Soil report and bill of quantities: why you need a soil report before pricing
A soil report (geotechnical report) is the document that describes what lies below the ground surface and how a building should be founded on it. For the bill of quantities (BOQ) it is one of the most important sources: it decides whether there is a raft foundation or piles, how deep the excavation goes, which subbase layers are needed and which tests go into the bill.
Why you need a soil report before pricing
The US Federal Highway Administration defines subsurface investigation as the most important stage of geotechnical design. In Israel too, Israeli Standard SI 940 Part 1 deals with the design, construction and supervision of building foundations and geotechnical works. For the quantity surveyor, without the report there is no basis for the large items below the floor slab. A line like that stays marked "missing" rather than receiving a number that only looks reasonable.
A manual from the same administration recommends making the subsurface information available to all bidders, so that they can price fairly, and notes that an inadequate investigation can lead to claims. That is also the point of a good bill of quantities: contractors price better when everyone sees the same information.
What the report contains
- General data. Location and topography, neighbouring buildings and infrastructure, and the planned building: floors, basements and excavation depth.
- The soil profile. The layers by depth, the test results in each layer, and the groundwater level.
- Foundation recommendations. A raft foundation, piles or a combination, with allowable loads by diameter and depth.
- Excavation and shoring. Data for designing the shoring wall, and a support method according to the excavation depth.
- Floor slabs, fill and drainage. What is laid under the floor slab, how the space behind the walls is filled and how it is drained.
- Support during construction. When to call the geotechnical consultant to the site, and which tests are required.
Most reports also include a disclaimer: the borings represent a very small volume of the ground, and conditions may differ during construction. So quantities derived from the report are an estimate that is measured again during construction, and it is worth stating that in the bill.
Trial borings, field tests and groundwater
The SPT test
The most common field test in the reports is the standard penetration test (SPT): a sampler is driven into the ground with a 63.5 kg hammer, and the blows are counted. The number helps characterise the layers, and the sample recovered is used to identify the soil, but not for advanced laboratory tests. In many reports the test is carried out about every 2 m along the borehole.
Number and depth of borings
The geotechnical consultant sets them according to the building and the site. For medium-sized buildings the borings usually go to a depth of about 16 to 22 m, and deeper for towers or in a soft soil profile. On a small urban plot there are sometimes only one or two borings, so some reports require an additional boring after an existing building is demolished. In some cases the report asks for trial borings with a piling rig before the works, to check whether the piles can be drilled "dry". The Standards Institution of Israel offers geological supervision of trial borings and test pits, and the laboratory tests include, among others, Atterberg limits, moisture content, grading and swelling.
Groundwater
The water level is usually recorded during drilling, at the end of it and about 24 hours later, and an observation pipe or a piezometer is installed for monitoring. When the water is close to the bottom of the excavation, the report passes the decision to a hydrologist. The bill may then include piezometers, dewatering and a floor slab designed for uplift. An observation well may need a drilling licence from the Water Authority, so it is worth checking this in advance.
Monitoring during excavation
In a deep excavation next to existing buildings, the report sometimes requires a condition survey of the neighbouring buildings before the works, a baseline reading, and movement readings at every stage: inclinometers in the shoring wall and settlement points on the neighbouring buildings. These are items that can be measured and put into the bill.
Soil types in Israel, in brief
On the coastal plain kurkar is common: a partly cemented sand in which cemented slabs of different sizes are scattered without order, with lenses of sand between them. So its properties change even within the same plot. In the kurkar ridges, layers of kurkar alternate with layers of hamra, an ancient soil (palaeosol). On some sites there is a clay layer above the kurkar, and some clays swell and change volume with moisture.
Expansive clay needs special design. A study from the Technion documented buildings built on clay in which the columns between the piles and the floor beams cracked, and another study showed that swelling can create tension in the pile reinforcement. That is why reports on this kind of ground call for suspended floor slabs on void formers, soil replacement or deeper piles.
The Negev has areas of loess, and the hill regions have limestone and dolomite rock in which voids (karst) may appear. Uncontrolled fill is a problem soil in every region: one report noted that such fill behind walls can settle by a few per cent of its thickness. This division is general only, and there is no way to know what lies under a particular plot without a report for that plot.
Which drilling method suits which soil
Bored piles are usually drilled "dry" as long as no groundwater or collapsing soil is met. When the sides of the bore are unstable, they are supported with a casing or with bentonite slurry, and the concrete is poured through a pipe from the bottom up. In the CFA (continuous flight auger) method, the concrete is pumped through the hollow stem of the auger, with no casing and no slurry. The method usually suits soil and weak rock, and less so loose sand with shallow groundwater or limestone with voids. In hard rock other tools are used, and on sites with limited access or karstic rock, micropiles sometimes appear.
The drilling method affects the bill. When the report leaves the choice open until a trial boring, it is worth pricing both methods, or deciding in advance how a switch between them will be priced.
What the soil report changes in the BOQ
| Recommendation in the report | What changes in the bill |
|---|---|
| A raft foundation instead of piles | Deeper excavation and a subbase under the raft; sometimes an additional row of ground anchors |
| Piles and a suspended floor slab | A piling chapter, and void formers under the slab instead of a subbase |
| Pile length to be set on site | A quantity in linear metres that is an estimate and is measured again during construction |
| A clay layer under the subbase | A thicker subbase or soil replacement, and compaction tests |
| Shallow groundwater | Piezometers, dewatering and a slab designed for uplift; sometimes a different concrete specification |
| Movement monitoring | Inclinometers and survey points, as items in the bill |
Each of these lines has to be checked against the project's own geotechnical report and with the geotechnical consultant and the structural engineer. More on the items themselves on the page excavation and shoring BOQ.
What a professional quantity surveyor checks in the report
A quantity surveyor who comes from the field, an engineer or a registered building technician who worked on site, like Yaakov Avital, reads the report together with the structural drawings and asks where they do not agree. For example: soil replacement that the report requires for the whole footprint of the building while the drawing marks it for only part of it, or a pile depth that appears in two places in the same report with two different numbers. Every gap like this is recorded as a question to the geotechnical consultant and the structural engineer before the tender. A full example is on the page professional BOQ.
Good to know. This page explains general principles only. It does not replace a soil report, and it does not replace the recommendations of the project's geotechnical consultant and structural engineer.
The report directly affects the quantity takeoff for excavation, fill, piles and drainage. So the quantity calculation file should state which report, and which revision of it, it refers to.
Frequently asked questions
Who prepares a soil report?
A soil and foundation consultant (geotechnical consultant), following a testing programme the consultant prepares. In many cases the consultant also stays with the project while the foundations are built, and approves them on site.
What is an SPT test?
A standard penetration test in a borehole: the blows needed to drive a sampler into the ground are counted. The result helps characterise the layers and goes into the soil profile in the report.
Is an old soil report still valid?
Some reports have a validity clause, for example three years, and a design change, such as an added basement or a change in the excavation level, usually requires an update. It is worth checking this with the geotechnical consultant before pricing.
Can a BOQ be prepared without a soil report?
Most of the bill can be prepared, but the lines that depend on the ground stay marked "missing" until the report arrives. More about the role on the page quantity surveyor for construction projects.
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Contact me for a quote* Created with AI for marketing, for illustration only. Not reviewed by Yaakov Avital and not professional advice. Every project must be checked on its own by a professional quantity surveyor and an engineer.