Various types of reinforcement carry the tensile stresses in the concrete. The main reinforcement is applied in the wall where the biggest bending moments occur, in the inner corner between the wall and the floor, and in the toe of the floor. The bending moment in the wall decreases once the cut in the wall is made higher to determine the forces. At the top of the wall, the bending moment is 0. By dividing the height of the wall and determining the required reinforcement per segment, a lot of reinforcement can be saved. Besides, compression reinforcement, distribution reinforcement and bollard reinforcement are needed to carry and distribute loads properly.
A construction pit of temporary sheet piles with a strut frame makes it possible to excavate approximately 2.5 metres of the soil and lower the water level. After excavating several teams can work in shifts to apply the formwork, processing the reinforcement bars and to pour concrete. Once the construction and backfill have been finished, the temporary sheet piles can be removed.
Anchored sheet pile wall
The steel anchored sheet pile wall is the second traditional quay wall structure that is used for the comparison. The design as shown in Figure 4 is in reality designed and constructed for the Flevokust haven. Because it concerns a validated design, no constructive calculations have been made.
The construction consists of permanent sheet piles with an average length of 21 metres. Two grout anchors per 3-metre quay wall carries the bending moments in the sheet pile resulting in shorter sheet piles.
Construction starts with the installation of sheet piles into a load-bearing layer. Lowering the water level and backfilling sand on the existing soil including preload speeds up the settlement process. When soil is sufficiently settled, the grout anchors can be installed.
Reinforced soil structure
The third design is the innovative reinforced soil structure. The retaining function of the design is derived from the use of a high density polyethylene (HDPE) reinforcement, geotextile and sand. Uniaxial geogrids such as HDPE reinforcement can carry high tensile loads applied in one direction. The elongated perforated structure allows the backfill material to interact with the reinforcement through frictional resistance. Meanwhile the aperture structure of geogrids could cause the backfill material to washout. Geotextile provides a barrier to confine the backfill material. Open graded sand is desired to ensure a drained effect of the backfill.
The design is checked for internal and external stability in accordance with the CUR-198 guidelines. In order to ensure the local internal stability, it is important to know that a reinforced soil structure can internally fail due to two reasons. The first being that the tensile strength of the reinforcement is exceeded causing the reinforcement to break. The second is that the reinforcement can be pulled out due to insufficient bonding when the reinforcement length is not sufficient to transfer the tensile force to the backfill material. A check on pulling out is disregarded because this is only decisive in situations where very short reinforcement lengths are used.
Initially, the tensile force must be determined for each reinforcement layer. There are four types of loads that directly affect the tensile force, taking into account the self-weight, surcharge loads, concentrated horizontal and vertical loads. The self-weight of sand and surcharge loads causes a vertical force in the construction. This vertical force results in a horizontal force due to the active ground pressure because the sand is enclosed by the geotextile. Concentrated vertical loads due to a bearing are not applied to this design.
The total tensile force Ti;d is the sum of all the tensile forces due to self-weight, surcharge loads Ty;i;d and concentrated horizontal loads Th;i;d. Simplified, the equation is as follows:
Calculating the tensile forces due to selfweight and surcharge loads Ty;i;d is done by multiplying the active ground pressure factor K1;d with the reinforcement layer height hi (0.6 metres) and the vertical effective stress v;i;d.
The vertical effective stress v;i;d is derived by the sum of the vertical forces divided by the effective width.
Determining the influence of the concentrated horizontal load of the bearing in the considered layer, gives the following equation. Basically, the load is spread linearly on the depth depending on the active shear wedge and the distance between the point of engagement (centre bearing) to the facing of the reinforced soil structure.
As shown in Figure 5, varied geogrid types are used that differ in tensile strength. The tensile force in the geogrids increases once they are lower in the structure due to the increasing ground pressure. The unfavourable horizontal loads are carried by the top layers of the reinforcement resulting in higher required tensile strengths. A deviation in geogrid type can also be found below the water level where the active earth pressure on the geogrids is reduced due to the saturated conditions, resulting in a lower effective weight of the soil fill.
The global internal stability can be calculated with the compound method. A shear wedge with a fixed angle produces a load that needs to be caried by the intersected reinforcement layers. The global internal stability check has not led to a normative load case.
Checking the design for external stability resulted in an analysis of consolidation, settlements, tilt stability, vertical load bearing and horizontal sliding. A geogrid length of 13.6 metres provided enough resistance against all these failure mechanisms.
The global circular shear failure mechanism as a final check showed to be normative in determining the geogrid lengths, resulting in a 15-metre-long reinforcement.
The construction consists of 17 layers of soil, each 0.6 metres high; two layers for the embedding depth and 15 for the required retaining height, including settlement compensation. Settlement calculations showed that a settlement of 0.72 metres occurs, resulting in an extra layer of reinforced soil of 0.6 metres to meet the settlement requirement.
The construction of a reinforced soil structure in this case is as following. A construction pit of temporary sheet piles with a strut frame makes it possible to excavate approximately 3 metres of the soil and lower the water level. Then the reinforced soil structure can be built layer by layer. A steel mesh formwork is repeatedly applied followed by rolling out and extracting geogrids and geotextile, and applying and compacting the backfill material. Finally, the geogrids and geotextile are folded back to enclose the backfill material.
The total costs for all three designs i.e. the concrete, steel and reinforced soil structure can be divided into two categories, construction costs and material costs. Focussing on the material costs, the limited use of steel within the reinforced soil structure results in a solution with the lowest material costs (total material costs 1.950.000 EUR). In the case of both the conventional structures, only the costs of steel are more expensive (cantilever wall: 1.971.000 EUR and sheet pile wall: 2.010.000 EUR) than the total material costs of the reinforced soil structure. For each construction, the backfill material costs are approximately 1 million EUR.
Compared to the material costs, the construction costs are somewhat different. The respectively high construction costs of the cantilever (1.720.000 EUR) and reinforced soil structures (1.106.000 EUR) are caused by using temporary sheet piles to create a construction pit. Therefore, the sheet pile wall is a less labour-intensive construction method resulting in lower construction costs.
Nevertheless, the total investment cost of the geogrid reinforced soil structure is still significantly lower than the total investment costs of either conventional structures. The material and construction costs show that the total investment cost for the soil structure is approximately 3.1 million EUR compared to 5.9 million EUR and 4.1 million EUR for the cantilever wall and sheet pile wall respectively.
During the total lifetime of a project, for each material or construction process it is possible to determine the societal cost to compensate the environmental effects. Using the Environmental Cost Indicator (ECI), the effects can be determined by multiplying the quantified emissions of a material or process per functional unit with the total amount. The outcome of this calculation is for each material or process an environmental impact expressed in euros. It is important to note that all materials are calculated with a life span of 100 years.
The ECI can be divided into different system phases or impact categories. The dividing by lifecycle phase is shown in Figure 8. The production phase of the materials has the highest contribution in the total ECI. Sand mining and transportation is for all three constructions the main cause of this high ECI. This is due to the relatively high density and the large volumes of sand used, and the large number of transport movements required. Both conventional structures further increase these ECIs within this phase due to the large amount of steel. During the production of steel, a vast amount of heat is necessary to deform the material, which in turn effects the Global Warming Potential (GWP).
The environmental impact during construction is almost equal to each other. The three structures include almost the same amount of sand. Processing the sand has in all cases the highest impact and effects the Global Warming Potential (GWP), Acidification (AP) and Human Toxicity (HT) the most.
The last phase assesses to what extent the materials can be reused or recycled for the next production system. Sand and concrete can easily be reused or recycled. Sand is an extremely circular product and mining of new sand can be avoided by reusing the product. Meanwhile, according to the Dutch National Environmental Database, 45% of steel in the sheet pile will be lost during its lifetime due to corrosion. This negative fund is taken into account by reproducing the lost steel. The environmental costs of reproducing the corroded steel do not outweigh the positive funds of reusing sand.
The construction processes other than the application of the materials, such as excavating the soil, water extraction and the temporary sheet piles cover around 50,000 EUR for the cantilever wall and the reinforced soil structure. In case of the sheet pile wall, these costs are only 25,000 EUR by not applying temporary sheet piles.
Instead of using concrete and steel as main materials, the retaining function of the reinforced soil structure is derived from the use of polymers. However, like steel and concrete, polymers also have major environmental impact. High density polyethylene (HDPE) and polyethylene (PE) – the polymers that are used – are mainly obtained from petroleum, yet the reinforced soil structure has significantly lower environmental costs. The low ECI of these polymers originates in the very limited volume that is used. Thin layers of stretched HDPE collectively have a low volume.
The environmental effects can also be expressed in 13 impact categories as shown in Figure 9. Global Warming Potential (GWP), Human Toxicity (HT) and Acidification (AP) are the most notable categories indicated in shades of blue. GWP is caused by greenhouse gasses, such as CO2, methane and nitrous oxide. This category is expressed in an equivalent with CO2 as reference. Greenhouse gasses hold warmth that results in a (faster) rising temperature on earth. Human toxicity includes the emissions of toxic substances that are exposed to human beings. This exposure finds its way by breathing or consuming products like meat and fish. Acidification arises after releasing sulphur oxides. The acidification of soil and water has a negative influence on ecosystems.