Q: Tunnel boring machines, called TBMs for short or “moles,” feature circular plates on the front called shields. Disc-shaped cutters on the shield tear through rock and dirt, which passes through openings in the shield onto a conveyor belt inside the TBM that deposit it behind the machine.  The types of TBM shields vary according to whether the mole is to dig into soft, wet ground or hard rock. Shield diameters range from Aker Wirth’s 26.3-foot (8.03 m) shield on its TBM used for Switzerland’s Linth-Limmern power stations to Hitachi Zosen’s “Big Bertha,” with its diameter of 57.5 feet (17.5 m).  Moles designed to dig tunnels below the water table also feature chambers at the front to pressurize the ground being dug through. For large jobs, multiple TBMs may be called for. The Channel Tunnel required 11. For a surface level tunnel, this is no problem. If the tunnel is to be bored deep underground, an access shaft, usually circular, is drilled and lined with concrete. The TBM is lowered into it, and the tunnel is excavated from that point.  For long tunnels, multiple access shafts are drilled. When the tunnel is finished, the access shafts may become ventilation shafts and/or emergency exits. If they are not so employed, they are left in place for the life of the tunnel. Segments of pre-cast concrete are erected behind the TBM to form a ring at pre-determined intervals. The sprayed concrete, or shotcrete, coats and stabilizes the tunnel walls. The shotcrete may include steel or polypropylene fibers to strengthen it, much the way steel rebar is used to strengthen cast concrete.  The shotcrete also contains an accelerant to help it stick to the tunnel walls and dry quickly.
A: Choose the right tunneling machine for the job. Get the machine into position. Erect tunnel supports as the machine bores through. Spray concrete on the excavated walls between the support rings.

Article: It's the blue square icon with a white ″f″ inside. You'll usually find it on the home screen. It's at the bottom-right corner of the screen. It's toward the top of the menu, though you may have to scroll down a little to see it. Look for the green and gray storefront icon. If you don't see this option, you may have to tap See More. Marketplace will automatically display items for sale near the location you've added to your Facebook account. To change the location, tap Change next to ″Top Picks in (your city).″ A list of categories will appear. If you're searching for something in particular, tap the Search Marketplace bar at the top of the screen, type your search terms, then tap the arrow to start the search. In addition to items and vehicles, you can also browse service listings (such as plumbers and lawyers), jobs, rentals, Facebook groups for buying and selling, and selected deals of the day.  To check out items near you, tap Local Listings, then select a category to see what's available. To browse vehicles for sale, tap Vehicles, then select your desired filters (make, type, price) to see the listings. Browsing for jobs, services, and rentals is easy. Just select a category, and then refine the results using the drop-down menus at the top of the list. Details about the selected item will appear.  Swipe left on an item's main photo to see additional photos (if available). Scroll down to see a description, the set price, the item's location on the map, and information about the seller/owner. If you're browsing services, you'll have to select the type of service you're looking for (e.g., wedding planning, air conditioning services) before a list of professionals will appear. The steps to do this depend on the type of Marketplace listing:  If you're interested in an item, tap Ask for Details (below the photo). This sends a message to the seller asking if the item is still for sale. If you have a more specific question about an item or want to make an offer, tap Message to type your own message. To apply for a job, tap Apply Now. To contact a property owner about a rental or home for sale, tap Contact. If you're not sure whether you want to buy an item (or just want to save it so you can find it easily later), tap Save beneath the item's photo to bookmark it. To see your saved items, return to the main Marketplace page, tap You, and then tap Saved.
Question: What is a summary of what this article is about?
Open Facebook on your iPhone or iPad. Tap the ≡ menu. Tap Marketplace. Set your location (optional). Tap Shop. Select a category. Tap a listing to check it out. Contact the seller or owner. Save an item for later (optional).

Q: Tunnel boring machines, called TBMs for short or “moles,” feature circular plates on the front called shields. Disc-shaped cutters on the shield tear through rock and dirt, which passes through openings in the shield onto a conveyor belt inside the TBM that deposit it behind the machine.  The types of TBM shields vary according to whether the mole is to dig into soft, wet ground or hard rock. Shield diameters range from Aker Wirth’s 26.3-foot (8.03 m) shield on its TBM used for Switzerland’s Linth-Limmern power stations to Hitachi Zosen’s “Big Bertha,” with its diameter of 57.5 feet (17.5 m).  Moles designed to dig tunnels below the water table also feature chambers at the front to pressurize the ground being dug through. For large jobs, multiple TBMs may be called for. The Channel Tunnel required 11. For a surface level tunnel, this is no problem. If the tunnel is to be bored deep underground, an access shaft, usually circular, is drilled and lined with concrete. The TBM is lowered into it, and the tunnel is excavated from that point.  For long tunnels, multiple access shafts are drilled. When the tunnel is finished, the access shafts may become ventilation shafts and/or emergency exits. If they are not so employed, they are left in place for the life of the tunnel. Segments of pre-cast concrete are erected behind the TBM to form a ring at pre-determined intervals. The sprayed concrete, or shotcrete, coats and stabilizes the tunnel walls. The shotcrete may include steel or polypropylene fibers to strengthen it, much the way steel rebar is used to strengthen cast concrete.  The shotcrete also contains an accelerant to help it stick to the tunnel walls and dry quickly.
A:
Choose the right tunneling machine for the job. Get the machine into position. Erect tunnel supports as the machine bores through. Spray concrete on the excavated walls between the support rings.