9 Supervision of execution and monitoring (p. I)

The execution of any type of diaphragm wall requires careful supervision and monitoring of the work. The following items shall be supervised and controlled during the various phases of construction:

a) preliminary work prior to the excavation phase:

  • position of the wall;
  • materials;
  • reinforcement cages and other elements to be inserted;

b) wall construction:

  • excavation method, dimensions and alignment;
  • cleaning the excavation;
  • forming the joints;
  • placing the reinforcements or other elements;
  • concreting.

All items may not be applicable to each type of wall.

Tables 3 to 7 list the controls to be performed for the various types of diaphragm walls.

Control of the execution shall be in accordance with the project specifications.

Controls for special execution techniques, such as the use of polymers as a supporting fluid, are not considered in this clause.

Table 3 — Controls for cast in situ concrete diaphragm walls
Construction phase or item Control Comments
Setting-out – Position of wall – From lay-out drawings and fixed setting out points
– Position and level of guide-walls – Shall be checked before they are concreted and
again after removal of the formwork
– Position of panels and joints – The position of the joints shall be marked on the guide-walls
Water – Suitability for use – Usually not required for potable water
Bentonite – Source of supply – Delivery documents shall be checked for each load
Fresh bentonite suspension – Fluid loss, filter cake, pH – Shall be checked at the start of work, and as required thereafter
– Density, Marsh value – At least once per shift
– Gel strength – To be carried out when deemed to be necessary
Concrete – Composition, consistency and compressive strength – Suitability tests according to ENV 206
Delivery of reinforcement cages – Number, diameter and location of bars
– Welds and couplings
– Vertical and horizontal lengths, width
– Spaces for concreting pipes
– Location of formwork
Items to be checked for each cage with respect to the working drawings
Excavation – Position of tool – Visual observation
– Verticality and twist of excavation – Shall be checked for each panel during and at the
end of excavationa
– Ground profile  
– Depth of excavation – Visual observation
– Bentonite suspension level  
Bentonite suspension to be placed in the excavation – Fluid loss, filter cake, pH, density, Marsh value – Shall be carried out at least once per panel or shift. See Table 1 ("fresh" or "ready for re-use" characteristics)b
– Gel strength – To be carried out when deemed to be necessary
Cleaning – Depth of excavation – To be checked after cleaning at a minimum of three locations per panel or at specified spacing
Bentonite suspension before inserting reinforcement cage or other elements – Density, Marsh value, sand content – Shall be carried out for each panel. See Table 1 ("Before concreting" characteristics) andc
– Gel strength – To be carried out when deemed to be necessary
Placing stop ends – Alignment of stop end elements and composition – To be checked before insertion
– Verticality, position and depth – The stop end should be against the end of excavation
Lowering reinforcement cages – Rigidity of cages – To be checked during lifting for first cage of each type
Prior to lowerinq
– Reference number
– Orientation with respect to the exposed face of the wall
– Location and number of the spacers
To be checked for each cage
During lowering
– Position and verticality
– Connections between vertical elements
To be checked for each cage, especially the length of laps
End of lowering
– Level and position
To be checked for each cage
Concreting – Length of the concreting pipes, length and position of each individual element – To be checked for each panel
– Concrete delivery certificate – To be checked for each load. Applies only to ready-mix concrete (in accordance with ENV 206)
– Concrete appearance – To be checked by visual observation
– Concrete consistency – To be checked at the beginning of each panel and when required (in accordance with ENV 206)
– Concrete strength – One sample shall be taken at least every 100 m3 of concrete from one single sourced
– Method of starting concreting – To be made for each panel according to 8.8. The first load through each concreting pipe shall be placed in its entirety, without interruption
– Depth of concrete in relation to the volume of concrete poured – To be checked after each concrete load or set of loads. The top of the concrete at each concreting pipe shall be kept at a uniform level. A corresponding graph should be established for panels of depth greater than 20 m, or when the consumption of concrete is significantly different from the theoretical volume
– Position and level of the reinforcement cage. – To be checked in accordance with the tolerances of 8.2.2
– Concrete level before shortening the concreting pipe(s) , number and length of pipe elements being removed, immersion of pipes – To be checked each time pipes are removed
– Time between mixing and start of pouring – To be checked for each load
– Duration of concreting – To be recorded for each panel
– Concrete temperature – May be appropriate in the case of extreme weather conditions (see ENV 206)
– Final concrete level – To be checked for each panel
Extracting stop ends – Application of the extraction procedure established on the first panels of each type – Applies to all panels (see 8.6)
Trimming – Level of cut-off and quality of concrete at cut-off level – Quality of concrete to be checked for each panel by visual observation
Exposed face – Protrusions – To be checked by visual observation
a Verticality and twist of the excavation shall be controlled in order to remain within the required tolerances. The monitoring frequency shall be increased for cases presenting high risk, such as where boulders or obstructions are present in the ground, or where concrete from the adjacent panel is encountered. These controls shall normally be by visual observation or simple measurements (e.g. position of grab cables). Special measures (e.g. inclinometers) to determine the excavated profile may be necessary in some cases, e.g. in deep panels, T-panels or circular cofferdams.
b The control of the characteristics of the bentonite suspension should be more frequent in special cases such as in the presence of organic content in soils, or chemically active groundwater, or when the joint between panels is formed by cutting into the hardened concrete of the previously cast adjacent panel.
c The samples for determining the characteristics of the bentonite suspension before concreting shall be taken near the bottom of the panel. They can be taken either from the desanding circuit or by a sampling device, whichever is more convenient and appropriate to the technique used for excavation and mud replacement.
d Where the concrete is produced under a continuous and nationally certified quality assurance system, different requirements for concrete sampling on site may be agreed. The minimum number of cylinder or cube specimens in a sample is four.

EN 1538:2000 Execution of special geotechnical works — Diaphragm walls