57.—(1) Before beginning to construct the specified works and again at the written request of the Agency following their completion, Network Rail shall at its own expense and to the reasonable satisfaction of the Agency, conduct a survey of the condition of so much of the river walls on each side of the river Thames as is situated within 100 metres of the centreline of the Blackfriars Bridge and submit a report to the Agency upon the condition of such walls.
(2) If any defects are identified in the initial survey conducted pursuant to sub-paragraph (1), being defects which may be affected by the construction of the specified works, Network Rail shall monitor the defects not less than once every 4 weeks where tides permit during the construction of the specified works in accordance with such reasonable requirements as the Agency may specify or, if the Agency reasonably requires having regard to the results of any such monitoring, at such lesser intervals as the Agency may specify.
(3) Before beginning to construct the specified works and at 2-weekly intervals thereafter until their completion or at such greater intervals as the Agency may agree for the period following the completion of foundation works comprised in the specified works, Network Rail shall at its own expense and to the reasonable satisfaction of the Agency survey the levels of the foreshore within 200 metres of the specified works and submit a report upon those levels to the Agency.
(4) The survey required under sub-paragraph (3) shall be undertaken at points corresponding to the intersection of grid lines spaced at 10 metre intervals parallel to the river walls and at 50 metre intervals at right angles to the river Thames.
(5) If during the construction of the specified works, any defects in the river walls deteriorate or scouring or siltation of the foreshore exceeds an average of 150 millimetres from the initial measurements taken on any two adjacent grid lines 50 metres apart, Network Rail shall immediately cease the construction of the specified works and not re-commence the construction otherwise than in accordance with such reasonable requirements as may be specified by the Agency which may include changes to working methods and the completion of mitigation works.
(6) If in the circumstances of any particular case it is reasonable to do so, having regard to the nature of the particular works concerned, the Agency shall at the written request of Network Rail substitute for the distances, periods or intervals specified in this paragraph, such lesser distances and such greater periods and intervals as are appropriate.