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INTRODUCTION
In 2019–2022, a consortium of the Gdańsk University of Technology and the geotechnical contractor Budokop Geotechnika Sp. z o. o. (Leader) carried out the “DPDT-Auger” research project on screw displacement piles. An inspiration to launch the project was the development of a new prototype version of pile auger invented at the Gdańsk University of Technology by the main author of this article. The DPDT (Displacement Pile Drilling Tool) was patented in Poland as PL 235442 B1 [1] in 2020.
Screw displacement piles and columns are currently one of the most popular technologies for piling and soil improvement on account of their technical and economic advantages. However, their drawback is the high resistance encountered when screwing the auger into the ground. These resistances occur mainly in non-cohesive soils, making it difficult to obtain longer piles with deeper embedment in load-bearing layers and necessitating the use of high rotational power drilling rigs. In cohesive soils, on the other hand, resistances have lower values, but cause the generation of excessive pore water pressure and degradation of soil structure (mainly cohesion), which may result in the reduction of pile load capacity.
The prototype version of the DPDT auger was supposed to generate lower screwing resistance in non-cohesive soils and less disturbance to cohesive soil structure. These assumptions were initially checked and confirmed in model tests, even before the auger was patented. This is why the auger required verification on the natural scale (in the field conditions) and in terms of pile load capacities and Q-s characteristics. An additional advantage is the simplicity of the auger construction, Fig. 1, reducing the costs of its production and regeneration.
Figure 1.
Construction of the prototype DPDT drill auger according to Patent No. PL 235442 B1, [1]
In the research project, a prototype version of DPDT auger was tested in comparison with other displacement augers commonly used in practice, mainly SDP (Screw Displacement Pile). The project consisted of 7 stages. In Stage 1, augers and piles were tested on a model scale, as described, among others, in [2]. In Stages 2, 3 and 4, full-scale tests were performed on experimental field plots. Auger and pile tests were combined with in-situ and laboratory subsoil tests. Other project tasks (Stages 5, 6 and 7) included analysis of obtained test results and developing reliable methods for calculating and predicting bearing capacity, settlement characteristics and performance parameters, including FEM numerical methods (omitted in this paper but described in [3]) and the development of recommendations for control and acceptance of piling works.
This article focuses mainly on presenting the final results of the project, observed correlations and regularities as well as empirical methods developed for calculating and predicting the bearing capacity, settlement characteristics Q-s and the performance parameters of screw displacement piles.
FIELD TESTS OF AUGERS AND PILES
In the field investigations, 6 experimental plots were organized in several locations in northern Poland. In total, over 80 screw displacement piles were made and tested by Budokop Geotechnika. These included 24 research piles and the rest were anchor piles. The geotechnical subsoil structure on the plots was determined using exploratory boreholes, CPTU [4] and DMT [5] soundings and laboratory tests of soil samples (Project Stage 3). Stage 2 was devoted to testing the DPDT and DPDT-S augers (shortened length version) and traditional SDP auger in terms of resistance during penetration into the ground. For comparative purposes, approximately one half of the test and anchor piles were made with DPDT and DPDT-S augers and the other half with the SDP auger. Four test piles were made in each plot, which after obtaining full concrete strength, were subjected to static load tests (Stage 4). All three of the tested augers had a diameter of D = 0.40 m, Fig. 2.
Figure 2.
Displacement pile augers tested in the research project: a) DPDT auger, b) DPDT-S auger (shortened length version), c) SDP auger, [6]
Complete results of full-scale tests on screwing resistance are included in the Report [6] and partly in [7]. A comparative example of the resistances of DPDT, DPDT-S and SDP augers from one of the test plots (No. 2) is shown in the form of graphs in Fig. 3, where the torque is MT, the unit number of rotations is nR and the drilling times is tD. In the context of screwing resistance, field test results generally confirmed the results of model tests performed in Stage 1, described in [2]. When screwing into the ground, prototype DPDT and DPDT-S augers generated lower values of torques MT, while higher unit rotations number nR than the corresponding SDP auger. The differences described above became more visible when augers reached the lower, load-bearing subsoil layers, where the piles were ended. The increased number of generated rotations nR of DPDT and DPDT-S augers causes an extension of pile installation time by 30–40% when compared to SDP auger.
Figure 3.
A graphical comparison of the screwing parameters of DPDT, DPDT-S and SDP augers (Test plot No. 2)
Nevertheless, the the performance of prototype auger was assessed positively due to the reduced MT value. This advantage is the most important, as it enables achieving greater penetration depths and thus allows for making piles of greater lengths and greater load capacities. It also gives a greater guarantee of effective pile installation in various ground conditions.
Field static pile load tests were the subject of Stage 4. Tests were carried out with additional pile instrumentation, using vibrating wire extensometers. The aim was to determine the bearing capacity and Q-s characteristics with separate identification of the pile shaft and pile base resistances. Examples of comparative results of static pile tests conducted in experimental plot No. 2 are shown in Fig. 4. Both, these results and other experimental plots showed that DPDT (DPDT-S) and SDP piles are characterized by similar soil interaction parameters (capacity and Q-s characteristics).
Figure 4.
Comparative results of pile load tests from experimental plot No. 2, [6]
CORRELATIONS OBTAINED FROM FIELD RESEARCH AND DEVELOPED CALCULATION METHODS
The search for and determination of various correlations between the field test results of subsoil, augers and piles was the subject of Stage 5. This article describes the correlations between in the situ subsoil tests and pile bearing capacities and between the in situ subsoil tests and pile screwing resistance. Based on these correlations, computational and prognostic empirical methods for piles made with DPDT and SDP augers were also developed and presented. Due to their extensiveness and complexity, the correlations between the screwing pile auger resistances, the load capacities and the Q-s characteristics, as well as the corresponding calculation methods, have been described in a separate publication [8].
Correlations between CPT soundings and piles bearing capacity
Values of soil unit resistances along the pile shaft ts [kPa] and under the base qb [kPa] were determined from pile load tests. The aim was to find a correlation between the mentioned pile resistances and the resistance of the CPT cone qc. For this purpose, the representative (equivalent) cone resistances qcs and qcb were calculated according to the scheme shown in Fig. 5 (similar to the one proposed in [9, 10]). The values of qcs and qcb were calculated as the harmonic mean of the qc values read from the zones corresponding to the shaft and base of a given pile, with additional consideration of weighting factors (formulas in Fig. 5). In the case of layered soils (cohesionless with cohesive), the qcs values should be calculated separately for each layer (for more, see [6]).
Figure 5.
Scheme for determining representative values of cone resistances qcs and qcb
Fig. 6 shows the correlation results with regard to the ultimate ts;ult and qb;ult resistances, which were determined by displacement criteria – for the shaft resistance ss;ult = 15 mm, and for the base resistance ss;ult = 40 mm (10 % of pile diameter D). The above correlations could be unified for piles made by using both DPDT and SDP auger types on account of their closeness, but had to be separated into cohesionless and cohesive soils, and into saturated and moist (unsaturated) soils. Very little research data was obtained in the case of base resistances for cohesive soils, hence the correlation for qb;ult/qcb in Fig. 6d is very approximate and subject to high uncertainty (R2 = 0.16). The discussed correlation was also described with the power function, mainly due to the preservation of the same description as in the other correlations shown in Fig. 6. In the future, this correlation will still need to be verified with further research results.
Figure 6.
Correlations between ultimate unit resistances of DPDT and SDP piles and representative CPT(U) cone resistances for cohesionless and cohesive soils
Empirical formulas were created to calculate the ultimate unit resistances ts;ult and qb;ult of the soil based on representative cone resistances qcs and qcb. These formulas, which are listed in Tab. 1, can be used for an engineering purposes to calculate the pile load capacity in accordance with EC7 recommendations [11].
Correlations between DMT soundings and piles bearing capacity
Similar correlations as those described in section 3.1. but between the soil resistances ts and qb and the resistances measured on the membrane of the Marchetti dilatometer (DMT, [4]) were also analysed. In this case, representative parameter Δp [kPa] was adopted, calculated as the difference between resistances p1 and p0 measured directly on the dilatometer membrane during soil testing (Δp = p1 – p0). As for cone resistance qc, a scheme for calculating the representative Δps and Δpb values presented in Fig. 7 was adopted. These values should also be calculated as harmonic means and in the case of a stratified subsoil, separately for cohesionless and cohesive soil layers (for more, see [6]).
Figure 7.
Scheme for determining representative values of DMT membrane resistances ps and pb
The results of the correlation between the ultimate unit resistances of pile ts;ult and qb;ult and the representative resistances Δps and Δpb of the dilatometer membrane are shown in Fig. 8. They turned out to be very similar to those presented in Fig. 6, which refers to the resistances of a CPT cone qc. Again, these correlations could be unified for piles made with DPDT and SDP augers, but they have to be divided into cohesionless and cohesive soils as well as saturated and moist (unsaturated) soils.
Figure 8.
Correlations of ultimate unit resistances around DPDT and SDP piles and representative DMT membrane resistances
As in Figure 6, the qb;ult/ Δpb correlation in Figure 8d results from a very small amount of research data and will therefore need to be verified with further research tests.
Empirical formulas were created from the above correlations to calculate the ultimate unit resistances ts;ult and qb;ult of the soil. These formulas, which are listed in Tab. 2, can be used for engineering purposes to calculate the pile load capacity in accordance with EC7 recommendations [11].
The load capacity calculation of SDP and DPDT screw displacement piles is based on the standard EC7 formula [11]:
{R_{c;cal}} = {R_{s;cal}} + {R_{b;cal}} = \sum\limits_i {{A_{s;i}} \cdot {t_{s;ult,i}} + {A_b} \cdot {q_{b;ult}}}
where:
As;i, Ab, –pile shaft area in the “i” section and pile base area respectively
qb;ult, ts;ult,i – ultimate unit soil resistances under the base and along the pile shaft, determined from the formulas in Table 1 or Table 2.
List of formulas for calculating ultimate resistances ts;ult and qb;ult of the soil around DPDT and SDP piles, based on CPT(U) soundings
2) reference stress should be taken as pref = 1 kPa
Areas Ab and As should be calculated assuming the nominal diameter D of the pile-forming auger.
Q-s characteristics of screw displacement piles
From the results of field pile load tests, generalized formulas of t-z and q-z transfer functions were derived for calculating the Q-s characteristics of piles according to the proposal of Gwizdała [12]. The discussed power functions are presented below:
{t_s}\left( {{s_s}} \right) = {t_{s;ult}} \cdot {\left( {{{{s_s}} \over {{z_{s;f}}}}} \right)^{0.28}} \le {t_{s;ult}}\,\left[ {{\rm{kPa}}} \right],\,{z_{s;f}} = 15\,{\rm{mm}}{q_b}\left( {{s_b}} \right) = {q_{b;ult}} \cdot {\left( {{{{s_b}} \over {{z_{b;f}}}}} \right)^{0.50}} \le {q_{b;ult}}\,\left[ {{\rm{kPa}}} \right],\,{z_{b;f}} = 0,1D
The resistance values of ts;ult and qb;ult in the above formulas are obtained from calculation procedures presented in sections 3.1 or 3.2. The values of the exponents α= 0.28 and β= 0.50 were determined from statistical analyses, based on which it was established that they can be averaged and standardized for SDP and DPDT piles and for all soils. As a consequence, the transfer functions (2) and (3) can also be adapted to total pile resistances:
{Q_s}\left( {{s_s}} \right) = {Q_{s;ult}} \cdot {\left( {{{{s_s}} \over {{z_{s;f}}}}} \right)^{0.28}} \le {Q_{s;ult}}\,\left[ {{\rm{kPa}}} \right],\,{z_{s;f}} = 15\,{\rm{mm}}{Q_b}\left( {{s_b}} \right) = {Q_{b;ult}} \cdot {\left( {{{{s_b}} \over {{z_{b;f}}}}} \right)^{0.50}} \le {Q_{b;ult}}\,\left[ {{\rm{kPa}}} \right],\,{z_{b;f}} = 0.1D
In the presented method, the predicted pile settlement curve QN(s) in load-bearing soils is obtained. It is a sum of the shaft resistance Qs(s) and the base resistance Qb(s) (Fig. 9).
Figure 9.
An example of calculating and determining the Q-s characteristic of a DPDT pile
In engineering calculations, shortening of the pile shaft, of axial stiffness EA, can also be taken into account in a simplified way according to the formula:
{\delta _t}\left( s \right) = {{{Q_N}\left( s \right) + {Q_b}\left( s \right)} \over {2 \cdot EA}} \cdot {h_N}
where: hN –pile length in bearing soil layers (Fig. 9).
Correlations between the CPT sounding and the screwing resistance of DPDT and SDP pile augers
Finding correlations between the results of in situ subsoil tests and SDP, DPDT and DPDT-S screwing pile auger resistances was one of the most important project tasks, as it is an important technological issue. A certain proposal regarding the prediction of screwing resistance for the SDP auger in cohesionless soils based on the CPT cone resistance qc has already been presented in [9] and [13]. The value of their predicted MT torque was divided into components MTs and MTb and made dependent on several additional factors, such as the distance between helixes on the auger, the speed of auger penetration, the vertical force of pressure and the current value of the auger embedment in the bearing layer. Unfortunately, the proposal turned out to be too complicated for engineering and practical purposes.
Instead, connections were sought between the torque MT values generated during auger peneration and the only qc resistance of a CPT(U) cone. Other parameters of the auger screwing process, such as unit auger rotations number nR or the vertical pressure force QTv were omitted, whilst recognising that these values are related to the value of torque MT.
A direct comparison of the torque MT value with the cone resistance qc values at the same depths would be inappropriate. Therefore, the resistance qc had to be appropriately averaged and recalculated according to the diagram in Fig. 10 to obtain a substitute qcr graph, hereinafter referred to as a graph of dimensionless representative cone resistance. The qcr value is referenced to the same depth as the auger tip, but represents the qc resistances collected over the entire length of the auger. According to the diagram in Fig. 10, augers should be divided along their length into 2 or 3 sections with hi lengths, to which appropriate wi weights were assigned, determining the impact of individual sections on generating the resistance torque MT.
Figure 10.
Scheme for determining representative cone resistance values qcr from CPT probing graph and for individual pile auger types
In the conversion procedure, the actual qc graph should first be replaced by a step graph with qc;j values averaged over 0.5 m or 0.25 m segments. Then, the representative resistance values qcr;j should be calculated at subsequent depths every 0.5 m or 0.25 m according to the formula:
{q_{c;r;j}} = {{\sum\limits_i {{w_i} \cdot {q_{c;j;i}} \cdot {h_i}} } \over {1MPa \cdot \sum\limits_i {{w_i} \cdot {h_i}} }}
where:
qc;i;j – averaged cone resistance value qc over the hi length of the auger segment at zj auger penetration depth.
The torque values MT measured in the tests were linked (combined) with calculated representative cone resistances qcr, determining the correlations shown in Fig. 11. The results come only from measurements on the test piles at experimental plots, as CPT probing was carried out only in these pile locations. The correlation points presented in Fig. 11 show individual trend lines for SDP, DPDT and DPDT-S augers.
Figure 11.
Comparison of MT/qcr trend lines for all three augers from all experimental field plots
Formulas were created from the trend functions presented in Fig. 11 to calculate the torque MT values based on representative, dimensionless cone resistance values qcr. Due to the visible significant point dispersion in the graph, the formulas were multiplied for safety by a correction factor of 1.25.
The final versions of the derived formulas are shown below:
for SDP auger:
{M_T} = 49.5 \cdot {({q_{cr}})^{0.46}}
for DPDT auger:
{M_T} = 33.28 \cdot {({q_{cr}})^{0.56}}
for DPDT-S auger:
{M_T} = 39.63 \cdot {({q_{cr}})^{0.48}}
The above formulas are shown in graph forms in Fig. 12. Ultimate resistance values qcr for individual augers and for a pile drilling rig with a maximum torque MT;ult = 200 kNm were also marked.
Figure 12.
Derived MT dependencies on qc;r for the tested SDP, DPDT and DPDT-S pile augers
After performing calculations and preparing the qc;r;j and MT;j diagrams based on the actual (original) cone resistance qc from CPT sounding, one can approximately determine whether and to what depth it will be possible to install a displacement pile in a given place, using the given auger type and given pilling rig.
CONCLUSIONS
The results of the “DPDT-Auger” research project should be assessed positively, and therefore the project should be considered justified. The field research tests and analyses allow for the formulation of several conclusions, the most notable of which are as follows:
The prototype, proprietary DPDT and DPDT-S pile augers generate lower torque values MT than a standard SDP auger during penetration. However, the torque decrease takes place at the cost of an increased rotations number nR and an extended time of pile driving tD. The overall balance of screwing resistance should be considered favourable for the DPDT and DPDT-S augers, as the generation of a lower torque value allows for greater drilling depths and longer pile lengths, as well as for more successful crossing over stronger soil layers.
Piles made with SDP and DPDT (DPDT-S) augers have very similar interaction characteristics with various subsoil types. Statistically, the bearing capacity and stiffness of both pile types are comparable.
The research and results analyses carried out for the project enabled the detection and numerical definition of various correlations between the in situ subsoil tests and the results of the auger and pile tests. The derived correlations allowed the development of empirical computational and prognostic methods that can be used in the design and execution of screw displacement piles in practice. They have already been implemented by project consortium member, Budokop Geotechnika.
The correlations and calculation methods presented in this article refer to piles made with the D = 0.40 m shaped augers shown in Fig. 2. The proposed formulas may also be used in the case of other auger diameters, but only after appropriate recalculations (adjustments).
The presented calculation methods are relatively recent, preliminary versions. They are still in the development phase and in the future will certainly be subject to verification, correction or modification in accordance with subsequent research.
