Ground penetrating radar technologies for the monitoring of highways

Kulizhnikov A.M. doctor of technical sciences, professor (GP RosdorNII); Beloserov A.A., leading engineer (GP RosdorNII),  postgraduate (Arkhangelslkiy GTU)

It is known that in different seasons of the year (spring, summer, autumn, winter) and in different sections of the road (in excavations, high and low embankments, taking into consideration different orientation to the parts of the world etc.) vary the action of the automobile loads as well as the influence of natural climatic factors on the road construction. In this case take place the so called calculated periods of year (spring, autumn) and the weakened sections of motor roads (for example, the sections of roads which are located in the lowered places and have low altitude of embankments or passing through excavations). Specifically at this time and here is observed high humidity of the soils of the road bed. In this case the strength and the density of the ground is lower, thus the resistibility of road constructions to automobile loads decreases.

In connection with that mentioned the monitoring after the current internal state of the road constructions is necessary. For example, the monitoring after a change in the humidity and the density of the ground, after the motion and the infiltration of underground waters, after the depth of freezing and thawing soils, the presence and the behavior of the local loosening in road bed and the lower coating. These data will be claimed, for example, with the design of repair work and also with the setting of the limitations of the passage of heavy automobile loads in the period of slush.

Many of the mentioned survey works can be executed with the application of the ground penetrating radar technologies. The operation of GPR is based on the use of classical principles of ground penetrating radar. By the transmitting antenna of instrument are emitted the pulses of the short duration, which have 2... 3 half-periods of quasi-harmonic signal and sufficiently wide radiation spectra. The pulse radiated into the investigated environment is reflected from the boundaries of the permittivity alterations, is received by wide band antenna, is amplified and represented in the form of radar gram on the display of notebook. By processed and interpreted radar gram are determined the types of materials and grounds, boundary layers, are revealed the zones of local loosening and infiltration of soils and so on.

With the survey works on motor roads we have used the ground penetrating radars with AB -150, AB -250, AB -400 and AB -1200 antenna units developed by “NII priborostroeniya” and OOO "LogiS" (Zhukovskiy city). In this case the AB -1200 antenna unit was used for monitoring of coating and upper layer of the base of pavement, and the AB -150, AB -250 and AB -400 antenna units were used for survey of lower layers of the base of road coating, soils of the earth bed and underlying mound of the basement. The depth of sounding with a change in the frequency of antennas from 1200 MHz to 150 MHz respectively varied from 0,5 to 20,0 m with the resolution from 0,01 to 0,35 m.

Depending on the form of survey works were assigned the methods of work procedures.

Monitoring inspections of the humidity of the soils on the extent of motor road. In this case continuous scanning lengthwise most frequently was carried out.  The length of the continuously written file varied from 200 m to 2... 3 km. In this case the characteristic sections of motor roads were marked, for example, the sections of low mounds, boundary of excavations, places of the culverts laying-out, defects on the coating, etc. In the sections which needed detailed inspection the scanning in the transverse direction was carried out.

This kind of monitoring was carried out on the main M-10 "Russia" highway in the territory of Tver (picture 1) and Novgorod regions. It is known that with the aid of the GPR with the single channel antenna unit it is possible to determine the humidity of soil in several cases:

• by radar gram with the known thickness of constructional layers of the road coating and soils of the earth bed;
• according to the dielectric permittivity in case of the presence of local objects (culverts, engineering communications) in the basement or body of the earth bed;
• with a change of the distance between the receiving and transmitting antennas.

In the bad section of motor road Moscow - Saint Petersburg (km 252+000... 254+500) that crosses the populated area were detected the very closely arranged local objects (pipes of underground communications) at depth 1,5... 2,0 m from the surface of the covering that made it possible to determine the humidity of the soils of the working layer of the earth bed.

Humidity was determined as follows:

• by radar gram first was determined the dielectric permittivity of the environment which was located higher than the local objects;
• on the basis of the dielectric permittivity was determined the humidity  of soils (by mass) on dependence E = 3,2 + 1,1 W; where E is the dielectric permittivity, W is the humidity of soil (by mass).

The values of humidity obtained by scanning corresponded sufficiently well to the results of the determination of the humidity of soil made simultaneously with the executed thermo weight method.

On the graphs of a change of the humidity of soils along the length of the motor road (piń. 1) are given the results of the determination of the humidity with the description of marks. Points on the graph are the sections of the roads in the longitudinal section on which was determined the humidity of soil. The analysis of the indicated graphs make it possible to conclude that in the places of the increased humidity of soils are fixed sags, track, cracks, dents in the covering of the road.

The soil of the earth bed and the underlying soils are here composed by the plastic sandy loam which has humidity of soil on the boundary of fluidity 21,8 %. Consequently, in those sections where the humidity of soil already exceeds 0,7 from the humidity on the boundary of fluidity (15,3 % by mass) it is necessary to perform work for its decrease. Alongside the 2,5 km investigated in the fall of 2003 were revealed 5 sections of the roads (with a total length of 250 m) on which the humidity exceeded 15,3%.

Interesting results were obtained also in the airport "Domodedovo" (pic. 2).  In the work took part also Kamenetskiy L.B.. The built cartogram makes it possible to define the sections of different humidity of sands both in the depth and along the length of airport construction. As our studies showed, one of the basic reasons for crack formation on the surface of concrete slabs were the drainages put not under the joints of slabs but on their middle.

Monitoring observations of a change in the humidity of soils in the typical sections within the annual cycle. For the observations of the road constructions within the annual cycle were selected the most typical sections of motor roads (mounds in the leading marks, the typical road soil of the earth bed and underlay basement of the mound, the most frequently met conditions of moistening etc.) and also the trouble sections where is clearly present the destruction of road constructions without obvious reasons.

The length of the fixed section was 20... 50 m lengthwise to the entire width of the earth bed. Were recorded the continuous files by length 20... 50 m with drawing of GPR lengthwise along the right and left edges of roadway and if movement conditions allowed along the axis. With towing of GPR in the transverse direction under the specific operating conditions the record was carried out only in the center of section, in the unfavorable – with several passages of GPR with the assigned pace 2... 5 m. For purposes of an increase in the accuracy of interpretation before beginning monitoring works in each section one control boring with the sampling of soil was carried out.

The following indices were determined at the stage of interpretation: the travel speed and the permittivity of different layers, the position of the level of ground waters and the boundary of freezing or thawing to the date of the measurements.

In the sections which are located in the trouble operating conditions were determined the zones of sag deformations and loosing soils,  the zones of the infiltration of surface and ground water, the zones of heterogeneous soils, the position of sliding curve in landslide sections, the zones of the over damped soils, sinkhole deformations and washouts.

During the year according to the results of GPR survey it is possible to determine a change in the humidity of the soils as well as calculate with the known materials and the thickness of the construction layers of the pavement the module of elasticity on the surface of the construction; by the position of the level of ground water in the different periods of year it is possible to make judgments about the work of surface drain and drainages etc.

The inspections of the constructions of the motor roads M-8 "Kholmogory" in the territory of the Archangel region were carried out in the period from August 2001 through June 2003. In the work participated also the graduate student S.N.Burda.

Within the long-lasting observation of the state of the test section of the motor road M-8 "Kholmogory" were apprehensionbasis of the true determination of the dielectric permittivity of the environment (see the table).

Table

Monitoring observations in the karsted sections were conducted in the section of motor road M-8 "Kholmogory" which passes along coast slope in the Vinogradovskiy district of the Archangel region. The large part of the territory of this section (length of 1,3 km) is formed by the carbonate rocks which facilitate the development of karsts. The apprehensions of the workers of the administration of "Arkhangelskavtodor" were based on a large quantity of the gap surface forms within the limits of the right-of-way and their possible influence on the technical state of the object.

For determining the changes in the elements of the construction of motor road and in the geological structure of the close areas the surveys were carried out in different periods of year. Continuous GPR longitudinal scanning was carried out in the entire section of the motor road on both lines of traffic and on the edges of the right-of-way as well as the detailed scanning on the surface of the road covering and on the curbs in the transverse directions with the pace 2 m.

1.5-year monitoring made it possible to reveal a number of the essential aspects which are not the subject to identification by traditional methods or identified by them, but at the last stage of its development, when events can acquire the irreversible nature.

A number of the basic conclusions is noted according to the results of works:

• the monitoring didn’t allow to reveal the substantial changes in the geological structure of the section during the period of the inspection. This tells us about the stability of karst and possibility of further trouble-free exploitation of the object in case of keeping up a number of operational recommendations;
• in the basement of the mound were discovered the massifs of limestone, and also sections vulnerable to erosion;
• In the body of the mound were discovered the slips of soil, zones of the sag of soil and zones with the lowered density of the material of the mounds (picture 3) confirmed by boring, carried out on the basis of the information obtained according to the results of the geophysical shaping.

In the zones of the sag of soil is fixed the anomaly of speed which we have interpreted as void. Moreover this phenomenon was observed independently of the season. The sizes of the discovered voids had an area in the plan up to 4 square meters and a thickness 0,3... 0,8 m. It was proposed to fill these voids with sand-cement solution. For purposes of a cessation of erosion was recommended the restoration of the system of longitudinal and transverse drain which would considerably decrease the probability of erosion.

The monitoring  of freezing and thawing of the ground. The GPR works have been executed in the sections of motor road Moscow-Archangel where the monitoring works were realized in 7 sections which are located in 3 districts of the Archangel region. The distance from the first section to the latter was 250 km. With the identical periods of closing the sections of roads, in the spring of 2002 it was determined that alongside 250 km the depth of the thawing of road construction varies from 0,9 m (northern section of road) to ,6 m (southern section of road) from the surface of the road covering. The depth of thawing which exceeded the thickness of operating layer made it possible to conclude that in the southern section it was already possible to open the traffic while according to the season time the traffic could be opened in 1,5 weeks.

By the depths of freezing or thawing of the grounds revealed with help of GPR methods it is possible to set the periods of the open traffic or to determine the correctness of the motor road non-act period. Moreover, by the depth of freezing it is possible to correctly estimate the frost resistance of pavement.

With monitoring of motor roads GPR technologies are highly productive, economically effective, reveal the true causes of the roads destruction and in no way render any unfavorable environmental effects. According to the results of the monitoring works the effective methods of repair works are assigned which increase the operational reliability of highways. Moreover after revealing sections with the increased humidity of the road bed soils of it is possible to give address recommendations regarding the securing of a water drainage and the draining of the road bed soils. Monitoring works make it possible even now to govern operationally the periods of closing and opening of the roads as well as limitations of load.