Sewer Installation by Pipejacking in the Urban Areas of Hong Kong Part II Performance of Works, Lessons Learned and Improvements Proposed

Transcription

1 Sewer Installation by Pipejacking in the Urban Areas of Hong Kong Part II Performance of Works, Lessons Learned and Improvements Proposed This paper discusses the performance of pipejacking works carried out under DSD Contract No DC/2000/11, including TBM utilisation, jacking force, rate of penetration and friction resistance, tolerance in alignment, TBM daily production rate, disc cutter replacement pattern and ground settlement. As-built records for three pipejacking drives under soft, hard and mixed ground conditions respectively were highlighted. Lessons learned and improvements proposed are also given. The paper should be read in conjunction with Part I: Planning Design, Construction and Challenges. Keywords: TBM Utilisation, Rate of Penetration, Tolerance in Alignment, Disc Replacement, Daily Production Rate, Ground Settlement, Sonic Soft-ground Probing, Options of TBM Driving The Hong Kong Institution of Engineers Transactions, Vol 14, No 1, pp31-43 Wilson W S MOK Black & Veatch Hong Kong Limited Maxwell K W MAK Drainage Services Department Felix H T POON Black & Veatch Hong Kong Limited TRANSACTIONS PAPER Introduction Pipejacking techniques have been commonly adopted in the urban areas of Hong Kong for the installation of pipelines, such as sewers, drains and watermains, in recent years. Problems such as excessive ground settlements, out-of tolerance in line and level, frequent change of disc cutters, and malfunction of the TBM are often encountered during the course of works, affecting the adjacent utilities, services, structures, roads and footways, and more importantly, the daily production. In the past, many observations on the pipejacking techniques were based on overseas projects, with little discussions on the local practices. It is hoped that, through the successful completion of DSD Contract No DC/2000/11 Wan Chai East and North Point Sewerage Trunk Sewers, practitioners can gain better understanding of such techniques on the ground conditions in Hong Kong. This paper discusses the performance of the pipejacking works observed for the 4 km long trunk sewers, completed in 21 shallow and deep drives, under different depths and ground conditions, by the 4 nos slurry operated TBMs, in two of the most busiest urban areas in the Hong Kong Island. Analyses of the measured data and comparisons with the predicted values, with regard to ground behaviours and responses, are given. Improvements proposed, and different options of TBM driving to suit different site constraints, and the need in programming, are highlighted. Recommendations, based on the lessons learned and experiences gained, are also made. Performance of Pipejacking Works TBM Utilisation Apart from the capacity to control the line and level and limit the ground settlement, the performance of a TBM in a drive is also reflected by the excavation (working) time and advancement. For the 4 nos TBMs adopted, the Herrenknecht AVN 600 TBM had the highest average excavation time, which was followed by the Herrenknecht AVN 1800T TBM, the Herrenknecht 1200TC TBM and the Lovat mts 2000 TBM. A breakdown of the total time period for each TBM is shown in the four pie charts in Fig 1, and the average contract excavation time/the excavation time of each drive versus the drive length is shown in Fig 2. The TBM utilisation for each drive is summarised in Table 1. The significant difference in excavation time could be due to the different configuration in each TBM and different ground conditions encountered. It is noted that the low excavation time for one of the TBMs had been greatly affected by the slow progress in the first drive due to the learning stage, the long time replacement of disc cutters in a number of drives, and the stalled TBM due to obstruction in a drive which resulted in the requirement of a rescue shaft for changing the cutting wheel before the resumption of the works. TBMs also performed significantly different in excavation time even in similar ground conditions and curvature. The drives in soft ground usually had better excavation time than those in mixed or hard ground, with the highest 77% and the lowest 21%. The presence of a high content of hard materials sometimes caused damage to the TBM. Frequent repair and maintenance was often required, significantly affecting the TBM performance. However, there was no indication that the straight alignment had better excavation time and less downtime than the curved alignment. For classification purposes, the soft ground is defined as a ground made of clayey, silty and/or sandy materials such as marine deposits and alluvium; the mixed ground generally consists of soft materials with rock in part of the excavation face; the hard ground is a ground consisting of rock in the full excavation face. Such classifications were based on visual inspection of the materials collected in the muck tank after the completion of jacking each pipe. Jacking Force, Rate of Penetration and Friction Force The average jacking force required to tunnel through filled ground for shallow drives ranged from 70 to 220 T, with an average rate of penetration of 110 to 175 mm/min. This rate was reduced to 70 to 90 mm/min when hard materials were encountered. For deep drives in mixed to hard ground, the average jacking force was generally in the range of 290 to 480 T, giving an average rate of penetration of 30 to 90 mm/min. In soft ground such as marine clay or alluvial deposits, the rate of penetration could be as high as 350 mm/min under a jacking force of 300 T. A tunnel through a full face of moderately to slightly decomposed granitic (M/SDG) bedrock had recorded a penetration of 18 mm/min under a consistent jacking force of 300 to 330 T. The average rate of penetration versus the average jacking force for different drives is shown in Fig 3. There was no indication that a higher average jacking force had been required for curved drives in soft ground, although a jacking force of more than 750 T had been used at certain locations of the 404 m S-curve alignment and some of the curved alignments. A higher jacking force was usually required to push the pipeline after the TBM had stopped in ground for some time for repair or changing of disc cutters. The average friction resistance derived from the jacking force on the drives ranged from 1.2 to 4.8 kn/m 2 for clayey materials such as marine deposits, from 1.9 to 2.8 kn/m 2 for sandy materials such as alluvium and 31

2 Figure 1 TBM Time Allocation Charts Figure 2 Average Contract Excavation Time/Excavation Time of a Drive versus Drive Length Figure 3 Average Rate of Penetration versus Average Jacking Force for Different Pipejacking Drives 32

3 % of Time Available for Different Activities Ground Pipe TBM Survey Slurry Change of Drive Radius (m) Length (m) Conditions Excavation Positioning Downtime Check Mixing Cutters Others Herrenknecht AVN 1800T TBM WC2-WC Soft WC2-WC4 400 & Straight 145 Soft NP5-NP3 500 & Straight 404 Soft, mixed & hard NP5-NP7 500 & Straight 370 Mixed to hard NP8-NP7 Straight 220 Soft, mixed & hard NP8-NP9 Straight 170 Hard NP15-NP16 Straight 58 Soft Average Herrenknecht AVN 1200TC TBM WC5-WC Soft with little mixed WC5-WC5A Straight 40 Soft with little hard WC5-WC Soft WC8-WC Soft NP2-NP Soft NP2-NP3 Straight 60 Soft, mixed & hard Average Herrenknecht AVN 600 Microtunneller WC8-WC9 Straight 53 Soft Lovat mts 2000 TBM NP12-PS1 Straight 107 Soft NP12-NP13 Straight 148 Soft, mixed and hard NP12-NP Soft, mixed and hard NP10-NP11 Straight 307 Soft with little mixed NP10-NP Soft, mixed and hard NP14-NP Soft NP14-NP Soft, mixed and hard Average Table 1 Summary of TBM Time Allocation for Each Drive completely weathered granite, and from 0.3 to 1 kn/m 2 for moderately to slightly weathered granitic bedrock. These resistances were much less than the predicted 8 kn/m 2 for wet sand and 12 kn/m 2 for firm clay (based on recommendations given in [1]). This could be due to the full effect of the bentonite and polymer lubrication applied to the pipeline during the course of driving. It is noted that the resistance built up significantly when obstructions were encountered and when the TBM had stopped in ground for a long time due to malfunction or repair, resulting in the surrounding ground moved / collapsed to the pipeline. For the 2 nos longest curved deep drives where irregular, alternating soil and rock composition was encountered, the friction on the pipeline did not increase significantly and a resistance in the range of 3.4 kn/m 2 and 4 to 7 kn/m 2 respectively had been recorded. Tolerance in Alignment The contract requirement for tolerance in alignment, based on the Tunnelling Specification [2], is 50 mm for line and 35 mm for level. Of the 4 km long pipejacking drives (which consisted of 66% of soft, 14% of mixed and 20% hard materials), 21.3% for line (44% in soft; 17% in mixed; 39% in hard) and 19.4% for level (55% in soft; 12% in mixed; 33% in hard) exceeded the tolerance, generally by 2-50 mm. However, more than 100 mm was recorded at some locations in a few drives where the ground conditions were alternating. For the straight sections (1.1 km long), both the line and level exceeded the tolerance by 16% whereas for the curved sections (2.8 km long), the line and level showed an out-of-tolerance by 24% and 20% respectively. Despite the above, only 1.7% of the pipelines exceeded the tolerated 0.5 angular deflection at pipe joint, with 1.9% for curved drives and 1% for straight drives. On completion of the pipejacking works, the respective pipe joints were inspected visually and no damage in the pipes was found. Infiltration tests were also carried out in all pipe joints and the results were within the limit as specified in the General Specification [3]. For TBM performance, the Herrenknecht AVN 600 TBM generally had good control in both line and level in soft materials. As only one drive was completed by this TBM, its capacity in controlling the line and level in mixed to hard ground conditions had not been tested. The Herrenknecht AVN 1200TC TBM also showed good control in curved sections in soft ground. However, the presence of mixed ground caused out-of-tolerance in another drive. Difficult control in line and level was observed in the two straight sections where soft marine clay and mixed and hard materials were present respectively. For the 1.6 km long drives completed by the Herrenknecht AVN 1800T TBM, their out-of-tolerance generally ranged from 20 to 42% for line and from 14 to 37% for level for curved sections (with an average of 28% and 24% respectively), and ranged from 0 to 47% for line and from 9 to 27% for level for straight sections (with an average of 26% and 24% respectively). For the Lovat mts 2000 TBM, it showed better control in the 562 m long straight sections with an average out-of-tolerance of 11% for both line and level. However, the out-of-tolerance increased to 32% for line and 28% for level for curved sections. It is noted that a TBM with 3 nos steering jacks always required a longer length to correct the line and level in order to avoid causing damage to the jacking pipes. The summary of the as-built alignment of all pipejacking drives is given in Table 2. For shallow drives with a depth up to 8 m, the line and level of the pipelines are well within the tolerance as the ground contained no or very few hard materials. However, in very loose ground, the control of such was always a problem. 33

4 Drive Herrenknecht AVN 1200TC TBM % Out of Tolerance Total Diameter Depth to (Contract) Curvature (m) Length Ground Condition Encountered (mm) Invert (m) (m) Line Level (50 mm) (35 mm) NP2 - NP All soft 0 0 NP2 - NP Straight m soft, 10 m mixed and 20 m hard 0 24 WC5 - WC m soft and 25 m mixed WC5 - WC5A Straight m soft, 10 m mixed WC5 - WC All soft 3 1 WC8 - WC All soft 4 0 Herrenknecht AVN 1800T TBM Table 2 Summary of As-built Alignment of Pipejacking Drives Average Value: NP15 - NP Straight 58 All soft 0 9 NP5 - NP Straight Straight m soft, 130 m mixed and Straight 84 m hard NP5 - NP Straight-500-Straight m mixed and 175 m hard NP8 - NP Straight m soft, 70 m mixed and 65 m hard NP8 - NP Straight 170 All hard 6 27 WC2 - WC All soft WC2 - WC Straight Straight 145 All soft Average Value: Herrenknecht AVN 600 TBM WC8 - WC Straight 53 All soft 5 0 Average Value: Lovat mts 2000 TBM NP10 - NP Straight m soft and 10 m mixed 0 0 NP10 - NP m soft, 37 m mixed and 53 m hard NP12 - NP m soft, 30 m mixed and 20 m hard NP12 - NP Straight m soft, 20 m mixed and 20 m hard 0 9 NP12 - PS Straight 107 All soft NP14 - NP m soft, 53 m mixed and 119 m hard NP14 - NP All soft 13 0 Average Value: Average Value of All Drives: For deep drives, with a straight section, tunnelling through soft ground or a ground with few hard materials, the line and level could be generally controlled within the tolerance, except in marine clay and alluvial deposits where significant deviations always occurred. Depending on the locations, sizes and hardness of materials encountered in the excavation face, the line and level of a pipeline could also fluctuate significantly along its length, as the TBM always tended to move faster towards soft materials. The decision making of the TBM operator as to how and when the jacking force and the steering jacks should be adjusted also affected the control of the pipeline alignment. The driving in hard ground or a ground with alternating soil and rock composition would easily cause the TBM to tilt up as a result of the likely escape of rock debris or flour below its invert during the excavation process. The presence of rockhead in the invert of the TBM would also cause the pipeline to slant upward as the disc cutters were unable to cut rock effectively in such a small contact area, causing the TBM to tilt up as a result. This situation appears to be unavoidable but a suitable combination of extension and retraction of the steering jacks in the TBM and slow advancement of the pipeline could minimise the excessive deviation. It is also essential that, under hard ground conditions, the annulus of the pipeline is fully filled with lubricant at all times to prevent settling of small rock fragments, which could form a wedging action inducing high frictional force during the jacking operation and causing damage to the pipes. The out-of-tolerance in alignment in a pipeline required the checking of the hydraulic performance and luckily they were still acceptable. As a comparison, if the tolerance is changed to 75 mm for line and 50 mm for level, then the out-of-tolerance for the completed pipelines would be reduced to about 10% for both line and level. This value generally matches with those obtained from DSD Contract No DC/95/05 [4] and DSD Contract No DC/98/06 [5]. TBM Daily Production Rate The average TBM production (progress) rate for all the drives was 4.2 m/day. This rate was dictated by the time required for repair and maintenance during the course of work such as the extension of ventilation duct, disconnection and connection of power cables, extension of compressed air pipe, installation of jacking pipes, application of lubricant, 34

5 change of disc cutters, survey check, and downtime such as mechanical fault, electrical fault, unblocking of slurry charge and discharge pipes, breakdown of the hydraulic system, malfunctioning of the computer system, and the worst, the stoppage of the TBM in ground for more than 8 months, as highlighted under the Section TBM Utilisation. The daily production for a pipejacking drive (NP5-NP3) under mixed ground conditions is shown in Fig 4. The average daily production rate for each pipejacking drive versus the drive length, the maximum daily production rate versus the drive length and the average contract production rate/the average production rate of each drive versus the drive length are shown in Figs 5 to 7. The overall production for each pipejacking drive, completed by the four TBMs, and for each TBM, is shown in Figs 8 to 10 and 11 respectively. Replacement of Disc Cutters A total of 254 nos steel alloy disc cutters were used for the 21 drives, averaging about 0.5 set of cutters for each shallow drive and 1.5 sets for each deep drive. A disc cutter had to be replaced when it was damaged or had a wear of about mm. Based on the records, the damage of some of the disc cutters was caused by the dynamic impact of the TBM, as a result of the sudden change in ground conditions from soft to hard. The damage also occurred in hard driving under high jacking forces. It was observed that the damaged disc cutters generally appeared in the form of breakage of part of the disc, fractured rings and distortion in the bearing system. No discs had to be replaced for the Herrenknecht AVN 1200TC TBM during the course of each drive, but a number of drives required the replacement of some disc cutters before commencement. For the Lovat mts 2000 TBM, no replacement of disc cutters was required for the drives in soft ground. However, the average replacement frequency for each disc cutter, in the range of times, was recorded in Figure 4 Daily Production for Drive NP5 - NP3 For the Herrenknecht AVN 600 TBM, the average daily production rate was 2.5 m/day. It is considered that, due to the limited size of the TBM, in particular the configuration in the cutting wheel, its capacity in tunnelling through full face rock could be a problem. For the Herrenknecht AVN 1200TC TBM, the average production rate for the drives was generally in the range of 6-9 m/day (except 3.3 m/day for the drive which had to slow down in order to pass through the narrow gap between the Mass Transit Railway Corporation (MTRC) pedestrian tunnel and the existing trunk utilities accurately), with a maximum length of 22 m recorded in a single day (12 hour shift) when tunnelling through alluvial deposits. For comparison purposes, this average rate is much better than the 3-5 m/day recorded by DSD Contract No DC/95/05, using the same model of TBM and under similar ground conditions [6]. The Herrenknecht AVN 1800T TBM had an average production rate of 5 m/day, ranging from 3.8 m/day for a curved drive tunnelling through a ground with high rock content, to 9.7 m/day for a drive in soft ground. A number of drives advanced more than 10 m/day in soft ground, with a maximum length of 18 m. This rate is slightly higher than the 8.5 m/day recorded in DSD Contract No DC/95/05. It is also 66% higher than the 6 m/day recorded in DSD Contract No DC/98/06 [6] that a smaller TBM (1500 mm in diameter) in the same series was used. A tunnel through M/SDG bedrock, with strengths of MPa, achieved 4 m/day on average, which generally matches with the production rate achieved in both DSD Contract Nos DC/95/05 and DC/98/06. This suggests its capacity in tunnelling through rock may have been optimised. The drives completed by the Lovat mts 2000 TBM showed the average daily production rate with significant fluctuations, from 1.2 m for a curved drive tunnelling through a ground comprising soft, mixed and hard materials, to 17 m for also a curved drive in soft ground, with an average of 3.2 m/day. The best recorded length in a single day was 36 m (15 hour shift) in this drive. In a number of drives, the centre disc cutters were found to be damaged but could not be replaced from the rear of the cutting wheel, affecting the excavation capability. Based on the above, it would appear that this TBM is good in soft ground but could have difficulty in effectively tunnelling through a high content of hard materials. Figure 5 Average Daily Production Rate for Each Pipejacking Drive versus Drive Length Figure 6 Maximum Daily Production Rate for Each Pipejacking Drive versus Drive Length 35

6 Figure 7 Average Contract Production Rate/Average Production Rate of a Drive versus Drive Length Figure 10 Overall Production of Each Drive Using Lavat mts 2000 TBM Figure 8 Overall Production of Each Drive Using Herrenknecht AVN 1200TC TBM Figure 11 Overall Production of Each TBM Figure 9 Overall Production of Each Drive Using Herrenknecht AVN 1800T TBM highly variable ground made up of soft, mixed and hard materials, for each drive. The Herrenknecht AVN 1800T TBM also did not require any replacement of disc cutters for the drives in soft ground. However, it showed an average frequency of 1-3 times for each disc cutter in highly variable ground, and 1.9 times in hard ground. Gauge disc cutters usually required more frequent replacement than centre disc cutters. This is because, during each rotation of the cutting wheel, the gauge disc cutters have to travel a longer path for excavation, thus causing higher consumption. For the Herrenknecht AVN 1800T TBM, the cutters were replaced in a ratio of 1 (for centre discs) to 2.5 (for gauge discs), while a 1 to 4 ratio was recorded for the Lovat mts 2000 TBM. Depending on the ground conditions encountered, particularly the location, thickness and hardness of materials, some of the disc cutters had to be replaced again after driving for a short length. This situation occurred in some long curved drives with highly variable ground conditions and the replacement of disc cutters in a 4m drive length had been experienced in the worst case. The replacement of more than 30 nos disc cutters during the course of works in three drives had been recorded. For effective monitoring, a numbering system was adopted for the disc cutters in the cutting wheel, with a number assigned to each disc cutter 36

7 Figure 12 Disc Cutter Replacement Pattern for a Pipejacking Drive TBM Total No of Disc Used Herrenknecht 5 53 m 19% 38% 43% AVN 600 TBM Herrenknecht m 6% 11% 83% AVN 1200TC TBM Herrenknecht m 30% 19% 51% AVN 1800T TBM Lovat mts 2000 TBM m 15% 10% 75% Table 3 Service Life of Disc Cutters Average Service Distance Ground Composition Hard Mixed Soft location, and their replacement records were properly maintained. The TBM disc cutter replacement pattern for a pipejacking drive under typical mixed ground conditions is shown in Fig 12. This figure shows that 31 nos disc cutters had been replaced to complete the drive, corresponding to 2.82 sets. Including the original set of disc cutters, each set of disc cutters had an average service life of 400 m 3 (equivalent to 36 m 3 per disc cutter). The average service life of each set of disc cutters for each TBM, in terms of driving distance, is also given in Table 3. It is noted that the disc cutters in the Herrenknecht AVN 600 TBM still remained in a satisfactory condition after completing the 53 m long drive. Ground Settlement The average measured surface ground settlement, induced by the pipejacking works, above the centre line of the pipeline for most of the drives, is generally less than that predicted. The summary of settlement caused by the pipejacking works is given in Table 4. Settlements induced by the pipejacking works were predicted based on the invert normal probability distribution curve recommended by O Reilly and New (1982) [7]. This method generally predicts a narrow but relatively large settlement trough for shallow tunnels, and a wider settlement trough with small settlement for deep tunnels. It was assumed that adequate face pressure would be provided and thus the tunnel face loss should be negligible. The overcuts by TBMs were thus adopted as the predicted volume loss. The measured settlement on the monitoring markers varied and this could be due to different ground conditions above the jacked pipeline throughout its length, different speed of TBM advancement and the presence of existing utilities and services at the respective locations. As the settlements varied along the jacked pipeline, average values of the recorded settlements at monitoring locations along the centre line and left and right hand sides of the pipeline (at 5 m offset) were adopted in establishing the settlement trough. For two drives, the settlements at the left and right hand sides were greater than that in the centre line, causing the trough width undefined. Such phenomenon could be due to the presence of existing utilities/services at the respective locations and depths below the settlement monitoring markers, impeding the measurement of the actual magnitude. The transverse settlement trough width for each drive was back calculated from the measured data. It was found that the trough widths ranged from 18 to 30 m for the shallow drives, with an average of 19 m, and from 30 to 60 m for the deep drives, with an average of 35 m. Except for one drive, these troughs were all flatter than those predicted, with an average of 8 m and 24 m for shallow and deep drives respectively. This was because although the settlement above the centre line of the pipeline was less than the predicted value, the settlements at the left and right hand sides were greater than the predicted value. The average volume loss for the deep drives, 3.97%, was less than the predicted 4.46%, while for the shallow drives, the average volume loss, 4.86%, was greater than the predicted 3.15%. However, it is interesting to note that although the average volume loss was greater than that predicted for shallow drives, the measured settlements above the centre line of the pipeline were generally less than the predicted values as the widths of settlement troughs were underestimated. Besides the overcut of the TBM assumed in the calculation, other ground losses such as those discussed in the following paragraphs may also have certain contributions to such phenomenon. A number of drives for which the measured ground settlement exceeded the predicted value can be attributed to no slurry having been injected to balance the water pressure in the excavated face as a result of sudden change in ground conditions not immediately noticeable to the TBM operator. In some cases, the presence of hard materials in part of the excavation face caused face loss due to the adjacent soft materials moving faster into the excavation chamber of the TBM, resulting in more settlement. It was observed that large settlements usually occurred during hard and prolonged driving as a result of obstruction, the presence of existing underground voids, and the migration of the surrounding soil into the receiving shaft due to the failure of the treated soil block outside the receiving eye, when the TBM was approaching. Large settlement was also found at the location beyond the treated soil block outside the jacking shaft when the complete TBM had not been pushed into the ground and started to inject slurry to balance the total horizontal soil and water pressure in the excavation face. There were some occasions that voids were found at locations of significant settlement detected by the monitoring markers. A review on the records however did not indicate any abnormality in the pipejacking works, such as over-excavation or excessive loss of slurry. It was suspected that such voids existed in the ground for some time and the pipejacking works just triggered the incidents. The voids were then backfilled with sandy materials or grout before the resumption of works. The results of ground settlement monitoring for a pipejacking drive, that illustrated the presence of voids below the concrete carriageway, are shown in Fig 13, and the condition of a void is shown in Fig 14. For almost all the drives, ground settlement was negligible at a distance beyond half of the depth of the jacked pipeline ahead of the TBM. The Figure 13 Results of Surface Ground Settlement Monitoring for a Pipejacking Drive Beneath Voided Ground 37

8 Drive Diameter (mm) Depth to Invert (m) Total Length (m) Average Overburden SPT N Value at Shaft Location Ground Condition Encountered Measured Surface Settlement (mm) Min Max Average Predicted Surface Settlement (mm) Herrenknecht AVN 600 TBM WC8 - WC All soft Herrenknecht AVN 1200TC TBM NP2 - NP All soft NP2 - NP m soft, 10 m mixed and 20 m hard WC5 - WC m soft and 25 m mixed WC5 - WC All soft WC5 - WC5A m soft and 10 m mixed WC8 - WC All soft Herrenknecht AVN 1800T TBM WC2 - WC All soft WC2 - WC All soft NP5 - NP m soft, 130 m mixed and 84 m hard NP5 - NP m mixed and 175 m hard NP8 - NP m soft and 70 m mixed and 65 m hard NP8 - NP All hard NP15 - NP All soft Lovat mts 2000 TBM NP10 - NP m soft and 37 m mixed and 53 m hard NP10 - NP m soft and 10 m mixed NP12 - NP m soft, 30 m mixed and 20 m hard NP12 - PS All soft NP12 - NP m soft, 20 m mixed and 20 m hard NP14 - NP m soft, 53 m mixed and 119 m hard NP14 - NP All soft Average Value of All Drives 11.0 Table 4 Summary of Settlement Caused by Pipejacking Works settlement, however, developed rapidly after the TBM had passed through. An average of 74% and 77% of the settlement occurred during and immediately after the completion of pipejacking works for the shallow and deep drives respectively. However, due to the time required for dismantling the fixtures inside the completed pipeline, such as slurry charge and discharge pipes, electricity cables, water hose, ventilation hose, and the automatic lubrication system, further settlement developed as a result of the tendency of the ground to close in the annulus of the pipeline before the grouting material could be applied. It is noted that the settlement for shallow drives (which were mainly carried out in sandy ground) usually stopped within 3-4 weeks after the completion of pipejacking works. For deep drives in highly variable ground, particularly with clayey materials, the settlement would continue slowly over a much longer time. In some cases, more than 8-10 months would be required before the ground became stabilised. There is no direct relationship between jacking force and surface ground settlement as such depends very much on the material encountered in the excavation face, the type and density of material in the overburden, the surcharge load, and the TBM capacity. However, the presence of underground utilities and services above the jacked pipeline would lead to under-measure of the surface ground settlement. This could be because if settlement occurs, such utilities and services, due to their rigidity, may not settle in the same magnitude as the underlying ground. It is noted that the settlement increased slowly under a high jacking speed. 38

9 locations of desilting, access and ventilation openings, can be tailor made to suit site constraints. If precast rings are used to construct the shaft wall, the construction time could be reduced significantly. However, watertightness at the successive construction joints under a high water head should be looked at, as the water stop that can be installed in-situ is usually small and does not have an embeddment in concrete. This may require a mass concrete wall outside as a compensation measure. The access, ventilation and desilting openings of permanent shafts should be located in one traffic lane as much as possible to minimise the disturbance to traffic during maintenance. Figure 14 Condition of a Void below Concrete Carriageway Suggested Areas for Improvement Alignment Design Notwithstanding maintenance issues for the permanent works, the minimum radius of a curved section is governed by the allowable angular deflection at the pipe joint for the respective pipe size. Therefore, the larger the pipe size is, the smaller would be the allowable curvature of the pipeline in order that its structural integrity and watertightness can be maintained. Sufficient straight section, say 15 m, should be allowed between any two curves for proper transfer of load. A straight section is also required outside the jacking shaft to enable the launching of the TBM to be completed in ground before the steering jacks can be operated to change alignment. TBM Design The performance of disc cutters affects the TBM s capacity of excavation and rate of advancement. The cutter change in the excavation chamber is labour-intensive, costly and, particularly in soft ground, a risk for the maintenance team. It is important that rock cores are obtained for determination of their strength before finalisation of the design of the disc cutters. It is also important that all the disc cutters, particularly those in the centre, can be replaced at the rear of the cutting wheel, in the middle of the drive, at the right time. Recycled discs made by local shops should be used with care due to quality control. It is understood that TBM manufacturers have developed a fully automatic hardware and software system for detecting hard material in front of TBMs by means of sonic soft-ground probing. This system probes ahead for density contrasts in the soil and visualises the results (Fig 15). With these results, the TBM operator can plan the works in advance and adopt corresponding measures when the TBM is moving close to the obstruction. Hydraulic Design A marked difference in invert levels for incoming and outgoing pipes in permanent shafts, of say 200 mm, should be allowed to avoid the possible backflow situation due to the out-of-tolerance problem, if any, in the downstream pipeline. Smaller sewers with steeper gradient should be used instead of large sewers with flatter slope to ensure flow capacity and self-cleansing velocity, due to the likely irregular profile after the completion of pipejacking works. Pipejacking Design The main characteristic of the Hong Kong soils is their high variability over a short length. Appropriate type and mix of bentonite or polymer should be used as lubricant and adjustment made immediately, for the ground encountered in order to reduce the frictional force along the jacked pipeline and to stabilise the overcut formed by the TBM, minimising the ground settlement. For long drives, the main jacking station always experiences high jacking force to push the pipeline forward, particularly when excavating in hard rock. The installation of an intermediate jacking station near or at the rear of a TBM (if no telescopic section is equipped), in addition to others provided in the pipeline, would ensure sufficient capacity for excavation. This would also help to minimise shock loading in the cutting wheel during operation, which is liable to create excessive wear and/or damage to the disc cutters. Permanent Shaft Design Rectangular shafts are easier to construct than circular shafts, at locations where existing utilities and services are congested. Their size and shape, in particular the intermediate landing platforms, benching details and Figure 15 Detection of Hard Material in front of TBM by Means of Sonic Soft Ground Probing (Courtesy of Herrenknecht AG) It is also understood that an electronic tool monitoring system, by means of a sender electrically connected with a power supply, can be installed in the disc cutters for detecting the wear limit. This could warn the TBM operator in time, such that the maintenance intervals can be controlled economically, cutter life used optimally and labour cost for time-consuming interventions reduced. As-built Records for Pipejacking Drives The as-built records showing the jacking force, torque pressure, speed, level, line and ground composition, for three pipejacking drives under soft, mixed and hard ground conditions respectively, are shown in Figs 16 to 18. Options of TBM Driving The following options of TBM driving (for both straight and curved pipelines) have been successfully completed in the contract. 39

10 Figure 16 As-built Records for Pipeline Driving through Soft Ground Conditions Figure 17 As-built Records for Pipeline Driving through Mixed Ground Conditions With Completion of Jacking and Receiving Shafts in Advance (For Normal Driving between 2 Shafts) (Fig 19) This is the traditional method, in that the pipeline will only be jacked after the completion of the two temporary shafts at each end. This would ensure the watertightness between the permanent shaft and the completed pipeline. With Completion of Intermediate Temporary Shaft in Advance (With 3 or More Shafts in a Line) (Fig 20) This would require the construction of the receiving and launching eyes at the piles of the temporary shaft before the jacked pipeline passes through. The condition of the TBM can be inspected in the shaft and the damaged disc cutters changed before the pipeline is pushed forward. The shaft is then flooded with water to balance the groundwater pressure inside and outside, and a double layer rubber seal is installed at both eyes to minimise the loss of lubricant and migration of the fines in the surrounding soil when the jacking continues. A wooden planking system is provided on top of the pipeline to prevent floatation. It is, however, noted that in so doing, the friction resistance on the pipes would be increased. Alternatively, the temporary shaft could be temporarily backfilled for such purposes. The benefit of using this option is the flexibility of programming which could make the re-sequencing of works easier should any site problems be encountered. In addition, there will be a cost saving in not moving and setting-up the TBM plant at the intermediate temporary shaft location. Without Completion of Intermediate Temporary Shaft in Advance (With 3 or More Shafts in a Line) (Fig 21) When timing is a key factor for completing the works, the jacked pipeline could pass through an intermediate shaft, without completing its construction in advance, ie excavation. In order to achieve this, the piling works for the shaft are completed first, leaving a window at the incoming and outgoing sides of the pipeline where the ground outside is fully stabilised by grout (from about 1 m above the crown level of the pipeline to the same toe level as the piles), so as to achieve watertightness. Immediately after the completion of the jacking operation, the cavities between the jacked pipeline and the grout zones (due to overcut of TBM) are to be filled up by bentonite cement grout from the jacking pipes, so as to close them up and prevent ingress of water. The shaft excavation can start at any time, after the jacking operation has been completed. Steel lagging plates are provided at the window locations while excavation proceeds. The pipeline inside the shaft is then cut for the construction of the permanent shaft. In the worst scenario, the jacked pipeline can also be completed before the pile installation, particularly when waiting for the timely diversion of 40

11 existing utilities/services to give room. This would also give the option to move the shaft location along the completed pipeline if any unexpected situations arise; however, there may be difficulty to effectively treat the ground at the window locations. With Completion of Permanent Shaft(s) in Intermediate Temporary Shafts and Receiving Shaft in Advance (With 3 or More Shafts in a Line) (Fig 22) The reinforced concrete structure for the permanent shaft is completed first, with the receiving and launching openings prefixed and formed. A mass concrete filling is then provided across the shaft to confine the alignment of the jacked pipeline and prevent the escape of lubricant when the pipeline is passing through. This could also enable the inspection of the condition of the disc cutters and the necessary replacement. The gap between the temporary and permanent shafts at the opening locations also needs to be sealed up by concrete to prevent ingress of water causing drawdown outside the temporary shaft and then attracting unnecessary ground settlement. The speed of the TBM, when close to the temporary shaft, is to be reduced and the position of the TBM is double checked and adjusted, as necessary, before the TBM is pushed in. The jacking force and the speed of driving is controlled in such a manner that the vibration induced (generally less than 25 mm/sec based on the measured data on three drives) will not damage the permanent shaft. Figure 22 Pipejacking through an Intermediate Shaft with Completion of Permanent Shaft Figure 18 As-built Records for Pipeline Driving through Hard Ground Conditions Figure 19 Pipejacking Carried out between the Completed Jacking and Receiving Shafts Figure 20 Pipejacking through an Intermediate Shaft with Completion of Temporary Works in Advance Upon the completion of the pipejacking works, the gap between the permanent shaft and the pipeline (with Hi-rib, which is a special type of construction joint material, provided to increase bonding) is properly sealed up by non-shrinkage epoxy grout. For the case in the receiving shaft, the internal size of the permanent shaft must be large enough to allow the removal of the longest section of the TBM, and the internal fixtures and top slab of the shaft are to be completed later on. This option can be adopted effectively at shaft locations where public complaints in respect of lengthy occupation of works area causing loss of business and affecting environments are a major problem. Such would allow the construction of the permanent shaft immediately after the completion of the intermediate shaft, the reinstatement of the occupied area (except for the receiving shaft location where the top slab of the permanent shaft is to be placed after the retrieval of the TBM) and the release of the site to the public, at a much earlier stage. There is no restriction on the number of intermediate shafts that the TBM and pipeline can pass through. However, typical TBMs could generally operate 4 nos intermediate jacking stations. Therefore, the TBM should be specially designed for long drives such that sufficient nos of intermediate jacking stations can be installed and operated, to account for the anticipated ground conditions and the length of driving. Conclusions Figure 21 Pipejacking through an Intermediate Shaft without Completion of Temporary Works in Advance Certain observations and conclusions may be drawn from the experiences gained on this contract. 1) The average production rate for the drives completed by the 4 nos TBMs ranged from 2.5 to 14.7 m/day. For planning purposes, a production rate of 4 m/day is considered to be a reasonable lower 41

12 bound figure for the commonly encountered ground conditions in Hong Kong. 2) More than 20% of the deep sewers exceeded the contract tolerance for line and level, despite the fact that the excavation face had been conditioned by slurry and sometimes by the addition of polymer, as necessary. In consideration of the steering capacity of the TBMs available in the market, the allowable angular deflection of the jacking pipes, and the procedure of correcting the out-of-tolerance, a tolerance of 50 mm for line and 35 mm for level may not be applicable to all Hong Kong soils for pipejacking works. Therefore, the respective tolerance should only be used with extreme care, with adjustment as necessary. 75 mm for line and 50 mm for level could be a more realistic tolerance that the TBM deployed can practically control the alignment of pipeline in the ground likely to be encountered. 3) Empirical values for estimation of the friction on pipeline are conservative. The jacking resistance could be significantly reduced by application of suitable bentonite and polymer lubricants. However, tunnelling through a highly variable ground, particularly with irregular, alternating soil and rock composition, increases the excavation resistance significantly. For design purposes, a recommended value of 6 kn/m 2 should be sufficient for the highly variable ground conditions in Hong Kong. 4) With the proper application of slurry to balance the excavation face, ground settlements associated with the pipejacking works can be generally controlled within 20 mm for typical ground conditions in Hong Kong. However, settlements could increase significantly when obstructions are encountered. Large settlements could also occur when the TBM stops in ground for a long time for repair or replacing disc cutters. 5) In this contract, settlement values predicted by the invert normal probability distribution curve, with the TBM overcut as the assumed volume loss, were generally higher than the average measured settlement at the centre line of the pipeline for most of the drives. The widths of the settlement troughs for both deep and shallow drives were however underestimated. For shallow tunnelling through sandy ground, the settlements developed rapidly and should be completed within a month while in deep tunnels or clayey ground, the settlements could take a much longer time for completion. 6) The chemical composition of the disc cutters was not made available due to commercial reasons. Given the different degree of wear and damage in the disc cutters, including fractured rings, the difference in rock penetration over the length of driving, and the frequency of replacement, with respect to the ground conditions encountered, it appears that the cutters may be too brittle for the nature of the rock. This situation could be improved by coring rock samples at different locations of the site for laboratory analysis of their strength before finalising the disc cutter design. 7) For large contracts involving long drives in highly variable ground, it is preferable to install a fully automatic sonic soft ground probing system for detecting hard material in front of TBMs and an electric tool monitoring system for detecting the wear limit of disc cutters, such that the risk associated with TBM stoppage, as a result of obstruction, could be minimised, and the replacement of disc cutters could be made at the right time. 8) All 4 nos TBMs had a different degree of deviation in line and level in the pipelines completed. By virtue of the ground conditions encountered, a TBM with 4 nos steering jacks appears to have better control in the curved alignment in most cases, particularly under mixed to hard ground conditions. 9) There is no indication that the performance of straight drives is superior to curved drives. The only noticeable exception is that straight sections have better control in line and level than curved sections. 10) Four options of TBM driving had been successfully adopted for different pipejacking drives (with both straight and curved sections), under different ground conditions. This gives clients and contractors flexibility in designing and programming the works, particularly when site constraints are encountered. 11) Based on the problems encountered and the lessons learned, certain areas for improvements are suggested. It is believed that, through the proper implementation of such improvements, the performance of pipejacking works could be greatly enhanced. Acknowledgements The authors wish to express their gratitude to the Drainage Services Department of the Government of Hong Kong SAR, for permission to extract the materials from the respective Project, to publish this paper. The assistance provided by Mr Derek Arnold, Director of Black & Veatch Hong Kong Limited, and Mr K C Leung and Mr Ivan S W Wong of the Resident Site Staff in analysing the field data, are also appreciated. Special acknowledgement is given to Herrenknecht AG for his permission to extract the photographs from technical brochures in making Fig 15. References 1. Thomson, J., Pipejacking and Microtunnelling, pp. 182, Black A&P., U.K., (1993). 2. The British Tunnelling Society and The Institution of Civil Engineers, Specification for Tunnelling, pp. 36, & 106, Thomas Telford, U.K., (2000). 3. General Specification for Civil Engineering Works, Volume 1, pp , Civil Engineering Department, Government of the Hong Kong SAR, Hong Kong (1992). 4. Field data of DSD Contract No. DC/95/05 Central, Western and Wan Chai West Sewerage Trunk Sewers. 5. Field data of DSD Contract No. DC/98/06 Aberdeen and Ap Lei Chau Trunk Sewers. 6. Mok, W. W.S., Performance of Trenchless Techniques for Sewer Construction in Hong Kong, HKIE Transactions, Volume 9, No. 1, pp , The Hong Kong Institution of Engineers, Hong Kong (2002). 7. O Reilly, M.P. and New, B.M., Settlement above Tunnels in the United Kingdom, Their Magnitude and Prediction, pp , Proceedings Tunnelling 82 Symposium, Institute of Mining and Metallurgy, U.K. (1982). About the Authors Wilson W S MOK BASc BA CEng RPE(Civil & Geotechnical) CPEng CSci MICE MIMMM MIEAust MHKIE wilsonwsmok@netvigator.com Ir Wilson Mok graduated from the University of Windsor in Canada and has over 28 years practical working experience in a wide variety of geotechnical and civil engineering projects in both design office and site. He is particularly experienced in dealing with design and construction associated with geotechnical investigations and instrumentation, deep excavations, tunnels, ground improvements, settlement analysis, reclamation, site formation, slope preventive measures, foundations and sewerage works. He has been involved in the design, supervision and administration of more than 16 km of trunk sewers construction in the urban areas of Hong Kong, using different types of trenchless techniques from relining, TBMs to hand tunnelling, and has carried out extensive research on such. He is currently employed by Black & Veatch Hong Kong Limited as Senior Resident Engineer on a DSD contract. 42

13 Maxwell K W MAK BSc Dip HE Delft PCLL MA Arb CEng MIEAust MICE MHKIE MIHT kwmak@dsd.gov.hk Ir K W Mak is currently Chief Engineer in Drainage Services Department. In the past few years, he has participated in numerous large-scale and prestigious drainage and sewerage projects, including the Wan Chai East and North Point sewerage project, the first tertiary sewage treatment plant at Ngong Ping, Lantau Island, as well as the 6 km long rainwater-intercepting tunnel in Tsuen Wan. Previously, he spent six years in the then Works Bureau dealing with slope and water policies. During his earlier years with the Drainage Services Department, Ir Mak received post-graduate training in river engineering in the Netherlands, and had then worked in the Shenzhen River Regulation Project. He obtained a Master of Arts in the area of dispute resolution and arbitration. On top of his engineering engagements, Ir Mak acquires legal qualification and is a non-practising Barristerat-law of the High Court of the HKSAR. Felix H T POON BEng MSc CEng MICE MHKIE poonfelix@gmail.com Ir Felix Poon graduated from the University of Hong Kong with First Class Honours and joined Black & Veatch Hong Kong Limited in Since then, he has participated in various projects including design and construction of pipe-jacked sewage tunnels, deep shafts, water transfer tunnel, box and large diameter pipe culverts, river channels and other drainage and sewerage features. He had been actively involved in DSD Contract No DC/2000/11: Wan Chai East and North Point Sewerage Trunk Sewers, during the stages from design, tendering to construction, in that he was posted as an assistant resident engineer responsible for supervision of the respective construction by trenchless technique, and the analysis of the field data obtained from the works. He is currently an engineer in the design office working on a drainage improvement project in Northern New Territories and a water transfer tunnel project on Lantau Island. 43