purpose style bucket.  Round Nose and General Purpose buckets are very similar in design, weight, and the number of sheaves; the main difference is the bowl corners on a Round Nose bucket are rounded.  This allows for better penetration in harder material.  A Round Nose bucket does require more labor for manufacturing therefor; the cost will be higher than a General Purpose Bucket.   When choosing which style bucket, it is important to consider the density of the material that will be dredged. 

For environmental dredging applications, a cable level cut bucket is the preferred style of bucket.  Cable Level buckets close within plus or minus 2 inches and do not scoop like a traditional clamshell bucket.  This allows for the removal of only the contaminated sediment.  An environmental level cut bucket will also have features such as a low turbidity design and baffles to contain sediment while releasing water.

Cable Dredging Bucket Models:

RN - Round Nose Dredging Bucket

GP - General Purpose Dredging Bucket

CLC - Cable Level Cut Dredging Bucket