10.1 Post-construction evaluation and monitoring
Post-construction monitoring may be considered important in two specific ways, i.e.:
- confirming that the works are performing correctly and that any performance criteria have been satisfied; and
- providing information on the performance and design of the works which may be of value in future projects.
However, careful consideration should be given to the need for any post-construction evaluation and monitoring given implications for any future remedial works and any regime should be planned to be both simple and robust.
10.1.2 Evaluation of embankments/fills
10.1.2.1 Methods of evaluation
It may be necessary to monitor both the settlement and stability of the fill and underlying foundation after construction subject to the form of specification adopted and the anticipated long-term performance of the fill and /or foundation. Any requirements should be clearly set out in the Geotechnical Design Report and the contract documentation e.g Appendix 6/12 in SHW . See also BS EN 1997-1:2004, Section 4.
The extent and method of monitoring depend on the nature of the ground, the significance of the project/fill and the accuracy required.
This may range from simple levelling to a comprehensive range of instruments installed to measure accurately the deformations occurring in the embankment and foundation.
Any trigger levels and associated action plans should be clearly set out in the Geotechnical Design Report. The duration and scope of any post-construction monitoring should be re-assessed following observations made during construction and the results should be evaluated and interpreted on a regular basis.
10.1.2.2 Monitoring techniques
The following techniques are commonly employed on site:
- surface levelling stations to measure settlement of the fill surface;
- settlement plates to measure settlement of the fill thickness;
- magnetic extensometers to measure settlement at incremental depths within the fill;
- settlement gauges to measure settlement of the underlying foundation;
- piezometers to measure the water level within the fill; and
- inclinometers to record any lateral movements within the fill/foundation.
Further information on available techniques can be obtained from standard publications such as Dunnicliff  and from instrumentation suppliers.
10.1.3 Monitoring of slopes
Where experience or stability analysis gives reasonable assurance of stable conditions in a slope no special measures are recommended for monitoring stability. However, it is good practice to make periodic inspections, particularly in the early months after completion when the surface might be subject to erosion before grass cover is established.
These inspections should include the following observations.
- a) Deformation. Settlements in the upper part of the slope and bulging towards the toe may indicate incipient failure by a rotational shear slide (see Annex A).
- b) Cracking. A series of cracks in the vicinity of and sub-parallel to the crest of a slope may indicate sliding, as do en echelon cracks at the lateral boundaries of incipient movement. Hexagonal or random pattern cracking indicates drying shrinkage.
- c) Fissuring. Opening of joints and fissures in a rock slope indicates incipient translational or toppling failure (see Annex A).
- d) Seepage. Water carrying soil particles seeping from a slope face indicates internal or seepage erosion (see Annex A).
- e) Gullying. Channels eroded on a slope face indicate the need for protection against surface erosion.
Inspections should be made after periods of heavy rainfall, snow or severe frost. Clay slopes should be inspected during or immediately after rainfall following a period of dry weather to assess the effects of water entering surface cracks. Inspection of the position and inclination of pegs driven into the slope may be used as a simple means of detecting gross deformations.
Where there are concerns about the short or long term stability of cutting slopes it may be desirable to install instrumentation to give warning of incipient instability, to enable remedial measures such as the installation of drainage, grouting or anchoring to be undertaken before the stage of failure is reached.
NOTE Suitable methods of monitoring slopes are described in 10.1.2.2.
10.1.3.2 Water pressure
Pore pressures behind a cutting slope can have a critical effect on stability, so it may be desirable to monitor pore pressure changes during and after excavation of a cutting to check the validity of assumptions made at the design stage and to ensure that critical conditions of high pore pressures are not developing.
In homogeneous permeable soils pore pressures may be monitored by plumbing water levels in simple standpipes (see BS 5930:1999+A1). In layered soils or soils of moderate to low permeability the response time of standpipes to changes in pore pressure may be considered inadequate to detect critical conditions in sufficient time to take remedial action. In these cases pore pressures should be monitored by properly sealed and protected piezometers (BS 5930:1999+A1) with their tips located in each critical soil layer at a number of locations along the slope. Water levels in the piezometers may be monitored by plumbing down the riser pipe or by connecting a series of piezometers to a gauge house by means of pneumatic, hydraulic or electrical transmission and recording systems (see Dunnicliff ). Precautions should be taken against damage to a piezometer installation from construction and maintenance operations and from the effects of frost and vandalism.
10.1.3.3 Monitoring surface and sub-surface movements
Monitoring of ground surface movement in both horizontal and vertical planes may be carried out by field survey methods. The particular methods used should depend on the accuracy required.
For short term schemes when a high degree of accuracy is not required, simple measurements taken on metal pins or pegs driven into the soil may be taken by normal levelling, tachometric survey methods or short range electronic distance measuring equipment (EDM), with the measurements referred to one or more stable base line stations set some distance from the affected area.
Where a higher order of accuracy is required (±5 mm or better) and the measurements should be repeated at regular intervals over a long period of time, a properly designed monitoring scheme will be necessary. Consideration should then be given to use of one or a combination of the following methods:
- a) precise levelling using a geodetic level and Invar staff;
- b) triangulation using first order theodolites (reading to one second of arc);
- c) trilateration with special EDM equipment.
These measurements should be taken from stable survey monuments, preferably with fixed centring for the instruments or referred to deep bench marks or datum points. The targets should be designed to provide a unique point to which the measurements can be taken during repeated visits.
Photogrammetry may be used for monitoring purposes, but when a high degree of accuracy is required the ground control would need to be established by methods a), b) and c).
10.2 Incorporation of GFR into health and safety file
The geotechnical feedback report (see 6.5) should be incorporated into the project Health and Safety File along with design reports, as-built drawings, etc. This enables the asset management process to start.
For simple projects feedback may be limited, apart from inclusion in the Health and Safety File.
NOTE The relevance of retaining or distributing further information will depend on the scope, extent and future maintenance requirements of the project.
10.3 Formal handover
All relevant information on the earthworks covering ground investigations, design, construction and monitoring should be assembled prior to completion and formally handed over to the asset owner's maintenance team.