When the investor gives a new piece of land for design, the engineer must advise the owner what he needs to know from that land. The result of the geological survey is an indispensable input for the design documents. Without this data, it is like a blind man touching an elephant, and the law does not allow it. The name of that job is Survey Task Preparation. As a job, there are many skills involved. Here, we will summarize them into a topic for easy practice.
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Minimum knowledge
– Save money💰 for the investor
so that the survey volume is just enough for technical requirements, do not do more. Unless the boss has extra money!
– Knowledge of Soil Mechanics, foundation
– Survey experience: common practical work in the market that survey contractors can do
– Experience in foundation design
– Standards: live and work according to the Law
Step 1: Determine the scope of the survey
The scope is what to survey, how large or small the scale is.
Starting from the general architectural plan of the building, usually there are (many) main blocks and the exterior of the house. Outside the house there are often auxiliary works: power stations, underground tanks, …; infrastructure: yard, road, manhole …
The principle is that any item that needs a foundation (eg house), ground (eg road) must have data for design. That means it must be surveyed.
Step 2: Plan the type of foundation in mind
This step is based on experience, showing the professional feeling of the design engineer.
For example, a 20-storey building in Hanoi would be difficult to make a shallow foundation and immediately think of a pile foundation. But if the house is built on a mountain, a shallow foundation can be made like a rock. If it is a pile foundation, the pile tip must be in good soil. If the soil is not too deep, it is normal to use a driven pile, but if it is too deep, it is a bored pile. So we need deeper geological data.
Outdoor constructions often have less load, so the foundation is also shallower and smaller. In my head, I immediately thought that this part needs less survey than indoor constructions.
Soil and foundation data of neighboring constructions also constitute Experience. Collect data🔍. If you have never done any construction in that area, ask your colleagues, if you are stuck, ask Google.
Step 3: Determine the number of boreholes
To survey for depth data, the geologist must drill into the ground at the points required by the design.
Of course, the boreholes should not be made close together across the entire survey area. The minimum number of boreholes should be limited. The minimum number depends on the maximum distance Dmax between boreholes allowed by the Standard.
Dmax depends on the following factors:
– Type of proposed foundation (in step 2)
+ Shallow foundation: TCVN 9363:2012 Appendix D.
+ Pile foundation: TCVN 10304:2014 Section 5, Appendix D.
– Design stage to be carried out: basic design (TKCS), technical design (TKKT) or construction drawing design (TKBVTC). The basic design has a smaller quantity (larger Dmax)
– Is the upper load-bearing structure (expected in step 2) complicated, is the load large or small, is it evenly distributed or easily causes subsidence, …
– Reference data collected in step 2: is there a lot of information, does the information show that the geology of that area is complicated?
How complicated is not subjective but according to the standard: Appendix 2 TCVN 4419:1987 is divided into complexity levels I, II, III.
After determining the required standard Dmax corresponding to your project, draw circles with diameter Dmax on the total plan of the project. Draw until the area in all these circles covers the survey area. At that time, we have the minimum number of boreholes.
Practical examples
💎example 1: for shallow foundation design
The purpose of the survey for the TKBVTC stage should follow Appendix D of TCVN 9363:2012, table D.1. The construction site in Phu Quoc, based on information collection of surrounding structures with uncomplicated geology, the geological complexity level is level I. Refer to table D.1 for a building under 9 floors, the survey grid is 70x70m. That is, Dmax=70m.
Draw circles with a radius of 70m on the total ground as shown above. To cover all the works that need to be surveyed, the number of boreholes is 23.
💎Example 2: for pile foundation design
According to TCVN 10304:2014, Appendix F, the work is of level II importance (manufacturer).
Based on the surrounding geological data, according to Appendix D.1, the complexity of the ground conditions is type 1: the ground consists of many layers almost parallel to each other, within each layer the soil properties are homogeneous.
According to Table D.1, Dmax=50m. The designer considers that the number of boreholes is a bit too much, because the factory floor is very large while there are only 1-2 floors, the column foot load is not very large. Because Appendix D is not mandatory (with the word reference). Should be taken according to Appendix D.1 of TCVN 10304:2014 with column foot load, Dmax=70m.
Step 4: Determine the depth of the survey borehole
Similar to the Quantity section, the drilling depth also needs to be at least Lmin, just enough to meet the requirements of the standard. The general rule is that Lmin is greater than the compression depth of the foundation (the settlement stop position according to the calculation of the foundation settlement). This requires the knowledge and experience of the design engineer to judge. The standard also stipulates certain cases:
– Shallow foundation: TVCN 9363:2012 section 5.3.7.2, appendix C for the TKCS stage
Lmin is greater than the minimum settlement depth (compression zone) by 1-2m
– Pile foundation: TCVN 10304:2014 section 5.11
Because it is difficult to predict the settlement depth of the pile foundation at this stage (unless the engineer feels super), the standard stipulates that Lmin is taken from the expected pile tip depth Lcỏ, depending on the size of the pile cap (pile group): small, large cap, pile raft. Also do not forget to note that Lmin must penetrate the weak soil layers below.
Estimate the pile tip depth Lcỏ, so that the surveyor can visualize. Because while drilling, the standard penetration test (SPT) is also conducted at each depth. NSPT data can be used as an indicator for the surveyor about Lcỏ.
+ Driven/pressed piles: experience shows that Lcợp at NSPT>20 is good enough to stop the pile tip
+ Bored piles: the pile tip usually rests on gravel or very good soil layers, Lcợp is deeper than the starting point meeting NSPT=100 depending on the expected design load capacity for a pile.
+ Piles against rock:
TCVN 9363:2012 section 5.3.7.3. Must drill continuously in unweathered rock at least 3m or at least 5m from the time of drilling to touch the rock, if encountering weathered rock or Karst caves, continue drilling through.
– Basement excavation: TCVN 9363:2012 section 5.3.7.6
Usually Lmin is at least 2-3 times the depth of the excavation hole
The above shows that Lmin is not an absolute number of meters, but a relative number of how much deeper than the point “feeling the soil is good enough”. It also means that the depth of drilling is decided on site depending on the depth at which the point is encountered.
The best thing is to have survey results data of surrounding works. The survey depth of that data can be used as the basis for the absolute number of Lmin for the new work, helping the surveyor visualize more easily (and also make it easier to quote).
Return to the 2 examples above to determine the survey depth:
💎Example 1: shallow foundation
From experience in calculating settlement according to surrounding geology, with the construction load, settlement stops at a depth of 13m. According to TCVN 9363:2012 section 5.3.7.2, the survey drilling can be stopped at 15m (plus 2m more). To carefully control the NSPT value encountered at 15m to avoid encountering a weak soil layer below this depth causing adverse settlement, because the construction has a long basement. Using NSPT to control helps the surveyor understand the conditions for stopping drilling more easily.
💎Example 2: pile foundation
According to section 5.11 TCVN 10304:2014, drill at least 10m below the pile tip because the pile caps are expected to bear a load of no more than 3MN per column leg. The pile tip is specified when it begins to encounter NSPT≥30. So Lmin is equal to the starting point of NSPT≥30, down another 10m, the surveyor can stop drilling.
Based on the surrounding geological data, 15m is the starting point of NSPT≥30. Take 25m as a reference value for Lmin for the Survey Contractor to use as a basis for initial quotation in his survey outline.
Step 5: Parameters needed to derived from Tests
After finishing the 2 most difficult steps, which are determining the survey volume (quantity x depth). Now comes the physical indicators that the design engineer needs to test. Same principle💰, just do enough testing.
🗂Minimum indicators:
Every house must do it, including the results from laboratory testing (from samples taken directly from the borehole) and on-site, as shown in the table below
Note that it is very difficult to take samples of sandy soil layers in their original state to the laboratory for testing: just pull them up and the soil particles will fall apart. Therefore, the assessment of compactness, deformation characteristics, and strength should use SPT tests to infer which is more suitable for practice.
Some field tests always need to be done:
+ SPT standard penetration test: drill to where it is done, very convenient and cheap. But its results have many applications: to divide soil layers, determine the good or bad physical properties of the soil. It is also the basis for stopping the survey drilling at the current location or not (as in step 4).
+ Monitoring groundwater levels, measuring static water levels to provide information on surface water regime, stable groundwater levels, and aquifer locations. The measurement results are used for designing foundations, excavations, basement walls, and proposing measures to dry the bottom of the excavation for construction.
+ Experiments to determine soil resistance are performed in boreholes at different depths to provide information for lightning protection and grounding design of the Electrical Department.
🗂Optional indicators:
Design is required. Because the cost💰 is quite large, it is necessary to consider carefully when TCVN requires it.
For example, when a project has many basements and requires data for the excavation problem, it is necessary to add some more indicators than the minimum such as:
+ Water suction test from the borehole to determine the flow, permeability coefficient, hydraulic slope and the ability to generate hydrodynamic pressure… to serve the design of anti-retention and waterproofing for the wall and bottom of the excavation, the design and construction of lowering the groundwater level.
+ Triaxial compression test for soil samples in the laboratory, to determine the effective value of cohesion c and internal friction angle φ. The best is the CD test (drained consolidation), if it is more economical, it is also necessary to do CU (undrained consolidation).
+ Laboratory unconfined compression test with unloading – reloading diagram to determine deformation module indexes for Hardening Soil model if Plaxis software must be used for excavation problem.
Step 6: Write the survey task
This is the product of the thinking process of the above steps, the design consultant must establish it as a legal basis. From there, the Investor sends the Survey contractor to establish a Survey Outline (Outline is different from the Task, ahem), estimate the survey basket price, and conduct the Survey.
The task includes the written part (description) and the visual part (location, depth of the designated survey boreholes…)
All are in the file 🎁 attached with the examples above.
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👓Reference documents:
1. TCVN 4419:1987 Survey for construction – Basic principles
2. TCVN 9363:2012 Survey for construction – Geotechnical survey for high-rise buildings
3. TCVN 10304:2014 Pile foundation – Design standards
