AN EXPERIMENT IN WOOD PRESERVATION

BY

Mr. M. MEIGS

United States Civil Engineer

In 1889, the writer was engaged under Major, now General A. Mackenzie, retired, in designing and building of the United States dry dock at Keokuk, Ia., which is attached to the Des Moines Rapids Canal. This dock is 400 feet long at bottom and 100 feet wide, and is filled by gravity from the canal and discharged into the river, which, at all ordinary stages, is below the level of the bottom of the dry dock.

The grillage on which the boats are supported when the dock has been emptied consists of timbers, in local parlance "logs," crossways of the dock at intervals of 16 feet. These logs rest upon sills consisting of 12 by 12 inch pine timbers, bedded in the pavement of the dock bottom, and projecting above the surface of the dock 3 inches. The photograph, Fig. 1, shows these features very clearly, and the dock is described in detail in the Report of the Chief of Engineers, 1886, Vol. II, pg. 1477.

The material below the level of the dock was originally soil about 4 feet thick, below which was ledge rock. The soil was all removed and replaced by gravel, dredged from a bar in the river near by; the pavement consisting of coursed rubble, hammer dressed in blocks about 10 inches long and 6 inches wide, and 8 to 10 inches deep, closely laid on edge, but with no cement between the joints.

The sills, eight in number, extend the length of the dock 400 feet, or 4,800 lineal feet of sill in all. (The writer is thus particular about the surroundings of the sills, as on this must be based any judgment as to the experiment itself.) They rest upon concrete walls the same length as the sills. In putting the sills in place there was first dug a trench in the gravel about 18 inches deep and 14 to 16 inches wide, and the sills were suspended temporarily over this trench, which was afterwards filled up with cement concrete until the cement reached the bottom of the sills. The pave

ment was then laid close to the timbers, the top of the pavement being kept 3 inches below the top of the sill.

Holding-down bolts were necessary at intervals to overcome the upward pull of the sills and logs attached thereto, and these bolts were about 30 inches long of -inch iron. The bolts had a nut and washer at one end and a right-angled bend at the bottom, forming a hook to prevent them from pulling out of the concrete. The bolts being inserted in the timber before it was put in position, as the work of concreting the foundation proceeded, these bolts were firmly embedded in the concrete and performed their office very well for a long time. Of late years, however, some of these sills have shown in places a tendency to lift, and to hold them down, all the spare scrap iron about the place was piled at intervals on them. It was found on inspection that some of the hooks or lugs had wasted and had finally broken off, allowing the bolt to draw up through the concrete and the sill to lift; therefore, it became necessary this season to take up some of the timbers and reset them, and it gave an excellent opportunity to inspect the wood. Each one needing it, in pieces about 20 feet long, was raised; the trench it. occupied in the pavement was cleaned out; holes were bored through the timbers, 6 feet apart, large enough to permit of inserting an ordinary quarry drill. New holes were then drilled in the concrete, new 3-inch bolts inserted with a large washer and nut at the top end, and the bolts were grouted with Portland cement, which renewed the anchorage of the grillage to the concrete foundation very successfully.

Now, when those sills were first put in, in 1889, the writer tried an experiment in wood preservation. Obviously, those long sills would be difficult and expensive to replace, and any increase in the life of the timber would be a considerable gain. The method of preservation adopted was to bore the timber nearly through, at intervals of 4 feet, with a 11⁄2 inch auger, fill the hole nearly full of sulphate of copper, and then plug the hole with a plug about 1 or 11⁄2 inches long. This simple and inexpensive protection seems to have had unusually good results. In every timber bored for inserting the new anchor bolts, and about twenty-five of the 20-foot timbers were thus bored, the wood 11⁄2 inch below the surface seems now, after twenty-one years of partial burying in the ground, to be in as good condition as when first put down.

The preservation has been so good that test holes were bored at random in many other timbers which did not require repair, to see

if all were alike, and in every case the auger brought out bright fresh chips like a new stick. A few of the timbers in the 4,800 linear feet of the whole lot were very sappy on the top side, and this sap wood in some cases has fallen away to the depth of the heart wood, but the whole of the heart wood of the timber is still sound and good and perfectly well fulfills its office.

Unfortunately, the writer did not look forward far enough to leave one of the sills untreated with the copper sulphate as a control experiment, and the question now arises "What would have

Fig. 1. U. S. S. Arthur Hider in dry dock at Keokuk, Iowa; showing construction of grillage and location of sills

been the present condition of these timbers had no effort been made to increase their life by using sulphate of copper?" It looks to the writer as if the ordinary white pine and Norway timbers, such as those used, would by now have been completely rotted out. The timber is exposed to sun and rain; is alternately wet and dry, and while sometimes a dockage may not be made for several months, at other times the dock is filled and emptied two or three times in a week. The timbers are really exposed somewhat, the same as a railroad tie is, though it projects above the surface of the ground

more than the latter does on a well kept up railroad, and it seems to the writer the exposure the timbers have had is more severe than any tie ever gets because damper, and more exposed to the weather. Moreover, these sills are of pine and the usual railroad tie is of oak.

Altogether, the preservation of the timber appears remarkable, and those engineers and others who have inspected it have been surprised at its condition. One timber that was removed during the repairs was not put back, but a new one was substituted in its place, and the old timber kept for a specimen. This timber, split in two by wedges, is the subject of Fig. 2. The grain of the wood

Fig. 2. Sill at United States dry dock, Keokuk, Iowa, preserved by sulphate of copper. Laid in 1889, taken up in 1909, and shows no deterioration

is perfect, the color unimpaired by its twenty-one (nearly twentytwo) years of partial interment. The borings taken from the new holes for the anchor bolts were saved and mixed together, and a sample of these borings has been sent to Major Connor, Corps of Engineers, United States Army (the editor of this journal), for examination.

It will be noticed that many of the chips are green, showing a copper stain. These chips came from holes that were nearest the sulphate of copper pockets. Traces of the stain could in many

cases be seen at least 2 feet from the place where the copper was inserted, showing that the whole wood was permeated.

The cross logs were also treated with the copper sulphate. These logs are exposed completely to the air and are not in nearly so good condition as the sills. The application of the sulphate was, however, different, and seems to have had much to do with the preservative action of the copper. In the case of logs, holes were bored in the center of the ends, filled, and then plugged as on the sills. Now, these end holes do not cut across the annual layers of wood, as a hole bored cross-wise would do. A hole bored radially communicates with each layer of wood from the center out, while one parallel to the growth of the tree seems much less favorably situated for capillary distribution of the chemical in a damp state. This may account for the smaller preservation of these cross timbers.

These notes are submitted by the writer to his brother engineers for what they are worth and for discussion. Possibly the timbers would have lasted without treatment. So far, as many as have seen the timber have been of the opinion that the sulphate of copper is the cause of this timber preservation, and that without it they would have rotted away. What do you think?