Geohydrological Aspects of the Reclamation of the Markerwaard Polder

GEOHYDROLOGICAL ASPECTS OF THE RECLAMATION OF THE MARKERWAARD POLDER. THE NETHERLANDS. GEOTECHNICAL INVESTIGATIONS.

By A.J. van Bruchem Heidemij Consultancy Division Haarlem, The Netherlands

Abstract After reclamation of the Markerwaard polder, the changes in the groundwater regime will result in a settlement of compressible holocene layers (clay and peat layers). Within the framework of the geotechnical investigations we have studied the spatial distribution of the settlements to be expected. To this end the study area was divide into subareas, which are, within certain limits, uniform as regards the profile structure of the holocene layer and the hydrological situation (map of geotechnical subtypes). These calculations were carried out using the settlement theories of Terzaghi - Koppejan, modified to incorporate stress-dependent compression constants. On the basis of the computational results we prepared 12 diagrams for the whole study area showing the relationship between the reduction in piezometric head in the first aquifer and the final settlement. Using these diagrams, the map of geotechnical subtypes, and the piezo metric head reduction known from the geohydrological investigations, a map was prepared on which the expected land subsidence (settlements) are indicated.

1) General Hydrologie model investigations have shown that reclamation of the Markerwaard polder in the Netherlands will cause changes in the ground water regime on the mainland in a 5-10 km wide strip along the eastern coast of North-Holland Province. These changes comprise decline of the piezometric head of the deeper groundwater and consequent minor declina tions of the phreatic surface, especially in summer (Vos et al 1984). As part of the investigations in North-Holland into the geohydrologic aspects of the reclamation of the Markerwaard polder, a study has been made of the behaviour of the subsoil in relation to the changes in the groundwater regime. This study, designated as 'geotechnical investiga tions', was done in close co-operation between the Geological Service of the Netherlands (RGD), the Delft Soil Mechanics Laboratory (LGM) and Heidemij Consultancy Division.

2) Problem definition Under the influence of the decrease in piezometric head of the groundwater in the upper pleistocene aquifer and the decline of the phreatic surface, the pore pressure in the holocene layer will drop. As a result, the effec tive stress in this layer will increase, as is shown schematically in figure 1. This increase in the effective stress in the holocene layer will result in settlement of compressible soil layers (clay and peat layers), so that

899 Principal drawing "S- * reduction in the pore pressure caused by a piezometric head reduction in the upper aquifer

sand reduction of pore pressure rise in effective stress clay lower peat sand [ pleistocene ) piszomtrk heed refaction in the uppar aquifer subsidence of the ground surface will occur. The objective of the geotechnical investigations can thus be summarized as follows: 1. determining the magnitude of the settlements and ground surface subsi dence to be expected in the study area; 2. determining the rate at which these settlements will occur.

3) Set-up of the investigations The magnitude of the settlement occurring through the changes in the groundwater regime is dependent on: the drop in piezometric head in the upper aquifer; - the declanation of the phreatic surface; the present hydrological conditions; the geotechnical properties of the soil layers in the holocene layer (e.g. permeability and compressibility) The rate at which the settlement will occur depends on: the rate at which the piezometric head in the upper aquifer decreases and the rate at which the phreatic surface subsides. the structure of the holocene layer; the permeability and the compressibility of the various soil layers in the holocene layer.

In view of the above, it will be clear that soil investigations can give adequate information on the structure of the holocene layer, the hydrolo gical conditions, and the geotechnical properties of the various soil layers in one single spot, but that there will be substantial variations within a large study area. For this reason the problem has been approached area-wise; the study area was divided into subareas which are, within certain limits, uniform as regards: the structure of the holocene layer; the hydrological conditions; - the geotechnical properties of the various soil layers.

900 The Geological Service then prepared a basic map on which the study area was divided according to the structure of the holocene layer (Westerhof et al, 1984). This map was mainly based on information on file with the Geological Service, the Soil Mechanics Laboratory, and Heidemij. In places for which insufficient data were available, the Soil Mechanics Laboratory carried out additional soil investigations. These investigations showed that 11 typical soil layers can be distinguished in the holocene layer; in each of these layers the relevant geotechnical properties are fairly uniform. In addition, it was possible to establish clear relationships between the different geotechnical parameters (Hannink et al, 1984). With the information thus obtained about the geotechnical properties of the soil layers and further information on the hydrological conditions in the study area, 160 profile types were distinguished within the holocene layer. On the basis of these profile types, Heidemij then refined the basic map to a map of geotechnical subtypes. Figure 2 shows one of the profile types; the figure clearly demonstrates the heterogeneity of the holocene layer in the study area.

4) Computations For each profile type the relationship was determined between the reduc tion in piezometric head in the upper aquifer and the final settlement.

Fig. 2 Ar example of a profile type legend Az06 T I peat ( at the surface)

2| peat (deeper layers)

3| lower peat

t.W 7T~1 clay ( unit weight 13-14,5 kN/m, lutum »25% )

~5~~\ clay ( unit weight 14,5-16 kN/m, lutun-25% )

6 | clay 1 deep layer of velsen, unit weight 13-14,5 kN/m, lutum» 25% )

7 | clay ( deep layer of velsen, unit weight 14,5-16 kN/m, lutum 25% )

T1 sandy clay ( unit weight 16-17 kN/m, lutum 12-25% )

,3 "9~1 clayey sand ( unit weight 17-18 kN/m\ lutum 8-12% ) 12 1Q | holocene sand ( clay containing, unit weight 18-19 kN/m, lutum 0-8% )

11 | holocene sand ( silt containing, unit weight 18-20 kN/m )

12 I pleistocene sand

901 The final settlement is defined as the settlement occurring 10.000 days after reclamation of the polder. To determine this relationship, settle ment computations were made for piezometric head reductions of 0.25, 0.50, 0.75, 1.00 and 1.50 m in the upper pleistocene aquifer. The hydrological investigations had shown that such piezometric head reductions are possi ble in the study area. The decline of the phreatic surface for each profile type was input as a percentage of the piezometric head reduction. This percentage was determined by two model investigations for 9 profile types which can be regarded as representative for the study area: 1. determination of the influence of the piezometric head reduction on the level of the phreatic surface in a stationary situation without rain fall or evaporation; 2. determination of the level of the phreatic surface during the growing season of a particular crop, as a function of the profile type and meteorological circumstances. In this non-stationary model a discharge relation is used that has been determined in the stationary model investigation (Heidemij consulting division; rapport nr. 662-83/1, 1983). From the settlement computations it was found that the settlement behaviour of the study area can be represented in 12 diagrams, which give the relationship between the piezometric head reduction in the first aquifer and the final settlement. The Soil Mechanics Laboratory carried out an analysis of the reliabi lity of the soil parameters used; this analysis was used to define for each relationship diagram a band representing the 90 % reliability inter val. Figure 3 shows such a diagram. For the settlement calculations use was made of compressibility con stants that were, unlike those in the usual theories, within certain limits dependent on the effective stress and the increment of the ef fective stress (Hannink et al, 1984).

Fig. 3 Relationdiagram piezofRstrk head réduction - settleaent 0.30 .

0.25

15 0.20 .5 0 "g 0.15 M

S o.io G

0.0 5 _(

0.00- 0.00 0.25 0.50 0.75 1.00 1.25 1.50 piezometric head reduction in metres

902 Medemblik L. Lake Ussel

North-Holland jsnhfvEnkhuizen ,, , . Bovenkarspel '"^;»> Hoogkarspel ,#, __ jJîfflÉKïilr •ffeiTlt til"'11 tiffin1 WrTSEr É^^^f^j

Hoorn 11

I Border Lake f 1 1 % l 0 1 2 3 5 km

/ Legend

/ average subsidence 0- 30mm

\ average subsidence 30- 60mm

L \ llliijll average subsidence 60- 90mm

to^SBÈÈLf ) HlnËPl avera9fc subsidence 90- 120mm

;nr ":;;' Edam|prjl»\ ^^

Purmerendi-f Volendam"™fw \ /

ÉÈms \ \ Polder I t-J-*^ W \ Markerwaard / / ••W? Marke"JmL^ \ (Planned) // MonnickendamiPt* f (Jw'^ ) ,/ y/

^^~~~~~7/ \ y/y/ Flevoland

^Vf.- .:H;H Amsterdam J y^ —J*W^ •*••• /

f=:filifiitt^lFfc^ |ÏV\ ) of the p0'c'er Markerwaard. Figure 4 ,..!..,,.n.:.:3H; nTîWrrr\ \ \ V X^

Fig. 4-Calculated subsidence caused by reclamation of the polder Markerwaard. 903 The rate of decrease of the piezometric head in the groundwater in the upper aquifer has a strong influence on the rate of the settlements. The Soil Mechanics Laboratory performed time-settlement calculations for the assumption that the piezometric head reduction will take place over a period of 5 years (Hannink et al, 1984).

5) Results On the basis of the map of geotechnical subtypes, the piezometric head reductions determined in the geohydrologic investigations, and the dia grams presenting the relationship between piezometric head reduction and the final settlement, a map was produced, on which the lowerings of the ground surface (settlements) to be expected are indicated (see figure 4). The map shows that the greatest settlements are to be expected immediately north of Volendam and in the vicinity of Wijdenes (0.09 to 0.12 m). Three to five km more inland settlements are expected of 0.03 to 0.06 m. In peat areas the lowerings of the ground surface may be somewhat larger than indicated on the map, as a result of shrinkage and oxidation of organic matter in peat profiles, due to lowering of the phreatic surface. This may cause an extra lowering of the ground surface of up to 0.02 m. It should be noted, however, that in the present situation the ground surface in peat areas is already subsiding due to oxidation of organic matter. Recla mation of the Markerwaard polder will accelerate this process by 1 to 2 mm per year for a number of years. Since the rate of the piezometric head reduction in the upper aquifer has a much greater influence on the rate of settlement than the differen ces in the profile types, it was possible to represent the time-settlement relation for the whole study area by only 4 diagrams (Hannink et al, 1984). The computations show that within 5.5 to 8 years after the start of pumping the settlements will reach 90 % of their final value.

Acknowledgements This study forms part of a project study by five research institutes. The author wishes to thank Rijkswaterstaat (Dutch Public Works Department) for their permission to present the results of the study, and all members of the project-team for the pleasant co-operation.

References :

1. Claessen, F.A.M. 1984 An integrated study to forecast and to prevent disadvantageous effects in the province of North Holland resulting from a -change in the ground water regime after the reclamation of the Markerwaard. Proc. Symp. On Land Subsidence March 19-25, 1984 Venice

2. Vos, G. Claessen, F.A.M. van Ommen, H. 1984 Geohydrological compensatory measures to prevent land subsidende, resulting from the construction of the polder Markerwaard, in the Netherlands. Proc. Symp. On Land subsidence March 19-25, Venice.

3. Westerhoff, W.E. de Mulder, E.F.J. 1984 The Markerwaard reclamation project. Quaternatery geological frame work of the study area (North Holland and Western part of the lake IJssel, the Netherlands). Proc. Symp. On Land subsidence March 19-25, 1984, Venice.

904 4. Hannink, G. Talsma, K.W. 1984 Geotechnical consequences to the enviroment, polder Markerwaard. Proc. Symp. On Land subsidence March 19-25, 1984, Venice.

5. Carrée, G.J. Hulsbergen, J.G. 1984 Method for damage estimates of buildings in subsiding areas. Proc. Symp. On Land subsidence March 19-25, 1984, Venice.

6. Heidemij consultancy division; Rapport nr. 662-83/1, 1983). Een onderzoek in Noord-Holland naar de geohydrologische aspecten van de aanleg van de Markerwaard. Cultuurtechnisch onderzoek.

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