[{"data":1,"prerenderedAt":462},["ShallowReactive",2],{"blog-en-wrijvingsgetal-sondering-rf-uitleg":3},{"id":4,"title":5,"body":6,"date":452,"description":453,"extension":454,"meta":455,"navigation":456,"path":457,"readingMinutes":458,"seo":459,"stem":460,"__hash__":461},"blog_en\u002Fblog\u002Fwrijvingsgetal-sondering-rf-uitleg.md","CPT Friction Ratio (Rf): What It Tells You About Soil",{"type":7,"value":8,"toc":429},"minimark",[9,29,37,46,58,73,80,98,156,162,171,197,204,261,267,271,295,311,315,336,340,345,356,360,375,382,399,406,417],[10,11,12,13,17,18,21,22,25,26,28],"p",{},"The friction ratio R",[14,15,16],"sub",{},"f"," is the ratio of local sleeve friction f",[14,19,20],{},"s"," to cone resistance q",[14,23,24],{},"c",", expressed as a percentage. After q",[14,27,24],{},", it is the most important parameter for deriving soil type from a CPT (cone penetration test). A low friction ratio roughly indicates sandy layers, while a high friction ratio points to clayey or organic layers. In this article you will read how the friction ratio of a CPT is built up, how to interpret it and where to find it in a GEF file.",[30,31,33,34,36],"h2",{"id":32},"cpt-friction-ratio-how-rf-is-determined","CPT friction ratio: how R",[14,35,16],{}," is determined",[10,38,39,40,42,43,45],{},"During a CPT, the cone measures two things: the cone resistance q",[14,41,24],{}," at the tip and the local sleeve friction f",[14,44,20],{}," along the friction sleeve directly above the cone. The friction ratio follows from these two:",[10,47,48,49,51,52,54,55,57],{},"R",[14,50,16],{}," = f",[14,53,20],{}," \u002F q",[14,56,24],{}," × 100%",[10,59,60,61,63,64,66,67,72],{},"Because both values are in MPa, the unit cancels out and a percentage remains. The friction ratio is therefore not a separate measurement but a derived quantity. On a CPT plot it usually appears as the third curve next to q",[14,62,24],{}," and f",[14,65,20],{},", plotted against penetration length or against depth relative to NAP (the Dutch ordnance datum). If you want to revisit the basics first, read ",[68,69,71],"a",{"href":70},"\u002Fen\u002Fblog\u002Fwat-is-een-sondering","what a CPT is",".",[30,74,76,77,79],{"id":75},"what-do-high-and-low-rf-values-tell-you","What do high and low R",[14,78,16],{}," values tell you?",[10,81,82,83,85,86,88,89,91,92,94,95,97],{},"In sand, the friction on the sleeve is small relative to the high cone resistance, so R",[14,84,16],{}," is low. In clay, q",[14,87,24],{}," is low while the sleeve experiences relatively high friction, so R",[14,90,16],{}," is higher. In peat, q",[14,93,24],{}," is very low and R",[14,96,16],{}," is often at its highest. The general pattern:",[99,100,101,120],"table",{},[102,103,104],"thead",{},[105,106,107,111,116],"tr",{},[108,109,110],"th",{},"Soil type",[108,112,113,114],{},"q",[14,115,24],{},[108,117,48,118],{},[14,119,16],{},[121,122,123,135,146],"tbody",{},[105,124,125,129,132],{},[126,127,128],"td",{},"Sand",[126,130,131],{},"High",[126,133,134],{},"Low",[105,136,137,140,143],{},[126,138,139],{},"Clay",[126,141,142],{},"Low to moderate",[126,144,145],{},"Higher",[105,147,148,151,154],{},[126,149,150],{},"Peat",[126,152,153],{},"Very low",[126,155,131],{},[10,157,158,159,161],{},"This is an indication only. A silty sand or a sandy clay falls between these extremes, and the same R",[14,160,16],{}," can belong to different soil types. A CPT therefore does not replace a borehole with a sample description.",[30,163,48,165,167,168,170],{"id":164},"rf-and-qc-together-the-robertson-classification",[14,166,16],{}," and q",[14,169,24],{}," together: the Robertson classification",[10,172,173,174,176,177,181,182,184,185,187,188,190,191,193,194,196],{},"The friction ratio only becomes truly useful in combination with q",[14,175,24],{},". The Robertson classification plots both in a chart and divides the data points into soil behaviour zones. You can read how this works in ",[68,178,180],{"href":179},"\u002Fen\u002Fblog\u002Fgrondsoorten-herkennen-sondering-robertson","identifying soil types with the Robertson classification",". For a Robertson indication you therefore need R",[14,183,16],{}," (or f",[14,186,20],{}," to calculate R",[14,189,16],{},") in addition to q",[14,192,24],{},". If the friction is missing from a file, only an indication based on q",[14,195,24],{}," alone remains.",[30,198,200,201,203],{"id":199},"what-should-you-watch-for-when-interpreting-rf","What should you watch for when interpreting R",[14,202,16],{},"?",[205,206,207,228,243,252],"ul",{},[208,209,210,219,220,222,223,225,226,72],"li",{},[211,212,213,214,216,217,72],"strong",{},"Low q",[14,215,24],{},", erratic R",[14,218,16],{}," You divide by q",[14,221,24],{},". At very low cone resistance, a small measurement inaccuracy in f",[14,224,20],{}," already causes large spikes in R",[14,227,16],{},[208,229,230,233,234,236,237,239,240,242],{},[211,231,232],{},"Thin layers."," The friction sleeve sits above the cone tip, so f",[14,235,20],{}," relates to a slightly different depth range than q",[14,238,24],{},". In thin sand or clay layers, R",[14,241,16],{}," is therefore less reliable.",[208,244,245,248,249,251],{},[211,246,247],{},"Transitions."," At the boundary between layers, R",[14,250,16],{}," can show a value that occurs in neither of the two layers.",[208,253,254,257,258,260],{},[211,255,256],{},"Missing or truncated data."," Check that f",[14,259,20],{}," has been recorded over the entire penetration length and that there are no missing values in the column.",[10,262,263,264,266],{},"Use R",[14,265,16],{}," as a trend over multiple measuring points, not as a single value at one depth.",[30,268,270],{"id":269},"finding-the-friction-ratio-in-a-gef-file","Finding the friction ratio in a GEF file",[10,272,273,274,278,279,63,281,283,284,286,287,290,291,72],{},"In a GEF file, the measured columns are described in the ",[275,276,277],"code",{},"#COLUMNINFO"," lines. There you can see which column is penetration length, cone resistance, local sleeve friction or friction ratio, and in which unit. Not every file contains all columns: some have only q",[14,280,24],{},[14,282,20],{},", others also R",[14,285,16],{}," and pore pressure u",[14,288,289],{},"2",". The structure of these headers is covered in ",[68,292,294],{"href":293},"\u002Fen\u002Fblog\u002Fgef-bestand-uitgelegd","the GEF file explained",[10,296,297,298,302,303,305,306,310],{},"In ",[68,299,301],{"href":300},"\u002Fen","CPT Converter"," you open a GEF file directly in your browser and see the plot with an indicative soil profile (Robertson, or based on q",[14,304,24],{}," alone). This is useful for quickly checking that the columns are what you expect. If you want the values in a spreadsheet, ",[68,307,309],{"href":308},"\u002Fen\u002Fblog\u002Fgef-openen-converteren-excel-csv","opening GEF and converting it to Excel, CSV or AGS4"," describes the options.",[30,312,314],{"id":313},"friction-ratio-in-old-paper-cpts","Friction ratio in old, paper CPTs",[10,316,317,318,320,321,323,324,326,327,331,332,72],{},"On a paper CPT, q",[14,319,24],{},", f",[14,322,20],{}," and R",[14,325,16],{}," appear as separate curves side by side. If you digitise such a CPT, check afterwards which columns are in the resulting file and whether they match the original. How to get from scan to file is described in ",[68,328,330],{"href":329},"\u002Fen\u002Fblog\u002Fsondering-digitaliseren-naar-gef","digitising an old CPT to GEF",". To find out whether an old CPT is usable for a renovation, read ",[68,333,335],{"href":334},"\u002Fen\u002Fblog\u002Foude-sonderingen-hergebruiken","reusing old CPTs",[30,337,339],{"id":338},"frequently-asked-questions","Frequently asked questions",[341,342,344],"h3",{"id":343},"what-is-the-friction-ratio-in-a-cpt","What is the friction ratio in a CPT?",[10,346,12,347,349,350,352,353,355],{},[14,348,16],{}," is the local sleeve friction f",[14,351,20],{}," divided by the cone resistance q",[14,354,24],{},", times 100%. It gives an indication of soil type.",[341,357,359],{"id":358},"is-a-high-friction-ratio-always-clay","Is a high friction ratio always clay?",[10,361,362,363,365,366,368,369,371,372,374],{},"No. A high R",[14,364,16],{}," points to clayey or organic soil, but measurement inaccuracy at low q",[14,367,24],{}," and layer transitions can also produce high values. Always assess R",[14,370,16],{}," together with q",[14,373,24],{}," and its variation with depth.",[341,376,378,379,381],{"id":377},"do-i-need-rf-for-a-pile-calculation","Do I need R",[14,380,16],{}," for a pile calculation?",[10,383,384,385,387,388,395,396,398],{},"The pile calculation relies mainly on q",[14,386,24],{},", as described in ",[68,389,391,392,394],{"href":390},"\u002Fen\u002Fblog\u002Fconusweerstand-qc-paalberekening","cone resistance q",[14,393,24],{}," and pile calculation",". R",[14,397,16],{}," mainly helps to identify the soil layers and to support the profile.",[341,400,402,403,405],{"id":401},"can-i-calculate-rf-myself-if-it-is-missing-from-my-gef","Can I calculate R",[14,404,16],{}," myself if it is missing from my GEF?",[10,407,408,409,167,411,413,414,416],{},"If f",[14,410,20],{},[14,412,24],{}," are in the file in the same unit, you can calculate R",[14,415,16],{}," with the formula above. If needed, export the file to Excel or CSV and calculate the column there.",[418,419,420],"cta",{},[10,421,422,425,426,428],{},[211,423,424],{},"Old CPT on paper?"," With ",[68,427,301],{"href":300}," you can turn it into a GEF file in a few minutes that any calculation software can read.",{"title":430,"searchDepth":431,"depth":431,"links":432},"",2,[433,435,437,439,441,442,443],{"id":32,"depth":431,"text":434},"CPT friction ratio: how Rf is determined",{"id":75,"depth":431,"text":436},"What do high and low Rf values tell you?",{"id":164,"depth":431,"text":438},"Rf and qc together: the Robertson classification",{"id":199,"depth":431,"text":440},"What should you watch for when interpreting Rf?",{"id":269,"depth":431,"text":270},{"id":313,"depth":431,"text":314},{"id":338,"depth":431,"text":339,"children":444},[445,447,448,450],{"id":343,"depth":446,"text":344},3,{"id":358,"depth":446,"text":359},{"id":377,"depth":446,"text":449},"Do I need Rf for a pile calculation?",{"id":401,"depth":446,"text":451},"Can I calculate Rf myself if it is missing from my GEF?","2026-10-23","CPT friction ratio: Rf is local sleeve friction divided by cone resistance. Learn what high and low values mean and how to find Rf in a GEF file.","md",{},true,"\u002Fblog\u002Fwrijvingsgetal-sondering-rf-uitleg",11,{"title":5,"description":453},"blog\u002Fwrijvingsgetal-sondering-rf-uitleg","TNNQuOcNmz4_R6LkeyuFCxAS0LpDfQoyJ6zQTCclQR4",1792756834799]