Almost every vacant plot needs earth filling or leveling before construction, but the common problem is filling before thinking — filling to match the neighboring plot, filling without compaction, or filling higher than necessary and blowing the budget, then dealing with settlement, cracked floors and standing water later, which costs many times more than planning correctly from the start. This article follows the actual sequence of work: site plan → levels → cut & fill → compaction and testing → settlement prevention.

What Is a Site Plan, and Why Do It Before Filling?

A site plan is a drawing showing the building position, setbacks from the property boundary, access points, drainage direction and ground levels across the whole plot. It is one of the documents required for a building permit application and the starting drawing for all earth filling and grading work, because the ground level of each zone on the plot (house, roads, parking, garden) is set from this plan.

An accurate site plan requires 2 sets of input data:

  • Actual site level data: from a topographic survey that records existing ground levels, boundaries, structures, and the levels of public roads and drains. Read more in the article What Is a Topographic Survey?
  • Subsurface soil data: from soil investigation, to know how much load from the fill and the building the existing ground can carry, and how much it will settle after filling.

If this step is skipped and filling starts right away, what usually follows is ground levels that don't relate to the road and drains, so water cannot drain off the plot, or fill that is too high or too low and the earthwork has to be redone.

What Is the +0.00 Level, and How Is It Set?

The +0.00 level is the project's zero reference level, usually set at the ground-floor finished floor level (FFL) of the building. All other levels in the construction drawings are written as plus or minus values relative to this point, such as an internal road level of −0.60 or a second-floor level of +3.50. Setting +0.00 appropriately requires considering at least 4 factors:

  • Level of the public road in front of the plot: the house floor should be reasonably higher than the road so that water from the road does not flow back into the house.
  • Local flood records: the highest recorded flood level is key data for deciding how high to raise the floor. Large projects or land near waterways should have a flood study to determine the 100-year flood level.
  • Public drain level: water from the plot must flow into the public drain by gravity. If the plot is lower than the drain, a pump sump is needed, adding to both construction and maintenance costs.
  • Future settlement: in soft-ground areas such as Bangkok and its vicinity, the ground around the house gradually settles while the pile-supported house stays where it is. Levels should therefore allow for this difference from the design stage.

How Do You Plan Cut & Fill Economically?

The principle of cut & fill is to use soil cut from the high zones of the plot to fill the low zones, keeping cut and fill volumes as close as possible. The biggest cost in earthwork is hauling soil on and off site; if the earthwork can be balanced within the plot, it reduces truck trips, imported fill costs and disposal costs.

Principle of cut and fill grading: cut soil from high zones to fill low zones within the plot, place and compact fill in layers with density testing of every layer, and reference all levels to +0.00 The left view is a section through a sloping plot. The existing ground line, from a topographic survey, is high on the left and low on the right. The design ground line is horizontal. The left zone, where existing ground is above the design level, is the cut zone; the right zone, below it, is the fill zone. Soil from the cut zone is used to fill the low zone within the plot in roughly equal volumes. Before filling, organic topsoil must be stripped and removed, then fill is placed and compacted in layers of about 20 to 30 centimeters each. The building's ground-floor finished floor level is the +0.00 reference level. The ground surface slopes to drain toward the road and the public drain. The right view shows density testing of each layer by sand cone per ASTM D1556, compared with the maximum dry density from a Proctor test per ASTM D698 or D1557; only when a layer passes is the next layer placed. The bottom strip summarizes the sequence: set +0.00 from the site plan and survey, balance cut and fill, and strip organic topsoil before filling over it. Cut & Fill grading: balancing soil on the plot Not to scale +0.00 Gr. floor Use cut soil to fill the low zone Cut Fill Compact in 20–30 cm layers Design grade (Design Grade) Existing grade (topo survey) Strip organic topsoil before filling Road Public drain Compact + test each layer 20–30 cm Sand Cone ASTM D1556 % compaction vs Proctor D698 / D1557 Pass → Next layer 1 Site plan + topo Set +0.00 from road, drain, flood 2 Cut ≈ Fill Less soil hauling 3 Strip organic topsoil No soft layer under fill
Principle of cut & fill — cut soil from high zones to fill low zones, balanced within the plot, placed and compacted in layers, with all levels referenced to +0.00 (conceptual illustration, not a construction drawing)

The cut & fill planning steps used in practice are to overlay the level map from the topographic survey on the design ground levels of the site plan, then calculate the cut and fill volumes for each zone. The result is the quantity of soil to import or haul away, which can be used to budget the earthwork before work begins, instead of a guessed lump sum.

A point to watch is that not all soil cut from the plot can be reused as fill — organic soil, topsoil with roots or mud must be excavated and removed before filling over it; otherwise it becomes a soft layer trapped under the fill that compresses later.

What Compaction and Testing Does Fill Need?

Correct filling means placing and compacting fill in layers (typically about 20–30 cm per layer) and spot-testing the density of each layer before placing the next, not dumping a single pile of soil and smoothing the surface. The main tests used for quality control of fill are:

  • Field Density Test (Sand Cone) — ASTM D1556: tests the in-place density of the fill and compares it as a percentage of the maximum dry density from the laboratory, against the criteria in the project specification. Read the detailed test method in the article What Is a Field Density Test?
  • Proctor Compaction Test — ASTM D698 / D1557: a laboratory compaction test to find the maximum dry density and optimum moisture content of that fill material, used as the reference value for the field density test.
  • CBR (California Bearing Ratio): for internal roads or parking areas, used to assess the strength of the subgrade for pavement structure design.

These test results are objective evidence for accepting fill work from the contractor — pass or fail is decided by numbers, not by stepping on the ground. SPN provides all these field tests with engineer-certified reports; see details on the soil investigation and testing page.

Settlement After Filling — Why Does It Happen and How Can It Be Prevented?

Settlement after filling comes from 2 mechanisms that must be distinguished, because the ways to prevent them differ:

  • Compression of the fill itself: caused by filling without compaction or with compaction below the criteria; the fill gradually densifies over time. This can be prevented from the start by compacting in layers and testing density as described in the previous section.
  • Settlement of the original ground beneath the fill: common in Bangkok and its vicinity, where thick soft clay layers are present. The weight of the fill itself makes the soft clay gradually expel water and settle continuously for several years; the thicker the fill, the greater the settlement. No amount of compaction helps with this mechanism; it must be handled in design, for example by choosing a fill level no higher than necessary, placing the fill in advance so the ground settles before construction, or using pile foundations to transfer the building load to a stiff layer.

The answer to how your plot will settle, and by how much, lies in the soil data from the soil investigation, which engineers use to estimate settlement and choose the most cost-effective solution before work begins. Read in-depth solutions in the article How to Fix Settlement on Filled Land and the overall project design picture in our guide to RC structural and civil engineering design.

⚠️ This article provides general information

This content explains the principles of planning earth filling and grading for decision support only; it is not a design specification for any particular project. Fill levels, compaction layer thickness and test criteria for each job must be set by the design engineer based on the actual survey and soil data for that plot.