Uplift Risk Maps for Sewerage
Renewal Planning in Deltaic Cities
Mattijs BORST a and Ella van der HOUT b
a
B3 Engineering, Berkel en Rodenrijs, The Netherlands
b
Municipality of Rotterdam, Project Management and Engineering, The Netherlands
Abstract. A method was developed to identify areas where uplift is likely to occur during sewerage renewal. The results were presented as a map that can be used by generalist civil engineers in the early stage of the sewerage renewal planning process. Keywords. Uplift, risk maps, sewerage renewal
1. Introduction
This article describes the development of uplift risk maps for sewerage renewal in the low-lying parts of Rotterdam.
2. Problem Description 2.1. The Sewerage System
Rotterdam is the second largest city in the Netherlands, with almost 620,000 inhabitants and a land surface of 20,600 ha. The total length of the sewerage system exceeds 2,500 km. Based on a technical lifetime of 50 years, every year on average 50 km sewerage needs to be renewed. 2.2. The Hydrogeological System
Several city quarters are located in so-called polders: land, surrounded by dikes, with the surface level below mean sea level (MSL). The deepest polders have a surface level of 6 m below MSL.
The topmost soil layer in the Dutch delta usually consists of peat and clay, both functioning as a confining layer. Underneath this confining layer lies a coarse layer of sand: the first confined aquifer.
Since the water pressure in the first confined aquifer is influenced by rivers and inflow from
adjacent higher areas, the water pressure may be above surface level in the deep polders.
In the natural situation, there will be some seepage (1 to 2 mm/day) from the first confined aquifer towards the phreatic groundwater. 2.3. The Uplift Mechanism
In the case of sewerage renewal the thickness and subsequently the weight of the confining layer is temporarily reduced by excavation. This leads to an increased risk of uplift occurrence.
When the water pressure in the first confined aquifer exceeds the weight of the remaining confining layer, uplift may occur in the form of a boil at the weakest point of the confining layer.
At the location of the boil, the resistance of the confining layer is reduced towards zero, leading to a significant groundwater flow from the first confined aquifer through the boil into the excavation. This is an undesired situation.
To avoid this situation, the uplift risk should be assessed in the design phase. If necessary, the water pressure in the first confined aquifer can be lowered by groundwater abstraction (depressurization).
2.4. The Mossenbuurt Case
In 2011, the sewerage management department of the Municipality of Rotterdam decided to renew the sewerage system in the Mossenbuurt neighborhood. Based on experience from
Geotechnical Safety and Risk V T. Schweckendiek et al. (Eds.) © 2015 The authors and IOS Press.
This article is published online with Open Access by IOS Press and distributed under the terms of the Creative Commons Attribution Non-Commercial License.
doi:10.3233/978-1-61499-580-7-197
previous projects, a timeline and a budget were assigned to the project. The inhabitants were informed about the preparation of works and the time schedule. The Project Management and Engineering department received the task to execute the project within the given time frame and budget. During a late stage of the design phase, the project turned out to contain a significant risk of uplift. The first confined aquifer needed to be depressurized.
Under the Dutch Water Law, permission is required for major groundwater abstractions. The process for getting such a permit typically takes 3 to 8 months including preparations.
The sewerage design for the Mossenbuurt was therefore modified towards a more shallow system that could be created without depressurization of the first confined aquifer. This design modification led to a delay of several months in the project schedule.
2.5. Need for Uplift Risk Maps
The Mossenbuurt case highlighted a weak point in the conventional project approach: the uplift risk was only assessed in a late stage of the design phase, despite its severe impact on the project’s timeline and budget.
In the project evaluation of the Mossenbuurt, the sewerage management department asked the authors to develop tools that provide insight in the uplift risk. The tools should be presented as maps to be used by generalist civil engineers in the preliminary design phase.
With the uplift risk map, the sewerage management department should have a first indication on whether an area is sensitive for uplift risk. This should lead to a reduction of the risk of unexpected delays and exceeding of budgets.
3. Uplift Risk Assessed Traditionally
The calculation of uplift risk is usually done by geotechnical engineers during a late stage of the design phase.
In an early stage of the design phase, the sewerage engineers determine the diameters and depths of the new sewerage system.
Based on the depths of the new sewerage system, the excavation depths can be determined accurately. Furthermore, the geology of the project area can be explored by cone penetration tests (CPT) or drilling. The water pressure in the confined aquifer can be interpolated from long-term head measurements at monitoring wells throughout the city.
The design standards have been defined in the Dutch national standard NEN9997, which is an implementation of the Eurocode 7: Geotechnical Design. Following the design standard, the geotechnical engineer can perform the unity check for uplift risk, yielding a clear answer: Yes, the uplift risk is acceptable, or: No, the uplift risk is not acceptable.
4. Development of the Uplift Risk Maps 4.1. Need for Simplifications
Since the target audience for the uplift risk maps is generalist civil engineers, the format should be straightforward. This can only be achieved by making several simplifications. The uplift risk map should be based on readily available data, without the need to perform additional fieldwork or need for a detailed design.
4.2. Study Area
The deep polders north of the river Meuse are especially prone to uplift. The sewerage management department asked to create uplift risk maps for the quarters Prins Alexander, Hillegersberg-Schiebroek and Overschie, an area covering a total of 5,200 ha.
4.3. Excavation Geometries
The first simplification concerns the geometry of the sewerage system. In cooperation with the sewerage management department, two representative excavation geometries were derived as shown in Table 1.
M. Borst and E. van der Hout / Uplift Risk Maps for Sewerage Renewal Planning in Deltaic Cities
Table 1. Representative excavation geometries
Standard Shallow
Depth below surface level 2.5 m 1.8 m
Width at bottom of trench 2.5 m 1.5 m
Slopes 1 in 1 Vertical
Length at bottom of trench Unlimited 10 m
The standard geometry can be used for modern sewerage systems with separated systems for stormwater and for wastewater. Within the shallow geometry, a combined sewerage system can be created. The vertical slopes assume usage of trench shoring systems.
In this article the standard geometry is discussed. In engineering practice, comparison of the uplift risk map for the two geometries shows whether depth optimization is useful. In the Mossenbuurt case, the change towards the shallow geometry was enough to reduce the uplift risk to an acceptable level.
In practice, the depth of the sewerage system is more differentiated. In general, the depth will increase towards the pumping station, since the sewage water flows towards the pumping station under gravity flow. Sewerage systems often contain culverts at crossings with other pipes and cables.
4.4. Hydrogeological Profile
The city of Rotterdam is situated in the Rhine delta. The area north of the river was originally covered by a thick layer of low peat. The upper meters of peat have been removed to be sold as an energy source, leaving shallow lakes in the area.
With the introduction of the steam engine, several of these lakes could be dewatered, thus creating the deep polders suitable for agriculture or urbanization. The surface level in the study area nowadays is typically 4 to 6 m below MSL.
The current soil profile starts with the remains of the Holocene peat layer (Nieuwkoop Formation) to a depth of MSL -9 to -10 m. The peat layer is followed by a clay layer (Echteld Formation) to a depth of MSL -13 to -16 m. The peat and clay layers function as confining layers. In the Lage Land neighborhood, the clay layer has been eroded by flow in tidal creeks, leading to a channel belt that is filled with fine sand. These particular channel belts are deposited right on top of the underlying
Kreftenheye sand layer. These channel belts have a typical width of 80 to 120 m. For the overview picture it is important to note that several other channel belts are hydraulically separated from the underlying Kreftenheye sands by the remains of the original Echteld clay layer.
The topmost layer of Pleistocene age is the Kreftenheye Formation, mainly consisting of coarse sand and gravel. From a hydrogeological viewpoint, this layer is very important as the main regional aquifer. The thickness of this layer is typically 15 to 20 m. The bottom of this layer is found at MSL -30 to -35 m.
The deepest relevant geological formation is the Waalre Formation, which extends from MSL -35 to -105 m. This fluviatile layer consists of clay and sand. From a hydrogeological perspective this layer is regarded as a confining layer because of the Kedichem clay sublayers. 4.5. Analysis of Available Geological Data The Geological Survey of the Netherlands (GS-TNO) has published several (hydro)geological maps and models of the Rotterdam area. These include the printed 1:50,000 edition (1998), but also the more recent Regis-II and GeoTOP subsurface models.
The spatial extents of the channel belts have been drawn based on their archive of geological borings (with only 10 borings in the study area) and possibly information from other sources.
The city of Rotterdam has an extensive archive of CPT tests (approx. 12,000 in the study area). The extents of the mapped channel belts according to GS-TNO have been verified based on the available CPT tests. The match between those two sources was poor. As a result, creation of an uplift risk map based only on the geological maps did not give any reliable results.
The alternative approach, although more time consuming, was to assess all available CPT tests to find the top of the first confined aquifer and the thickness of the overlaying soil profile. At least at the locations of the CPT tests this will yield very reliable results.
4.6. Volumetric Weights
In the early days of CPT testing usually only the cone resistance was measured, which was M. Borst and E. van der Hout / Uplift Risk Maps for Sewerage Renewal Planning in Deltaic Cities 199
enough to determine bearing capacities for piles. Since the introduction of the sleeve friction cone, the friction ratio could be calculated, which is very important for distinguishing peat and clay.
Since the majority of the available CPT tests had no friction ratio, the whole soft soil profile was treated as a single layer, with representative volumetric weights for the whole soft soil profile as shown in Table 2.
Table 2. Volumetric Weights ()
Value [kN/m³] Representative for Soil Profile
Low 11 100% Peat
Mid 13.5 50% Peat / 50% Clay
High 16 Sandy Clay
The use of the low, mid and high values is explained further in paragraph 4.10.
4.7. Top of First Confined Aquifer
From a geotechnical perspective the topmost sandy layer should be assessed for uplift risk, followed by an assessment of any deeper aquifers.
From a hydrogeological perspective it is useful to differentiate between isolated thin sand layers and major aquifers, based on the different consequences of uplift probability.
The isolated thin sand layers (channel belts) in the study area have a typical thickness of 1 to 3 m and a typical hydraulic conductivity of 2 to 10 m/day. Transmissivity may therefore be between 2 and 30 m2/d.
The Kreftenheye formation has a typical thickness of 20 m and a typical hydraulic conductivity of 40 to 60 m/day. Transmissivity for this layer is in the order 800 to 1,200 m2/d.
The discharge needed to depressurize an isolated thin sand layer is negligible compared to the discharge needed to depressurize the underlying Kreftenheye formation.
However, when there is no confining layer between the thin sand layer (channel belts) and the underlying Kreftenheye formation, the thin sand layer has been treated as part of the Kreftenheye formation, since the former cannot be depressurized separately.
4.8. Piezometric Head
The Municipality of Rotterdam has an extensive network of monitoring wells throughout the city with monthly head measurements. For the surrounding municipalities head measurements were gathered from GS-TNO.
According to the Eurocode, variations in groundwater levels should be taken into account. In this case, the 90th percentile has been used as a representative upper value.
The potentiometric head in the first confined aquifer is governed by the regional hydrogeological system: infiltration from rivers, inflow from deeper aquifers and adjacent higher areas, seepage to the phreatic layer. In the absence of major groundwater extractions or infiltrations, the head pattern is very smooth.
The potentiometric head at the measuring wells was interpolated using a kriging algorithm. 4.9. The Use of Safety Factors
The Eurocode prescribes the use of partial safety factors in the uplift verification. A partial factor of 0.9 should be used on the weight of the soil. Or: to avoid uplifting, the water pressure should not exceed 90% of the soil pressure.
However, in reality uplifting will only occur when the water pressure exceeds (100% of) the soil pressure. For predicting the actual failure, a partial factor of 1.0 has been used.
4.10. Color Scheme
In the uplift risk map, a color scheme is used to indicate the uplift risk. The uplift risk can be solved by the application of depressurization dewatering. In the color scheme (Table 3), the uplift risk is therefore described by the need for depressurization dewatering.
Table 3. Color Scheme
Color Need depressurization dewatering?
Red Yes
Orange Expected: yes, but needs verification
Yellow Expected: no, but needs verification
Green No
Grey Not determined
Red means: even when assuming a heavy soil profile (high volumetric weight) and without M. Borst and E. van der Hout / Uplift Risk Maps for Sewerage Renewal Planning in Deltaic Cities
any additional safety factor, depressurization is necessary to avoid uplift for the considered excavation.
Green means: even when assuming a light soil profile (low volumetric weight) and with the prescribed safety factor, no depressurization is needed to avoid uplift for the considered excavation.
Orange and yellow indicate intermediate areas, for which in the indicative project stage of the uplift risk map only an expectation can be given that will need to be verified in a later stage.
Orange stands for: uplift expected, Yellow for: uplift not expected. In order to get a more accurate impression of the actual uplift risk additional CPT’s or borings can be performed. With these methods, the composition of the soft soil layers and the top of the first confined aquifer can be determined.
5. Results
Figure 1 shows the uplift risk map for the southern part of the quarter Prins Alexander.
Each colored square indicates a CPT test. The CPT density shows a strong variation throughout the study area, since the tests have been performed for construction and infrastructural projects.
In areas with a dense pattern of CPT tests the extent of the channel belt is clearly delineated. In areas with few CPT tests and different colors, the uplift risk map can be used to get a first impression, but additional fieldwork should be carried out in order to reduce the uncertainty in the prediction.
The top of the first confined aquifer is the parameter that causes most of the short-distance variation in uplift risk.
Figure 2 contains a more detailed view for the Lage Land neighborhood, being the central part in the Prins Alexander quarter.
The authors have experimented with interpolation of the uplift risk maps to produce a full coverage of the study area. The top of the first confined aquifer does not change gradually, but rather stepwise. In order to capture these transitions correctly, the CPT density should be high in areas close to the channel belt boundary.
Figure 1. Uplift risk map for the Prins Alexander quarter
Figure 2. Uplift risk map (detail) for the Lage Land neighborhood
The right part of Figure 2 would have sufficient data for delineating the extent of the channel belts accurately, while the data in the left part is rather poor for this purpose.
Any interpolation strongly suggests a large degree of certainty, even when the underlying data density is rather poor. This consideration led to the use of a presentation that clearly shows any lack of data for the least predictable parameter (top of first confined aquifer as derived from CPT data).
6. Final Remarks
The uplift risk map project illustrates the possibilities for indicating uplift risk in an early stage, without the need to perform extensive fieldwork or the need to perform detailed calculations.
In order to avoid any misinterpretation of the uplift risk maps, the map does not provide an interpolated full coverage of the study area, but only colored squares on locations with CPT data.
At these locations, the uncertainty about the soil profile is negligible. For areas with low CPT density or high variability in uplift risk, the end users are encouraged to consult a hydrogeologist or geotechnical engineer before drawing any conclusions.
The sewerage management department has started to use the uplift risk map as a part of their planning process. In a follow-up project the authors will prepare an uplift risk map for the Hoogvliet quarter.
The analysis of the CPT tests provides a clear picture of the extents of the channel belts. This information may be valuable in specialisms like archaeology as well.
M. Borst and E. van der Hout / Uplift Risk Maps for Sewerage Renewal Planning in Deltaic Cities
