Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

Recovery of Skin Friction of Cambridge Gault Clay with Time EffectTsunenobu NOZAKI
General Manager, International Construction Design & Planning Department, Giken Ltd., Tokyo, Japan
Email: nozaki@giken.com

ABSTRACTThe link between pile wall adhesion factor (soil shear strength modification factor) α and undrained shear strength Cu
was widely analyzed by a number of researchers. The adhesion factor is used to calculate the ultimate skin friction ofpiles. However, how much the skin friction increases from installation stage i.e. mobile skin friction, to the maximum
value i.e. static skin friction is not widely researched. Due to the uncertainty, the static skin friction is often
underestimated, especially, when the skin friction during pile installation is low. This paper features the tendency of therecovery of the skin friction and time effect on the recovery. As a research model, over consolidated clay, called
Cambridge Gault Clay was studied.
Key words: Press-in Method, Skin Friction, Clay

1. Outline of the project

2. Structural type and piling method

1.1. PlaceGrand Arcade is located in the heart of Cambridge city

2.1. Site condition
The development involved constructing a basement

centre. Cambridge is a university city and the county town

and the sheet pile perimeter wall was installed adjacent toof Cambridgeshire, England, on the River Cam

existing buildings. The scope description specified

approximately 80 km north of London. Cambridge is“design and installation of temporary and permanent

home to the world-renowned University of Cambridge,

works steel piling and perimeter protection all sheet

which was founded in 1209. The university includes

piling to be virtually silent and virtually vibration free as

King's College Chapel, Cavendish Laboratory, and theGiken system or similar”. Various types of sheet piles

Cambridge University Library, one of the largest legal

were utilized depending on locations, which were from

deposit libraries in the world.

6.6m to 15.5m long and 600mm wide U sheet piles, 6.0m
to 14.0m long and 580mm wide Z sheet piles, and 8.1m
to 11.0m long Zero sheet piles.

1.2. Background and objectives of the project
The Grand Arcade development site extends to 2.8ha ofland in the historic core of Cambridge. It contained arange of listed and unlisted buildings in varying ages. The

2.2. Ground condition
In the Cambridge area, over-consolidated clay known

development composed of four main areas.

as “The Gault Clay” is underlying made ground and

New Robert Style Department Store

alluvial sand & gravel layers.

50 retail units set in two arcades linked by Central

The Gault Clay in the Cambridge area, is a highAtrium

plasticity (LL > 70%, PI > 40%), over-consolidated stiff

New Magistrate Court

to very stiff, slightly silty, very closely fissured CLAY

Replacement 600 space car parkwith bands of very stiff to hard clay marl. The stratum isalso known to have a high calcium carbonate content andit is understood that the carbonate is present as silt sized- 497 -Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

particles within the clay matrix, and only provides alimited cementing effect.
The Gault Clay formation was laid down under marine
conditions and was overlain by about 150 m to 400 m ofChalk that has subsequently been eroded. Following theglacial period, which would have reloaded the strata but
probably not in excess of its previous loading, furthererosion has occurred followed by deposition of theexisting Terrace Deposits. It is very uncertain what theactual thickness of Chalk/Gault that has been eroded
might be. However, pre-consolidation pressures in rangeof 3400 kPa and 7000 kPa indicates OCR (overconsolidated ratio) of 10, which supports a postulated
depth of burial/exhumation in the range of 170 m to 700m.
Sandwiched between the Lower Greensand and theChalk (previously), the Gault Clay will have experienced
a similar geological history to these strata which not only
includes vertical changes in stress but changes inhorizontal stress due to tectonic processes such as theAlpine Orogeny. Therefore, conventional methods for
estimating in situ stresses based on vertical stresschanges only, will not necessarily be appropriate for thismaterial.
Typically, the water level lies between 0.5 m below
and 1.0 m above the surface of the clay. The typical SPT
N values and undrained shear strength of the Gault Clayare plotted as shown in Fig. 1 and Fig. 2 respectively.

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SPT N Value vs Depth: The Gault ClayProceedings of the First International Conference on Press-in Engineering 2018, Kochi

driving assistance. The water jet temporarily and locallysoftens soil at the pile toe, and lubricates the pile surface.
Thus, the installation resistance is reduced, resulting in
improvement of drivability and reduction of pile damage.
Once the sheet piles were installed, they were left inplace as the permanent basement walls.
3. Press-in piling3.1. Layout
The plan layout of the retaining walls are shown in boldlines in Fig. 3.

Fig. 3 Project Layout Plan

3.2. Productivity
The piling work was carried out from February toSeptember in 2005 by utilizing 3 different types of SilentFig. 2Pilers. Due to the complexity of the job site, the piling

Undrained Shear Strength vs Depth: The Gault Claywork was carried out intermittently. The typical2.3. Structural type
Due to the presence of the stiff Gault Clay, a raft typeproduction rate was between 200m and 300m a day of
total driving pile length excluding downtime.

foundation was designed for the project and the majorityof the building load is carried by the raft slab. To
construct a basement, 421.2m of perimeter wall was

3.3. Piling data
Sometimes, the issue of ground disturbance is raised

installed utilizing steel sheet piles. The retained height ofwhen considering water jetting to install piles. Giken Ltd.

the sheet pile wall in the temporary condition ranged

empirically knows that impact of water jetting isfrom 3.35m to 7.7m and 85% of the perimeter wall was

temporary and local with proper water pressure andpropped with 1 to 3 levels of raking strut system. In thevolume control. However, there had not been researchpermanent condition, the sheet pile walls were cladded

data on the use of water jetting in over consolidated stiffwith 900mm thick reinforced concrete to provide verticalcohesive soil. Therefore, Giken Ltd. carried out

load transfer and a grade 3 basement environment.

experimental piling to figure out the impact of water
jetting on Cambridge Gault Clay as described in Fig.4.

2.4. Piling method
In order to install sheet piles into the stiff Gault Clay,Skin friction was monitored during and after the pilewater jetting was employed for the Press-in Method as ameasure the skin friction, an unclutched 14m long AZ48

installation to quantify the recovery of skin friction. To- 499 -Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

Rft = αCuAs (kN)sheet pile was installed. 200kN skin friction was(1)recorded at the final depth. The surface area of AZ48
sheet pile is 1.90m2/m and the embedded depth of theα; Pile Wall Adhesion Factor (Soil Shear Strengthpile was 13.5m, which represents a unit mobile skinmodification factor) for Each Soil Layer (see Fig. 5)2friction of 7.8kN/m .
Then after 24 hours, the pile was extracted and theskin friction was measured at 950kN, which represents aunit skin friction of 37kN/m2. This indicates that themobile skin friction became static friction, increased by
475% with time effect.
Fig. 5α Factor versus Cu for Clay according to Tomlinson

Cu; Average Undrained Shear Strength of Each
Layer (kPa)As; Embedded Area of the Shaft of the Pile incontact with the soil (m2)Here, As = LgD = 1.90 x 13.5 = 25.7m2
Lg; Girth Length of Single Pile (m)D: Embedded Pile Depth (m)Rft = αCuAs (kN)

(Average Cu= 0.5 x 105 x 25.7=105kPa)= 1,349 kN
The actual skin friction Rf measured on the job site
was as follows: 200kN: during pile installation (15% of Rft)
950kN: 24 hours after installation (70.4% of Rft)
This sharp increase in the skin friction within a shortperiod indicates that the water jetting did not soften theFig. 4

Gault Clay as initially thought. Moreover, the skin

Sideview of Experimental Pilingfriction will even gradually recover further after theinitial 24 hours.
4. Back analysis
To determine the Gault Clay static friction, we carried

5. Concluding remarksout reverse calculation of the obtained geotechnicalinformation.

This study reveals that the recovery of skin friction was
faster than initially thought. However, although skin

The average undrained compressive strength Cu of thefriction rapidly increased in the first 24 hours after the pile

Gault Clay was 105kPa over the embedded pile length.

installation, it is concurred that a further increase occurs

The theoretical static skin friction Rft is worked out bydays or months after installation. Therefore, further

the following formula.

research is required to determine a more precise adhesion
factor of Gault Clay after water jetting.

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