Developed between 1979 and 1981, Dos Santos sandwich belt high-angle (SBHA) conveyors began commercial operation in 1984. Since then some 150 systems have gone into successful operation throughout the world with most still in productive operation many years later. They have been successful with numerous, varied bulk materials from light, friable wood chips and grains to heavy, coarse copper and iron ores.

Throughput rates have varied widely, from 272kgs/hr to 6,000t/h. Conveying angles have varied from 30°– 90° (vertical). Indeed, 35% of the installations elevate the bulk continuously vertically. These characteristics make the system ideal for haulage from underground, particularly at tunnelling projects where the excavated muck must be elevated within the tight spaces of an access vault or shaft to the surface.

Muck disposal from the largest tunnel boring machines (TBMs) is now predominantly by conventional trailing conveyors. The large continuous volumes must be elevated to the surface, then conveyed and discharged at a holding area, to be removed to permanent fill locations. For high volumetric rates, the elevating systems to the surface have been pocket belts and sandwich belt high-angle (SBHA) and vertical conveyors.

The simplicity of the pocket belts, hanging vertically between the lower and upper terminals has been compelling. However, they have great difficulty in fully discharging the muck from the pockets. On the other hand, sandwich belt systems suffer the perceived complexity of equipment: idler rolls that must be accessed and occasionally replaced throughout the beltline. The resounding advantage however, is that SBHA conveyors use conventional smooth-surfaced belts that discharge the material completely because they can be scraped clean.

The very sticky muck to be excavated at the Paris Metro extension project led the client to specify the exclusive use of SBHA conveyors for the continuous, vertical haulage requirements. There is a recurrence of other tunnelling projects specifying the exclusive use of SBHA conveyors for the same reason – very wet and sticky materials.

Sandwich Belt High-Angle Conveyors

The Dos Santos SBHA conveyor was developed by the author more than 40 years ago while working for various firms including Dravo Corp, Continental Conveyor and Equipment Co, and Dos Santos International. The first commercial installation went into operation in Wyoming, USA in 1984.

Sandwich Belt Principle

Dos Santos SBHA conveyors represent a logical evolution and optimisation of the sandwich belt concept. Basically, the sandwich belt approach employs two ordinary rubber belts which sandwich the conveyed material. Additional distributed force on the belt provides hugging pressure to the conveyed material in order to develop sufficient friction at the material-to-belt and material-to-material interface to prevent sliding back at the design conveying angle.

Figure 1 is a realistic model of the belt sandwich. An ample belt edge distance assures a sealed material package during operation, even if belt misalignment occurs. This model also illustrates the interaction of forces within the sandwich. The applied or induced hugging load is distributed across and along the carrying belt sandwich. Of that hugging pressure, only the middle pressure hugs the material load while the outer pressure merely bears against the material free edges of the belt. Both belt surfaces apply their frictional traction on the material. From this model, one can calculate the required material hugging pressure that will ensure the material does not slide back due to the tangential gravity loads. This is expressed by the equation:

Dos Santos SBHAs

Following the extensive study of past sandwich-belt conveyors and governing theories and constraints, a broader scope effort was undertaken in 1979 to develop the first SBHA conveyor to meet the needs of mining and bulk materials’ handling. The resulting conveyors are evolutionary and conform entirely to the governing theory, to the constraint equations and to the development criteria. These sandwich-belt conveyors fulfill all established operational requirements and can conform to a wide variety of applications.

Advantages of Sandwich Belts

Dos Santos SBHA conveyors are designed to offer numerous benefits including:

  • Virtually unlimited capacity, (greater than10,000t/h possible).
  • High lifts (up to 300m) and conveying angles (90°).
  • Flexibility in planning and application (single- to multipleruns).
  • Simplicity of approach, very high availability and low maintenance costs.
  • High conveying speeds (same as conventional conveyors, to and beyond 5m/s).
  • Available belts/hardware up to 3m-wide allow capacities > 10,000t/h.
  • Belts are easily cleaned and quickly repaired.
  • They enable spillage-free operation.

Selected Sandwich-Belt Installations

Since 1984, more than 100 Dos Santos SBHA conveyor units have gone into operation throughout the world.

Project DS 005 was the first commercial Dos Santos SBHA conveyor for construction and it was also the first vertical installation. Part of the Los Angeles metro expansion of the late 1980s, the high-angle conveyor elevated the excavated earth continuously from under the street to a truck loading bin above. Though it was designed to load directly into the bin, for reasons of truck access and traffic flow, a connecting conveyor was added in order to locate the bin further away from the intersection of the two busy streets. The surge capacities of the hopper below and the bin above allowed independent discontinuous excavation and truck loading without interrupting the continuous elevating of the high-angle conveyor. The system was designed to begin operation during the early excavation, requiring only 25m of lift. It was then extended down as the depth increased in 1.219m increments until reaching the design maximum depth and the corresponding design lift of 32.3m.

(The record highest vertical lift for a sandwich conveyor of 104m was recorded at an Illinois coal mine in 1993).

Though the system proved successful there were lessons learned. For subsequent such systems we decided to have wider belts (not less than1,200mm), thicker damage-resistant wear covers and rubber-disc centre rolls at idlers to soften the indents of the large lumps as they travel along the transition curves.

Elevating TBM Muck

Project DS 037 on the Chicago TARP (Tunnel and Reservoir Plan) project was the first sandwich-belt installation to elevate tunnel muck from a TBM. The entire excavating and muck haulage system consisted of the TBM, a trailing conveyor below ground, the vertical sandwich-belt conveyor system to lift the material to the surface and finally, a transfer and stacking system consisting of a grasshopper conveyor and radial stacker. As with subsequent systems, the major equipment (drives and take-up systems) was located on the surface, at the head end, so it could be easily accessed and serviced. Only the intermediate structure and tail pulleys were located underground. Also, the intermediate structure served as the support and guidance for the trailing conveyor’s return belt strands that were elevated to its belt storage unit located on the surface, 70m above.

SBHA Conveyors at The Paris Metro Expansion

In 1997, the author founded Dos Santos International and continued to offer SBHA conveyors. But Continental Conveyor of Winfield, Alabama also continued to offer its version.

The Paris Metro expansion project finally acknowledged the distinct advantage of SBHA conveyors and specified them exclusively. That advantage is the ability to handle and fully discharge very wet and sticky material. This is because the smooth-surfaced rubber belts can be continuously scraped clean. This benefit had long been demonstrated in the many Dos Santos sandwich-belt installations in other industries, particularly in municipal waste, where many units elevate municipal sludge, sludge/sawdust mix, and some industrial sludges. Nearly all of these installations conveyed the sticky material vertically.

In contrast, the tunnelling industry had continued to struggle with handling excavated material with its pocket-belt elevating systems. At its discharge end, the pocket belt required a long overlap distance with the surface receiving conveyor and a series of eccentric rolls that beat the back of the pocket belt in order to dislodge the caked material from the pockets and onto the outgoing conveyor below.

The two Dos Santos units supplied so far for the expansion of the Paris Metro share a common basic design but have minor differences. Essentially, for the common design rate of 800t/h, the width of the belt is 1,400mm and this is compatible with the specified material size which it has handled admirably. The speed of the belt is 3m/s.

Equipment Summary

In anticipation of larger material, the belts are armored with 10mm-thick carrying covers of Grade 1 rubber, the best resistant grade against wear, impact and cutting. Additionally, along the transition curves, all CEMA D6 idlers have rubber-disc centre rolls and steel wing rolls. The rubber-disc centre rolls soften the ride of the larger lumps as they are urged radially by the outer belt against the inner belt. All pulleys are rubber lagged: plain at non-drive pulleys and diamond grooved for traction at the head drive pulleys. The non-drive pulleys are also crowned.

The combination of lagging and crowning promotes good belt alignment. The lagging is also softer and more forgiving on those occasions when material enters the pinch between the belt and the pulley face.

Both belts are driven equally by shaft-mounted drives at the head/discharge pulleys. Driving both belts shares the drive tension equally and facilitates better belt alignment than driving only one belt while the other merely follows.

Variable frequency control at both drives facilitates equal load-sharing, provides soft starts, and allows variation of belt speed as deemed appropriate in response to the actual material and flow characteristics. Tension control is by near-constant pressure hydraulics. A tensioning cylinder at each belt pulls on the take-up pulley carriage and operates within a narrow band of hydraulic pressure, pumping at the lower operating limit and stopping at the higher operating limit.

Arrangement and Structure

The common design is for current and long-term requirements. Because tunnelling and construction projects tend to be of short duration, lasting several years at most and typically less than two years, it is difficult to justify the cost of custom, dedicated equipment. It was thus important to provide a common design that would address both current and future requirements.

Accordingly, Dos Santos designed the system to easily decrease or increase vertical lift to a possible low of 19.6m and a possible high of 43m. This can be achieved by subtracting or adding vertical structure (and belt length) in runs that are multiples of 1,676mm, the length coverage of each gently pressed sandwich (GPS) hugging pressure module. Thus the system and equipment must be designed for the maximum lift.

The system is modular to facilitate extension and contraction, with most equipment located on the surface for easy access and maintenance. The intermediate structure is of simple parallel channel sections and hangs down freely, suspended from the head end. At the bottom, the loading station and approach structure is supported at the loading end (left side) on grade and at the vertical (right side) by the hanging intermediate structure. Strategically-bolted splice points along the vertical facilitate the adding and subtracting of vertical structure as required for each subsequent installation. With this arrangement and structure, the lower loading section and vertical intermediate sections are kept simple with the most vital equipment located at the head end.

Early Operation and Adjustments

The Paris Metro Expansion is vindicated in its exclusive specification of SBHA conveyors for muck-elevating duties. However, operations did not go smoothly immediately. The Dos Santos team knew the muck would be sticky but no one anticipated just how wet and sticky.

The project specifications did not provide sufficient guidelines to cope with the material found. So, it must be concluded that no one was prepared, including those who supplied conventional conveyors for the project. The muck could best be described as ‘goop’ or very wet goop.

Material flows through the chute-work dragged or plugged. Ironically, more water was sprayed on the material to keep it moving. Chute covers were removed to allow water spray with hoses. The original supply included a profiled deflector plate as is common to guide the discharge. This too became an obstacle and was replaced with a hanging chain curtain. The flailing motion of the latter made it self-cleaning.

Though elevating the muck was never the problem, scraping the belts clean proved challenging. Ultimately, scrapers were replaced with more effective sprung-blade versions which follow the belt’s dirty surface.

The changes and adjustments mentioned greatly improved the sandwich-belt operation and it continues operating without interruption. Production and material flow problems continued at the TBM, especially at the inclined section of the discharge conveyor where globs of wet clay tended to slide and stagnate.

This did not challenge the design capacity of the vertical Dos Santos sandwich units. Taking advantage of the VFD speed control, the operating speed was reduced from 3m/s to 2.4m/s (from 50Hz to 40Hz). This handled production very well while reducing wear and tear associated with belt speed.

With the remaining work at the Paris Metro expansion and the others that have followed suit, the opportunity now presents itself for a quantum leap in sandwich-belt high-angle conveyors at construction and tunnelling projects, especially for the high volumes produced by the largest TBMs.