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1. Strength of Timber.

2. Preservation of Timber.

3. Strength and Preservation of Timber.

1. A stick of mine timber, due to the cellular nature of wood, may be considered as a bundle of parallel tubes. It resists pressure against the ends much better then pressure brought against the sides. Strong end pressure on a stick will cause in to split lengthwise with subsequent failure. Heavy pressure against the sides will squeeze the cells together, compressing the stick. It unsupported, the stick will bend and later fail by splitting or cracking.

2. Moisture affects the strength of timber. Drying stiffens and strengthens the wood fibers, and it has stated that a dry stick has double the strength of the same stick when green. However, seasoning commonly results in some checking, splitting, warping and twisting. For these reasons, and the fact that the timber is likely to be wet at some time, it is safer not to assume any increase in strength over the green conditions. Peeling and proper seasoning may reduce the weight some 40 to 43 % as compared with green timber. Experiments show that trees cut in winter have fully as much sap in them as trees cut in spring or summer.

3. The causes of the destruction of mine timber are commonly given as follows: decay, fire and crushing, insects and waste.

4. Decay is caused by low forms of plant life or fungi which break down the wood substance and use it as food. Fungi will live under conditions of a wide range of temperature, from nearly freezing to about 120⁰ F., but the optimum conditions for growth are between 70⁰ and 90⁰ F. The wood must be just moist, for both saturated and dry conditions inhibit growth. Sufficient air for growth seems to be obtained in any part of a mine. The most effective way of preventing the growth of fungi is to poison their food supply by impregnating the wood with some chemical or combination of chemicals.

5. Guniting (торкретирование) the exposed surface of timber with Portland cement and sand sprayed from a cement gun acts as an effective protection against fire, and for this reason many mines treat timber in shafts and at shaft stations. Expanded metal lath or chicken wire is first nailed to the timber before the coating of cement is applied. However, as a protection against decay, guniting is effective only as long as the coating of cement remains unbroken and covers the entire of the timber.

6. Timber is prepared for treatment with a preservative by proper seasoning. In green timber must be treated, it can be conditioned by steaming and than applying a vacuum.

7. Methods of treatment are:

(1) Brush treatment or painting the timber with a preservative or merely dipping it in a tank of preservative;

(2) Open-Tank Method, in which the timber is immersed in a tank of hot preservative and it is then in a tank of cold preservative;

(3) Pressure Processes, in which preservative is forced into the cells of wood.

8. Preservatives used are creosote, zinc chloride, sodium fluoride and other chemicals.

Note: 1. to apply coat by dipping – наносить покрытие окунанием.

2. guniting with Portland cement – обработка портланд цементом.

Дата добавления: 2015-10-31