A double-deep rack is basically a selective pallet rack that is 2 pallet positions deep. This rack is loaded and unloaded from the front, requiring stocking and picking activities in the same aisle.

A variant on standard adjustable beam racking, Double-deep Pallet Racking, as the name implies, allows pallets to be stored two deep but still accessible from the same aisle.

A double deep racking layout involves back to back rows of 2 deep selective rack. Each pick slot now has a front and a back pallet. To access the back pallet, requires a deep reach attachment on the lift truck. Double deep racking is created by placing two double sided runs together. Pallets are stored two deep and are accessed using special trucks with pantograph or telescopic forks.

Advantages:
-Increased space utilization by reducing the number of access aisles and using the space saved to accommodate additional racking, a Double-deep configuration provides a highly space-efficient storage system.
-Good aisle-to-rack ratio
-Density of storage because there are fewer aisles. Doubles the density of racking between the aisles.
-Provides double the storage space per SKU to accommodate faster movers.
-Deep reach attachment is less expensive than alternative of purchasing swing reach trucks, and going to Very Narrow Aisle (VNA) single selective layout.
-Flexibility - reach trucks equipped with deep reach attachment can still fulfill all other reach truck functions.
- Increased storage capacity
-Greater use of floor space by reducing number of access aisles

Disadvantages:
-special lift truck which will incurred more cost
-Last in, first out (LIFO) picking from location
-Less selectivity
-Crossbars needed
-A double deep rack decreases picking efficiency if different SKUs are stored within the same location.
-Doubles the density of racking between the aisles, which causes aisles space to become narrower.
-Not suitable for rapid stock rotation – the first pallet in is the last one out
-Each lane dedicated to one SKU which leads to space loss in case of SKU's with odd number of pallets

Conditions to use double deep racking:
· when the storage requirement for a SKU is five pallets or greater
· when product is received and picked frequently in multiples of 2 pallets.
·The double deep rack is used in projects that require moderate picking throughput. When additional storage capacity is needed and expansion space is limited, an existing selective rack can be replaced with a double deep rack, which increases storage capacity by decreasing the number of aisles within the warehouse.

More information:
If the back pallet is required, the front pallet must be moved to another location or brought down to the floor and then back to the location after retrieving the back pallet. The Warehouse Management System (WMS) must control the utilization of these locations to minimize double handling and honeycombing caused by partially filled locations or the inability to utilize the entire location. Again, location capacity is usually defined by the number of storage containers. Although the speed of access to all of the pallet positions is restricted, with an efficient stock management system this can easily be compensated for to take advantage of the increased storage density.

http://www.eseprojects.co.uk/palletstor-racking.asp#double-deep
http://www.pearsonknight.co.uk/double_deep.html http://www.industrysearch.com.au/Products/Dexion_Keylock_Double_Deep_Pallet_Racking-10254 http://www.pacificwesteel.com/index.cfm?2,2
http://www.warehousesystemsinc.com/double_deep.asp