Stunning increases in unpaid loans by local contractors and foreign borrowers have more than tripled the volume of "problem loans" at Riggs National Bank in the past year and wiped out any growth in profits for Washington's biggest bank.

Riggs lost more than $5.1 million in interest last year because big borrowers failed to pay on time, leaving the bank holding more than $103 million in what bankers call "past due and nonperforming assets."

Unpaid loans to contractors are 30 times what they were a year ago, and past due foreign loans total 20 times as much as last year when Riggs' bad loans amounted to $29 million, the banks' financial reports show.

As a result, Riggs problem-loan ratio today is more than twice that of the average bank its size and three times as high as that of American Security Corp., its closest rival in Washington banking.

The dramatic deterioration in the bank's loan portfolio is not mentioned until page 24 of Riggs annual report to shareholders, which was made public yesterday.

Chairman of the Board Joe L. Allbritton says nothing about the growth in bad loans in his glowing report to stockholders on what has happened since he bought control of Riggs a year ago. According to Allbritton, Riggs "is strongly positioned to maintain and expand our traditional role of preeminence in Washington banking."

In fact the slightly smaller American Security bank made more money than Riggs in 1981 because Riggs' profits increased only $11,000 despite a $550 million growth in the amount of money it manages.

The $5.1 million in interest that Riggs lost due to bad loans was $4 million more than the loss in the previous year and appeared to be the main reason the bank's profits failed to grow.

Even if Riggs has additional losses on the problem loans, bank records indicate that the institution's stability would not be affected because of money set aside to cover potential losses.

When asked, a spokesman for the bank said the jump in problem loans is "absolutely not" related to the change in control of the bank that occurred last year when Texas financier Allbritton purchased more than 40 percent of Riggs' stock from former chairman Jorge Carnicero and other investors. Allbritton's personal press representative, George Beveridge, could not be reached for comment.

The report to shareholders says the increase in overdue residential construction loans is a "result of the current economic environment." Past due foreign loans soared "as a result of changes in the international political and economic climate."

Virtually all of the increase in problem loans resulted from business borrowing, because the volume of past due and otherwise delinquent consumer loans went down slightly during the year.

"The big thing is residential construction," where the amount of loans on which the bank has stopped trying to collect interest jumped from $938,000 to $30.8 million in one year, said Vincent C. Burke, chairman of the executive committee.

"Historically our bank has been very large in residential construction lending. We anticipate little, if any, loss on that $31 million because it is all fully secured."

The Riggs annual report discloses dramatic increases in four different kinds of borrowing problems, ranging from overdue loans to property on which the bank had foreclosed.

A year ago, Riggs was holding $4.98 million worth of real estate acquired through foreclosure because the owners could not repay their loans. During 1981 the value of the real estate the bank had to take over jumped 124 percent to $11.2 million. The percentage increase is big but "the amount of money is still small," Burke said.

Riggs' latest report shows $21.4 million in past due foreign loans, $20.3 million more than the $1.1 million the bank had a year earlier. Burke said most of the overdue foreign loans are guaranteed by the U.S. government.

What Riggs calls "nonperforming assets" have jumped from $19.8 million to $64.3 million in the past year. That figure includes $31.7 million of loans on which the bank is not collecting interest, another $21.3 million on which the bank has given lenders more time to pay and the $11.2 million in real estate repossessed by foreclosure.

Another $39.2 million in other loans that are past due but have not been written off bring the bank's total overdue lending to more than $100 million, up from $29 million in just one year.

The "nonperforming" loans amount to 3.8 percent of Riggs' outstanding loans, twice the average 1.85 percent problem loan ratio of banks its size. The problem loan rate is 1.16 percent at American Security, which has $19.3 million of loans in that category.