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Stiff geogrids over wet mortar columns – for soil improvement of a newly designed waterfront promenade and for load shielding of an existing quay wall

1. Summary

The following article describes the redesign of an existing harbour promenade in the Lower Elbe area in order to increase the attractiveness of the harbour area, open up the former exclusively commercial harbour area and integrate the harbour into city life. This requires raising the ground level. Due to the low-bearing soil layers in the harbour promenade area, additional measures are necessary to minimise the new structural loads on the existing quay facilities and to reduce the associated long-term settlement to an acceptable level and prevent damage to the new promenade.

2. Introduction

As part of an engineering consortium, Ramboll, together with landscape planners (Bruun & Möllers) and architects (bof architekten), has been commissioned to redesign a harbour promenade in the Lower Elbe area. As the project is still in the planning stage and various decisions on implementation cannot yet be made by the client, the project is presented anonymously below.

In addition to the redesign of the harbour promenade (open-air facility), the planning content includes the creation of new access points through the existing flood protection wall, the redesign of the harbour master's office and the repair of the reinforced concrete flood protection wall, which is showing visible signs of ageing.

3. Current situation

The planning area covers an area of around 7,000 m². In addition to the high volume of tourist traffic, a special feature of the project is its location. The planning area is located on the water side of both the public flood protection system and the upstream property protection system. As a result, the planning area is statistically flooded around 13 times a year as a result of storm surges.

The flood protection wall upstream of the public flood protection system, which is in need of repair, serves as property protection and at the same time compensates for a difference in elevation of around 2.5 metres between the city and the harbour. The flood protection wall also acts as a barrier between the city and the harbour. This is particularly important during the storm surge season from 15 September to 31 March, as some of the flood protection gates and flood protection flaps are permanently closed during this period (see Figure 3-1).

Fig. 3-1: Closed HWS wall during the storm surge season

The HWS wall shows visible signs of ageing and therefore needs to be repaired as part of the overall project. The same applies to the harbour master's house, which is located between the quay and the HWS wall and has been severely damaged by storm surges.

Until the 1990s, the harbour itself was used exclusively for commercial purposes. As part of a larger urban development project, a shift towards tourist use has been initiated in recent years and is to be further promoted by the redesign of the harbour promenade.

4. Planning implementation

The core element of the outdoor facility planning involves raising the terrain by around 2.5 m and creating an upper promenade, see Figure 4-3. This will compensate for the existing difference in terrain on the water and land sides of the storm surge barrier and, in combination with new storm surge barrier gates, enable new barrier-free access to the harbour. A total of three new access points are planned. Due to the relocation of the HWS gates to the higher level, permanent closure during the storm surge season will no longer be necessary, as the higher promenade will then only be flooded once every five years, statistically speaking.

Fig. 4-1: Area of terrain elevation (area outlined in red dotted line)

Further elements of the plan include the reconstruction of the harbour master's office mentioned above, the construction of public toilets and a restaurant. In addition, flood-proof glass elements are planned for the area of the storm surge barriers and the storm surge barrier wall will be given a high-quality design.

Fig. 4-2: Area of the terrain elevation (section)

Due to the very inhomogeneous, low-bearing capacity and settlement-sensitive building ground, measures to reduce settlement are also necessary. The aim here is to reduce the mathematically expected deformations in the building ground caused by the redesign to a tolerable level that will not damage the new harbour promenade with its granite stone steps. To this end, Ramboll developed and investigated a soil improvement method using wet mortar columns in combination with a load transfer layer as the preferred option. In addition to significantly reducing settlement, the wet mortar columns also relieve some of the load on the existing quay structure thanks to the 2.5 m higher embankment.

Fig. 4-3: Visualisation of the plan

5. Soil improvement using wet mortar columns

5.1 General

The selected and preferred method for ground improvement using wet mortar columns involves the construction of unreinforced concrete columns in a uniform grid (spacing) to be determined, see Figure 5-1.

Fig. 5-1: Schematic representation of the load transfer of columns with a load transfer layer using the example of CMC columns [2]

The columns are inserted into the subsoil by soil displacement, see Figure 5-2. The base of the column is located in the load-bearing subsoil, which means that a large part of the additional loads caused by the elevation of the terrain are transferred directly there and thus cause hardly any measurable settlement. In addition, a dimensionally stable load transfer layer made of geogrid layers serves to better transfer the surface load to the columns and increase the terrain's resistance to failure.

The significantly relieved, low-bearing capacity subsoil between the wet mortar columns is improved by the soil displacement during column construction, and the possible settlement between the columns is noticeably reduced.

One system established in Germany for the production of wet mortar columns is the so-called CMC columns (Controlled Modulus Columns) from Menard, see Figure 5-2.

Fig. 5-2: Schematic diagram of the process for producing columns for ground improvement (here: CMC columns)

5.2 Settlement calculations

The current plan is to arrange the wet mortar columns (diameter 32 cm) in a uniform square grid of 2 x 2 m, see Figure 5-3. Precast concrete head plates are attached to the column heads before the geogrid is laid in order to increase the load concentration on the columns and shorten the distance between them. A load distribution layer approximately 45 cm thick, consisting of two orthogonally laid geogrid layers (e.g. NAUE Secugrid 80/20 R6 or similar) in combination with 15 cm thick sand interlayers, will be installed above the column head plates, see Figure 5-4.

Fig. 5-3: Top view of the arrangement of the columns and their spacing, possible head plates on the right

Within the granular embankment, the high difference in stiffness between the wet mortar columns and the surrounding soft soil creates a vaulting effect between the column heads. This causes a load concentration on the column heads in the sand cushion, see Figure 5-5. Based on the vaulting approach published in EBGEO, a load transfer of approximately 60 per cent to the columns is calculated, i.e. approximately 40 per cent of the load at the column head level remains on the settlement-sensitive soil layers between the columns despite the column foundation. As a result, the settlements cannot be "completely" reduced mathematically despite the column foundation.

Fig. 5-4: Representation of the load transfer layer with two geogrid layers above the wet mortar column (here: CMC) with head plate

Due to the uniform arrangement of the wet mortar columns in a 2.0 m x 2.0 m square grid, the expected settlements are reduced by a factor of approximately 3 according to Priebe's method, because the vaulting effect in the sand body relieves the soft layer and the remaining significantly lower load on the dimensionally stable geogrid causes elongation and sagging in the middle of the field between the columns. It can be assumed that a large part of the mathematically predicted settlements will probably occur within the load-distributing geogrid layers above the column heads during the construction period.

Fig. 5-5: Schematic diagram of the vaulting effect within the fill and deformation pattern of the load-distributing geogrid layer

However, in the case of unreinforced columns with a small diameter and high axial stiffness in relation to the surrounding soil, care must always be taken to ensure that the columns do not fail due to buckling. The calculation method according to Alber (2013) showed that a single column in the project area with the most unfavourable ground profile would not fail due to buckling at a maximum

axial load of 500 kN/m² (full load transfer to the columns) and that the deformations are within the elastic range. Thus, buckling of the columns is not to be expected.

Fig. 5-6: Area of the planned ground improvement (marked in green)

As part of the redesign of the east promenade, the construction of an extensive granite staircase and the new construction of two buildings is planned in the southern part of the project area, see Figure 4-3. In order to transfer the structural loads to the deeper, load-bearing ground, deep foundations are required for the staircase and the harbour master's office from a structural engineering point of view, so as not to place any further load on the existing quay. For this purpose, 39 bored piles (70 cm in diameter) are arranged under the strip foundations of the staircase and under the harbour master's office.

In addition to the vertical structural loads, the bored pile foundation must also transfer significant horizontal loads due to the adjacent and planned future elevated outdoor area. The horizontal loads cannot be transferred via the wet mortar columns, as these are sensitive to bending stress.

In addition, the elevation of the terrain requires an inspection of the existing 60-year-old quay facility, as despite the bored pile foundation of the staircase and despite soil improvement by wet mortar columns with geogrid layers above the column heads, residual additional earth pressures on the quay facility can significantly affect the stability of the quay facility. Additional reinforcement measures may be necessary after completion of the geostatic review.

Literature

1. Alber: "Proof of the internal stability of pile-like load-bearing elements in the ground", Bautechnik 90 (2013), issue 12
2. N. Meyer, A. Emersleben, J. F. Kirstein: "Test loads on CMC column groups – influence of the load distribution layer on the stress on the subsoil and the columns"
3. DYNIV GmbH: Brochure "Ground improvement with new technologies"
4. Priebe: "Design of vibro replacement", Ground engineering 2005
5. Priebe: "The Design of vibro replacement", GeTec Ground engineering 1995