Ground improvement concept and selection for an onshore gas supply facilities in Malaysia

An onshore natural gas supply facility is proposed to be constructed in Bintulu, Sarawak to meet the local industrial demand. The project site is mostly flat, with compressible layer identified at several locations. To mitigate the risk of uneven settlement, RC piled foundation is selected to support facilities on the soft ground. However, for infrastructure such as roads, drains, and underground firewater lines, fully piled foundations will drastically increase the project duration and cost. Alternative mitigation measures to address the issues shall not hinder the access for other activities and construction sequence. In this paper, the approach taken to mitigate site-specific subsurface conditions is explained. Readers will be guided through the stages of ground improvement, from the inception of the project, the methodology used, analysis conducted, assessment and the way forward. Several options were explored considering factors such as estimated consolidation period, depth and rate of settlement, and stability analysis etc. The Remove and Replace, was selected as most suitable solution. Preliminary field results show that the targeted settlement at some areas were achieved earlier than anticipated. This enable the field installations such as firewater pipe to be laid earlier, contributed to the good project progress. Monitoring program is continued to ensure all identified areas met the accepted criteria.


Introduction
A proposed gas plant facility is intended to be constructed at Bintulu, Sarawak.The site is an undeveloped forest with uneven terrain and a water body.The site is generally flat, with the existing ground level ranging from RL2.5m to RL15.31m.The higher ground elevation is observed in the southwest of the project site, and the ground slopes towards the lower ground at the east side.The topography survey plan shows two swampy areas, with the first area located at the west side, the elevation ranges from RL8.51m to RL9.50m.As the proposed final platform level is at RL8.0m, no backfilling is required at this area.On the other hand, the second area is located at the east side, the elevation ranges from RL1.62m to RL4.67m, thus backfilling is required.For more details, refer to figure 1 on the site location and its specific features.
To accommodate the plant layout and equipment arrangement, the final platform level is set at RL8.0m at the south and southwest, and it slopes down to RL5.7m towards east and southeast of the project site, as depicted in figure 2. Based on the SI data, the soft ground area is located at the east side of the plot, as shown in figure 3. The soft ground area is mainly composed of soft compressible soil, organic materials, peat, clay, and decayed wood.The remaining plot area generally comprises residual soil with hard layer (SPT-N value more than 50) at shallow depth.
The soil at the upper layer of soft ground has high organic content with low shear strength.Decomposition of the organic material can cause settlement over the time.Apart from that, below the ground, there are thick and soft compressible soil layer ranges from 3.0m to 11.5m which also can cause soil settlement.Therefore, ground improvement is necessary to prevent excessive soil settlement which may affect the integrity of the plant facilities.Zoning of soft soils is shown in figure 4.This is determined based on the soil profile as shown in figure 5

Objective
To address the soft ground issue, the ground improvement is required to prevent excessive soil settlement which may affect the integrity of the plant facilities.The selection of ground improvement method shall meet project schedule by considering construction work sequence and activities.

Methodology
With the available soil investigation (SI) information and data, soft ground areas are identified.The following workflow is established to evaluate, assess and determine on the fit-for-purpose ground improvement option.

Assessment criteria, selection & ranking
The following assessment criteria crafted to evaluate the options to meet the objectives.a. Technically acceptable (i.e., meeting project technical requirement) b.Practicality to site specific (i.e., availability of resources, equipment, competency etc) c.Overall project schedule (i.e., sequence of work and its impact to overall project activity & schedule) d.Others (i.e., Typical advantages & disadvantages, design impact etc) From the above assessment, option 2, i.e., R&R with counterweight berm was selected in view of schedule advantage and meeting all the technically requirement.Detailed analysis was performed to determine R&R thickness based on expected consolidation duration and total soil settlement depth.

Analysis
Analysis was performed for option 2. The soft ground was further clustered to few zonings, i.e., G5a, G5b and G5c, as shown in figure 4, based on the average thickness of soft compressible layer.For analysis purposes, the zoning was further divided based on platform level & the required net fill height (NFH) as shown in table 2.
Table 2. Soft soil zoning with platform level & NFH [2] .Settlement analysis and stability analysis were performed in accordance with design criteria stipulated in Project Technical Specification.

Zone
For settlement analysis, the Terzaghi 1-Dimensional Consolidation Method [1] based on the result of oedometer test was adopted in estimating the settlement for different NFH by the following equations: Figure 9. Net fill height (NFH) equation [5] For stability analysis, the stability of the embankment was assessed using limit equilibrium analysis [1] with consideration for different potential failure surfaces (circular and block failures) by using Spencer's Method [1] and Plaxis 2D software.The stability analysis was performed for both Section 1 and 2 as indicated in figure 4. The following cases in table 3 were considered for the stability analysis.
Table 3. Stability analysis by cases with target minimum FOS [2] .

Stability analysis
Stability analyses were performed for R&R thickness of 1.0m, 1.5m and 3.0m.From the assessment, R&R thickness of more than 1.5m would pose threat to stability of adjacent structures, challenges in constructability, mainly due to groundwater intrusion as the project site was adjacent to a river and near to the coastline.Deeper soil replacement work would not be cost effective as well.In contrary, with R&R thickness of 1.0m, it was unable to achieve the required FOS for stability.R&R thickness of 1.5m was therefore selected as optimum replacement depth.Both global stability check (slip circle) and localised stability check of the slope were evaluated.Analysis results for R&R of 1.5m were explained in the following paragraph.
Refer to figure 10 and figure 11 below on the analysis location for Section 1. Global stability represented the whole embankment stability from top of the whole embankment to its toe if failure would impact the whole embankment structure, whilst local stability was at limited portion of the slope i.e. within its toe or top of slope only and the impact failure will be at localised area only.Both stability check using the same concept of identifying the potential slip circle and evaluate the factor of safety of the critical slip circle.Since the local slip circle is too small, the slip circle could not be seen in the diagram below.Table 5. Factor of Safety for embankment stability [1] .

Section
Section 1 (Pipe rack Area 5.7mRL) Based on the summary of analysis result as shown in table 5, the R&R thickness of 1.5m was adopted as the factor of safety are meeting the requirements.

Field implementation
Unsuitable soil at zones G5a, G5b and G5c were excavated and removed to the designed level.Controlled filling was carried out at soft ground area with counterweight berm to enhance the stability of the embankment permanently.Following activities were implemented at site: a. Controlled filling i.
Fill material was spread in layers of 300mm maximum thickness of loose material before compaction.ii.
The filling at site shall not exceed 0.8m high per week.iii.
During filling, the ratio of (δ / ρ) which is the ratio of lateral displacement (δ) over settlement (ρ) shall not exceed 0.2 [1] .If δ / ρ reached 0.2, the filling at site shall stop and the Engineer shall be informed immediately.b.Settlement Monitoring Rod Settlement Marker (RSG) and inclinometer were installed at site to monitor the actual settlement rate.

Preliminary settlement monitoring result
Settlement monitoring instruments were installed at different locations to assess the settlement behaviour of soft ground with the embankment fill.Settlement monitoring result is shown in table 6.The measurement was taken to check the degree of consolidation status and to correlate with the estimation in the settlement analysis.6. Settlement monitoring result [1] .*RSG that found damaged and new RSG to monitor the settlement.** Completed means meeting the settlement rate of <2.0mm/day [3] ***RSG-6, RSG-8, RSG-13 & RSG-14 data are not tabulated due to located beyond the ground improvement zone

Conclusion
From the preliminary results, area G5a, where underground firewater lines are to be laid, had achieved the required settlement rate of < 2mm/day [3] , thus it is ready for the installation of firewater line.The selected R&R with counterweight berm ground improvement method has achieved its objective.Soil settlement monitoring program is to be continued to record the long-term settlement.

Figure 3 .
Figure 3. Extend of soft ground area and the identified assets within the zone.

Figure 13 .
Figure 13.Local 1 and Local 3 embankment stability analysis for Section 2.

Figure 14 .
Figure 14.Local 2 and Local 4 embankment stability analysis for Section 2.

Table 1 .
Technology, option assessment, ranking and selection.
a Based-on contractor's previous experience