The Key Differences
-
Purpose, and the level of digging you need to get to the bottom of things\
-
How deep and reliable the information you get is\
-
The level of intrusion needed to get the job done\
-
The types of risk they're designed to address\
-
What sort of outputs and documentation you can expect\
-
Their value in informing design and construction decisions
What's Involved in a Site Investigation
A site investigation is a process you go through to get a better understanding of what's going on beneath your feet - ie soil profile, strength and groundwater conditions. Its main aim is to try and reduce uncertainty, particularly when it comes to foundation design, drainage strategy and the like, as well as assessing ground-related risk. It's a factual exercise that focuses on what's actually there, rather than just someones opinion.
What's Involved in a Site Inspection
A site inspection is just a visual look around at the surface and what's easily visible - it's about topography, access constraints, what sort of drainage you have, and so on. It can identify potential issues, but unfortunately it can't give you any idea of what's going on beneath the surface. Its real value lies in the context it sets - giving you a general idea of what you might need to look out for.
Info You Can and Can't Get from Each Approach
A quick look around might suggest that you've got a load of made ground, but only a proper ground investigation can give you any idea of just how deep it goes and what it's made of. Similarly, surface water features might suggest drainage issues, but its the subsurface testing that gives you an idea of just how permeable the ground is - and what the groundwater's going to do. A visual assessment gives you a general idea of the risk - but it's the investigations that put a number on it
When a Site Inspection is Enough
A site inspection is probably all you need at the early stages of a project or when the level of risk is low - things like feasibility studies, picking a site, and doing an initial planning review.
When a Site Investigation is Required
You need a site investigation when the decisions you're making are heavily dependent on how the ground performs - like foundation design, checking for structural loading, assessing the risk of contamination, coming up with a drainage strategy that needs to know how permeable the ground is, and so on - or when the groundwater conditions are going to affect the stability of the excavation.
How Inspections and Investigations Work Together
The inspection phase helps to guide the investigation by flagging up any likely risks or constraints - then the investigation data goes and gives you a much deeper understanding of what's going on, and helps inform the design. Together they provide a balanced approach to managing risk and keeping costs in check.
Common Misconceptions
-
A site walkover tells you everything - no it doesn't, surface clues are nice to have but they're just that - clues\
-
You only need to go for an investigation when problems show up - more often than not the problems are down there in the ground and you can't see them\
-
Surface signs always reflect what's going on beneath - not necessarily, surface appearances can be deceptive and hide a completely different profile