Putzmeister shotcrete equipment at the Fresnillo mine. Putzmeister shotcrete equipment at the Fresnillo mine.

Fresnillo plc’s namesake underground mine, located in the state of Zacatecas, Mexico, and in operation for almost five centuries, lies in an area with rich mining resources. Silver and gold are the primary products taken from the Fresnillo mine, along with lead and zinc.

One of the mine’s principal contractors on site is Diseños y Construcciones Martínez (DICOM), a specialist in mine development. DICOM handles the Fresnillo mine’s drilling and blasting, ground support and mucking, electrical, water and ventilation installation and mine infrastructure.

Depending on the length of advance per day and type of ground, rock support in the mine variously involves anchor bolts, wire mesh and shotcrete. DICOM applies mechanized, wet-mix shotcrete, working with Putzmeister machinery.

DICOM owns four Putzmeister SPM 4210 Wetkret dual drive concrete spraying units, which can be powered either electrically or from their diesel engines when working in areas without an electrical supply.

The SPM 4210 Wetkret series has a spraying arm with a vertical spraying reach of 10 m, as well as a concrete pump with a capacity of 20m3/h. Its proportional remote control (cable and wireless) permits full regulation of the concrete output as well as adjustments to the predefined additive dosage. This enables the machine operator to cope with the changing working conditions in the mine. A hydraulically-operated synchronized peristaltic additive pump is equipped with an automatic dosing device proportional to the concrete flow, thereby assuring spraying accuracy and quality.

DICOM also has four Putzmeister Mixkret 4 series low-profile concrete mixers, each with a capacity of 4 m3, for transporting pre-mixed shotcrete from the concrete plant to the work site. The Mixkret 4’s are equipped with an Integrated Continuously Variable Drive (ICVD) system, which eliminates gear shifting and takes full advantage of engine power at all times, maintaining steady traction and making operation easier for the operator. The Mixkret units used by DICOM incorporate an optional additive discharging system, with a capacity of 120 l, to optimize shotcrete additive logistics.

DICOM mainly uses Sika products when it comes to additives for the shotcrete mix. These include retardants for transport, because the concrete plant is located outside the mine and underground work faces can be as far as 10 km away from the portal. Plasticizers and accelerants also are added for spraying.

DICOM formerly used the dry-mix method for shotcreting, but switched to wet spraying with the Putzmeister systems because of the multiple advantages associated with this method, which are listed as:

  • Improved worker safety—operators don’t work in unstabilized areas.
  • The reduced quantity of dust leads to a healthier working environment.
  • There is less material rebound than with dry spraying.
  • A higher level of support is achieved using the wet spraying process.
  • Mechanized concrete spraying is more efficient, maintenance costs are reduced, less manpower is required, and work cycles are shorter.