Earthworks and ground modelling: giving shape before planting

Ground modelling defines how you move through the garden and how water moves. It also determines walls, steps, the volume of fill and the relationship with the house.

The existing levels and the design are confirmed on the ground. Fixed reference points prevent errors from building up during excavation.

The cut-and-fill balance affects haulage and cost. Excavated material is not automatically suitable as structural fill or planting soil.

Layers are compacted according to their function. Root zones should not be given the same compaction as a paving base.

Paths need comfort and accessibility. Surfaces drain without carrying water towards buildings or neighbours.

Gradients that are too gentle can create puddles; excessive ones speed up erosion.

The choice depends on the difference in level, the space and the soil. Banks take up area and need stabilising with planting or construction.

Walls require a foundation, drainage and structural checking.

The fertile layer is protected and put back at the right stage. Prolonged storage in tall heaps reduces its biological quality.

Finished levels should allow for the thickness of paving, the prepared soil and the root collar of the trees.

Ground modelling is completed before the plants are set out, but it can take local adjustments during planting.

Balancing the material excavated against the material needed reduces haulage, but not all the soil can be reused. Organic soil, clay, rock, rubble and structural fill behave differently and go to different destinations.

Fill is placed in layers according to its function. A paving base needs compaction; a root zone needs stability without losing porosity. Mixing these requirements creates hard soils where trees cannot spread.

Banks take up space and need a slope compatible with the soil, the water and the vegetation. Planting does not immediately stabilise ground modelling that has been poorly designed.

Where there is not enough width, a retaining wall may be needed, with a foundation, drainage and structural checking. The decision should also consider future access for weeding, pruning and inspection.

Raising or lowering the level next to a tree changes oxygen, water and roots. Keeping the trunk on the drawing is not enough when the ground modelling cuts into or buries the root zone.

The original level is kept wherever possible. Unavoidable work next to important specimens requires a specific solution and technical supervision.

Levels are checked during excavation, filling, base and finishing. Measuring does not wait until completion.

Fixed survey reference points, sections and records make it possible to correct deviations while there is still room to do so. Vegetation softens the landform, but it does not correct an unsafe gradient or water with nowhere to go.

Ground modelling should work in heavy rain, not just on a dry day. Low points, discharges from roofs and natural drainage lines are built into the levels.

Banks are given slopes compatible with the soil and with maintenance. When the difference in level calls for retention, the wall needs its own drainage and structural design. Vegetation helps to protect the surface, but it does not correct an unstable mass of earth.

Reusing excavated material reduces haulage, provided its quality suits the function. Topsoil, clay, rock and waste cannot be used interchangeably.

The volumes are recorded and the surplus is disposed of lawfully. Soil intended for planting is stored and handled so as to keep its structure and organic matter.

• Garden drainage
• Soil, water and drainage

1. Principles of surveying, ground modelling and landscape drainage.
2. Diaplant’s method for checking levels and earthworks.