Prediction Of Ground Settlement Response To Sheet Pile Extraction (P. Mc Gough)

Peter McGough. Prediction of Ground Settlement in Response to Sheet Pile Extraction. Proceedings of the BGA International Conference on Foundations, Dundee, Scotland,24 – 27 June 2008. IHS BRE Press, 2008.

Prediction of Ground Settlement Response to Sheet Pile Extraction

Peter McGough Principal Consultant, PGM Geotechnical Pty Ltd, Perth, Australia

SUMMARY: A new design method for predicting the volume loss and resultant ground surface settlement trough in response to sheet pile extraction is presented in this paper. The method is based on field data from several sites within Perth, Western Australia where sands and soft alluvial silts and clays are predominant. The data from these sites have been analysed, and a design method developed to predict ground surface settlement in response to sheet pile extraction based on the sheet pile cross-section, depth of sheet pile, and soil type. The design method has been compared against other published data on extraction-induced settlement and has shown excellent correlation. The effect of soil type on the choice of adhesion factor and its effect on the correlation is also discussed in detail. Keywords: Design, Extraction, Settlement, Sheet Piling .

INTRODUCTION Sheet piling is commonly used in combination with strutting to form the temporary support system for construction of permanent underground structures. It is well known that ground settlement can be introduced by sheet pile installation. The ground settlement introduced by sheet pile installation is typically very distinct, and is usually confined to within 1-2m of the sheet pile. This induced settlement can be very damaging to adjacent pavements and brittle concrete or masonry structures as illustrated in Figure 1. Settlement due to sheet pile installation is not only confined to vibratory piling, but can also occur when press-in or self-boring piling systems are used.

What is lesser known, and apparently unpublished, is that a significantly greater amount of ground settlement can occur upon removal of sheet piles after the permanent works are complete. This settlement can extend to a distance approximately equal to the depth of the pile installed. As a result, a substantial amount of damage can occur to nearby structures due to removal of sheet piles. The relative impact is illustrated in Figure 2 which shows the same area in Figure 1 after sheet pile removal.

During the New MetroRail (NMR) Project in Perth, Western Australia, a large

database of ground surface settlement measurements in response to sheet pile extraction was obtained. This database revealed that the extent of settlement upon removal was substantial in comparison to that experienced during installation.

Peter McGough

. Fig. 1: Settlement within 1-2m of piles upon installation

Fig. 2: Settlement and damage caused from sheet pile removal in same area as Fig. 1

A literature search found only one published example of ground settlement

response to sheet pile extraction. [Meijers and Tol (2006)]. The search highlighted that there was little known about the topic, and that there was no method for predicting the ground surface response to sheet pile removal. Thus, using the database of measurements obtained from the NMR project, the author has developed an empirical method for predicting ground surface settlement in response to sheet pile removal.

BACKGROUND

Approximately 1 km of sheet piles were installed on the NMR Project. Sizes ranged from PU 16 to PU 32. The location of the sheet piled excavations, and the predominant geology is schematically illustrated in Figure 3. Full details of the strutting and sheet pile design on the project can be found in Sigl et. al. (2007).

Extent of Installation Damage (1-2m)



Extent of Removal Damage (15m)

Extent of Removal Damage (15m)


Fig. 3: Schematic geology and location of sheet pile excavations As highlighted in the introduction, settlement due to sheet pile installation was noted in all areas, but was confined to within close proximity of the sheet piles. Significant settlements upon sheet pile removal were experienced when the first sheet piles were removed prior to the TBM launch. When the remainder of the sheet piles were removed, the potential impact on nearby structures was a concern, and thus detailed monitoring was undertaken to assess the effect of sheet pile removal. Measurements of ground surface settlement in response to sheet pile removal were taken at the Perth Yard Receival Box/ Cut and Cover Areas, Esplanade Station and TBM Launch Box, Foreshore Area 2 and Foreshore Area 7. The measured surface settlement from all the areas is shown in Figure 4, (after McGough and Williams (2007)). Analysis of the data indicates that the average volume of the settlement depression ranged from 0.22m3 to 0.75m3 per metre length, averaging approximately 0.46m3 per metre length of wall.

EMPIRICAL RELATIONSHIP FOR PREDICTION OF GROUND SURFACE SETTLEMENT IN RESPONSE TO SHEET PILE REMOVAL

Using the NMR Project data, an empirical relationship for estimating ground surface settlement in response to sheet pile removal has been developed. The relationship has been developed by applying a log normal best fit curve to the measured settlement data at each location on the NMR project, and then integrating this best fit curve to establish the volume loss on sheet pile removal. The lognormal best fit curve was of the form: Settlement (mm) = A* Ln (m) - B, where m is the distance from the wall in metres. The individual parameters A and B were found to vary consistently with the measured volume loss as illustrated in Figure 5. Thus it was found that the shape and depth of the ground settlement curve was a function of the volume loss on sheet pile removal, and the measured ground settlement curve could therefore be obtained, or predicted, by multiplying the volume of the sheet pile removed by an appropriate factor to account for soil adhesion and geology

Sheet Piled Excavations

Peter McGough

y = 32.615Ln(x) - 90.933

Volume = 0.46 m3 per metre of wall

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t (m

m)

Launch Box North Wall

Esplanade Station

Foreshore Area 2

Perth Rail Yard

Freeway Dive

Range of volume loss = 0.22 to

0.75 m3 per metre of wall

Fig. 4: Measured settlement in response to sheet pile removal on NMR Project (after McGough and Williams, 2007)

B = 0.5948Ln(VL) + 3.1994

A = 0.0072Ln(VL) + 0.0197

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Volume Loss (m3) per metre width of wall

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s / A

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B /

A

Volume/A

B/A

Log. (B/A)

Log. (Volume/A)

Fig. 5: Relationship between measured volume loss and parameters for lognormal best fit curves The empirical relationship for predicting the quantity and shape of the ground

settlement curve in response to sheet pile removal is expressed in the form: Settlement (mm) = A* Ln (m) - B, where m is the distance from the wall in metres.


The procedure for predicting the ground settlement curve is described below: Step 1: Determine the cross sectional area of the pile per metre length of wall, X. i.e. for example X for PU 32 =0.242m2

Step 2: Determine the depth of the pile, L. i.e. for example L = 20m Step 3: Determine the volume of sheet pile per metre length of wall,

VP= X * L, i.e for this example VP = 0.242 *20 = 0.48 m3.

Step 4: Allow for an adhesion/geology factor (f) [for a this example use 1.5]

then calculate the Expected Volume Loss, VL = VP * f, i.e for this example VL = 0.72 m3 per metre length of wall.

Step 5: Determine the settlement curve parameters A and B from the equations

A = VL / [0.0072 * Ln(VL) + 0.0197] and B = A * [0. 5948 * Ln(VL) + 3.1994]

i.e for this example of 0.72m3, A= 41.7, and B = 125.6 Step 7: Plot the expected settlement curve in the form

Settlement (mm) = A* Ln (m) - B, where m is the distance from the wall in metres.

An example of the use of the empirical formula to predict the settlement curve due to sheet pile removal for a 20m deep PU 32 sheet pile is shown in Figure 6.

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20m Long PU 32

Fig. 6: Predicted settlement response to sheet pile removal for above example

This empirical model has been verified against the data presented by Meijers and

Tol (2006) and has been able to perfectly predict the shape of the settlement curve on extraction using and adhesion factor of 0.45 as shown in Figure 7. This adhesion factor is less than experienced on the NMR project as the Meijers and Tol experimental data

Peter McGough

involved installation of a sheet pile and subsequent extraction soon afterwards, without the construction process assumed in the empirical model. The experimental data reported by Meijers and Tol (2006) also involved the settlement of a full trough, whilst the empirical data, obtained from the NMR Project, assumes that all settlement occurs in the half space (half of full settlement trough) due a permanent wall being constructed adjacent the sheet piles before removal. Thus a minimum adhesion factor of 0.9 could have been expected from the Meijers and Tol (2006) data if a permanent wall was constructed adjacent the sheet piles prior to removal. This factor of 0.9 is comparable to that experienced from back analysis of the NMR data.

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Meijers and Tol AZ26 Fig 7

Meijers and Tol AZ26 Fig 8

14.65m Long AZ26

Fig. 7: Predicted and measured settlement response to sheet pile removal

It should be noted that this empirical relationship is notionally only valid for predicted volume losses between 0.1 and 1.0 m3 per metre length of wall, and for the situation where a permanent structure has been constructed adjacent the sheet piles and the adjacent cavity has been backfilled with compacted granular material. This cavity also should be small (in the order of 1-2m) such that additional compaction of the backfill does not effect the prediction.

The NMR Project data has been investigated further with respect to each individual area; the effect of geology (and other contributing factors) on the adhesion/geology factor is assessed in the following section.

DETAILED ASSESSMENT OF ADHESION/GEOLOGY FACTOR The NMR project data has been assessed on a site by site basis and an adhesion/geology factor has been fitted at each site. The adhesion/geology factor was found to range from 0.9 to 2.2. The reasons for this variation were initially unknown and thus a detailed assessment of the reasons for this variation has been undertaken at five separate


construction areas on the project which encompassed 8 different design sections where sheet pile details, installation depths and geology/soil types were different.

Details of the sheet pile types and depths being considered in this section are summarised in Table 1.

Table 1: Design sections and pile details

Section Approximate Chainage

Sheet Pile Section

Ground Level

(m AHD)

Sheet Pile Toe Level

(m AHD)

Excavation Depth

(m) TBM Launch box 545 to 580 PM PU 20 RL +2.0 RL -22 12.5 to 14.6

Perth Rail Yard Receival Box / Cut & Cover

350 to 372 PB PU 24 RL +11.25 RL -8.75 12.1

372 to 410 PB PU 24 RL +11.25 RL -6.75 9.5

410 to 428 PB PU 16 RL +11.25 RL -4.75 8.6

Esplanade Station 580 to 690 PM PU 24 RL +2 RL -20 12.5

Area 2 709 to 797 PM PU 24 RL +2 RL –16.5 10.7

762 to 797 PM PU 32 RL +2 RL -19.0 8.5

Area 7 1347 to 1380 PM PU 16 RL +6 RL -4.0 5.6

Esplanade Launch Box Sheet piles on the northern side of the launch box were the first removed; just before tunnelling commenced. This design section is shown in Table 1. The sheet piles were installed through fill sands (MG1), very soft silt/clay (SRA)/stiff sandy slays (UGU) and clayey sands (GFU), underlain by soft to firm alluvial silts and clays (LGU) as illustrated in Figure 8.

Fig. 8: Geological section at Esplanade Launch Box

The gap between the sheet piles and the permanent structure was 3.5 metres. This

was backfilled with cemented stabilised sand which formed the pressure seal as the TBM launched. This sheet pile removal was not observed, but there was no adhesion expected due to the cemented nature of the fill. Internal compaction of the backfill was also not expected and thus all volume loss was expected to be the result of sheet pile volume extraction and compression of the existing soils due to vibration on removal.

The data obtained for each for the northern side of launch box is shown in Figure 9. It is apparent that the volume loss on the flanks of the northern launch box is small

Launch Box and Tunnels

Sheet Piles +2mRL to -22mRL

+0 mRL

-10 mRL

-22 mRL

Peter McGough

and the response to sheet pile removal is essentially a 2D phenomenon with little lateral influence. Using the empirical design method an adhesion/geology factor of 2.2 predicted the maximum settlement for the launch box north. The results also indicate that the range of settlement was consistent over the two tunnels and between then, indicating that the overall response to removal was relatively uniform.

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Distance from Sheet Pile (m)G

roun

d M

ovem

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mm

2.2*PU20 Volume, Launch BoxNthBTS 1 Alingment

Centreline

BTS 2 Alignment

East Flank

West Flank

Fig. 9: Measured and back analysed response to sheet pile removal at Esplanade Launch Box

Perth Rail Yard Within Perth Rail Yard, a cut and cover section of the works was constructed between CH 350 and 428 PB. This cut and cover section was divided into three design sections with different sheet pile depths and types as shown in Table 1. The sheet piles were installed through fill sands, clean wind blown medium dense sands (Spearwood Sands), lenses of stiff sandy clays (UGU) which were underlain by dense slightly clayey sands (GFU) as illustrated in Figure 10. The gap between the sheet piles and the permanent structure was 1.5 metres. This was backfilled in layers with compacted clean sand.

Observation of the sheet pile removal process showed no adhesion of clay or sand to the piles and thus all volume loss on sheet pile removal could be attributed to the volume of the sheet pile and some compaction in the natural soil and backfill.

The data obtained from each of the three design areas is shown in Figure 11. It is

apparent that the volume loss in the deeper Receival Box is greater than the shallower cut and cover section which is line with expectations. It was determined that using the empirical design method an adhesion/geology factor of 1.65 predicted the maximum settlement of 0.64 m3 per metre length of the wall for the Receival Box. A similar factor may be applicable to the other two design sections but there is insufficient data to confirm this.


Fig. 10: Geological section at Perth Rail Yard

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1.65 VL Perth Yard ReceivalBox

Perth Yard Receival Box

1.65 VL Perth Yard Ch 360-398

Perth Yard Ch 360-398

1.65 VL Perth Yard Ch 398-416

Perth Yard Ch 398-416

Fig. 11: Measured and back analysed response to sheet pile removal at Perth Railway Yard

If a similar adhesion/geology factor of 1.65 is applied to the 1.5m wide, 12.1m

deep sand fill between the sheet pile and the permanent wall, the additional volume of settlement is 0.11m3 per metre length of wall. If these two volume losses were combined into an equivalent half space, the equivalent adhesion/geology factor for a total settlement of 0.75 m3 per m would be 1.94. This factor is very similar to that observed in north face of the Esplanade Launch Box where a full half space was interpreted (due to cemented backfill) and where loose sand fill and very soft silts were encountered. Esplanade Station Sheet piles on the east and west of the station were removed upon completion of construction. This design section is shown in Table 1. The sheet piles were installed through fill sands (MG1), stiff sandy clays (UGU) and dense clayey sands (GFU)

+10

+00

-10 mRL Sheet Piles -4.75 to -8.75 mRL

Permanent Structure

Peter McGough

underlain by soft to firm alluvial silts and clays (LGU) as illustrated in Figure 12. The fill ramp at the top of the section was removed down to +2mRL prior to piling.

Fig. 12: Geological section at Esplanade Station

The gap between the sheet piles and the permanent structure was 1.5 metres as per

Perth Yard. This was backfilled with compacted clean sand. Sheet pile removal was observed as having a consistent amount of smeared alluvial silt/clay adhesion along the length of the sheet pile that could be removed with a hand scraper. The data obtained for Esplanade Station is shown in Figure 13.

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Esplanade Station Field Data

1.0*PU24 Volume, EsplanadeStation

Photographic data

Fig. 13: Measured and back analysed response to sheet pile removal at Esplanade Station

The interpreted adhesion/geology factor for the Esplanade Station data is 1.0,

which is less than the other two areas presented so far. This is most likely due to the presence of relatively dense clayey sands and stiff sandy clays in the majority of the sheet pile profile which have not compacted upon vibration.

Permanent Structure

Esplanade Station Sheet Pile Toe -20 mRL


Area 2 Sheet piles on the east and west of the cut and cover section were removed upon completion of construction. This section was the most extensively monitored section due to its close proximity to adjacent buildings and its location within paleochannel sediments which consists of very soft to soft alluvial silts and clays. There are two design sections within Area 2 as shown in Table 1. The sheet piles were installed through fill sands, alluvial sands (GFU) and alluvial silts and clays UGU and SRA as illustrated in Figure 14. The geological section shows the variability of strengths within each of the units and also the variability across the design section.

Fig. 14: Geological section at Area 2


Perth Yard and Esplanade Station. This was backfilled with compacted clean sand. Small amounts of alluvial silt/clay adhering to the pile were observed during sheet pile removal at this location.

The data obtained for Area 2 is shown in Figure 15. It is apparent that the settlement is different in the two design areas, and is a function of the sheet pile volume, with both design sections containing thick sequences of soft marine silts/clays overlain by sand. The measured adhesion/geology factors of 0.9 and 1.0 are consistent with the Esplanade Station data. The reason for the similar results to Esplanade Station is most likely due to the geology; with the presence of silt and clays in the majority of the sheet pile profile which have not compacted upon vibration.

This phenomenon is supported by the inclinometer data which shows consistent

lateral movements through the profile after sheet pile removal, which reflects a relaxation into the void created by sheet pile removal. Thus it is concluded that where there is a substantial presence of cohesive materials such as clays, silt/clay mixtures or clayey sands an adhesion/geology factor of 1.0 is applicable to the prediction of settlement in response to sheet pile removal.

Area 2 Sheet Pile Toe –16.5 to -19.5 mRL

Permanent Structure

Peter McGough

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Ch 709-762, PU 24, 18.5m

0.9*PU 24 Volume

CH 762-797, PU32, 21m

1.0*PU 32 Volume

Fig. 15: Measured and back analysed response to sheet pile removal at Area 2

Freeway Dive Structure Area 7 During a previous phase of sheet pile removal in the adjacent Area 6, distortion of a nearby freeway bridge was experienced resulting in the need for rapid corrective jacking. The damage to the bridge deck and bearings is shown in Figure 16. Given the experience at Area 6 and the very close proximity of the Area 7 sheet piles to the main north freeway in Perth (with high speed traffic passing within a few metres of the piles), the consequence of any significant unforseen settlement in Area 7 was very high and as such this area was extensively monitored during sheet pile extraction. The pile depth and required pile type within Area 7 are shown in Table 1. The sheet piles were installed through clean reclaimed fill sands (MG1) as illustrated in Figure 17.

Fig. 16: Differential settlement across a bridge join and bridge bearing damage caused from sheet pile removal adjacent a operating freeway bridge


Perth Yard and Esplanade Station. This was backfilled with compacted clean sand.


The data obtained for Area 7 is shown in Figure 18. An adhesion/geology factor of 2.0 predicted the maximum settlement of 0.32 m3 per metre width for Area 7

Fig. 17: Geological section at Area 7

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'Freeway Dive, PU 24, 18.5m

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Fig. 18: Measured and back analysed response to sheet pile removal at Area 7

It is apparent that the adhesion/geology factor of 2.0 in an engineered sand fill is

consistent with adhesion factors of 1.65 and 2.2 observed at Perth Yard and Launch Box North respectively, which are in predominantly sand profiles. It could be concluded from these observations that where loose to medium dense sands exist in the geological profile an adhesion/geology factor of 2-2.25 is applicable to the prediction of settlement in response to sheet pile removal. It is should be noted that this factor is only valid where there is a permanent wall within a 1-2 metres of the sheet pile and that the cavity between the wall and the sheet pile has been backfilled with an engineered fill.

Ch 1350 PM dn Ch 1400 PM dn

0 mRL

-10 mRL

-20 mRL

Design Pile Toe Level (-4mRL)

Peter McGough

SUMMARY An empirical design method for predicting the volume loss and resultant ground surface settlement curve in response to sheet pile extraction has been presented for the first time. The method is simple and only requires the cross sectional area of the pile, the proposed length and an adhesion/geology factor.

The method has been validated against case studies from the New MetroRail Project and from experimental data presented by other authors. The adhesion/geology factor has been assessed and it has been concluded that a factor of 2-2.25 is applicable to sandy soils and a factor of 1-1.5 is applicable to cohesive soils. The author believes that further research and case studies in differing geological environments would help refine the recommended adhesion/geology factor and thus make the method more robust over time. The proposed method is a considered a valuable design tool in predicting the potential impact caused by sheet pile removal, and managing the risk in construction projects.

REFERENCES

Sigl, O., Aikawa, F., Amon, A., Bo, H., and Stewart, D.. (2007). Temporary anchored and strutted sheet piling , Proc of Seminar on: New MetroRail City Project – Tunnelling & Underground Structures, Perth, 12 September 2007, pp 137-174

McGough, P.G. and Williams, M. (2007). Geotechnical Instrumentation and Monitoring, Proc of Seminar on: New MetroRail City Project – Tunnelling & Underground Structures, Perth, 12 September 2007, pp 81-136 Meijers, P., and Tol, A.F. van. (2006) The Raamsdonksveer Sheet Pile Test: Observed settlements due to installation of vibratory driven sheet piles. TRANSVIB 2006, International symposium on vibratory pile driving and deep soil vibratory compaction, 21-22 September 2006, Parijs

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Prediction Of Ground Settlement Response To Sheet Pile Extraction (P. Mc Gough)