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Method for cleaning and checking the base of diaphragm wall panels AG Berry, Arup, UK

AbstractBase cleaning is common for bear-ing piles, allowing the end-bearing capacity of the bored pile to be included in the design. However, the base cleaning of diaphragm wall panels is not common in the UK. This paper describes the author’s experience of a diaphragm panel wall base cleaning process that was developed for a load-bearing retaining wall for a large basement in east London. The extra vertical capacity achieved by the clean base allowed end-bearing to be taken into account in addition to skin fric-tion, providing substantial savings in the number of foundation piles required.

The base-cleaning process devel-oped consists of two stages. The first comprises scraping away ridges in the panel base left behind by the clamshell excavation bucket. Successful remov-al of the ridges is verified by means of a weight attached to a measuring tape wide enough to sit on top of any ridges.

The second stage involves remov-ing soft sludge from the panel base using a submersible pump placed at the base of the excavation. This is combined with the process of replac-ing the dirty support fluid in the panel with clean fluid to minimise the risk of suspended soil settling on to the base. The panel base cleanli-ness is verified by checking the hard-ness of the panel base using a quali-tative scale to assess the impact felt when a standard weight is lowered on to the base of the panel.

IntroductionBase cleaning of bearing piles for end bearing in London clay, and as a preparation for base grouting in Thanet sand, is a common practice in London. However, base cleaning and base grouting is less common for retaining walls as is use of these structures for load bearing.

During construction of a 600m perimeter diaphragm wall for a 15m-deep basement in east London, base grouting trials were performed on a number of diaphragm wall panels. Although the grouting trials were inconclusive because of leak-age problems in the injection system, the base-cleaning method developed was adopted for all the load-bearing panels. The base-cleaning method was developed by Arup in conjunc-tion with the contractor, a Costain/Bachy Soletanche joint venture.

Project backgroundThe site stratigraphy is given in Table 1 (below). The load bearing panels were founded in Thanet sand with toe levels generally varying between -17.5mOD and -26.0mOD. Thanet sand on the site comprises a very dense, green-grey, silty-fine sand, with the silt content increasing below approxi-mately -22.5mOD. Thanet sand is commonly used as a founding medium for bored piles in this area of London.

During design, a factor of safety is usually applied to the combined shaft capacity Qs and base capacity Qb, and a smaller factor of safety is applied to shaft capacity alone to allow for a serviceability check. This allows deflections at working load to be limited.

Figure 1 (above) shows the load-deflection curves for a deep founda-

tion with a cleaned base and poor-quality base. It can be seen that if the base quality is not controlled then the load deflection behaviour of the foundation can be very vari-able, preventing reliance on Qb in design and requiring a larger factor of safety to be applied to Qs alone.

The diaphragm wall panels were designed making reference to Eurocode 7 (British Standards Institution 2004) and BS8004 (British Standards Institution 1986) in consultation with the local building control officer.

In the case of this project, providing a clean, reliable panel base allowed the design to be based on the lower capacity from a global factor of safety of 2.5 applied to (Qs + Qb), or 1.5 applied to Qs alone. If the panel bases had not been cleaned then design would have been required to be based on a factor

of safety of 2.5 applied to Qs alone. The additional capacity available

from base cleaning was equivalent to approximately two-thirds of the vertical load capacity of a continuous flight auger (CFA) pile on the same site. Cleaning the panel base to increase vertical load capacity had a similar cost to constructing equivalent CFA pile capacity, but provided materials savings.

Base cleaningThe diaphragm wall was constructed in accordance with ICE SPERW (Institution of Civil Engineers 1996) and BS EN 1538:2000 (British Standards Insti-tution 2000). The diaphragm wall panels were excavated under ben-tonite using a toothed clamshell grab. The base cleaning for the diaphragm wall panels consisted of two processes:l removal of ridges left in the panel base by the grabl removal of sludge from the panel base and prevention of further sludge accumulating.

Ridge removal1. Ridges in the panel baseToothed clamshell grabs leave ridges in a panel base that could break off during concreting or collect soft material that settles out from the support fluid. Figure 2 (see overleaf) illustrates ridges left by the grab in made ground during a trial to ascertain their probable size and nature. The made ground at the site had a typical corrected SPT of 0 to 10 blows per 300mm and so is considerably softer than the Thanet sand (corrected SPT typically 50 to 100 blows per 300mm). It was therefore considered that the depth of the ridges observed in this trial would represent an upper bound.

The depths of the ridges were measured and it was found that except at the very edge of the bite their depth was similar to the distance between the tooth and the main bucket on a closed grab (see figure 3, overleaf). Measurement of the ridges left behind and the tooth width on the grab showed that the ridges were approximately 100mm wide. It can also be seen that the top of the ridges form a curved shape towards the edge of the panel. However, it should be noted that even in the relatively weak made ground the shape of the “panel base” is

LOAD

DISPLACEMENT

Good base

Qs+Qb

(Qs+Qb)/2.5 or Qs/1.5

Qs/2.5

Poor baseShaft only

Figure 1: Illustration of how base cleaning allows greater design loads

Stratum Top of stratum mOD

Made ground +5.3

Alluvium 0.0

Terrace gravel -2.0

Clayey Lambeth group -6.0

Sandy Lambeth group -11.0

Thanet sand -17.0

Chalk -31.0

Table 1: Site stratigraphy

COOLING PRIZE PAPER

30 GROUND ENGINEERING NOVEMBER 2009

COOLING PRIZE PAPERflatter than the circular arc

traced by the grab when it is closed in mid-air.

2. Ridge-removal techniqueA cleaning grab was developed with the contractor to scrape away these ridges and provide a flat base. The scraper initially developed consisted of stiffened flat plates that attached to the tooth connections of a standard clamshell rope grab, as illustrated in Figure 4. These plates were effective in removing the ridges, but progress was slow as the plates prevented the grab from closing fully and time was lost re-digging material dropped from the grab.

As an alternative to the scraper plates the contractor decided to use a standard rope grab with the teeth cut off (see figure 4). This grab could close completely, allowing much faster base cleaning. In addition, attaching a dedicated scraper grab saved time compared with attaching plates to a standard grab.

Figure 5 shows spoil scraped up by the toothless cleaning grab, which contains elongated chunks of material. Inspection of this material revealed it to be dense Thanet sand, providing confidence that the scraper grab was indeed removing the ridges as well as scooping up any loose material that had fallen to the base.

3. Ridge removal verificationThe trials of the scraper plates in made ground and observation of the scraped spoil in the Thanet sand gave confidence that the method was capable of removing base ridges in Thanet sand. However, it was necessary to develop a reliable method of verifying, under up to 31m head of bentonite, that in every base cleaned panel the ridges had been effectively removed. This was done by means of a weight welded to a stiff wire mesh, approximately 0.3m2, and attached to the end

of a measuring tape, as shown in figure 6. The width of the mesh caused the weight to sit on the top of the ridges, thus the depth to the shallowest part of the panel was measured.

Once the ridges had been scraped off then the weight was able to sit on the now-flat panel base, giving an increase in the measured depth. Base scraping was deemed to be complete when the depth, as measured with the “meshed” weight, had increased by the initial height of the ridges (as ascertained by measurement of the grab geometry and during the trials in made ground).

Each panel base was measured in at least five locations – at each edge of the panel, in the middle of the side bites and in the centre bite. Care was taken not to drag the weight sideways, but rather to lower it in from the top of the panel to provide more certainty as to which part of the base was being checked. The amount of material removed by the grab was also checked visually.

Sludge removal and prevention1. Sludge removal techniqueOnce the base ridges had been satisfactorily scraped away it was necessary to remove any loose sediment from the base of the panel and prevent further settlement of debris.

This was achieved by replac-ing the bentonite in the panel with “fresh” bentonite by means of a powerful submersible “Toyo” pump placed at the base of the panel. This pump sucked up the dense digging bentonite and removed loose sedi-ment from the base, while fresh ben-tonite was added at the top of the panel.

Once tests indicated that the sand content of the bentonite extracted from the base of the panel was less than 2% the pump was moved to a new part of the excavation and the process repeated.

Figure 2: Ridges left by the grab in made ground

Figure 4: Cleaning grabs: with scraper plates attached (left) and with teeth removed (right)

Figure 5: Spoil from the toothless grab. Ridge-shaped chunks of material have been outlined

Figure 3: Distance between tooth and main grab ≈ maximum ridge height.

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2. Sludge removal verificationVerification of successful sludge removal was achieved by sand content testing of the bentonite and also base hardness testing, using a qualitative, but repeatable, method developed by Arup for measuring the base hardness during construction of base grouted piles. The method involves lowering a heavy square weight on a tape, as shown in Figure 7, to the base and judging the hardness of the base depending on the impact felt when the weight touches the bottom.

The base is graded on a scale of 1 to 5, as described in Table 2 (see below). A hardness of 1 to 3 is considered acceptable. While not a quantitative measurement, assessment of base hardness by this method is easy to learn and has been shown to relate well to other indicators of base quality, such as ease of base grout injection, presence of soft clayey material on the weight when it is raised to the surface and observations of cores taken through the panel base.

Base hardness was tested in the same locations as the ridge removal verification and the panel depth was also checked for any rise in level that might indicate contamination of the base after the end of the scraping process. If a raise in base level was observed the panel was re-scraped. Where the base was not hard enough the Toyo pump was placed over the soft area for a further 5 to 10 minutes, after which the hardness was checked again. Generally this extra 10 minutes of pumping increased the hardness of the soft area from 4 to 2 due to remaining sludge being sucked away.

Because of the relatively large volume of a diaphragm wall panel and restrictions on site working hours it was not always possible to complete the entire base scraping, bentonite replacement and sludge removal, reinforcement placement and concreting all in one day. In such cases the cage placement was delayed until the day of the pour where possible as remediation of a base once the cage was in place would have been difficult.

On occasions when a cage was left in a base cleaned panel overnight ready for concreting early the next morning the clean bentonite was left re-circulating in the panel. Bentonite was sucked from the top of excavation and returned to the panel base via tremie pipes so the net flow of bentonite would be upwards, counteracting settlement of suspended material. The sand content of the bentonite at the bottom of the panel and the base hardness and depth were checked immediately prior to concreting.

Reliability of the cleanliness checksOn three out of 51 load-bearing pan-els, circumstances dictated that the panel was concreted despite a sub-standard base having been indicated by the checks. This was undesirable as it required a re-assessment of the panel capacity, however, these occa-sions did provide a chance to assess the reliability of the base cleanliness verification method.

Figure 8 shows a core taken through the base of a panel where an unacceptable base hardness of 4 had been detected; 20-30mm of clayey material can be seen between the concrete and the Thanet sand. Figure 9 shows a core taken through the base of a panel where a base level rise of 200mm was detected. In this core there was approximately 200mm of foreign material found between the concrete and Thanet sand.

Other cores were taken where no unusual depth changes were detected and the base hardness prior to concreting was in the range of

1 to 3 showed an acceptable contact between the concrete and the Thanet sand. This indicates that the verification procedure developed at this site to check the base was clean is sufficient to detect an unacceptable base.

ConclusionA method has been developed for cleaning and checking the base of diaphragm wall panels, which allows the design vertical capacity of the wall to be increased when compared with a standard dia-phragm wall panel. This method comprises two stages: creating a flat base by scraping away ridges, and preventing material from accumu-lating on the base. A wide weight attached to a measuring tape can be used to check successful removal of base ridges and a square “hardness” weight attached to a tape allows the degree of soft material on the base to be assessed.

Figure 8: 30mm of clayey material found between the concrete and the Thanet Sand where a soft base was detected

Figure 9: 200mm of coarse sand, clay and ceramic fragments found between the concrete and Thanet sand where a rise in base level was detected

Figure 7: Base hardness weight and tape

Figure 6: Meshed weight to sit on top of base ridges.

Table 2. Arup base quality grading scale

Grade Base quality

Observation by engineer with weighted tape

1 Hard Sharp impact

2 Firm Sudden impact felt, but some damping ofimpact vibrations

3 OK Distinct base detected with slight embedding ofweight into soil

4 Soft Significant embedding of weight making definitivebase difficult to detect

5 Very soft Weight sinks slowly into soil, making definitive basevery difficult to detect

n British Standards Institution. 1986. BS 8004:1986 Code of practice for foundations. British Standards Institution.n British Standards Institution. 2000. BS EN 1538:2000 Execution of special geotechnical works – diaphragm walls. British Standards Institution.n British Standards Institution. 2004. BS EN 1997-1:2004 Eurocode 7: Geotechnical design - part 1: general rules. British Standards Institution.n Institution of Civil Engineers in collaboration with the Highways Agency, Ove Arup & Partners, and the Federation of Piling Specialists. 1996. Specification for Piling and Embedded Retaining Walls. London, Thomas Telford.

REFERENCES