What was that about a league?

On pages 78 and 79 of "Mesopotamian Mathematics, 2100-1600 BC: Technical Constants in Bureaucracy," Eleanor Robson describes some of the workings of the brick makers.  

One of the tablets in her examples, Haddad 104, has a man walking 5 nindan (about 30 meters), digging up a basket of mud and carrying it back, mixing the material for the bricks, and moulding the bricks, then returning for another load.  The tablet counts digging as 1/3, mixing 1/6, and moulding 1/3 - this is how much of a sar volume of earth a man can handle in a day.

The distance that a man can carry things in a day would be 2,700 nindan, or rods, which would work out to 16,200 meters, or half of 3 leagues, about 32,400 meters - 1 league (or march) during each of 3 watches for a 6-hour work day, about 5 kilometers an hour.  

The factor for carrying is worked out by dividing 2,700 nindan by the distance of one trip, which comes out to 540 trips that can be made in one day.  The multiplier appears to be based on half the distance a porter is expected to walk in a day.

In Sumerian work planning documents, a basket of earth gets the same coefficient as a brick that is 20 fingers by 20 fingers by 6 fingers, making it the equivalent of about 1/1,620 of a volume sar (18 cubic meters) of earth.  A cubic meter of earth weighs about 1,220 to 1,905 kilograms, depending on the water content, so a basket should weigh between 13.5 and 21 kilograms, roughly.

People who carry baskets like that on their heads now carry about 70% of their body weight, max, which should make the basket about 30 to 50 kilograms.  The main suggestion in the texts is that the basket is really a weight of about 50 mina, or roughly 26 kilograms.  It looks like 1 person might be able to carry the equivalent of 3 or 4 bricks.

Using this figure for the volume of a basket of dirt and multiplying by 540 trips a day, 1 person could move 1/3 sar of earth a distance of 5 nindan in one work day.

So it turns out that the factor for carrying is actually 1/3.  The overseer takes the reciprocal for each: 3 for digging, 6 for mixing, 3 for moulding, and 3 for carrying, and adds them together to get 15 days that would be needed for 1 man to process 1 sar of earth.  The reciprocal of 15 is 4/60.

If the overseer determines that the man is making standard bricks, then he just takes the square side of a standard brick, 20 fingers by 20 fingers, and multiplies it by the height of a standard brick, 6 fingers, to get the volume of earth that is needed for each brick, except that he does that in nindan to keep his units straight.  He then takes the reciprocal of that and comes up with 1,620, the number of basic bricks that can be made from 1 sar volume of earth.

Multiplying 4/60 by 1,620, the overseer finds that he has 108, the number of bricks that can be produced by 1 man, which he then multiplies by 3, to get 324, the number of bricks to be produced in a day.