Infrastructure & site preparation

It is customary to first draw up a final design (FD) for such projects.  For large and/or complex projects, a preliminary design (PD) may be drawn up before the final design. 

The purpose of a final design is to determine the geometric dimensions of the design so that on completion of the project/in its final state, the requirements set with regard to, for example, stability and (delivery) height are met. 

When the final design is completed, a design for the construction phase is generally drawn up to execute the final design drafted.

The purpose of the design for the construction phase is: 

• To prevent instability of the underground and the embankments during the execution/realisation of the project. To ensure that sufficient excess height is present upon completion of the project so that there is adequate height over the lifespan (water barriers), that the residual settling requirement set (preparation for construction) or the requirements with regard to longitudinal and transverse flatness are met (roads and railways)
• To dimension the provisions that are necessary during the execution/realisation of the project in order to achieve the aforementioned goals, and to dimension provisions that ensure that the impact on the surroundings as a result of the execution of the project does not exceed the limits set. On the basis of the design for the construction phase, an execution plan, inspection plan and a monitoring plan can then be drawn up for the site engineering during the execution of the project

SOCOTEC can assist you with all the aforementioned design phases and with the site engineering. We have around 30 years and 20 years of experience respectively in the aforementioned areas, working for contractors, central government (public works and water management, water boards and local councils), engineering firms and project developers. 

Slope stability

By elevating (or placing a load on) the ground, or excavating (for example a construction pit), the balance of forces in the underground changes at the transition between the embankment (load)/excavation and the original ground level. This creates (additional) shear stresses in the underground. If the maximum shear stresses that can be absorbed by the underground are exceeded, the underground will collapse and become unstable.

Stability analyses can be useful for every phase of your project:

• In the design phase (final design) to determine the geometry (which slope, dimensions of any stability berm, etc.) so that a design can be obtained that meets the specified standards for final stability
• In preparation for the realisation phase (design for the construction phase), to determine the rate of elevation at which the embankment can be constructed in phases without instability occurring in the underground and possibly causing damage to the surroundings
• During the realisation phase itself (see Site Engineering)

With regard to the aforementioned provisions, depending on the phase of the project and on the circumstances, consider the following:

• Reduction of the angle of the slope (final design, design for the construction phase, realisation phase)
• (Temporary) installation of a stability berm (final design, design for the construction phase, realisation phase)
• Install vertical drains (design for the construction phase, realisation phase)
• Soil reinforcement work, for example in the form of a geotextile (final design, design for the construction phase, possibly in the realisation phase)
• Allowing more time between elevating successive layers (design for the construction phase, realisation phase)
• Application of light fill material (final design, design for the construction phase, realisation phase).

In addition to the aforementioned traditional arrangements/techniques, there are also innovative techniques such as soil nailing.

In practice, differences may arise between the behaviour of the underground predicted in the design phase and the actual behaviour of the underground during the realisation phase. These can lead to unforeseen deformations and in extreme situations even to instability of the underground (see Site Engineering). These uncertainties can be managed using the so-called observation method. Based on measured actual on-site pore pressures (see pore pressure meters), it is determined how much backfill material may be added to the embankment. If deviations are found between the actual and predicted behaviour of the ground, action can be taken. If there are negative deviations, it is often sufficient to adapt the rate of elevation. The motto here is “faster if possible, slower where necessary”. 

The calculations for the stability analyses are generally made with the validated Deltares D-series D-Geo Stability software. 

SOCOTEC can perform the stability analyses for you using the most commonly used Bishop, Uplift-Van (uplift model) and (Uplift) Spencer models.

For more advanced analyses, the validated Plaxis software can be used. 

Settlement 

Due to an increase in stress in the underground, vertical deformations (settlement) occur in compressible soil layers (clay, peat and/or silt layers).

Causes of stress increases in the underground include: 

• Embankments for preparing sites for construction, construction/widening of infrastructure and construction/reinforcement of water barriers
• Lowering of the water level, for example as a result of a lowering of the polder level or temporary dewatering
• Construction of a steel structure/work floor
• Settlement can also be caused by vibrations, for example due to the vibrating insertion of sheet piling or foundation piles. However, this is based on a different mechanism than the aforementioned increase in stress. For settlement as a result of vibrations, reference is made to the impact on the surroundings of construction pits

This settlement affects the following aspects: 

• The longitudinal and transverse flatness of a hard surface
• Residual settlement during preparation for construction (functionality/use of public space and new construction)
• Sufficient crest height of water barriers to withstand the required normative high water level over the life of the water barrier
• Limiting the impact on the surroundings so that no unacceptable damage occurs to local structures and infrastructure
• Settlement analyses can be useful for every phase of your project: the design phase (final design), preparation for realisation phase (design for the construction phase) or during the realisation phase itself (see Site Engineering)

During the design phase, SOCOTEC can investigate for you which settlement has to be taken into account for your project and whether any provisions are required to meet the settlement-related requirements upon completion of the project.

With a design for the construction phase (or possibly as part of the final design), SOCOTEC can examine what the aforementioned provisions are and which specifications they must meet. This can be done in combination with the establishment of a fill schedule. 

With regard to the aforementioned measures, you could consider (a combination of): 

• Observing a settlement / waiting period if there is sufficient time so that the majority of the settlement has occurred by the time the project is delivered
• Applying a temporary pre-load to accelerate the settlement process
• Using vertical drains to accelerate the settlement process
• A combination of the previous two measures
• Applying light filling material
• In special cases, using a stabilising construction, consisting of light filling material such as expanded polystyrene (EPS) or foam concrete

In order to control uncertainties in the properties and composition of the underground, and the calculation models used for the design (see Site Engineering) with regard to settlement, a so-called ‘back analysis’ is performed. The settlement parameters are recalculated until the calculated development over time of the settlement corresponds as closely as possible to the measured actual development of settling over time. The recalculated settlement parameters are then used to predict the expected final and residual settlement.

The calculations for the settlement analyses are generally made using the validated Deltares D-series software.

SOCOTEC can conduct settlement analyses for you according to the Koppejan models and the Den Haan (abc isotach) and Bjerrum isotach models. Preference is given to the isotach models since they are more advanced. SOCOTEC has some 15 years of experience with the more advanced isotach models.

In general, settlement analyses are performed deterministically. SOCOTEC has the experience to perform probabilistic settlement analyses for you. Depending on the type of probabilistic analysis, it can be investigated for you which parameter(s) has/have the greatest influence on the calculation of settling, or it can be investigated how likely it is that you will meet the intended residual settlement requirement.

In addition to Deltares software, SOCOTEC has developed a spreadsheet in-house that can be used to effectively calculate for you the final settlement resulting from a change in stress based on the results from large numbers (hundreds) of CPTs.

Environmental impact

Infrastructure and site preparation

As a result of embankments, for example for dike reinforcement, the construction of infrastructure or site preparation, excavations, for example for construction pits, or groundwater level lowering, deformations in the subsurface can occur.

The aspects mentioned above cause a certain degree of influence on the surroundings, such as influence on above- and underground infrastructure, archaeological finds in the subsurface or (foundations of) buildings. Depending on the degree of influence, damage may occur to the aforementioned objects.

SOCOTEC can advise you on this environmental impact. This can include making:

• Settlement analyses for groundwater level reductions
• Settlement and horizontal deformation analyses for embankments
• Horizontal deformation analyses (horizontal) for excavations

Predicting horizontal deformations can be done in two ways:

• Fast and indicative (weighted average over the height of the soil layer package subject to deformations) with analytical computational models (De leeuw, Mieusses and Bourgeses)
• More precisely with a Finite Element Model (Plaxis for example) where a detailed picture of the deformations of the different soil layers with different stiffnesses is obtained

For these analyses we use a geotechnical soil investigation as well as geotechnical laboratory investigations that we can perform ourselves. For the calculations we use the Dseries of Deltares (D-Settlement) and Plaxis 2D/3D.

Environmental monitoring
To minimize the impact on the environment as much as possible, environmental monitoring plays an important role during the execution of earthworks (excavations and embankments). With monitoring, uncertainties during execution can be identified so that, if necessary, risk-mitigating measures can be taken at an early stage.