DOI Seite / Zitierlink:
https://doi.org/10.11588/diglit.24#0017
— 11 —
The outer quay wall is subjected to a slightly increased pressure at the time when the
Reservoir level falls and the earth behind it is saturated ; but its mass is great enough to
prevent failure from this cause. The counterforts, too, are in all cases founded below
saturation-level, and are thus secure. It was therefore decided to support the whole
colonnade on steel girders, spanning the spaces between the counterforts, the space between
the tops of the girders and the under-side of the stone beams being packed with masonry in
cement. In addition, the girders were surrounded with a mass of masonry in cement to
prevent access of water.
The openings were made at the outside face of the colonnade (i.e. east of the columns)
between the counterforts. At the south end it was necessary to remove some thick paving
stones in order to effect an entrance. A trench one metre wide was taken out to a depth of
1-5 metres below the base of the existing stone beams, and timbered with boards and walings
stretched between the counterforts. In each bay two rolled steel girders, 14 inches by 6
inches, by 54 lbs. per foot, were placed on cement seatings, 50 centimetres apart from centre
to centre, so as to be centrally under the line of columns. Rubble masonry, in 3 to 1 Port-
land cement, was then built into the seating-spaces and into the entire excavation, to a
distance of one metre outside the centre line of columns. The packing between the girders
and the stones above was done with thin granite spalls, hammered into the cement. In some
cases, where the stone beams were fractured and extended further outwards than the normal
distance, small additional masonry piers were built on to the main mass to support them.
No attempt was made to restore sunken portions of the beams to position, as this would
have been attended with some risk, without resulting in any great advantage ; the under-
pinning has therefore preserved the colonnade precisely as it was found, with the addition
of security against further damage.
After the underpinning was accomplished, the whole foundation was thoroughly
grouted, both from the outside face (where suitable channels were formed in the masonry)
and from the top. Holes were drilled through the stones between each pair of columns,
and cement grout run in till no more was taken. In all, very nearly 4 tons of cement were
consumed by this grouting.
THE EAST COLONNADE.
(Plate IX.)
This colonnade is about 42 metres in length. It contains 17 columns, 16 of which were
built on a nearly continuous wall averaging 2*25 metres wide at base and 2-4 metres deep below
pavement-level. As the pavement is 103 metres above sea-level, there was a thickness of
about 3-6 metres of silt between the base of the old foundation and the saturation-level,
except at the south end, where the granite rock rises very nearly to the base of the old
masonry, though nowhere touching it; at the north end the rock is about 13 metres below
the surface, or 7 metres below the saturation-level.
The outer quay wall is subjected to a slightly increased pressure at the time when the
Reservoir level falls and the earth behind it is saturated ; but its mass is great enough to
prevent failure from this cause. The counterforts, too, are in all cases founded below
saturation-level, and are thus secure. It was therefore decided to support the whole
colonnade on steel girders, spanning the spaces between the counterforts, the space between
the tops of the girders and the under-side of the stone beams being packed with masonry in
cement. In addition, the girders were surrounded with a mass of masonry in cement to
prevent access of water.
The openings were made at the outside face of the colonnade (i.e. east of the columns)
between the counterforts. At the south end it was necessary to remove some thick paving
stones in order to effect an entrance. A trench one metre wide was taken out to a depth of
1-5 metres below the base of the existing stone beams, and timbered with boards and walings
stretched between the counterforts. In each bay two rolled steel girders, 14 inches by 6
inches, by 54 lbs. per foot, were placed on cement seatings, 50 centimetres apart from centre
to centre, so as to be centrally under the line of columns. Rubble masonry, in 3 to 1 Port-
land cement, was then built into the seating-spaces and into the entire excavation, to a
distance of one metre outside the centre line of columns. The packing between the girders
and the stones above was done with thin granite spalls, hammered into the cement. In some
cases, where the stone beams were fractured and extended further outwards than the normal
distance, small additional masonry piers were built on to the main mass to support them.
No attempt was made to restore sunken portions of the beams to position, as this would
have been attended with some risk, without resulting in any great advantage ; the under-
pinning has therefore preserved the colonnade precisely as it was found, with the addition
of security against further damage.
After the underpinning was accomplished, the whole foundation was thoroughly
grouted, both from the outside face (where suitable channels were formed in the masonry)
and from the top. Holes were drilled through the stones between each pair of columns,
and cement grout run in till no more was taken. In all, very nearly 4 tons of cement were
consumed by this grouting.
THE EAST COLONNADE.
(Plate IX.)
This colonnade is about 42 metres in length. It contains 17 columns, 16 of which were
built on a nearly continuous wall averaging 2*25 metres wide at base and 2-4 metres deep below
pavement-level. As the pavement is 103 metres above sea-level, there was a thickness of
about 3-6 metres of silt between the base of the old foundation and the saturation-level,
except at the south end, where the granite rock rises very nearly to the base of the old
masonry, though nowhere touching it; at the north end the rock is about 13 metres below
the surface, or 7 metres below the saturation-level.