of geotechnical investigations

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1 The importance of geotechnical investigations Michelle Wagner, Bob Kruger, Rebecca Zackavec, Marty Tellalian and Mark Butts, CB&I, USA, emphasise the importance of early and comprehensive geotechnical investigations for LNG tanks. A s global demand for LNG escalates, a critical component of the rapidly expanding infrastructure system is an accurate and cost effective storage tank design. A large contributing factor in both the tank and foundation design is the site s soil and seismic conditions. As such, a compelling argument can be made for a deliberate and attentive approach to geotechnical investigations. Done early in the project development lifecycle, a complete investigation can benefit both owners and tank contractors. It is therefore imperative that all stakeholders are aware of what comprises a complete investigation, when the investigation should be performed, and how each entity works together to create the tank and foundation bid design. Understanding these areas will highlight how fewer assumptions during the bid process enables value based engineering, allows for more accurate schedule and cost estimates, and prepares owners and project developers to make informed decisions when evaluating and selecting a tank contractor s bid. The role of geotechnical investigations An in depth geotechnical investigation focused directly beneath the tank footprint provides essential information, ultimately Reprinted Reprintedfrom from determining the impact that the local geology and seismicity will have on the final tank and foundation design. Properly done, this detailed investigation enables the contractors bidding on the storage tanks to answer the following necessary questions: nn What is the expected uniform and differential settlement? nn What are the possible alternatives for foundation and tank design? nn Are there any obstacles or features of the site that would impact the design or determine construction methods? nn What is the site s seismic classification, the corresponding liquefaction potential, and most efficient tank geometry? Geotechnical investigation components The information required to answer these questions is derived from the three components of a geotechnical investigation: nn The soil data report. nn The interpretative soil report. nn The site specific seismicity study. Combined, these three components create the geotechnical investigation. Because each section provides unique information, a cost effective and reliable

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3 Figure 1. Example soil profile at the location of an LNG tank. Figure 2. Example boring samples between depths of m. tank and foundation can only be designed if all components are complete. Soil data report The first component, the soil data report, consists of tabular and graphical results from both field and laboratory tests. The report includes the properties of each soil layer beneath the tank and within the zone of influence. Indicated on Figure 1 as a blue dotted line, the zone of influence encapsulates the soil impacted by the surface load exerted by the tank. Soil in this area can be prone to excessive consolidation if unable to bear the imposed tank loads. This consolidation is of particular concern when a tank settles unevenly, otherwise known as differential settlement. As this occurs, stress and deflection increase at the risk of compromising the tank s structural integrity and insulation systems. As differential settlement becomes more extreme, the potential for permanent damage to the tank increases. Since the foundation design can accommodate a certain amount of settlement, the soil properties quantified in the soil data report are crucial when calculating the predicted soil consolidation. These properties include the gradation, relative in place densities, consolidation rates, density and moisture content of each soil type, and depth of each soil layer. However, these samples, most commonly obtained from borings, must meet a variety of requirements. First, the depth of the samples must sufficiently cover the zone of influence, usually times the diameter of the tank. As LNG storage tanks increase in size, required boring depths will increase as well. Of the borings illustrated in Figure 1, only Borings 1 and 2, at 115 m and 125 m respectively, would fulfill this requirement for an 85 m diameter tank. Shallow borings are unable to sample the deeper soil layers and will reduce the accuracy of settlement calculations. Furthermore, the borings must be located directly beneath the tank footprint. While Boring 4 in Figure 1 is deep enough, its soil profile does not represent the soil underneath the storage tank. For example, Layer H, present in Boring 4, is not found in Borings 1 and 2, whereas the opposite is true of Layer I. Lastly, for a complete soil profile, enough borings must be taken within the tank footprint. This captures the potential variations in both soil types and soil layer depths that may exist beneath the tank. As shown in Figure 2, within a range of a few meters, noticeable differences in soil composition are apparent; these variations can translate to stressful differential settlement across the tank. By taking borings at the locations suggested in Figure 3, differential settlement can be calculated without excessive extrapolation. This allows the tank contractor to adjust the foundation design by varying certain pile lengths or using localised ground improvement. Though Figure 3 provides guidance on the spacing between borings, each project site is unique, and the appropriate location and depth for each boring will differ depending on site conditions. Therefore, enough flexibility should be given to the geotechnical engineer to collect samples that will adequately cover the demands of designing both the foundation and tank. Interpretative soil report The interpretative soil report is the second component of a geotechnical investigation and is built on the results captured in the soil data report. The interpretative report provides a qualitative analysis and discussion of the subsurface conditions, as well as the geotechnical engineer s observations and recommendations for foundation types and design parameters. Additionally, the soil report will provide direction from the geotechnical engineer to the tank contractor regarding alternative Reprinted from 2015

4 foundation options, such as ground improvement. This sophisticated analysis offers cost effective foundation possibilities that are not readily apparent when reviewing the raw information in the soil data report. Site specific seismicity study The final component of a geotechnical survey is a site specific seismic investigation and report, which outlines the magnitude of the ground motions from seismic activity and the influence of the soil. The seismicity study defines the ground motion hazard for the project location, determined by analysing many variables, including but not limited to regional seismicity, subsurface soil conditions, and proximity to known seismic faults. The study also assigns a classification from A - E, where Class A represents a site on hard rock and Class E represents a site on soft clay soil. As soils transition from stiff to loose, they are increasingly more prone to seismic instability and lateral motion. Additionally, the study provides information to quantify the liquefaction potential of the site, or the ability of the soil to shift and consolidate due to seismic activities. This calculation, derived from the provided acceleration rates and raw soil data, is then used to finalise the foundation design in conjunction with the settlement calculations. Timing of a geotechnical investigation The best timing for a complete geotechnical investigation is after the conceptual project design has been finalised, but prior to inviting tank contractors to bid designs. Early in a project development lifecycle, during the project conceptualisation and feasibility analysis, a preliminary geotechnical assessment will be performed as shown in Figure 4. At the most rudimentary level, these assessments commonly consist of a visual inspection of the site and a review of any previously documented information. While this assessment assists in determining plausible terminal or facility layouts, it is inadequate as the basis for storage tank designs or firm pricing during the bid stage. Furthermore, prematurely performing an investigation will result in data not specific to the tank s size, location, and zone of influence, thereby rendering it marginally useful. Rather, the complete geotechnical investigation, as indicated in Figure 4, should be performed so the results can be included with other customer specifications within the bid package. Delaying the availability of this information, to either later in the bidding process or even after awarding the contract, severely limits the ability to develop innovative technical solutions or pursue value based engineering. For example, soil improvement strategies, such as vibro compaction, can eliminate the need for a deep foundation, shortening the foundation construction schedule and decreasing material and equipment costs. However, to pursue this option, time during the bidding stage is needed for the tank contractor to locate and obtain estimates from specialty geotechnical subcontractors. Therefore, these potentially cost saving alternatives can only be achieved when complete geotechnical Figure 3. Suggested Boring spacing. information is made available as part of the bid package to allow sufficient time to create accurate designs. Utilising one geotechnical contractor To maximise the benefits from this early and complete investigation, a single geotechnical engineer should be used to conduct the full investigation. When third party geotechnical engineers must be located and contracted to perform the specialised skill of analysing the soil data, precious schedule time is lost while waiting for the supplementary reports. If a single party is used, fewer design basis assumptions are made, which results in more time to pursue value engineering options that benefit both the owner and tank contractor. Additionally, this approach improves consistency across the investigation and reduces the number of assumptions within each bid s design basis. Because third parties will produce reports and base recommendations on data which they did not originally collect, these recommendations may not be accurate, being either overly conservative or overly liberal. This can translate to a design inadequate for the location. The impact of assumptions on design, decisions, cost and schedule These incorrect assumptions can arise from incomplete investigations, late availability of information, and components generated from multiple parties. For example, if borings are not spaced adequately to cover the zone of influence, the soil profile can become overly generalised, resulting in a risk prone liberal design or a costly conservative bid. Incomplete seismicity studies can indicate a higher classification rating, such as D or E, and limit the tank geometry to a more expensive, yet sturdier, short and wide tank. Problematic to owners, these assumptions cloud the waters when deciding between bids. As layers upon layers of assumptions are made, the ability to identify what is assumed and what is known is hindered. Cost and schedule estimates decrease in Reprinted from 2015

5 Figure 4. Project development lifecycle and optimal timing of geotechnical investigation. accuracy while project risk increases, which makes awarding a contract significantly more difficult. Instead of an owner deciding between the most optimal designs with the best budget and schedule, they are instead betting on which tank contractor makes the most accurate assumptions. For example, assume only Borings 3 and 4 in Figure 1 were done prior to invitation to bid. A conservative assumption made by one company would include Layer H beneath the tank. If Layer H is a soft soil such as peat, the tank would be expected to experience large settlements, a high liquefaction potential, and would be assigned a Site Class E. All of these factors would require either a deep foundation with multiple piles or a short tank configuration. However, when compared to the actual soil conditions present beneath the tank, i.e., no Layer H, assumptions based on Borings 3 and 4 are much too conservative. Because more piles or a shorter tank are more costly and require longer construction schedules, the owner pays more for an unnecessary design. On the other hand, if Layer H was present beneath the tank, but a tank contractor assumed otherwise, the resulting tank and foundation would be under designed for the site. However, this error would not be discovered until after contract award requiring a new tank design, extending the engineering and construction schedules, and exceeding estimates and costs. By providing information to create an equal beginning design basis, fewer assumptions will be made about the soil profile and its expected performance, giving the owner a clearer understanding of the contractors bids. Potential savings of early action and communication Early timing of a comprehensive geotechnical investigation is crucial not only to reduce assumptions in design, but also to avoid the costly follow up investigation to fill information gaps. If adequate information is not provided during the bid process, a necessary additional survey is done after awarding the contract to complete the design. These additional surveys can add significant cost (potentially up to US$ ) and time (2-3 months at the front end of the engineering schedule) to the bid cycle. Additionally, lack of sufficient soil information can result in critical conditions being overlooked. Voids, soft soils, or buried debris can delay the construction schedule. On the contrary, if conditions such as those are known earlier via a complete geotechnical survey, mitigation strategies can be pursued sooner in project schedules at a lower cost. Altogether, the potential for cost and schedule savings can be significant. Though dependent on the site s conditions, a deep foundation design resulting from minimal or delayed information can require a 3-5 month construction schedule. Comparatively, a value design, such as a shallow foundation with ground improvement, may only take 2-3 months. Furthermore, the cost differential between these two designs may range from US$3-7.5 million/tank. Therefore, the potential combined savings to the owner could include: three months in the construction schedule, and any additional weeks saved by discovering obstacles prior to the start of construction, 2-3 months in engineering, up to US$ for avoiding a secondary investigation, and possibly up to US$7.5 million/tank as a result of value engineering. Overall, investing in a single and complete survey produces a positive return for both the project s finances and schedule. Minimising risk through geotechnical investigations The quality of the geotechnical investigation directly affects the accuracy of the tank and foundation designs, and therefore, the accuracy of the bid packages provided to the owner and project developers. Without complete information, possibly inaccurate assumptions must be made, which may be too liberal or conservative. Even supplementing client provided information by using a third party introduces new variability into the design basis and complicates the owner s decision. Ultimately, a complete, early, and singular party geotechnical investigation allows for a solid design basis that is common to all bidders. By eliminating design basis assumptions, owners are able to make more informed decisions when awarding the contract. Additionally, by eliminating the need to perform late investigations, problems are addressed earlier and more efficiently, engineering and construction schedules are shortened, and value engineering is pursued to create a cost efficient design. Furthermore, the potential for expensive and time intensive design adjustments after contract award is curtailed and minimised. Each of these benefits represents a reduction of risk to the project, to the owner, and to the contractor. By taking action early in the project development lifecycle, all involved stakeholders are able to contribute to a more efficient and accurate design process. T&T Reprinted from 2015