Back to the roots: The reliability of a pullout test for assessing the erosion resistance of grassed dike covers

Rens van der Meijden

Research output: Thesis β€Ί EngD Thesis β€Ί Academic

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Abstract

Grass cover erosion by wave overtopping is potentially a major failure mechanism for earthen dikes. Grass
cover erosion resistance (quantified by the critical velocity, π‘ˆπ‘ ) can be assessed using full-scale, destructive
tests with the wave overtopping simulator (WOS) in combination with the erosion model β€˜cumulative
overload method’ (COM). Although these tests provide valuable information, they are relatively expensive
and time consuming. Therefore, a small-scale grass pullout test (GPT) may be an attractive alternative.
With this method, multiple (typically 30) pullout tests within a test plot of roughly 10 m x 15 m are done,
after which the maximum pullout force for each test is translated to a representative tensile strength
parameter. This is currently the critical normal stress at the ground level, πœŽπ‘”π‘Ÿπ‘Žπ‘ π‘ ,𝑐 (0). From all tests
performed, a probabilistic design value for the weakest spot within the test plot is translated to the π‘ˆπ‘ via
a dedicated formula.
Given its easy execution and the relatively straightforward calculation of π‘ˆπ‘ , the GPT, developed in 2015,
is considered a quick and easy way to gain insight into the erosion resistance of a grass cover. However,
apart from five initial test sites with grass on clay, the π‘ˆπ‘ obtained with the GPT did not correspond well
with the π‘ˆπ‘ obtained with the WOS for grass on sand. Moreover, since pinpointing the cause for observed
discrepancies has been difficult and the relation between πœŽπ‘”π‘Ÿπ‘Žπ‘ π‘ ,𝑐 (0) and determinant vegetation
properties (e.g. root mass) has not been strong, the consistency of the method may be questioned.
This EngD project comprises a reliability analysis and subsequent improvement of the GPT as an estimator
for the π‘ˆπ‘ of grass covers. The study concerns the consistency of the tensile strength, the accuracy of the
π‘ˆπ‘ , as well as a critical reflection on the methodological steps. If necessary, these methodological steps
may be adapted to improve the reliability of the GPT. The ultimate aim of the EngD project is to generalize
the applicability of the GPT for all existing types of grass covers, comprising grass on clay, grass on sand,
and species-rich covers.
First, the consistency was assessed via the explainability of the tensile strength using a logical (i.e. in line
with literature) set of soil and vegetation parameters. It was found that the pullout force of an intact grass
sod, πΉπ‘–π‘›π‘‘π‘Žπ‘π‘‘, is reasonably consistent with the natural variation of root mass. This was to a lesser extent
the case for πœŽπ‘”π‘Ÿπ‘Žπ‘ π‘ ,𝑐 (0), which is derived through dividing πΉπ‘–π‘›π‘‘π‘Žπ‘π‘‘ by the surface area of the pulled grass
sod.
Next, the accuracy of the π‘ˆπ‘ was assessed via the correspondence between the π‘ˆπ‘ obtained with the GPT
and the WOS. This was done by comparing the π‘ˆπ‘ 𝑠 of the specific sites where both methods were applied
and by comparing the π‘ˆπ‘ distributions (i.e. mean and standard deviation) constructed based on all sites
for a specific grass cover type. There appears to be a significant discrepancy between the current GPT and
the WOS, whereby the GPT typically underestimates the π‘ˆπ‘ obtained with the WOS for all considered grass
cover types.
Subsequently, via a reflection on the steps in the current GPT method, several design adaptations were
proposed. Three design variants, comprising different combinations of the proposed design adaptations,
were compared by the correspondence between the estimated π‘ˆπ‘ and the π‘ˆπ‘ obtained with the WOS.
The preferred variant gave the highest accuracy for the site-specific π‘ˆπ‘ as well as for the π‘ˆπ‘ distributions.
Compared to the current GPT method, the adaptations of the selected variant consist of: the inclusion of
the self-weight of the pulled grass sod in the derivation of πΉπ‘–π‘›π‘‘π‘Žπ‘π‘‘, the use of (depending on the sample
skewness) either the 1st, 2.5th or 5th percentile of the sample distribution of πΉπ‘–π‘›π‘‘π‘Žπ‘π‘‘ as a design value for
the weakest spot, and the use of an adapted formula which directly translates πΉπ‘–π‘›π‘‘π‘Žπ‘π‘‘ to a π‘ˆπ‘ .
Finally, the methodology of the selected variant was further simplified by determining the minimum
number of tests required for a sufficiently accurate estimate of the π‘ˆπ‘ per test plot. It was found that at
least a sample size of n=20 is required to obtain a comparable consistency of the tensile strength (i.e.
πΉπ‘–π‘›π‘‘π‘Žπ‘π‘‘) and accuracy of the π‘ˆπ‘ as for n=30.
Ultimately, the adapted GPT method proposed in this EngD project provides a more accurate π‘ˆπ‘ for all
considered grass cover types (i.e. grass on clay, grass on sand, and species-rich covers) compared to the
current GPT method. Furthermore, due to its direct translation of πΉπ‘–π‘›π‘‘π‘Žπ‘π‘‘ to π‘ˆπ‘ , the results can be better
explained using the environmental (root) characteristics of a site. Moreover, due to the simplified
procedure of the adapted GPT method, the duration of the physical field tests is shorter than for the
current GPT method.
Original languageEnglish
Awarding Institution
  • University of Twente
Supervisors/Advisors
  • Augustijn, Dionysius C.M., Supervisor
  • Warmink, Jord Jurriaan, Co-Supervisor
Award date6 Jun 2024
Place of PublicationEnschede
Publisher
Print ISBNs978-90-365-6186-0
Electronic ISBNs978-90-365-6187-7
DOIs
Publication statusPublished - 6 Jun 2024

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