The GWR selected George Burge of Herne Bay as the major contractor, being responsible for undertaking 75 per cent of overall tunnel length, working from the western end.
Burge appointed Samuel Yockney as his engineer and manager.
Locally based Lewis and Brewer were responsible for the remainder, starting from the eastern side.
One of Brunel's personal assistants, William Glennie, was in overall charge until completion.

During December 1838,  construction commenced.
Work was divided into six sections; access to each was via a 25 foot (7.6 m) diameter ventilation shaft, which ranged in depth from 70 feet (21 m) at the eastern end to 300 feet (91 m) towards the western end.
The men, equipment, materials and 247,000 cubic yards (189,000 m³) of extract had to go in and come out of the shafts assisted by steam-powered winches.
The shafts were the safety exits from the tunnel.

Candles provided the only lighting in the workings and were consumed at a rate of one tonne per week, which was equalled by the weekly consumption of explosives.
Due to the considerable time required for men to enter and exit the workings, blasting  took place while they were in the tunnel.
This practice and water ingress exceeding the calculated volumes, has been attributed as causing most of the deaths that occurred.
About 100 navvies were killed during the tunnel's construction.
Additional pumping and drainage were required during and after its construction.
Large amounts of water entering the tunnel in the winter months impeded progress.

Once the eastern section had been blasted out, it was cut to form a gothic arch and left unlined.
The western section was excavated using picks and shovels and the walls were lined with brick.
Over 30 million bricks were used which were manufactured in nearby Chippenham and transported in horse-drawn carts.
Horses were used to remove much of the spoil.

The restrictions imposed by the site contributed to a delay in the tunnel's completion.
By August 1839, only 40 per cent of the works had been finished.
By summer 1840, the London Paddington to Faringdon Road section of the Great Western Main Line (GWML) had been completed, as was the track from Bath to Bristol Temple Meads.
The Box Tunnel was the last section of the GWML to be finished, although not for lack of effort on the part of Brunel.

During January 1841, Brunel came to an agreement with Burge and Yockney, to increase their workforce from 1,200 to 4,000 and the tunnel was completed in April 1841.
The completed tunnel was 30 feet (9.1 m) wide and capable of accommodating a pair of broad-gauge tracks.
When the ends of the tunnel were joined, there was less than 2 inches (50 mm) of error in their alignment.
Brunel was so delighted that he reportedly removed a ring from his finger and gave it to the works foreman.
The GWR selected George Burge of Herne Bay as the major contractor, being responsible for undertaking 75 per cent of overall tunnel length, working from the western end. Burge appointed Samuel Yockney as his engineer and manager. Locally based Lewis and Brewer were responsible for the remainder, starting from the eastern side. One of Brunel's personal assistants, William Glennie, was in overall charge until completion.
During December 1838,  construction commenced. Work was divided into six sections; access to each was via a 25 foot (7.6 m) diameter ventilation shaft, which ranged in depth from 70 feet (21 m) at the eastern end to 300 feet (91 m) towards the western end. The men, equipment, materials and 247,000 cubic yards (189,000 m³) of extract had to go in and come out of the shafts assisted by steam-powered winches. The shafts were the safety exits from the tunnel.
Candles provided the only lighting in the workings and were consumed at a rate of one tonne per week, which was equalled by the weekly consumption of explosives. Due to the considerable time required for men to enter and exit the workings, blasting  took place while they were in the tunnel. This practice and water ingress exceeding the calculated volumes, has been attributed as causing most of the deaths that occurred. About 100 navvies were killed during the tunnel's construction. Additional pumping and drainage were required during and after its construction. Large amounts of water entering the tunnel in the winter months impeded progress.
Once the eastern section had been blasted out, it was cut to form a gothic arch and left unlined. The western section was excavated using picks and shovels and the walls were lined with brick. Over 30 million bricks were used which were manufactured in nearby Chippenham and transported in horse-drawn carts. Horses were used to remove much of the spoil.
The restrictions imposed by the site contributed to a delay in the tunnel's completion. By August 1839, only 40 per cent of the works had been finished. By summer 1840, the London Paddington to Faringdon Road section of the Great Western Main Line (GWML) had been completed, as was the track from Bath to Bristol Temple Meads. The Box Tunnel was the last section of the GWML to be finished, although not for lack of effort on the part of Brunel.
During January 1841, Brunel came to an agreement with Burge and Yockney, to increase their workforce from 1,200 to 4,000 and the tunnel was completed in April 1841. The completed tunnel was 30 feet (9.1 m) wide and capable of accommodating a pair of broad-gauge tracks. When the ends of the tunnel were joined, there was less than 2 inches (50 mm) of error in their alignment. Brunel was so delighted that he reportedly removed a ring from his finger and gave it to the works foreman.
Box Tunnel